This document covers technologies designed to produce ocean alkalinity enhancement (OAE) from: 

1. Mining and distribution of natural rock-based alkaline minerals, including olivine and other silicate rocks as well as limestone and other carbonate minerals, either in open ocean (ocean liming) or coastal environments (coastal enhanced weathering) (Meysman & Montserrat, 2017; Renforth & Henderson, 2017; Lenton et al., 2018; Köhler et al., 2013; Köhler et al., 2010).
2. Production of hydroxide minerals, including lime/slaked lime (Kheshgi, 1995) from thermal calcination and magnesium hydroxide from synthetic weathering of olivine (Scott et al., 2021) and distribution in the open ocean.
3. Accelerated weathering of limestone (AWL) in ex-situ reactors to sequester non-fossil, point source CO₂ emissions. Note that AWL requires a concentrated source of CO₂ in seawater because carbonate minerals are oversaturated in seawater at ambient CO₂ concentrations. 
4. Production and addition of hydrated carbonate minerals to seawater for increased alkalinity (Renforth et al., 2022)

In contrast to recent reports (Gagern et al., 2019; EFI, 2020; Rackley, 2020), we consider these four mineral-based pathways of OAE together because of the common upstream and downstream considerations necessary to accelerate the development and testing of OAE pathways.

We consider all electrochemical-based technologies for mCDR, including alkalinity enhancement, in a separate road map due to their separate set of upstream and downstream considerations for electrochemical processes.

We also do not consider enhanced rock weathering in terrestrial ecosystems here despite its similarities with coastal enhanced weathering and its potential for gigaton-scale CDR (Beerling et al., 2020). Enhanced rock weathering in agricultural fields presents its own set of obstacles and development needs, many of which are distinct from those of the marine pathways considered in these road maps.

This document covers technologies designed to produce ocean alkalinity enhancement (OAE) from: 

1. Mining and distribution of natural rock-based alkaline minerals, including olivine and other silicate rocks as well as limestone and other carbonate minerals, either in open ocean (ocean liming) or coastal environments (coastal enhanced weathering) (Meysman & Montserrat, 2017; Renforth & Henderson, 2017; Lenton et al., 2018; Köhler et al., 2013; Köhler et al., 2010).
2. Production of hydroxide minerals, including lime/slaked lime (Kheshgi, 1995) from thermal calcination and magnesium hydroxide from synthetic weathering of olivine (Scott et al., 2021) and distribution in the open ocean.
3. Accelerated weathering of limestone (AWL) in ex-situ reactors to sequester non-fossil, point source CO₂ emissions. Note that AWL requires a concentrated source of CO₂ in seawater because carbonate minerals are oversaturated in seawater at ambient CO₂ concentrations. 
4. Production and addition of hydrated carbonate minerals to seawater for increased alkalinity (Renforth et al., 2022)

In contrast to recent reports (Gagern et al., 2019; EFI, 2020; Rackley, 2020), we consider these four mineral-based pathways of OAE together because of the common upstream and downstream considerations necessary to accelerate the development and testing of OAE pathways.

We consider all electrochemical-based technologies for mCDR, including alkalinity enhancement, in a separate road map due to their separate set of upstream and downstream considerations for electrochemical processes.

We also do not consider enhanced rock weathering in terrestrial ecosystems here despite its similarities with coastal enhanced weathering and its potential for gigaton-scale CDR (Beerling et al., 2020). Enhanced rock weathering in agricultural fields presents its own set of obstacles and development needs, many of which are distinct from those of the marine pathways considered in these road maps.

This document covers technologies designed to produce ocean alkalinity enhancement (OAE) from: 

1. Mining and distribution of natural rock-based alkaline minerals, including olivine and other silicate rocks as well as limestone and other carbonate minerals, either in open ocean (ocean liming) or coastal environments (coastal enhanced weathering) (Meysman & Montserrat, 2017; Renforth & Henderson, 2017; Lenton et al., 2018; Köhler et al., 2013; Köhler et al., 2010).
2. Production of hydroxide minerals, including lime/slaked lime^[2]Kheshgi, H. S. (1995) ‘Sequestering atmospheric carbon dioxide by increasing ocean alkalinity’, Energy, 20(9), pp. 915–922. doi: https://doi.org/10.1016/0360-5442(95)00035-F. from thermal calcination and magnesium hydroxide from synthetic weathering of olivine^[3]Scott, A., Oze, C., Shah, V. et al. Transformation of abundant magnesium silicate minerals for enhanced CO₂ sequestration. Commun Earth Environ 2, 25 (2021). https://doi.org/10.1038/s43247-021-00099-6 and distribution in the open ocean.
3. Accelerated weathering of limestone (AWL) in ex-situ reactors to sequester non-fossil, point source CO₂ emissions. Note that AWL requires a concentrated source of CO₂ in seawater because carbonate minerals are oversaturated in seawater at ambient CO₂ concentrations. 
4. Production and addition of hydrated carbonate minerals to seawater for increased alkalinity^[12]Renforth, P., Baltruschat, S., Peterson, K., Mihailova, B. D., & Hartmann, J. (2022). Using ikaite and other hydrated carbonate minerals to increase ocean alkalinity for carbon dioxide removal and environmental remediation. Joule, 6(12), 2674-2679. https://doi.org/10.1016/j.joule.2022.11.001

In contrast to recent reports^[4]Gagern, A., Rau, G. and Rodriguez, D. I. (no date) ‘Ocean Alkalinity Enhancement: Current state of knowledge and potential role of philanthropy’, p. 50. ^[5]Energy Futures Initiative. “Uncharted Waters: Expanding the Options for Carbon Dioxide Removal in Coastal and Ocean Environments.” December 2020. ^[6]Rackley, S.A. (2020). Ocean alkalinity enhancement: A preliminary research agenda and technology maturation roadmap. CarbonActionNow! Working paper, November 2020. , we consider these four mineral-based pathways of OAE together because of the common upstream and downstream considerations necessary to accelerate the development and testing of OAE pathways.

We consider all electrochemical-based technologies for mCDR, including alkalinity enhancement, in a separate road map due to their separate set of upstream and downstream considerations for electrochemical processes.

We also do not consider enhanced rock weathering in terrestrial ecosystems here despite its similarities with coastal enhanced weathering and its potential for gigaton-scale CDR^[7]Beerling, D.J., Kantzas, E.P., Lomas, M.R. et al. Potential for large-scale CO₂ removal via enhanced rock weathering with croplands. Nature 583, 242–248 (2020). https://doi.org/10.1038/s41586-020-2448-9 . Enhanced rock weathering in agricultural fields presents its own set of obstacles and development needs, many of which are distinct from those of the marine pathways considered in these road maps.

This document covers technologies designed to produce ocean alkalinity enhancement (OAE) from: 

1. Mining and distribution of natural rock-based alkaline minerals, including olivine and other silicate rocks as well as limestone and other carbonate minerals, either in open ocean (ocean liming) or coastal environments (coastal enhanced weathering^[1]Meysman FJR, Montserrat F. (2017) Negative CO₂ emissions via enhanced silicate weathering in coastal environments. Biol. Lett. 13: 20160905. http://dx.doi.org/10.1098/rsbl.2016.0905 ^[8]Renforth, P., and G. Henderson (2017), Assessing ocean alkalinity for carbon sequestration, Rev. Geophys., 55, 636–674, doi:10.1002/2016RG000533. ^[9]Lenton (2018) Assessing carbon dioxide removal through global and regional ocean alkalinization under high and low emission pathways. Earth System Dynamics 9, 339-357. https://doi.org/10.5194/esd-9-339-2018. ^[10]Köhler P, Abrams J F, Völker C, Hauck J, andWolf-Gladrow D A (2013) Geoengineering Impact of Open Ocean Dissolution of Olivine on Atmospheric CO₂, Surface Ocean pH and Marine Biology. Environmental Research Letters 8(1), 014009. https://doi.org/10.1088/1748-9326/8/1/014009 ^[11]Köhler P, Hartmann J and Wolf-Gladrow D A (2010) Geoengineering potential of artificially enhanced silicate weathering of olivine. Proceedings of the National Academy of Sciences of the United States of America 107(47), 20228–20233. https://doi.org/10.1073/pnas.1000545107 .
2. Production of hydroxide minerals, including lime/slaked lime^[2]Kheshgi, H. S. (1995) ‘Sequestering atmospheric carbon dioxide by increasing ocean alkalinity’, Energy, 20(9), pp. 915–922. doi: https://doi.org/10.1016/0360-5442(95)00035-F. from thermal calcination and magnesium hydroxide from synthetic weathering of olivine^[3]Scott, A., Oze, C., Shah, V. et al. Transformation of abundant magnesium silicate minerals for enhanced CO₂ sequestration. Commun Earth Environ 2, 25 (2021). https://doi.org/10.1038/s43247-021-00099-6 and distribution in the open ocean.
3. Accelerated weathering of limestone (AWL) in ex-situ reactors to sequester non-fossil, point source CO₂ emissions. Note that AWL requires a concentrated source of CO₂ in seawater because carbonate minerals are oversaturated in seawater at ambient CO₂ concentrations. 
4. Production and addition of hydrated carbonate minerals to seawater for increased alkalinity^[12]Renforth, P., Baltruschat, S., Peterson, K., Mihailova, B. D., & Hartmann, J. (2022). Using ikaite and other hydrated carbonate minerals to increase ocean alkalinity for carbon dioxide removal and environmental remediation. Joule, 6(12), 2674-2679. https://doi.org/10.1016/j.joule.2022.11.001

In contrast to recent reports^[4]Gagern, A., Rau, G. and Rodriguez, D. I. (no date) ‘Ocean Alkalinity Enhancement: Current state of knowledge and potential role of philanthropy’, p. 50. ^[5]Energy Futures Initiative. “Uncharted Waters: Expanding the Options for Carbon Dioxide Removal in Coastal and Ocean Environments.” December 2020. ^[6]Rackley, S.A. (2020). Ocean alkalinity enhancement: A preliminary research agenda and technology maturation roadmap. CarbonActionNow! Working paper, November 2020. , we consider these four mineral-based pathways of OAE together because of the common upstream and downstream considerations necessary to accelerate the development and testing of OAE pathways.

We consider all electrochemical-based technologies for mCDR, including alkalinity enhancement, in a separate road map due to their separate set of upstream and downstream considerations for electrochemical processes.

We also do not consider enhanced rock weathering in terrestrial ecosystems here despite its similarities with coastal enhanced weathering and its potential for gigaton-scale CDR^[7]Beerling, D.J., Kantzas, E.P., Lomas, M.R. et al. Potential for large-scale CO₂ removal via enhanced rock weathering with croplands. Nature 583, 242–248 (2020). https://doi.org/10.1038/s41586-020-2448-9 . Enhanced rock weathering in agricultural fields presents its own set of obstacles and development needs, many of which are distinct from those of the marine pathways considered in these road maps.

This document covers technologies designed to produce ocean alkalinity enhancement (OAE) from: 

1. Mining and distribution of natural rock-based alkaline minerals, including olivine and other silicate rocks as well as limestone and other carbonate minerals, either in open ocean (ocean liming) or coastal environments (coastal enhanced weathering^[1]Meysman FJR, Montserrat F. (2017) Negative CO₂ emissions via enhanced silicate weathering in coastal environments. Biol. Lett. 13: 20160905. http://dx.doi.org/10.1098/rsbl.2016.0905 ^[8]Renforth, P., and G. Henderson (2017), Assessing ocean alkalinity for carbon sequestration, Rev. Geophys., 55, 636–674, doi:10.1002/2016RG000533. ^[9]Lenton (2018) Assessing carbon dioxide removal through global and regional ocean alkalinization under high and low emission pathways. Earth System Dynamics 9, 339-357. https://doi.org/10.5194/esd-9-339-2018. ^[10]Köhler P, Abrams J F, Völker C, Hauck J, andWolf-Gladrow D A (2013) Geoengineering Impact of Open Ocean Dissolution of Olivine on Atmospheric CO₂, Surface Ocean pH and Marine Biology. Environmental Research Letters 8(1), 014009. https://doi.org/10.1088/1748-9326/8/1/014009 ^[11]Köhler P, Hartmann J and Wolf-Gladrow D A (2010) Geoengineering potential of artificially enhanced silicate weathering of olivine. Proceedings of the National Academy of Sciences of the United States of America 107(47), 20228–20233. https://doi.org/10.1073/pnas.1000545107 .
2. Production of hydroxide minerals, including lime/slaked lime^[2]Kheshgi, H. S. (1995) ‘Sequestering atmospheric carbon dioxide by increasing ocean alkalinity’, Energy, 20(9), pp. 915–922. doi: https://doi.org/10.1016/0360-5442(95)00035-F. from thermal calcination and magnesium hydroxide from synthetic weathering of olivine^[3]Scott, A., Oze, C., Shah, V. et al. Transformation of abundant magnesium silicate minerals for enhanced CO₂ sequestration. Commun Earth Environ 2, 25 (2021). https://doi.org/10.1038/s43247-021-00099-6 and distribution in the open ocean.
3. Accelerated weathering of limestone (AWL) in ex-situ reactors to sequester non-fossil, point source CO₂ emissions. Note that AWL requires a concentrated source of CO₂ in seawater because carbonate minerals are oversaturated in seawater at ambient CO₂ concentrations. 
4. Production and addition of hydrated carbonate minerals to seawater for increased alkalinity^[12]Renforth, P., Baltruschat, S., Peterson, K., Mihailova, B. D., & Hartmann, J. (2022). Using ikaite and other hydrated carbonate minerals to increase ocean alkalinity for carbon dioxide removal and environmental remediation. Joule, 6(12), 2674-2679. https://doi.org/10.1016/j.joule.2022.11.001

In contrast to recent reports^[4]Gagern, A., Rau, G. and Rodriguez, D. I. (no date) ‘Ocean Alkalinity Enhancement: Current state of knowledge and potential role of philanthropy’, p. 50. ^[5]Energy Futures Initiative. “Uncharted Waters: Expanding the Options for Carbon Dioxide Removal in Coastal and Ocean Environments.” December 2020. ^[6]Rackley, S.A. (2020). Ocean alkalinity enhancement: A preliminary research agenda and technology maturation roadmap. CarbonActionNow! Working paper, November 2020. , we consider these four mineral-based pathways of OAE together because of the common upstream and downstream considerations necessary to accelerate the development and testing of OAE pathways.

We consider all electrochemical-based technologies for ocean-based CDR, including alkalinity enhancement, in a separate road map due to their separate set of upstream and downstream considerations for electrochemical processes.

We also do not consider enhanced rock weathering in terrestrial ecosystems here despite its similarities with coastal enhanced weathering and its potential for gigaton-scale CDR^[7]Beerling, D.J., Kantzas, E.P., Lomas, M.R. et al. Potential for large-scale CO₂ removal via enhanced rock weathering with croplands. Nature 583, 242–248 (2020). https://doi.org/10.1038/s41586-020-2448-9 . Enhanced rock weathering in agricultural fields presents its own set of obstacles and development needs, many of which are distinct from those of the marine pathways considered in these road maps.

This document covers technologies designed to produce ocean alkalinity enhancement (OAE) from: 

1. Mining and distribution of natural rock-based alkaline minerals, including olivine and other silicate rocks as well as limestone and other carbonate minerals, either in open ocean (ocean liming) or coastal environments (coastal enhanced weathering.
2. Production of hydroxide minerals, including lime/slaked lime from thermal calcination and magnesium hydroxide from synthetic weathering of olivine and distribution in the open ocean.
3. Accelerated weathering of limestone (AWL) in ex-situ reactors to sequester non-fossil, point source CO₂ emissions. Note that AWL requires a concentrated source of CO₂ in seawater because carbonate minerals are oversaturated in seawater at ambient CO₂ concentrations. 
4. Production and addition of hydrated carbonate minerals to seawater for increased alkalinity

In contrast to recent reports, we consider these three pathways of OAE together because of the common upstream and downstream considerations necessary to accelerate the development and testing of OAE pathways.

We consider all electrochemical-based technologies for ocean-based CDR, including alkalinity enhancement, in a separate road map due to their separate set of upstream and downstream considerations for electrochemical processes.

We also do not consider enhanced rock weathering in terrestrial ecosystems here despite its similarities with coastal enhanced weathering and its potential for gigaton-scale CDR. Enhanced rock weathering in agricultural fields presents its own set of obstacles and development needs, many of which are distinct from those of the marine pathways considered in these road maps.

This document covers technologies designed to produce ocean alkalinity enhancement (OAE) from: 

1. Mining and distribution of natural rock-based alkaline minerals, including olivine and other silicate rocks as well as limestone and other carbonate minerals, either in open ocean (ocean liming) or coastal environments (coastal enhanced weathering).
2. Production of hydroxide minerals, including lime/slaked lime from thermal calcination and magnesium hydroxide from synthetic weathering of olivine and distribution in the open ocean.
3. Accelerated weathering of limestone (AWL) in ex-situ reactors to sequester non-fossil, point source CO₂ emissions. Note that AWL requires a concentrated source of CO₂ in seawater because carbonate minerals are oversaturated in seawater at ambient CO₂ concentrations. 
4. Production and addition of hydrated carbonate minerals to seawater for increased alkalinity

In contrast to recent reports, we consider these three pathways of OAE together because of the common upstream and downstream considerations necessary to accelerate the development and testing of OAE pathways.

We consider all electrochemical-based technologies for ocean-based CDR, including alkalinity enhancement, in a separate road map due to their separate set of upstream and downstream considerations for electrochemical processes.

We also do not consider enhanced rock weathering in terrestrial ecosystems here despite its similarities with coastal enhanced weathering and its potential for gigaton-scale CDR. Enhanced rock weathering in agricultural fields presents its own set of obstacles and development needs, many of which are distinct from those of the marine pathways considered in these road maps.

This document covers technologies designed to produce ocean alkalinity enhancement (OAE) from: 

1. Mining and distribution of natural rock-based alkaline minerals, including olivine and other silicate rocks as well as limestone and other carbonate minerals, either in open ocean (ocean liming) or coastal environments (coastal enhanced weathering).
2. Production of hydroxide minerals, including lime/slaked lime from thermal calcination and magnesium hydroxide from synthetic weathering of olivine and distribution in the open ocean.
3. Accelerated weathering of limestone (AWL) in ex-situ reactors to sequester non-fossil, point source CO₂ emissions. Note that AWL requires a concentrated source of CO₂ in seawater because carbonate minerals are oversaturated in seawater at ambient CO₂ concentrations. 

In contrast to recent reports, we consider these three pathways of OAE together because of the common upstream and downstream considerations necessary to accelerate the development and testing of OAE pathways.

We consider all electrochemical-based technologies for ocean-based CDR, including alkalinity enhancement, in a separate road map due to their separate set of upstream and downstream considerations for electrochemical processes.

We also do not consider enhanced rock weathering in terrestrial ecosystems here despite its similarities with coastal enhanced weathering and its potential for gigaton-scale CDR. Enhanced rock weathering in agricultural fields presents its own set of obstacles and development needs, many of which are distinct from those of the marine pathways considered in these road maps.

This document covers technologies designed to produce ocean alkalinity enhancement (OAE) from: 

1. Mining and distribution of natural rock-based alkaline minerals, including olivine and other silicate rocks as well as limestone and other carbonate minerals, either in open ocean (ocean liming) or coastal environments (coastal enhanced weathering).
2. Production of hydroxide minerals, including lime/slaked lime from thermal calcination and magnesium hydroxide from synthetic weathering of olivine and distribution in the open ocean.
3. Accelerated weathering of limestone (AWL) in ex-situ reactors to sequester non-fossil, point source CO₂ emissions. Note that AWL requires a concentrated source of CO₂ in seawater because carbonate minerals are oversaturated in seawater at ambient CO₂ concentrations. 

In contrast to recent reports, we consider these three pathways of OAE together because of the common upstream and downstream considerations necessary to accelerate the development and testing of OAE pathways.

We consider all electrochemical-based technologies for ocean-based CDR, including alkalinity enhancement, in a separate road map due to their separate set of upstream and downstream considerations for electrochemical processes.

We also do not consider enhanced rock weathering in terrestrial ecosystems here despite its similarities with coastal enhanced weathering and its potential for gigaton-scale CDR. Enhanced rock weathering in agricultural fields presents its own set of obstacles and development needs, many of which are distinct from those of the marine pathways considered in these road maps.

This document covers technologies designed to produce ocean alkalinity enhancement (OAE) from: 

1. Mining and distribution of natural rock-based alkaline minerals, including olivine and other silicate rocks as well as limestone and other carbonate minerals, either in open ocean (ocean liming) or coastal environments (coastal enhanced weathering^[1]Meysman FJR, Montserrat F. (2017) Negative CO₂ emissions via enhanced silicate weathering in coastal environments. Biol. Lett. 13: 20160905. http://dx.doi.org/10.1098/rsbl.2016.0905 ).
2. Production of hydroxide minerals, including lime/slaked lime^[2]Kheshgi, H. S. (1995) ‘Sequestering atmospheric carbon dioxide by increasing ocean alkalinity’, Energy, 20(9), pp. 915–922. doi: https://doi.org/10.1016/0360-5442(95)00035-F. from thermal calcination and magnesium hydroxide from synthetic weathering of olivine^[3]Scott, A., Oze, C., Shah, V. et al. Transformation of abundant magnesium silicate minerals for enhanced CO₂ sequestration. Commun Earth Environ 2, 25 (2021). https://doi.org/10.1038/s43247-021-00099-6 and distribution in the open ocean.
3. Accelerated weathering of limestone (AWL) in ex-situ reactors to sequester non-fossil, point source CO₂ emissions. Note that AWL requires a concentrated source of CO₂ in seawater because carbonate minerals are oversaturated in seawater at ambient CO₂ concentrations. 

In contrast to recent reports^[4]Gagern, A., Rau, G. and Rodriguez, D. I. (no date) ‘Ocean Alkalinity Enhancement: Current state of knowledge and potential role of philanthropy’, p. 50. ^[5]Energy Futures Initiative. “Uncharted Waters: Expanding the Options for Carbon Dioxide Removal in Coastal and Ocean Environments.” December 2020. ^[6]Rackley, S.A. (2020). Ocean alkalinity enhancement: A preliminary research agenda and technology maturation roadmap. CarbonActionNow! Working paper, November 2020. , we consider these three pathways of OAE together because of the common upstream and downstream considerations necessary to accelerate the development and testing of OAE pathways.

We consider all electrochemical-based technologies for ocean-based CDR, including alkalinity enhancement, in a separate road map due to their separate set of upstream and downstream considerations for electrochemical processes.

We also do not consider enhanced rock weathering in terrestrial ecosystems here despite its similarities with coastal enhanced weathering and its potential for gigaton-scale CDR^[7]Beerling, D.J., Kantzas, E.P., Lomas, M.R. et al. Potential for large-scale CO₂ removal via enhanced rock weathering with croplands. Nature 583, 242–248 (2020). https://doi.org/10.1038/s41586-020-2448-9 . Enhanced rock weathering in agricultural fields presents its own set of obstacles and development needs, many of which are distinct from those of the marine pathways considered in these road maps.

This document covers technologies designed to produce ocean alkalinity enhancement (OAE) from: 

1. Mining and distribution of natural rock-based alkaline minerals, including olivine and other silicate rocks as well as limestone and other carbonate minerals, either in open ocean (ocean liming) or coastal environments (coastal enhanced weathering^[1]Meysman FJR, Montserrat F. (2017) Negative CO₂ emissions via enhanced silicate weathering in coastal environments. Biol. Lett. 13: 20160905. http://dx.doi.org/10.1098/rsbl.2016.0905 ).
2. Production of hydroxide minerals, including lime/slaked lime^[2]Kheshgi, H. S. (1995) ‘Sequestering atmospheric carbon dioxide by increasing ocean alkalinity’, Energy, 20(9), pp. 915–922. doi: https://doi.org/10.1016/0360-5442(95)00035-F. from thermal calcination and magnesium hydroxide from synthetic weathering of olivine^[3]Scott, A., Oze, C., Shah, V. et al. Transformation of abundant magnesium silicate minerals for enhanced CO₂ sequestration. Commun Earth Environ 2, 25 (2021). https://doi.org/10.1038/s43247-021-00099-6 and distribution in the open ocean.
3. Accelerated weathering of limestone (AWL) in ex-situ reactors to sequester non-fossil, point source CO₂ emissions. Note that AWL requires a concentrated source of CO₂ in seawater because carbonate minerals are oversaturated in seawater at ambient CO₂ concentrations. 

In contrast to recent reports^{[4]Gagern, A., Rau, G. and Rodriguez, D. I. (no date) ‘Ocean Alkalinity Enhancement: Current state of knowledge and potential role of philanthropy’, p. 50. [5]Energy Futures Initiative. “Uncharted Waters: Expanding the Options for Carbon Dioxide Removal in Coastal and Ocean Environments.” December 2020. [6]Rackley, S.A. (2020). Ocean alkalinity enhancement: A preliminary research agenda and technology maturation roadmap. CarbonActionNow! Working paper, November 2020.} , we consider these three pathways of OAE together because of the common upstream and downstream considerations necessary to accelerate the development and testing of OAE pathways. 

We consider all electrochemical-based technologies for ocean-based CDR, including alkalinity enhancement, in a separate road map due to their separate set of upstream and downstream considerations for electrochemical processes.

We also do not consider enhanced rock weathering in terrestrial ecosystems here despite its similarities with coastal enhanced weathering and its potential for gigaton-scale CDR^[7]Beerling, D.J., Kantzas, E.P., Lomas, M.R. et al. Potential for large-scale CO₂ removal via enhanced rock weathering with croplands. Nature 583, 242–248 (2020). https://doi.org/10.1038/s41586-020-2448-9 . Enhanced rock weathering in agricultural fields presents its own set of obstacles and development needs, many of which are distinct from those of the marine pathways considered in these road maps.

This document covers technologies designed to produce ocean alkalinity enhancement (OAE) from: 

1. Mining and distribution of natural rock-based alkaline minerals, including olivine and other silicate rocks as well as limestone and other carbonate minerals, either in open ocean (ocean liming) or coastal environments (coastal enhanced weathering).
2. Production of hydroxide minerals, including lime/slaked lime from thermal calcination and magnesium hydroxide from synthetic weathering of olivine and distribution in the open ocean.
3. Accelerated weathering of limestone (AWL) in ex-situ reactors to sequester non-fossil, point source CO₂ emissions. Note that AWL requires a concentrated source of CO₂ in seawater because carbonate minerals are oversaturated in seawater at ambient CO₂ concentrations. 

In contrast to recent reports^,,, we consider these three pathways of OAE together because of the common upstream and downstream considerations necessary to accelerate the development and testing of OAE pathways. 

We consider all electrochemical-based technologies for ocean-based CDR, including alkalinity enhancement, in a separate road map due to their separate set of upstream and downstream considerations for electrochemical processes.

We also do not consider enhanced rock weathering in terrestrial ecosystems here despite its similarities with coastal enhanced weathering and its potential for gigaton-scale CDR. Enhanced rock weathering in agricultural fields presents its own set of obstacles and development needs, many of which are distinct from those of the marine pathways considered in these road maps.

Addition of carbonate minerals to stabilize seawater carbonate chemistry has already been occurring in aquaculture facilities for ~10 years (Bruno & Boyd, 2011), largely as a response to the hatchery failures ~2010 in the US Pacific Northwest and British Columbia from ocean acidification (Washington State Blue Ribbon Panel on Ocean Acidification, 2012). The Chalk-Ex experiment released crushed calcium carbonate in a ~1.5 km² patch in the Gulf of Maine in the early 2000s. However, this experiment's primary objective was to quantify particles' effects on upper ocean layer optical properties, not to achieve CDR. There are now a number of ongoing field trials, with more planned in the future. OAE's TRL is currently around a 6 (TRL, RMI The Applied Innovation Roadmap for CDR). See the mCDR Field Trial Database for information on current field trials.

Addition of carbonate minerals to stabilize seawater carbonate chemistry has already been occurring in aquaculture facilities for ~10 years^[1]Giri, Bruno J., and Claude E. Boyd. “Effects of Frequent, Small Doses of Calcium Carbonate on Water Quality and Phytoplankton in Channel Catfish Ponds.” North American Journal of Aquaculture 62, no. 3 (2000): 225–28. http://dx.doi.org/10.1577/1548-8454(2000)062%3C0225:EOFSDO%3E2.3.CO;2 , largely as a response to the hatchery failures ~2010 in the US Pacific Northwest and British Columbia from ocean acidification^[2]Washington State Blue Ribbon Panel on Ocean Acidification (2012): Ocean Acidification: From Knowledge to Action, Washington State’s Strategic Response. H. Adelsman and L. Whitely Binder (eds). Washington Department of Ecology, Olympia, Washington. Publication no. 12-01-015. . The Chalk-Ex experiment released crushed calcium carbonate in a ~1.5 km² patch in the Gulf of Maine in the early 2000s. However, this experiment's primary objective was to quantify particles' effects on upper ocean layer optical properties, not to achieve CDR. There are now a number of ongoing field trials, with more planned in the future. OAE's TRL is currently around a 6 (TRL, RMI The Applied Innovation Roadmap for CDR). See the mCDR Field Trial Database for information on current field trials.

Addition of carbonate minerals to stabilize seawater carbonate chemistry has already been occurring in aquaculture facilities for ~10 years^[1]Giri, Bruno J., and Claude E. Boyd. “Effects of Frequent, Small Doses of Calcium Carbonate on Water Quality and Phytoplankton in Channel Catfish Ponds.” North American Journal of Aquaculture 62, no. 3 (2000): 225–28. http://dx.doi.org/10.1577/1548-8454(2000)062%3C0225:EOFSDO%3E2.3.CO;2 , largely as a response to the hatchery failures ~2010 in the US Pacific Northwest and British Columbia from ocean acidification^[2]Washington State Blue Ribbon Panel on Ocean Acidification (2012): Ocean Acidification: From Knowledge to Action, Washington State’s Strategic Response. H. Adelsman and L. Whitely Binder (eds). Washington Department of Ecology, Olympia, Washington. Publication no. 12-01-015. . There are now a number of ongoing field trials, with more planned in the future. OAE's TRL is currently around a 6 (TRL, RMI The Applied Innovation Roadmap for CDR). See the mCDR Field Trial Database for information on current field trials.

Addition of carbonate minerals to stabilize seawater carbonate chemistry has already been occurring in aquaculture facilities for ~10 years^[1]Giri, Bruno J., and Claude E. Boyd. “Effects of Frequent, Small Doses of Calcium Carbonate on Water Quality and Phytoplankton in Channel Catfish Ponds.” North American Journal of Aquaculture 62, no. 3 (2000): 225–28. http://dx.doi.org/10.1577/1548-8454(2000)062%3C0225:EOFSDO%3E2.3.CO;2 , largely as a response to the hatchery failures ~2010 in the US Pacific Northwest and British Columbia from ocean acidification^[2]Washington State Blue Ribbon Panel on Ocean Acidification (2012): Ocean Acidification: From Knowledge to Action, Washington State’s Strategic Response. H. Adelsman and L. Whitely Binder (eds). Washington Department of Ecology, Olympia, Washington. Publication no. 12-01-015. . But for large-scale OAE in open-ocean environments, current technological readiness is largely limited to laboratory experiments and/or modeling studies (~3-5 on a technological readiness scale^[3]TRL definitions: https://www.nasa.gov/directorates/heo/scan/engineering/technology/txt_accordion1.html ). There are now a number of ongoing field trials, with more planned in the future. See the mCDR Field Trial Database for information on current field trials.

Addition of carbonate minerals to stabilize seawater carbonate chemistry has already been occurring in aquaculture facilities for ~10 years^[1]Giri, Bruno J., and Claude E. Boyd. “Effects of Frequent, Small Doses of Calcium Carbonate on Water Quality and Phytoplankton in Channel Catfish Ponds.” North American Journal of Aquaculture 62, no. 3 (2000): 225–28. http://dx.doi.org/10.1577/1548-8454(2000)062%3C0225:EOFSDO%3E2.3.CO;2 , largely as a response to the hatchery failures ~2010 in the US Pacific Northwest and British Columbia from ocean acidification^[2]Washington State Blue Ribbon Panel on Ocean Acidification (2012): Ocean Acidification: From Knowledge to Action, Washington State’s Strategic Response. H. Adelsman and L. Whitely Binder (eds). Washington Department of Ecology, Olympia, Washington. Publication no. 12-01-015. . But for large-scale OAE in open-ocean environments, current technological readiness is largely limited to laboratory experiments and/or modeling studies (~3-5 on a technological readiness scale^[3]TRL definitions: https://www.nasa.gov/directorates/heo/scan/engineering/technology/txt_accordion1.html ). There are now a few completed mesocosm experiments and field trials, with more planned in the future. See the mCDR Field Trial Database for information on current field trials.

Addition of carbonate minerals to stabilize seawater carbonate chemistry has already been occurring in aquaculture facilities for ~10 years, largely as a response to the hatchery failures ~2010 in the US Pacific Northwest and British Columbia from ocean acidification. But for large scale OAE in open-ocean environments, current technological readiness is largely limited to laboratory experiments and/or modeling studies (~3-5 on a technological readiness scale). There are now starting to be a few completed mesocosm experiments and field trials, with more planned in the future.

• The Chalk-Ex experiment released crushed calcium carbonate in a ~1.5 km² patch in the Gulf of Maine, although the primary objective of this experiment was to quantify the effects of particles on upper ocean layer optical properties, not to achieve CDR. 
• There has been at least one small-scale (~50m x 50m) OAE field trial (sodium hydroxide addition) on a coral atoll in the Great Barrier Reef in order to evaluate the ecosystem responses to ocean acidification mitigation. 
• Vesta has a demonstration pilot of coastal enhanced weathering of olivine in Southampton, NY, adding 500 cubic yards of olivine sand to the second phase of a beach nourishment effort.  
• Researchers at the Universiteit Antwerpen and Delft University of Technology are planning a 1000 ton coastal enhanced weathering field experiment in the North Sea in 2024.
• Mesocosm experiments have been conducted as part of the OceanNETs project in the European Union’s Horizon 2020 program.

Addition of carbonate minerals to stabilize seawater carbonate chemistry has already been occurring in aquaculture facilities for ~10 years, largely as a response to the hatchery failures ~2010 in the US Pacific Northwest and British Columbia from ocean acidification. But for large scale OAE in open-ocean environments, current technological readiness is largely limited to laboratory experiments and/or modeling studies (~3-5 on a technological readiness scale). However, there have been at least two field studies completed and 3 others in planning. 

• The Chalk-Ex experiment released crushed calcium carbonate in a ~1.5 km² patch in the Gulf of Maine, although the primary objective of this experiment was to quantify the effects of particles on upper ocean layer optical properties, not to achieve CDR. 
• There has been at least one small-scale (~50m x 50m) OAE field trial (sodium hydroxide addition) on a coral atoll in the Great Barrier Reef in order to evaluate the ecosystem responses to ocean acidification mitigation. 
• Vesta has plans for a demonstration of coastal enhanced weathering of olivine in Southampton, NY, adding 500 cubic yards of olivine sand to the second phase of a beach nourishment effort. Vesta is currently working on another demonstration in Puerto Playa, Dominican Republic. 
• Researchers at the Universiteit Antwerpen and Delft University of Technology are planning a 1000 ton coastal enhanced weathering field experiment in the North Sea in 2024.
• Mesocosm experiments have been conducted as part of the OceanNETs project in the European Union’s Horizon 2020 program.

Addition of carbonate minerals to stabilize seawater carbonate chemistry has already been occurring in aquaculture facilities for ~10 years, largely as a response to the hatchery failures ~2010 in the US Pacific Northwest and British Columbia from ocean acidification. But for large scale OAE in open-ocean environments, current technological readiness is largely limited to laboratory experiments and/or modeling studies (~3-5 on a technological readiness scale). However, there have been at least two field studies completed and 3 others in planning. 

• The Chalk-Ex experiment released crushed calcium carbonate in a ~1.5 km² patch in the Gulf of Maine, although the primary objective of this experiment was to quantify the effects of particles on upper ocean layer optical properties, not to achieve CDR. 
• There has been at least one small-scale (~50m x 50m) OAE field trial (sodium hydroxide addition) on a coral atoll in the Great Barrier Reef in order to evaluate the ecosystem responses to ocean acidification mitigation. 
• Vesta is planning a coastal enhanced weathering of olivine field trial aimed at removing 100 tons of carbon dioxide on a beach in the northern Caribbean. 
• Researchers at the Universiteit Antwerpen and Delft University of Technology are planning a 1000 ton coastal enhanced weathering field experiment in the North Sea in 2024.
• Mesocosm experiments have been conducted as part of the OceanNETs project in the European Union’s Horizon 2020 program.

Addition of carbonate minerals to stabilize seawater carbonate chemistry has already been occurring in aquaculture facilities for ~10 years^[1]Giri, Bruno J., and Claude E. Boyd. “Effects of Frequent, Small Doses of Calcium Carbonate on Water Quality and Phytoplankton in Channel Catfish Ponds.” North American Journal of Aquaculture 62, no. 3 (2000): 225–28. http://dx.doi.org/10.1577/1548-8454(2000)062%3C0225:EOFSDO%3E2.3.CO;2 , largely as a response to the hatchery failures ~2010 in the US Pacific Northwest and British Columbia from ocean acidification^[2]Washington State Blue Ribbon Panel on Ocean Acidification (2012): Ocean Acidification: From Knowledge to Action, Washington State’s Strategic Response. H. Adelsman and L. Whitely Binder (eds). Washington Department of Ecology, Olympia, Washington. Publication no. 12-01-015. . But for large scale OAE in open-ocean environments, current technological readiness is largely limited to laboratory experiments and/or modeling studies (~3-5 on a technological readiness scale^[3]TRL definitions: https://www.nasa.gov/directorates/heo/scan/engineering/technology/txt_accordion1.html ). However, there have been at least two field studies completed and 3 others in planning. 

• The Chalk-Ex experiment released crushed calcium carbonate in a ~1.5 km² patch in the Gulf of Maine, although the primary objective of this experiment was to quantify the effects of particles on upper ocean layer optical properties, not to achieve CDR^[4]Balch, W. M., Plueddeman, A. J., Bowler, B. C., and Drapeau, D. T. (2009), Chalk‐Ex—Fate of CaCO3 particles in the mixed layer: Evolution of patch optical properties, J. Geophys. Res., 114, C07020, doi:10.1029/2008JC004902. . 
• There has been at least one small-scale (~50m x 50m) OAE field trial (sodium hydroxide addition) on a coral atoll in the Great Barrier Reef in order to evaluate the ecosystem responses to ocean acidification mitigation^[5]Albright, R., Caldeira, L., Hosfelt, J. et al. Reversal of ocean acidification enhances net coral reef calcification. Nature 531, 362–365 (2016). https://doi.org/10.1038/nature17155 . 
• Vesta is planning a coastal enhanced weathering of olivine field trial aimed at removing 100 tons of carbon dioxide on a beach in the northern Caribbean^[6]Vesta / The Plan, https://www.projectvesta.org/plan#Phase-1. . 
• Researchers at the Universiteit Antwerpen and Delft University of Technology are planning a 1000 ton coastal enhanced weathering field experiment in the North Sea in 2024^[7]Comments from Dr. Filip Meysman, ‘Workshop on Ocean-based CDR Opportunities and Challenges, Part 1: Setting the Stage’, A Research Strategy for Ocean Carbon Dioxide Removal and Sequestration, U.S. National Academy of Sciences. 19th January 2021. Accessible at: https://www.nationalacademies.org/event/01-19-2021/a-research-strategy-for-ocean-carbon-dioxide-removal-and-sequestration-workshop-series-part-1 .
• Field experiments are also in planning as part of the OceanNETs project in the European Union’s Horizon 2020 program^[8]“About the Project.” OceanNETs, https://www.oceannets.eu/about-the-project/. .

Addition of carbonate minerals to stabilize seawater carbonate chemistry has already been occurring in aquaculture facilities for ~10 years^[1]Giri, Bruno J., and Claude E. Boyd. “Effects of Frequent, Small Doses of Calcium Carbonate on Water Quality and Phytoplankton in Channel Catfish Ponds.” North American Journal of Aquaculture 62, no. 3 (2000): 225–28. http://dx.doi.org/10.1577/1548-8454(2000)062%3C0225:EOFSDO%3E2.3.CO;2 , largely as a response to the hatchery failures ~2010 in the US Pacific Northwest and British Columbia from ocean acidification^[2]Washington State Blue Ribbon Panel on Ocean Acidification (2012): Ocean Acidification: From Knowledge to Action, Washington State’s Strategic Response. H. Adelsman and L. Whitely Binder (eds). Washington Department of Ecology, Olympia, Washington. Publication no. 12-01-015. . But for large scale OAE in open-ocean environments, current technological readiness is largely limited to laboratory experiments and/or modeling studies (~3-5 on a technological readiness scale^[3]TRL definitions: https://www.nasa.gov/directorates/heo/scan/engineering/technology/txt_accordion1.html ). However, there have been at least two field studies completed and 3 others in planning. 

• The Chalk-Ex experiment released crushed calcium carbonate in a ~1.5 km² patch in the Gulf of Maine, although the primary objective of this experiment was to quantify the effects of particles on upper ocean layer optical properties, not to achieve CDR^[4]Balch, W. M., Plueddeman, A. J., Bowler, B. C., and Drapeau, D. T. (2009), Chalk‐Ex—Fate of CaCO3 particles in the mixed layer: Evolution of patch optical properties, J. Geophys. Res., 114, C07020, doi:10.1029/2008JC004902. . 
• There has been at least one small-scale (~50m x 50m) OAE field trial (sodium hydroxide addition) on a coral atoll in the Great Barrier Reef in order to evaluate the ecosystem responses to ocean acidification mitigation^[5]Albright, R., Caldeira, L., Hosfelt, J. et al. Reversal of ocean acidification enhances net coral reef calcification. Nature 531, 362–365 (2016). https://doi.org/10.1038/nature17155 . 
• Project Vesta is planning a coastal enhanced weathering of olivine field trial aimed at removing 100 tons of carbon dioxide on a beach in the northern Caribbean^[6]Vesta / The Plan, https://www.projectvesta.org/plan#Phase-1. . 
• Researchers at the Universiteit Antwerpen and Delft University of Technology are planning a 1000 ton coastal enhanced weathering field experiment in the North Sea in 2024^[7]Comments from Dr. Filip Meysman, ‘Workshop on Ocean-based CDR Opportunities and Challenges, Part 1: Setting the Stage’, A Research Strategy for Ocean Carbon Dioxide Removal and Sequestration, U.S. National Academy of Sciences. 19th January 2021. Accessible at: https://www.nationalacademies.org/event/01-19-2021/a-research-strategy-for-ocean-carbon-dioxide-removal-and-sequestration-workshop-series-part-1 .
• Field experiments are also in planning as part of the OceanNETs project in the European Union’s Horizon 2020 program^[8]“About the Project.” OceanNETs, https://www.oceannets.eu/about-the-project/. .

Addition of carbonate minerals to stabilize seawater carbonate chemistry has already been occurring in aquaculture facilities for ~10 years^[1]Giri, Bruno J., and Claude E. Boyd. “Effects of Frequent, Small Doses of Calcium Carbonate on Water Quality and Phytoplankton in Channel Catfish Ponds.” North American Journal of Aquaculture 62, no. 3 (2000): 225–28. http://dx.doi.org/10.1577/1548-8454(2000)062%3C0225:EOFSDO%3E2.3.CO;2 , largely as a response to the hatchery failures ~2010 in the US Pacific Northwest and British Columbia from ocean acidification^[2]Washington State Blue Ribbon Panel on Ocean Acidification (2012): Ocean Acidification: From Knowledge to Action, Washington State’s Strategic Response. H. Adelsman and L. Whitely Binder (eds). Washington Department of Ecology, Olympia, Washington. Publication no. 12-01-015. . But for large scale OAE in open-ocean environments, current technological readiness is largely limited to laboratory experiments and/or modeling studies (~3-5 on a technological readiness scale^[3]TRL definitions: https://www.nasa.gov/directorates/heo/scan/engineering/technology/txt_accordion1.html ). However, there have been at least two field studies completed and 3 others in planning. 

• The Chalk-Ex experiment released crushed calcium carbonate in a ~1.5 km² patch in the Gulf of Maine, although the primary objective of this experiment was to quantify the effects of particles on upper ocean layer optical properties, not to achieve CDR^[4]Balch, W. M., Plueddeman, A. J., Bowler, B. C., and Drapeau, D. T. (2009), Chalk‐Ex—Fate of CaCO3 particles in the mixed layer: Evolution of patch optical properties, J. Geophys. Res., 114, C07020, doi:10.1029/2008JC004902. . 
• There has been at least one small-scale (~50m x 50m) OAE field trial (sodium hydroxide addition) on a coral atoll in the Great Barrier Reef in order to evaluate the ecosystem responses to ocean acidification mitigation^[5]Albright, R., Caldeira, L., Hosfelt, J. et al. Reversal of ocean acidification enhances net coral reef calcification. Nature 531, 362–365 (2016). https://doi.org/10.1038/nature17155 . 
• Project Vesta is planning a coastal enhanced weathering of olivine field trial aimed at removing 100 tons of carbon dioxide on a beach in the Dominican Republic^[6]Project Vesta / The Plan, https://www.projectvesta.org/plan#Phase-1. . 
• Researchers at the Universiteit Antwerpen and Delft University of Technology are planning a 1000 ton coastal enhanced weathering field experiment in the North Sea in 2024^[7]Comments from Dr. Filip Meysman, ‘Workshop on Ocean-based CDR Opportunities and Challenges, Part 1: Setting the Stage’, A Research Strategy for Ocean Carbon Dioxide Removal and Sequestration, U.S. National Academy of Sciences. 19th January 2021. Accessible at: https://www.nationalacademies.org/event/01-19-2021/a-research-strategy-for-ocean-carbon-dioxide-removal-and-sequestration-workshop-series-part-1 .
• Field experiments are also in planning as part of the OceanNETs project in the European Union’s Horizon 2020 program^[8]“About the Project.” OceanNETs, https://www.oceannets.eu/about-the-project/. .

Addition of carbonate minerals to stabilize seawater carbonate chemistry has already been occurring in aquaculture facilities for ~10 years^[1]Giri, Bruno J., and Claude E. Boyd. “Effects of Frequent, Small Doses of Calcium Carbonate on Water Quality and Phytoplankton in Channel Catfish Ponds.” North American Journal of Aquaculture 62, no. 3 (2000): 225–28. https://doi.org/10.1577/1548-8454(2000)062<0225:EOFSDO>2.3.CO;2. , largely as a response to the hatchery failures ~2010 in the US Pacific Northwest and British Columbia from ocean acidification^[2]Washington State Blue Ribbon Panel on Ocean Acidification (2012): Ocean Acidification: From Knowledge to Action, Washington State’s Strategic Response. H. Adelsman and L. Whitely Binder (eds). Washington Department of Ecology, Olympia, Washington. Publication no. 12-01-015. . But for large scale OAE in open-ocean environments, current technological readiness is largely limited to laboratory experiments and/or modeling studies (~3-5 on a technological readiness scale^[3]TRL definitions: https://www.nasa.gov/directorates/heo/scan/engineering/technology/txt_accordion1.html ). However, there have been at least two field studies completed and 3 others in planning. 

• The Chalk-Ex experiment released crushed calcium carbonate in a ~1.5 km² patch in the Gulf of Maine, although the primary objective of this experiment was to quantify the effects of particles on upper ocean layer optical properties, not to achieve CDR^[4]Balch, W. M., Plueddeman, A. J., Bowler, B. C., and Drapeau, D. T. (2009), Chalk‐Ex—Fate of CaCO3 particles in the mixed layer: Evolution of patch optical properties, J. Geophys. Res., 114, C07020, doi:10.1029/2008JC004902. . 
• There has been at least one small-scale (~50m x 50m) OAE field trial (sodium hydroxide addition) on a coral atoll in the Great Barrier Reef in order to evaluate the ecosystem responses to ocean acidification mitigation^[5]Albright, R., Caldeira, L., Hosfelt, J. et al. Reversal of ocean acidification enhances net coral reef calcification. Nature 531, 362–365 (2016). https://doi.org/10.1038/nature17155 . 
• Project Vesta is planning a coastal enhanced weathering of olivine field trial aimed at removing 100 tons of carbon dioxide on a beach in the Dominican Republic^[6]Project Vesta / The Plan, www.projectvesta.org/plan#Phase-1. . 
• Researchers at the Universiteit Antwerpen and Delft University of Technology are planning a 1000 ton coastal enhanced weathering field experiment in the North Sea in 2024^[7]Comments from Dr. Filip Meysman, ‘Workshop on Ocean-based CDR Opportunities and Challenges, Part 1: Setting the Stage’, A Research Strategy for Ocean Carbon Dioxide Removal and Sequestration, U.S. National Academy of Sciences. 19th January 2021. Accessible at: https://www.nationalacademies.org/event/01-19-2021/a-research-strategy-for-ocean-carbon-dioxide-removal-and-sequestration-workshop-series-part-1 .
• Field experiments are also in planning as part of the OceanNETs project in the European Union’s Horizon 2020 program^[8]“About the Project.” OceanNETs, www.oceannets.eu/about-the-project/. .

Addition of carbonate minerals to stabilize seawater carbonate chemistry has already been occurring in aquaculture facilities for ~10 years, largely as a response to the hatchery failures ~2010 in the US Pacific Northwest and British Columbia from ocean acidification. But for large scale OAE in open-ocean environments, current technological readiness is largely limited to laboratory experiments and/or modeling studies (~3-5 on a technological readiness scale). However, there have been at least two field studies completed and 3 others in planning. 

• The Chalk-Ex experiment released crushed calcium carbonate in a ~1.5 km² patch in the Gulf of Maine, although the primary objective of this experiment was to quantify the effects of particles on upper ocean layer optical properties, not to achieve CDR. 
• There has been at least one small-scale (~50m x 50m) OAE field trial (sodium hydroxide addition) on a coral atoll in the Great Barrier Reef in order to evaluate the ecosystem responses to ocean acidification mitigation. 
• Project Vesta is planning a coastal enhanced weathering of olivine field trial aimed at removing 100 tons of carbon dioxide on a beach in the Dominican Republic. 
• Researchers at the Universiteit Antwerpen and Delft University of Technology are planning a 1000 ton coastal enhanced weathering field experiment in the North Sea in 2024.
• Field experiments are also in planning as part of the OceanNETs project in the European Union’s Horizon 2020 program.

1. Carbon Capture

   Methods of OAE that rely on depositing minerals into the ocean to promote additional CO₂ uptake depend on how quickly the minerals dissolve relative to how quickly they sink (Fakhraee et al., 2023). Due to the vast quantities of alkaline rocks, and the capacity of the ocean to accommodate similarly vast quantities of bicarbonate ions, OAE has the theoretical potential to capture tens of gigatons of carbon dioxide annually^[1]Gagern, Antonius. “Ocean Alkalinity Enhancement: Current state of knowledge and potential role of philanthropy”. 9 September 2019. Meeting Proceedings Half Moon Bay, California. . However, given current technological readiness, this remains theoretical. Recent consensus reports cite the following as possible carbon dioxide removal potential from ocean alkalinity enhancement:

   • >0.1 – 1.0 Gt CO₂/year (NASEM 2022)
   • 1 – 15+ Gt CO₂/ year (NOAA 2023)

Technical, economic, social, political, and governance factors may also decrease this theoretical CDR potential, although the degree to which they limit the CDR remains to be determined. The most current cost estimates range from $25-160/ton CO₂ (NOAA 2023).

1. Sequestration Permanence

   OAE will result in additional CO₂ from the atmosphere being sequestered in the ocean as bicarbonate ions, which cannot exchange with the atmosphere (Renforth & Henderson, 2017). Currently, OAE is thought to have sequestration duration >20,000 years (NOAA 2023).

2. Accelerated Weathering of Limestone: Avoided Emissions and Hybrid Methods

   When AWL is used to sequester carbon dioxide resulting from the combustion of fossil fuels, it represents avoided emissions, not removal of atmospheric carbon dioxide (negative emissions) (Rau & Caldeira, 1999; Rau, 2011). However, there exist a number of hybrid possibilities to integrate AWL with various non-fossil sources of concentrated carbon dioxide to permanently sequester the carbon dioxide as bicarbonate ions in the ocean. These hybrid approaches include:

   • AWL coupled with direct air capture – captured CO₂ from a direct air capture (DAC) facility can be concentrated and reacted with limestone in the presence of seawater to trap the captured CO₂ as bicarbonate ions. 
     • The bicarbonate-rich effluent of DAC + AWL can also serve as a substrate to support algal growth (Rau, 2014); however newly-produced algal carbon must be sequestered from return to the atmosphere to generate negative emissions over relevant permanence timescales (> 100 years).
   • AWL coupled with bioenergy – CO₂ generated from the combustion of biomass (either terrestrial or marine-based) to generate energy can be concentrated and reacted with limestone in the presence of seawater to trap the CO₂ as bicarbonate ions. This process is analogous to bioenergy with carbon capture and storage (BECCS) (Hughes, 2012), except that the carbon is stored as bicarbonate ions in the ocean as opposed to being sequestered in geologic reservoirs. 

   We further consider AWL in these hybrid configurations that generate the removal of atmospheric carbon dioxide and sequestration as bicarbonate in the ocean.

1. Carbon Capture

   Methods of OAE that rely on depositing minerals into the ocean to promote additional CO₂ uptake depend on how quickly the minerals dissolve relative to how quickly they sink (Fakhraee et al., 2023). Due to the vast quantities of alkaline rocks, and the capacity of the ocean to accommodate similarly vast quantities of bicarbonate ions, OAE has the theoretical potential to capture tens of gigatons of carbon dioxide annually^[1]Gagern, Antonius. “Ocean Alkalinity Enhancement: Current state of knowledge and potential role of philanthropy”. 9 September 2019. Meeting Proceedings Half Moon Bay, California. . However, given current technological readiness, this remains theoretical. Recent consensus reports cite the following as possible carbon dioxide removal potential from ocean alkalinity enhancement:

   • >0.1 – 1.0 Gt CO₂/year (NASEM 2022)
   • 1 – 15+ Gt CO₂/ year (NOAA 2023)

Technical, economic, social, political, and governance factors may also decrease this theoretical CDR potential, although the degree to which they limit the CDR remains to be determined. The most current cost estimates range from $25-160/ton CO₂ (NOAA 2023).

1. Sequestration Permanence

   OAE will result in additional CO₂ from the atmosphere being sequestered in the ocean as bicarbonate ions, which cannot exchange with the atmosphere (Renforth & Henderson, 2017). Currently, OAE is thought to have sequestration duration >20,000 years (NOAA 2023).

2. Accelerated Weathering of Limestone: Avoided Emissions and Hybrid Methods

   When AWL is used to sequester carbon dioxide resulting from the combustion of fossil fuels, it represents avoided emissions, not removal of atmospheric carbon dioxide (negative emissions)^[5]Rau, G. H. and Caldeira, K. (1999) ‘Enhanced carbonate dissolution: a means of sequestering waste CO₂ as ocean bicarbonate’, Energy Conversion, p. 11. ^[6]Rau, G. H. (2011) ‘CO 2 Mitigation via Capture and Chemical Conversion in Seawater’, Environmental Science & Technology, 45(3), pp. 1088–1092. doi: 10.1021/es102671x. . However, there exist a number of hybrid possibilities to integrate AWL with various non-fossil sources of concentrated carbon dioxide to permanently sequester the carbon dioxide as bicarbonate ions in the ocean. These hybrid approaches include:

   • AWL coupled with direct air capture – captured CO₂ from a direct air capture (DAC) facility can be concentrated and reacted with limestone in the presence of seawater to trap the captured CO₂ as bicarbonate ions. 
     • The bicarbonate-rich effluent of DAC + AWL can also serve as a substrate to support algal growth^[7]Rau, G.H. (2014). Use of Carbonates for Biological and Chemical Synthesis. (U.S. Patent No. 8,828,706 B2). https://patentimages.storage.googleapis.com/f0/75/e4/4dd652a83ecd1c/US8828706.pdf ^[8]“Development of High Value Bioproducts and Enhancement of Direct-Air Capture Efficiency with a Marine Algae Biofuel Production System,” n.d., 2. ; however newly-produced algal carbon must be sequestered from return to the atmosphere to generate negative emissions over relevant permanence timescales (> 100 years).
   • AWL coupled with bioenergy – CO₂ generated from the combustion of biomass (either terrestrial or marine-based) to generate energy can be concentrated and reacted with limestone in the presence of seawater to trap the CO₂ as bicarbonate ions. This process is analogous to bioenergy with carbon capture and storage (BECCS)^[9]Hughes, Adam D., et al. “Does Seaweed Offer a Solution for Bioenergy with Biological Carbon Capture and Storage?” Greenhouse Gases: Science and Technology, vol. 2, no. 6, 2012, pp. 402–407., doi:10.1002/ghg.1319. , except that the carbon is stored as bicarbonate ions in the ocean as opposed to being sequestered in geologic reservoirs. 

   We further consider AWL in these hybrid configurations that generate the removal of atmospheric carbon dioxide and sequestration as bicarbonate in the ocean.

1. Carbon Capture

   Methods of OAE that rely on depositing minerals into the ocean to promote additional CO₂ uptake depend on how quickly the minerals dissolve relative to how quickly they sink (Fakhraee et al., 2023). Due to the vast quantities of alkaline rocks, and the capacity of the ocean to accommodate similarly vast quantities of bicarbonate ions, OAE has the theoretical potential to capture tens of gigatons of carbon dioxide annually^[1]Gagern, Antonius. “Ocean Alkalinity Enhancement: Current state of knowledge and potential role of philanthropy”. 9 September 2019. Meeting Proceedings Half Moon Bay, California. . However, given current technological readiness, this remains theoretical. Recent consensus reports cite the following as possible carbon dioxide removal potential from ocean alkalinity enhancement:

   • >0.1 – 1.0 Gt CO₂/year (NASEM 2022)
   • 1 – 15+ Gt CO₂/ year (NOAA 2023)

Technical, economic, social, political, and governance factors may also decrease this theoretical CDR potential, although the degree to which they limit the CDR remains to be determined. The most current cost estimates range from $25-160/ton CO₂ (NOAA 2023).

1. Sequestration Permanence

   OAE will result in additional CO₂ from the atmosphere being sequestered in the ocean as bicarbonate ions, which cannot exchange with the atmosphere. Currently, OAE is thought to have sequestration duration >20,000 years (NOAA 2023).

2. Accelerated Weathering of Limestone: Avoided Emissions and Hybrid Methods

   When AWL is used to sequester carbon dioxide resulting from the combustion of fossil fuels, it represents avoided emissions, not removal of atmospheric carbon dioxide (negative emissions)^[5]Rau, G. H. and Caldeira, K. (1999) ‘Enhanced carbonate dissolution: a means of sequestering waste CO₂ as ocean bicarbonate’, Energy Conversion, p. 11. ^[6]Rau, G. H. (2011) ‘CO 2 Mitigation via Capture and Chemical Conversion in Seawater’, Environmental Science & Technology, 45(3), pp. 1088–1092. doi: 10.1021/es102671x. . However, there exist a number of hybrid possibilities to integrate AWL with various non-fossil sources of concentrated carbon dioxide to permanently sequester the carbon dioxide as bicarbonate ions in the ocean. These hybrid approaches include:

   • AWL coupled with direct air capture – captured CO₂ from a direct air capture (DAC) facility can be concentrated and reacted with limestone in the presence of seawater to trap the captured CO₂ as bicarbonate ions. 
     • The bicarbonate-rich effluent of DAC + AWL can also serve as a substrate to support algal growth^[7]Rau, G.H. (2014). Use of Carbonates for Biological and Chemical Synthesis. (U.S. Patent No. 8,828,706 B2). https://patentimages.storage.googleapis.com/f0/75/e4/4dd652a83ecd1c/US8828706.pdf ^[8]“Development of High Value Bioproducts and Enhancement of Direct-Air Capture Efficiency with a Marine Algae Biofuel Production System,” n.d., 2. ; however newly-produced algal carbon must be sequestered from return to the atmosphere to generate negative emissions over relevant permanence timescales (> 100 years).
   • AWL coupled with bioenergy – CO₂ generated from the combustion of biomass (either terrestrial or marine-based) to generate energy can be concentrated and reacted with limestone in the presence of seawater to trap the CO₂ as bicarbonate ions. This process is analogous to bioenergy with carbon capture and storage (BECCS)^[9]Hughes, Adam D., et al. “Does Seaweed Offer a Solution for Bioenergy with Biological Carbon Capture and Storage?” Greenhouse Gases: Science and Technology, vol. 2, no. 6, 2012, pp. 402–407., doi:10.1002/ghg.1319. , except that the carbon is stored as bicarbonate ions in the ocean as opposed to being sequestered in geologic reservoirs. 

   We further consider AWL in these hybrid configurations that generate the removal of atmospheric carbon dioxide and sequestration as bicarbonate in the ocean.

1. Carbon Capture

   Methods of OAE that rely on depositing minerals into the ocean to promote additional CO₂ uptake depend on how quickly the minerals dissolve relative to how quickly they sink (Fakhraee et al., 2023). Due to the vast quantities of alkaline rocks, and the capacity of the ocean to accommodate similarly vast quantities of bicarbonate ions, OAE has the theoretical potential to capture tens of gigatons of carbon dioxide annually^[1]Gagern, Antonius. “Ocean Alkalinity Enhancement: Current state of knowledge and potential role of philanthropy”. 9 September 2019. Meeting Proceedings Half Moon Bay, California. . However, given current technological readiness, this remains theoretical. Recent consensus reports cite the following as possible carbon dioxide removal potential from ocean alkalinity enhancement:

   • >0.1 – 1.0 Gt CO₂/year (NASEM 2022)
   • 1 – 15+ Gt CO₂/ year (NOAA 2023)

Technical, economic, social, political, and governance factors may also decrease this theoretical CDR potential, although the degree to which they limit the CDR remains to be determined. The most current cost estimates range from $25-160/ton CO₂ (NOAA 2023).

1. Sequestration Permanence

   OAE will result in additional CO₂ from the atmosphere being sequestered in the ocean as bicarbonate ions, which cannot exchange with the atmosphere. The residence time of bicarbonate ions in the ocean is ~10,000 years, suggesting that OAE sequestration would be stable for ~10,000 years. 

2. Accelerated Weathering of Limestone: Avoided Emissions and Hybrid Methods

   When AWL is used to sequester carbon dioxide resulting from the combustion of fossil fuels, it represents avoided emissions, not removal of atmospheric carbon dioxide (negative emissions)^[5]Rau, G. H. and Caldeira, K. (1999) ‘Enhanced carbonate dissolution: a means of sequestering waste CO₂ as ocean bicarbonate’, Energy Conversion, p. 11. ^[6]Rau, G. H. (2011) ‘CO 2 Mitigation via Capture and Chemical Conversion in Seawater’, Environmental Science & Technology, 45(3), pp. 1088–1092. doi: 10.1021/es102671x. . However, there exist a number of hybrid possibilities to integrate AWL with various non-fossil sources of concentrated carbon dioxide to permanently sequester the carbon dioxide as bicarbonate ions in the ocean. These hybrid approaches include:

   • AWL coupled with direct air capture – captured CO₂ from a direct air capture (DAC) facility can be concentrated and reacted with limestone in the presence of seawater to trap the captured CO₂ as bicarbonate ions. 
     • The bicarbonate-rich effluent of DAC + AWL can also serve as a substrate to support algal growth^[7]Rau, G.H. (2014). Use of Carbonates for Biological and Chemical Synthesis. (U.S. Patent No. 8,828,706 B2). https://patentimages.storage.googleapis.com/f0/75/e4/4dd652a83ecd1c/US8828706.pdf ^[8]“Development of High Value Bioproducts and Enhancement of Direct-Air Capture Efficiency with a Marine Algae Biofuel Production System,” n.d., 2. ; however newly-produced algal carbon must be sequestered from return to the atmosphere to generate negative emissions over relevant permanence timescales (> 100 years).
   • AWL coupled with bioenergy – CO₂ generated from the combustion of biomass (either terrestrial or marine-based) to generate energy can be concentrated and reacted with limestone in the presence of seawater to trap the CO₂ as bicarbonate ions. This process is analogous to bioenergy with carbon capture and storage (BECCS)^[9]Hughes, Adam D., et al. “Does Seaweed Offer a Solution for Bioenergy with Biological Carbon Capture and Storage?” Greenhouse Gases: Science and Technology, vol. 2, no. 6, 2012, pp. 402–407., doi:10.1002/ghg.1319. , except that the carbon is stored as bicarbonate ions in the ocean as opposed to being sequestered in geologic reservoirs. 

   We further consider AWL in these hybrid configurations that generate the removal of atmospheric carbon dioxide and sequestration as bicarbonate in the ocean.

1. Carbon Capture

   Methods of OAE that rely on depositing minerals into the ocean to promote additional CO₂ uptake depend on how quickly the minerals dissolve relative to how quickly they sink (Fakhraee et al., 2023). Due to the vast quantities of alkaline rocks, and the capacity of the ocean to accommodate similarly vast quantities of bicarbonate ions, OAE has the theoretical potential to capture tens of gigatons of carbon dioxide annually^[1]Gagern, Antonius. “Ocean Alkalinity Enhancement: Current state of knowledge and potential role of philanthropy”. 9 September 2019. Meeting Proceedings Half Moon Bay, California. . However, given current technological readiness, this remains theoretical. Recent consensus reports cite the following as possible carbon dioxide removal potential from ocean alkalinity enhancement:

   • >0.1 – 1.0 Gt CO₂/year (NASEM 2022)
   • 1 – 15+ Gt CO₂/ year (NOAA 2023)

Technical, economic, social, political, and governance factors may also decrease this theoretical CDR potential, although the degree to which they limit the CDR remains to be determined. Cost estimates currently vary between studies with estimated ranges of $72-159/ton CO₂ (variation due to variations in source rock, means of mining and production{{2}), $60-110/ton CO₂^[3]Gattuso J-P, Williamson P, Duarte CM and Magnan AK (2021) The Potential for Ocean-Based Climate Action: Negative Emissions Technologies and Beyond. Front. Clim. 2:575716. doi: 10.3389/fclim.2020.575716 , and $<25-125/ton CO₂^[4]EFI Report “Uncharted Waters: Expanding the Options for Carbon Dioxide Removal in Coastal and Ocean Environments.” December 2020. . More work is necessary to produce ground-truthed cost estimates.

1. Sequestration Permanence

   OAE will result in additional CO₂ from the atmosphere being sequestered in the ocean as bicarbonate ions, which cannot exchange with the atmosphere. The residence time of bicarbonate ions in the ocean is ~10,000 years, suggesting that OAE sequestration would be stable for ~10,000 years. 

2. Accelerated Weathering of Limestone: Avoided Emissions and Hybrid Methods

   When AWL is used to sequester carbon dioxide resulting from the combustion of fossil fuels, it represents avoided emissions, not removal of atmospheric carbon dioxide (negative emissions)^[5]Rau, G. H. and Caldeira, K. (1999) ‘Enhanced carbonate dissolution: a means of sequestering waste CO₂ as ocean bicarbonate’, Energy Conversion, p. 11. ^[6]Rau, G. H. (2011) ‘CO 2 Mitigation via Capture and Chemical Conversion in Seawater’, Environmental Science & Technology, 45(3), pp. 1088–1092. doi: 10.1021/es102671x. . However, there exist a number of hybrid possibilities to integrate AWL with various non-fossil sources of concentrated carbon dioxide to permanently sequester the carbon dioxide as bicarbonate ions in the ocean. These hybrid approaches include:

   • AWL coupled with direct air capture – captured CO₂ from a direct air capture (DAC) facility can be concentrated and reacted with limestone in the presence of seawater to trap the captured CO₂ as bicarbonate ions. 
     • The bicarbonate-rich effluent of DAC + AWL can also serve as a substrate to support algal growth^[7]Rau, G.H. (2014). Use of Carbonates for Biological and Chemical Synthesis. (U.S. Patent No. 8,828,706 B2). https://patentimages.storage.googleapis.com/f0/75/e4/4dd652a83ecd1c/US8828706.pdf ^[8]“Development of High Value Bioproducts and Enhancement of Direct-Air Capture Efficiency with a Marine Algae Biofuel Production System,” n.d., 2. ; however newly-produced algal carbon must be sequestered from return to the atmosphere to generate negative emissions over relevant permanence timescales (> 100 years).
   • AWL coupled with bioenergy – CO₂ generated from the combustion of biomass (either terrestrial or marine-based) to generate energy can be concentrated and reacted with limestone in the presence of seawater to trap the CO₂ as bicarbonate ions. This process is analogous to bioenergy with carbon capture and storage (BECCS)^[9]Hughes, Adam D., et al. “Does Seaweed Offer a Solution for Bioenergy with Biological Carbon Capture and Storage?” Greenhouse Gases: Science and Technology, vol. 2, no. 6, 2012, pp. 402–407., doi:10.1002/ghg.1319. , except that the carbon is stored as bicarbonate ions in the ocean as opposed to being sequestered in geologic reservoirs. 

   We further consider AWL in these hybrid configurations that generate the removal of atmospheric carbon dioxide and sequestration as bicarbonate in the ocean.

1. Carbon Capture

   Methods of OAE that rely on depositing minerals into the ocean to promote additional CO₂ uptake depend on how quickly the minerals dissolve relative to how quickly they sink (Fakhraee et al., 2023). Due to the vast quantities of alkaline rocks, and the capacity of the ocean to accommodate similarly vast quantities of bicarbonate ions, OAE has the theoretical potential to capture tens of gigatons of carbon dioxide annually^[1]Gagern, Antonius. “Ocean Alkalinity Enhancement: Current state of knowledge and potential role of philanthropy”. 9 September 2019. Meeting Proceedings Half Moon Bay, California. . However, given current technological readiness, this remains theoretical. Recent consensus reports cite the following as possible carbon dioxide removal potential from ocean alkalinity enhancement:

   • >0.1 – 1.0 Gt CO₂/year (NASEM 2022)
   • 1 – 15+ Gt CO₂/ year (NOAA 2023)

Technical, economic, social, political, and governance factors may also decrease this theoretical CDR potential, although the degree to which they limit the CDR remains to be determined. Cost estimates currently vary between studies with estimated ranges of $72-159/ton CO₂ (variation due to variations in source rock, means of mining and production{{2}), $60-110/ton CO₂^[3]Gattuso J-P, Williamson P, Duarte CM and Magnan AK (2021) The Potential for Ocean-Based Climate Action: Negative Emissions Technologies and Beyond. Front. Clim. 2:575716. doi: 10.3389/fclim.2020.575716 , and $<25-125/ton CO₂^[4]EFI Report “Uncharted Waters: Expanding the Options for Carbon Dioxide Removal in Coastal and Ocean Environments.” December 2020. . More work is necessary to produce ground-truthed cost estimates.

1. Sequestration Permanence

   OAE will result in additional CO₂ from the atmosphere being sequestered in the ocean as bicarbonate ions, which cannot exchange with the atmosphere. The residence time of bicarbonate ions in the ocean is ~10,000 years, suggesting that OAE sequestration would be stable for ~10,000 years. 

2. Accelerated Weathering of Limestone: Avoided Emissions and Hybrid Methods

   When AWL is used to sequester carbon dioxide resulting from the combustion of fossil fuels, it represents avoided emissions, not removal of atmospheric carbon dioxide (negative emissions)^[5]Rau, G. H. and Caldeira, K. (1999) ‘Enhanced carbonate dissolution: a means of sequestering waste CO₂ as ocean bicarbonate’, Energy Conversion, p. 11. ^[6]Rau, G. H. (2011) ‘CO 2 Mitigation via Capture and Chemical Conversion in Seawater’, Environmental Science & Technology, 45(3), pp. 1088–1092. doi: 10.1021/es102671x. . However, there exist a number of hybrid possibilities to integrate AWL with various non-fossil sources of concentrated carbon dioxide to permanently sequester the carbon dioxide as bicarbonate ions in the ocean. These hybrid approaches include:

   • AWL coupled with direct air capture – captured CO₂ from a direct air capture (DAC) facility can be concentrated and reacted with limestone in the presence of seawater to trap the captured CO₂ as bicarbonate ions. 
     • The bicarbonate-rich effluent of DAC + AWL can also serve as a substrate to support algal growth^[7]Rau, G.H. (2014). Use of Carbonates for Biological and Chemical Synthesis. (U.S. Patent No. 8,828,706 B2). https://patentimages.storage.googleapis.com/f0/75/e4/4dd652a83ecd1c/US8828706.pdf ^[8]“Development of High Value Bioproducts and Enhancement of Direct-Air Capture Efficiency with a Marine Algae Biofuel Production System,” n.d., 2. ; however newly-produced algal carbon must be sequestered from return to the atmosphere to generate negative emissions over relevant permanence timescales (> 100 years).
   • AWL coupled with bioenergy – CO₂ generated from the combustion of biomass (either terrestrial or marine-based) to generate energy can be concentrated and reacted with limestone in the presence of seawater to trap the CO₂ as bicarbonate ions. This process is analogous to bioenergy with carbon capture and storage (BECCS)^[9]Hughes, Adam D., et al. “Does Seaweed Offer a Solution for Bioenergy with Biological Carbon Capture and Storage?” Greenhouse Gases: Science and Technology, vol. 2, no. 6, 2012, pp. 402–407., doi:10.1002/ghg.1319. , except that the carbon is stored as bicarbonate ions in the ocean as opposed to being sequestered in geologic reservoirs. 

   We further consider AWL in these hybrid configurations that generate the removal of atmospheric carbon dioxide and sequestration as bicarbonate in the ocean.

1. Carbon Capture

   Methods of OAE that rely on depositing minerals into the ocean to promote additional CO₂ uptake depend on how quickly the minerals dissolve relative to how quickly they sink (Fakhraee et al., 2023). Due to the vast quantities of alkaline rocks, and the capacity of the ocean to accommodate similarly vast quantities of bicarbonate ions, OAE has the theoretical potential to capture tens of gigatons of carbon dioxide annually^[1]Gagern, Antonius. “Ocean Alkalinity Enhancement: Current state of knowledge and potential role of philanthropy”. 9 September 2019. Meeting Proceedings Half Moon Bay, California. . However, given current technological readiness, this remains theoretical. Recent consensus reports cite the following as possible carbon dioxide removal potential from ocean alkalinity enhancement:

   • >0.1 – 1.0 Gt CO₂/year (NASEM 2022: A Research Strategy for Ocean-Based Carbon Dioxide Removal and Sequestration)
   • 1 – 15+ Gt CO₂/ year (NOAA 2022: Carbon Dioxide Removal Research)

Technical, economic, social, political, and governance factors may also decrease this theoretical CDR potential, although the degree to which they limit the CDR remains to be determined. Cost estimates currently vary between studies with estimated ranges of $72-159/ton CO₂ (variation due to variations in source rock, means of mining and production{{2}), $60-110/ton CO₂^[3]Gattuso J-P, Williamson P, Duarte CM and Magnan AK (2021) The Potential for Ocean-Based Climate Action: Negative Emissions Technologies and Beyond. Front. Clim. 2:575716. doi: 10.3389/fclim.2020.575716 , and $<25-125/ton CO₂^[4]EFI Report “Uncharted Waters: Expanding the Options for Carbon Dioxide Removal in Coastal and Ocean Environments.” December 2020. . More work is necessary to produce ground-truthed cost estimates.

1. Sequestration Permanence

   OAE will result in additional CO₂ from the atmosphere being sequestered in the ocean as bicarbonate ions, which cannot exchange with the atmosphere. The residence time of bicarbonate ions in the ocean is ~10,000 years, suggesting that OAE sequestration would be stable for ~10,000 years. 

2. Accelerated Weathering of Limestone: Avoided Emissions and Hybrid Methods

   When AWL is used to sequester carbon dioxide resulting from the combustion of fossil fuels, it represents avoided emissions, not removal of atmospheric carbon dioxide (negative emissions)^[5]Rau, G. H. and Caldeira, K. (1999) ‘Enhanced carbonate dissolution: a means of sequestering waste CO₂ as ocean bicarbonate’, Energy Conversion, p. 11. ^[6]Rau, G. H. (2011) ‘CO 2 Mitigation via Capture and Chemical Conversion in Seawater’, Environmental Science & Technology, 45(3), pp. 1088–1092. doi: 10.1021/es102671x. . However, there exist a number of hybrid possibilities to integrate AWL with various non-fossil sources of concentrated carbon dioxide to permanently sequester the carbon dioxide as bicarbonate ions in the ocean. These hybrid approaches include:

   • AWL coupled with direct air capture – captured CO₂ from a direct air capture (DAC) facility can be concentrated and reacted with limestone in the presence of seawater to trap the captured CO₂ as bicarbonate ions. 
     • The bicarbonate-rich effluent of DAC + AWL can also serve as a substrate to support algal growth^[7]Rau, G.H. (2014). Use of Carbonates for Biological and Chemical Synthesis. (U.S. Patent No. 8,828,706 B2). https://patentimages.storage.googleapis.com/f0/75/e4/4dd652a83ecd1c/US8828706.pdf ^[8]“Development of High Value Bioproducts and Enhancement of Direct-Air Capture Efficiency with a Marine Algae Biofuel Production System,” n.d., 2. ; however newly-produced algal carbon must be sequestered from return to the atmosphere to generate negative emissions over relevant permanence timescales (> 100 years).
   • AWL coupled with bioenergy – CO₂ generated from the combustion of biomass (either terrestrial or marine-based) to generate energy can be concentrated and reacted with limestone in the presence of seawater to trap the CO₂ as bicarbonate ions. This process is analogous to bioenergy with carbon capture and storage (BECCS)^[9]Hughes, Adam D., et al. “Does Seaweed Offer a Solution for Bioenergy with Biological Carbon Capture and Storage?” Greenhouse Gases: Science and Technology, vol. 2, no. 6, 2012, pp. 402–407., doi:10.1002/ghg.1319. , except that the carbon is stored as bicarbonate ions in the ocean as opposed to being sequestered in geologic reservoirs. 

   We further consider AWL in these hybrid configurations that generate the removal of atmospheric carbon dioxide and sequestration as bicarbonate in the ocean.

1. Carbon Capture

   Due to the vast quantities of alkaline rocks, and the capacity of the ocean to accommodate similarly vast quantities of bicarbonate ions, OAE has the theoretical potential to capture tens of gigatons of carbon dioxide annually. But given current technological readiness, this remains theoretical. Recent consensus reports cite the following as possible carbon dioxide removal potential from ocean alkalinity enhancement:

   • >0.1 – 1.0 Gt CO₂/year (NASEM 2022: A Research Strategy for Ocean-Based Carbon Dioxide Removal and Sequestration)
   • 1 – 15+ Gt CO₂/ year (NOAA 2022: Carbon Dioxide Removal Research)

Technical, economic, social, political, and governance factors may also decrease this theoretical CDR potential, although the degree to which they limit the CDR remains to be determined. Cost estimates currently vary between studies with estimated ranges of $72-159/ton CO₂ (variation due to variations in source rock, means of mining and production{{2}), $60-110/ton CO₂, and $<25-125/ton CO₂. More work is necessary to produce ground-truthed cost estimates.

1. Sequestration Permanence

   OAE will result in additional CO₂ from the atmosphere being sequestered in the ocean as bicarbonate ions, which cannot exchange with the atmosphere. The residence time of bicarbonate ions in the ocean is ~10,000 years, suggesting that OAE sequestration would be stable for ~10,000 years. 

2. Accelerated Weathering of Limestone: Avoided Emissions and Hybrid Methods

   When AWL is used to sequester carbon dioxide resulting from the combustion of fossil fuels, it represents avoided emissions, not removal of atmospheric carbon dioxide (negative emissions). However, there exist a number of hybrid possibilities to integrate AWL with various non-fossil sources of concentrated carbon dioxide to permanently sequester the carbon dioxide as bicarbonate ions in the ocean. These hybrid approaches include:

   • AWL coupled with direct air capture – captured CO₂ from a direct air capture (DAC) facility can be concentrated and reacted with limestone in the presence of seawater to trap the captured CO₂ as bicarbonate ions. 
     • The bicarbonate-rich effluent of DAC + AWL can also serve as a substrate to support algal growth; however newly-produced algal carbon must be sequestered from return to the atmosphere to generate negative emissions over relevant permanence timescales (> 100 years).
   • AWL coupled with bioenergy – CO₂ generated from the combustion of biomass (either terrestrial or marine-based) to generate energy can be concentrated and reacted with limestone in the presence of seawater to trap the CO₂ as bicarbonate ions. This process is analogous to bioenergy with carbon capture and storage (BECCS), except that the carbon is stored as bicarbonate ions in the ocean as opposed to being sequestered in geologic reservoirs. 

   We further consider AWL in these hybrid configurations that generate the removal of atmospheric carbon dioxide and sequestration as bicarbonate in the ocean.

1. Carbon Capture

   Due to the vast quantities of alkaline rocks, and the capacity of the ocean to accommodate similarly vast quantities of bicarbonate ions, OAE has the theoretical potential to capture tens of gigatons of carbon dioxide annually^[1]Gagern, Antonius. “Ocean Alkalinity Enhancement: Current state of knowledge and potential role of philanthropy”. 9 September 2019. Meeting Proceedings Half Moon Bay, California. But given current technological readiness, this remains theoretical. Technical, economic, social, political, and governance factors may also decrease this theoretical CDR potential, although the degree to which they limit the CDR remains to be determined. Cost estimates currently vary between studies with estimated ranges of $72-159/ton CO₂ (variation due to variations in source rock, means of mining and production{{2}), $60-110/ton CO₂^[3]Gattuso J-P, Williamson P, Duarte CM and Magnan AK (2021) The Potential for Ocean-Based Climate Action: Negative Emissions Technologies and Beyond. Front. Clim. 2:575716. doi: 10.3389/fclim.2020.575716 , and $<25-125/ton CO₂^[4]EFI Report “Uncharted Waters: Expanding the Options for Carbon Dioxide Removal in Coastal and Ocean Environments.” December 2020. . More work is necessary to produce ground-truthed cost estimates.

2. Sequestration Permanence

   OAE will result in additional CO₂ from the atmosphere being sequestered in the ocean as bicarbonate ions, which cannot exchange with the atmosphere. The residence time of bicarbonate ions in the ocean is ~10,000 years, suggesting that OAE sequestration would be stable for ~10,000 years. 

3. Accelerated Weathering of Limestone: Avoided Emissions and Hybrid Methods

   When AWL is used to sequester carbon dioxide resulting from the combustion of fossil fuels, it represents avoided emissions, not removal of atmospheric carbon dioxide (negative emissions)^[5]Rau, G. H. and Caldeira, K. (1999) ‘Enhanced carbonate dissolution: a means of sequestering waste CO₂ as ocean bicarbonate’, Energy Conversion, p. 11. ^[6]Rau, G. H. (2011) ‘CO 2 Mitigation via Capture and Chemical Conversion in Seawater’, Environmental Science & Technology, 45(3), pp. 1088–1092. doi: 10.1021/es102671x. . However, there exist a number of hybrid possibilities to integrate AWL with various non-fossil sources of concentrated carbon dioxide to permanently sequester the carbon dioxide as bicarbonate ions in the ocean. These hybrid approaches include:

   • AWL coupled with direct air capture – captured CO₂ from a direct air capture (DAC) facility can be concentrated and reacted with limestone in the presence of seawater to trap the captured CO₂ as bicarbonate ions. 
     • The bicarbonate-rich effluent of DAC + AWL can also serve as a substrate to support algal growth^[7]Rau, G.H. (2014). Use of Carbonates for Biological and Chemical Synthesis. (U.S. Patent No. 8,828,706 B2). https://patentimages.storage.googleapis.com/f0/75/e4/4dd652a83ecd1c/US8828706.pdf ^[8]“Development of High Value Bioproducts and Enhancement of Direct-Air Capture Efficiency with a Marine Algae Biofuel Production System,” n.d., 2. ; however newly-produced algal carbon must be sequestered from return to the atmosphere to generate negative emissions over relevant permanence timescales (> 100 years).
   • AWL coupled with bioenergy – CO₂ generated from the combustion of biomass (either terrestrial or marine-based) to generate energy can be concentrated and reacted with limestone in the presence of seawater to trap the CO₂ as bicarbonate ions. This process is analogous to bioenergy with carbon capture and storage (BECCS)^[9]Hughes, Adam D., et al. “Does Seaweed Offer a Solution for Bioenergy with Biological Carbon Capture and Storage?” Greenhouse Gases: Science and Technology, vol. 2, no. 6, 2012, pp. 402–407., doi:10.1002/ghg.1319. , except that the carbon is stored as bicarbonate ions in the ocean as opposed to being sequestered in geologic reservoirs. 

   We further consider AWL in these hybrid configurations that generate removal of atmospheric carbon dioxide and sequestration as bicarbonate in the ocean.

1. Carbon Capture

   Due to the vast quantities of alkaline rocks, and the capacity of the ocean to accommodate similarly vast quantities of bicarbonate ions, OAE has the theoretical potential to capture tens of gigatons of carbon dioxide annually But given current technological readiness, this remains theoretical. Technical, economic, social, political, and governance factors may also decrease this theoretical CDR potential, although the degree to which they limit the CDR remains to be determined. Cost estimates currently vary between studies with estimated ranges of $72-159/ton CO₂ (variation due to variations in source rock, means of mining and production), $60-110/ton CO₂, and $<25-125/ton CO₂. More work is necessary to produce ground-truthed cost estimates.

2. Sequestration Permanence

   OAE will result in additional CO₂ from the atmosphere being sequestered in the ocean as bicarbonate ions, which cannot exchange with the atmosphere. The residence time of bicarbonate ions in the ocean is ~10,000 years, suggesting that OAE sequestration would be stable for ~10,000 years. 

3. Accelerated Weathering of Limestone: Avoided Emissions and Hybrid Methods

   When AWL is used to sequester carbon dioxide resulting from the combustion of fossil fuels, it represents avoided emissions, not removal of atmospheric carbon dioxide (negative emissions)^,. However, there exist a number of hybrid possibilities to integrate AWL with various non-fossil sources of concentrated carbon dioxide to permanently sequester the carbon dioxide as bicarbonate ions in the ocean. These hybrid approaches include:

   • AWL coupled with direct air capture – captured CO₂ from a direct air capture (DAC) facility can be concentrated and reacted with limestone in the presence of seawater to trap the captured CO₂ as bicarbonate ions. 
     • The bicarbonate-rich effluent of DAC + AWL can also serve as a substrate to support algal growth^,; however newly-produced algal carbon must be sequestered from return to the atmosphere to generate negative emissions over relevant permanence timescales (> 100 years).
   • AWL coupled with bioenergy – CO₂ generated from the combustion of biomass (either terrestrial or marine-based) to generate energy can be concentrated and reacted with limestone in the presence of seawater to trap the CO₂ as bicarbonate ions. This process is analogous to bioenergy with carbon capture and storage (BECCS), except that the carbon is stored as bicarbonate ions in the ocean as opposed to being sequestered in geologic reservoirs. 

   We further consider AWL in these hybrid configurations that generate removal of atmospheric carbon dioxide and sequestration as bicarbonate in the ocean.

• OAE would likely provide localized reductions in ocean acidification, with expected benefit(s) to marine ecosystems (Gattuso et al., 2018; Feng (冯玉铭) et al., 2016).

• In certain areas, calcium or silica additions could act as fertilizers to support plankton populations (Adhiya & Chisholm, 2001).

• OAE would likely provide localized reductions in ocean acidification, with expected benefit(s) to marine ecosystems.

• In certain areas, calcium or silica additions could act as fertilizers to support plankton populations.

• OAE would likely provide localized reductions in ocean acidification, with expected benefit(s) to marine ecosystems^[1]Gattuso J-P, Magnan AK, Bopp L, Cheung WWL, Duarte CM, Hinkel J, Mcleod E, Micheli F, Oschlies A, Williamson P, Billé R, Chalastani VI, Gates RD, Irisson J-O, Middelburg JJ, Pörtner H-O and Rau GH (2018) Ocean Solutions to Address Climate Change and Its Effects on Marine Ecosystems. Front. Mar. Sci. 5:337. doi: 10.3389/fmars.2018.00337 ^[2]Feng (冯玉铭), Ellias Y, David P Keller, Wolfgang Koeve, and Andreas Oschlies. “Could Artificial Ocean Alkalinization Protect Tropical Coral Ecosystems from Ocean Acidification?” Environmental Research Letters 11, no. 7 (July 1, 2016): 074008. https://doi.org/10.1088/1748-9326/11/7/074008. .

• In certain areas, calcium or silica additions could act as fertilizers to support plankton populations^[3]Adhiya, Jagat, and Sallie W Chisholm. “Is Ocean Fertilization a Good Carbon Sequestration Option?,” Massachusetts Institute of Technology Laboratory for Energy and the Environment n.d., 70. .

• OAE would likely provide localized reductions in ocean acidification, with expected benefit(s) to marine ecosystems^[1]Gattuso J-P, Magnan AK, Bopp L, Cheung WWL, Duarte CM, Hinkel J, Mcleod E, Micheli F, Oschlies A, Williamson P, Billé R, Chalastani VI, Gates RD, Irisson J-O, Middelburg JJ, Pörtner H-O and Rau GH (2018) Ocean Solutions to Address Climate Change and Its Effects on Marine Ecosystems. Front. Mar. Sci. 5:337. doi: 10.3389/fmars.2018.00337 ^[2]Feng (冯玉铭), Ellias Y, David P Keller, Wolfgang Koeve, and Andreas Oschlies. “Could Artificial Ocean Alkalinization Protect Tropical Coral Ecosystems from Ocean Acidification?” Environmental Research Letters 11, no. 7 (July 1, 2016): 074008. https://doi.org/10.1088/1748-9326/11/7/074008. .
• In certain areas, calcium or silica additions could act as fertilizers to support plankton populations^[3]Adhiya, Jagat, and Sallie W Chisholm. “Is Ocean Fertilization a Good Carbon Sequestration Option?,” Massachusetts Institute of Technology Laboratory for Energy and the Environment n.d., 70. .

• OAE would likely provide localized reductions in ocean acidification, with expected benefit(s) to marine ecosystems^,.
• In certain areas, calcium or silica additions could act as fertilizers to support plankton populations.

• OAE may pose ecotoxicological risks from the release of elevated concentrations of trace metals from some minerals during mineral dissolution (Burns & Corbett, 2020).
  • The toxicity will depend on the source rock, the concentration of source rock applied, and the seawater chemistry (which determines bioavailability). These trace metal additions could also pose risks to human health if accumulation occurs through food webs (Bach et al., 2019).
  • Silicate rocks are likely to have higher metal concentrations than carbonate rocks, and thus may pose a greater ecotoxicological risk (Bach et al., 2019)
  • In-situ applications of alkaline materials (e.g. coastal enhanced weathering, ocean liming) may pose greater ecotoxicological risks than ex-situ applications (e.g. reactors for accelerated weathering of limestone) because of the ability to capture and treat the effluent from ex-situ reactors before release into the ocean. However, options exist to mitigate these effects e.g., through rapid dilution in the wake of vessels (Caserini et al., 2021)

• The potential for changes in water column particle concentrations, turbidity, and optical properties from dispersing fine particulates
• The potential for changes in seafloor deposition of particles and their effects on smothering or burial, food webs interactions, light availability and more
• Large-scale mining of silicate or carbonate rocks may pose environmental risks, similar to those typically associated with existing mines including ground vibrations from blasting, noise pollution, decreased air and soil quality, etc.

• Ecological and geochemical impacts of discharging high alkalinity/high pH waters, including precipitation (inorganic mineral formation) of carbonates. However, options to mitigate these effects exist e.g., through rapid dilution in ship's wakes (Caserini et al., 2021).
• OAE may have the potential to shift phytoplankton community composition from carbonate-shell producing plankton (coccolithophores) to silica-shell producing plankton (diatoms) or vice-versa depending on the form of alkalinity addition.
• Potential for bioaccumulation and biomagnification in the food chain (NASEM 2022).
• Dedicated ships/vessels to distribute minerals could cause environmental impacts such as additional noise pollution (Kaplan & Solomon, 2016), the potential transfer of nonindigenous species (Muirhead et al., 2015), and the addition of pollutants from shipping emissions (Dalsøren et al., 2013).

• OAE may pose ecotoxicological risks from the release of elevated concentrations of trace metals from some minerals during mineral dissolution (Burns & Corbett, 2020).
  • The toxicity will depend on the source rock, the concentration of source rock applied, and the seawater chemistry (which determines bioavailability). These trace metal additions could also pose risks to human health if accumulation occurs through food webs (Bach et al., 2019).
  • Silicate rocks are likely to have higher metal concentrations than carbonate rocks, and thus may pose a greater ecotoxicological risk (Bach et al., 2019)
  • In-situ applications of alkaline materials (e.g. coastal enhanced weathering, ocean liming) may pose greater ecotoxicological risks than ex-situ applications (e.g. reactors for accelerated weathering of limestone) because of the ability to capture and treat the effluent from ex-situ reactors before release into the ocean. However, options exist to mitigate these effects e.g., through rapid dilution in the wake of vessels (Caserini et al., 2021)

• The potential for changes in water column particle concentrations, turbidity, and optical properties from dispersing fine particulates
• The potential for changes in seafloor deposition of particles and their effects on smothering or burial, food webs interactions, light availability and more
• Large-scale mining of silicate or carbonate rocks may pose environmental risks, similar to those typically associated with existing mines including ground vibrations from blasting, noise pollution, decreased air and soil quality, etc.

• Ecological and geochemical impacts of discharging high alkalinity/high pH waters, including precipitation (inorganic mineral formation) of carbonates. However, options to mitigate these effects exist e.g., through rapid dilution in ship's wakes.
• OAE may have the potential to shift phytoplankton community composition from carbonate-shell producing plankton (coccolithophores) to silica-shell producing plankton (diatoms) or vice-versa depending on the form of alkalinity addition.
• Potential for bioaccumulation and biomagnification in the food chain
• Dedicated ships/vessels to distribute minerals could cause environmental impacts such as additional noise pollution, the potential transfer of nonindigenous species , and the addition of pollutants from shipping emissions.

• OAE may pose ecotoxicological risks from the release of elevated concentrations of trace metals from some minerals during mineral dissolution.
  • The toxicity will depend on the source rock, the concentration of source rock applied, and the seawater chemistry (which determines bioavailability). These trace metal additions could also pose risks to human health if accumulation occurs through food webs.
  • Silicate rocks are likely to have higher metal concentrations than carbonate rocks, and thus may pose a greater ecotoxicological risk
  • In-situ applications of alkaline materials (e.g. coastal enhanced weathering, ocean liming) may pose greater ecotoxicological risks than ex-situ applications (e.g. reactors for accelerated weathering of limestone) because of the ability to capture and treat the effluent from ex-situ reactors before release into the ocean. However, options exist to mitigate these effects e.g., through rapid dilution in the wake of vessels.

• The potential for changes in water column particle concentrations, turbidity, and optical properties from dispersing fine particulates
• The potential for changes in seafloor deposition of particles and their effects on smothering or burial, food webs interactions, light availability and more
• Large-scale mining of silicate or carbonate rocks may pose environmental risks, similar to those typically associated with existing mines including ground vibrations from blasting, noise pollution, decreased air and soil quality, etc.

• Ecological and geochemical impacts of discharging high alkalinity/high pH waters, including precipitation (inorganic mineral formation) of carbonates. However, options to mitigate these effects exist e.g., through rapid dilution in ship's wakes.
• OAE may have the potential to shift phytoplankton community composition from carbonate-shell producing plankton (coccolithophores) to silica-shell producing plankton (diatoms) or vice-versa depending on the form of alkalinity addition.
• Potential for bioaccumulation and biomagnification in the food chain
• Dedicated ships/vessels to distribute minerals could cause environmental impacts such as additional noise pollution, the potential transfer of nonindigenous species , and the addition of pollutants from shipping emissions.

• OAE may pose ecotoxicological risks from the release of elevated concentrations of trace metals from some minerals during mineral dissolution.
  • The toxicity will depend on the source rock, the concentration of source rock applied, and the seawater chemistry (which determines bioavailability). These trace metal additions could also pose risks to human health if accumulation occurs through food webs.
  • Silicate rocks are likely to have higher metal concentrations than carbonate rocks, and thus may pose a greater ecotoxicological risk
  • In-situ applications of alkaline materials (e.g. coastal enhanced weathering, ocean liming) may pose greater ecotoxicological risks than ex-situ applications (e.g. reactors for accelerated weathering of limestone) because of the ability to capture and treat the effluent from ex-situ reactors before release into the ocean. However, options exist to mitigate these effects e.g., through rapid dilution in the wake of vessels.

• The potential for changes in water column particle concentrations, turbidity, and optical properties from dispersing fine particulates
• The potential for changes in seafloor deposition of particles and their effects on smothering or burial, food webs interactions, light availability and more
• Large-scale mining of silicate or carbonate rocks may pose environmental risks, similar to those typically associated with existing mines including ground vibrations from blasting, noise pollution, decreased air and soil quality, etc.

• Ecological and geochemical impacts of discharging high alkalinity/high pH waters, including precipitation (inorganic mineral formation) of carbonates. However, options to mitigate these effects exist e.g., through rapid dilution in ship's wakes.
• OAE may have the potential to shift phytoplankton community composition from carbonate-shell producing plankton (coccolithophores) to silica-shell producing plankton (diatoms) or vice-versa depending on the form of alkalinity addition.
• Potential for bioaccumulation and biomagnification in the food chain
• Dedicated ships/vessels to distribute minerals could cause environmental impacts such as additional noise pollution, the potential transfer of nonindigenous species , and the addition of pollutants from shipping emissions.

• OAE may pose ecotoxicological risks from the release of elevated concentrations of trace metals from some minerals during mineral dissolution. 
  • The toxicity will depend on the source rock, the concentration of source rock applied, and the seawater chemistry (which determines bioavailability). These trace metal additions could also pose risks to human health if accumulation occurs through food webs. 
  • Silicate rocks are likely to have higher metal concentrations than carbonate rocks, and thus may pose a greater ecotoxicological risk
  • In-situ applications of alkaline materials (e.g. coastal enhanced weathering, ocean liming) may pose greater ecotoxicological risks than ex-situ applications (e.g. reactors for accelerated weathering of limestone) because of the ability to capture and treat the effluent from ex-situ reactors before release into the ocean. However, options exist to mitigate these effects e.g., through rapid dilution in the wake of vessels.
• The potential for changes in water column particle concentrations, turbidity, and optical properties from dispersing fine particulates.
• The potential for changes in seafloor deposition of particles and their effects on smothering or burial, food webs interactions, light availability and more
• Ecological and geochemical impacts of discharging high alkalinity/high pH waters, including precipitation (inorganic mineral formation) of carbonates. However, options to mitigate these effects exist e.g., through rapid dilution in ship's wakes.
• Large-scale mining of silicate or carbonate rocks may pose environmental risks, similar to those typically associated with existing mines.
• OAE may have the potential to shift phytoplankton community composition from carbonate-shell producing plankton (coccolithophores) to silica-shell producing plankton (diatoms) or vice-versa depending on the form of alkalinity addition.

• OAE may pose ecotoxicological risks from the release of elevated concentrations of trace metals from some minerals during mineral dissolution. 
  • The toxicity will depend on the source rock, the concentration of source rock applied, and the seawater chemistry (which determines bioavailability). These trace metal additions could also pose risks to human health if accumulation occurs through food webs. 
  • Silicate rocks are likely to have higher metal concentrations than carbonate rocks, and thus may pose a greater ecotoxicological risk
  • In-situ applications of alkaline materials (e.g. coastal enhanced weathering, ocean liming) may pose greater ecotoxicological risks than ex-situ applications (e.g. reactors for accelerated weathering of limestone) because of the ability to capture and treat the effluent from ex-situ reactors before release into the ocean. However, options exist to mitigate these effects e.g., through rapid dilution in the wake of vessels.
• The potential for changes in water column particle concentrations, turbidity, and optical properties from dispersing fine particulates.
• The potential for changes in seafloor deposition of particles and their effects on smothering or burial, food webs interactions, light availability and more
• Ecological and geochemical impacts of discharging high alkalinity/high pH waters, including precipitation (inorganic mineral formation) of carbonates. However, options to mitigate these effects exist e.g., through rapid dilution in ship's wakes.
• Large-scale mining of silicate or carbonate rocks may pose environmental risks, similar to those typically associated with existing mines.
• OAE may have the potential to shift phytoplankton community composition from carbonate-shell producing plankton (coccolithophores) to silica-shell producing plankton (diatoms) or vice-versa depending on the form of alkalinity addition.

• OAE may pose ecotoxicological risks from the release of elevated concentrations of trace metals from some minerals during mineral dissolution. 
  • The toxicity will depend on the source rock, the concentration of source rock applied, and the seawater chemistry (which determines bioavailability). These trace metal additions could also pose risks to human health if accumulation occurs through food webs. 
  • Silicate rocks are likely to have higher metal concentrations than carbonate rocks, and thus may pose a greater ecotoxicological risk
  • In-situ applications of alkaline materials (e.g. coastal enhanced weathering, ocean liming) may pose greater ecotoxicological risks than ex-situ applications (e.g. reactors for accelerated weathering of limestone) because of the ability to capture and treat the effluent from ex-situ reactors before release into the ocean. 
• The potential for changes in water column particle concentrations, turbidity, and optical properties from dispersing fine particulates.
• The potential for changes in seafloor deposition of particles and their effects on smothering or burial, food webs interactions, light availability and more
• Ecological and geochemical impacts of discharging high alkalinity/high pH waters, including precipitation (inorganic mineral formation) of carbonates. However, options to mitigate these effects exist e.g., through rapid dilution in ship's wakes
• Large-scale mining of silicate or carbonate rocks may pose environmental risks, similar to those typically associated with existing mines.
• OAE may have the potential to shift phytoplankton community composition from carbonate-shell producing plankton (coccolithophores) to silica-shell producing plankton (diatoms) or vice-versa depending on the form of alkalinity addition.

• OAE may pose ecotoxicological risks from the release of elevated concentrations of trace metals from some minerals during mineral dissolution^[1] “Antacids for the Sea? Artificial Ocean Alkalinization and Climate Change | Elsevier Enhanced Reader.” Accessed March 17, 2021. https://doi.org/10.1016/j.oneear.2020.07.016. . 
  • The toxicity will depend on the source rock, the concentration of source rock applied, and the seawater chemistry (which determines bioavailability). These trace metal additions could also pose risks to human health if accumulation occurs through food webs^[2] Bach LT, Gill SJ, Rickaby REM, Gore S and Renforth P (2019) CO₂ Removal With Enhanced Weathering and Ocean Alkalinity Enhancement: Potential Risks and Co-benefits for Marine Pelagic Ecosystems. Front. Clim. 1:7. doi: 10.3389/fclim.2019.00007 . 
  • Silicate rocks are likely to have higher metal concentrations than carbonate rocks, and thus may pose a greater ecotoxicological risk^[2] Bach LT, Gill SJ, Rickaby REM, Gore S and Renforth P (2019) CO₂ Removal With Enhanced Weathering and Ocean Alkalinity Enhancement: Potential Risks and Co-benefits for Marine Pelagic Ecosystems. Front. Clim. 1:7. doi: 10.3389/fclim.2019.00007
  • In-situ applications of alkaline materials (e.g. coastal enhanced weathering, ocean liming) may pose greater ecotoxicological risks than ex-situ applications (e.g. reactors for accelerated weathering of limestone) because of the ability to capture and treat the effluent from ex-situ reactors before release into the ocean. 
• The potential for changes in water column particle concentrations, turbidity, and optical properties from dispersing fine particulates.
• The potential for changes in seafloor deposition of particles and their effects on smothering or burial, food webs interactions, light availability and more
• Ecological and geochemical impacts of discharging high alkalinity/high pH waters, including precipitation (inorganic mineral formation) of carbonates. 
• Large-scale mining of silicate or carbonate rocks may pose environmental risks, similar to those typically associated with existing mines.
• OAE may have the potential to shift phytoplankton community composition from carbonate-shell producing plankton (coccolithophores) to silica-shell producing plankton (diatoms) or vice-versa depending on the form of alkalinity addition.

• OAE may pose ecotoxicological risks from the release of elevated concentrations of trace metals during mineral dissolution^[1] “Antacids for the Sea? Artificial Ocean Alkalinization and Climate Change | Elsevier Enhanced Reader.” Accessed March 17, 2021. https://doi.org/10.1016/j.oneear.2020.07.016. . 
  • The toxicity will depend on the source rock, the concentration of source rock applied, and the seawater chemistry (which determines bioavailability). These trace metal additions could also pose risks to human health if accumulation occurs through food webs^[2] Bach LT, Gill SJ, Rickaby REM, Gore S and Renforth P (2019) CO₂ Removal With Enhanced Weathering and Ocean Alkalinity Enhancement: Potential Risks and Co-benefits for Marine Pelagic Ecosystems. Front. Clim. 1:7. doi: 10.3389/fclim.2019.00007 . 
  • Silicate rocks are likely to have higher metal concentrations than carbonate rocks, and thus may pose a greater ecotoxicological risk^[2] Bach LT, Gill SJ, Rickaby REM, Gore S and Renforth P (2019) CO₂ Removal With Enhanced Weathering and Ocean Alkalinity Enhancement: Potential Risks and Co-benefits for Marine Pelagic Ecosystems. Front. Clim. 1:7. doi: 10.3389/fclim.2019.00007
  • In-situ applications of alkaline materials (e.g. coastal enhanced weathering, ocean liming) may pose greater ecotoxicological risks than ex-situ applications (e.g. reactors for accelerated weathering of limestone) because of the ability to capture and treat the effluent from ex-situ reactors before release into the ocean. 
• The potential for changes in water column particle concentrations, turbidity, and optical properties from dispersing fine particulates.
• The potential for changes in seafloor deposition of particles and their effects on smothering or burial, food webs interactions, light availability and more
• Ecological and geochemical impacts of discharging high alkalinity/high pH waters, including precipitation (inorganic mineral formation) of carbonates. 
• Large-scale mining of silicate or carbonate rocks may pose environmental risks, similar to those typically associated with existing mines.
• OAE may have the potential to shift phytoplankton community composition from carbonate-shell producing plankton (coccolithophores) to silica-shell producing plankton (diatoms) or vice-versa depending on the form of alkalinity addition.

• OAE may pose ecotoxicological risks from the release of elevated concentrations of trace metals during mineral dissolution. 
  • The toxicity will depend on the source rock, the concentration of source rock applied, and the seawater chemistry (which determines bioavailability). These trace metal additions could also pose risks to human health if accumulation occurs through food webs²⁹. 
  • Silicate rocks are likely to have higher metal concentrations than carbonate rocks, and thus may pose a greater ecotoxicological risk²⁹
  • In-situ applications of alkaline materials (e.g. coastal enhanced weathering, ocean liming) may pose greater ecotoxicological risks than ex-situ applications (e.g. reactors for accelerated weathering of limestone) because of the ability to capture and treat the effluent from ex-situ reactors before release into the ocean. 
• The potential for changes in water column particle concentrations, turbidity, and optical properties from dispersing fine particulates.
• The potential for changes in seafloor deposition of particles and their effects on smothering or burial, food webs interactions, light availability and more
• Ecological and geochemical impacts of discharging high alkalinity/high pH waters, including precipitation (inorganic mineral formation) of carbonates. 
• Large-scale mining of silicate or carbonate rocks may pose environmental risks, similar to those typically associated with existing mines.
• OAE may have the potential to shift phytoplankton community composition from carbonate-shell producing plankton (coccolithophores) to silica-shell producing plankton (diatoms) or vice-versa depending on the form of alkalinity addition.

• Risks associated with the expansion of mining activities for OAE
  • Demographic changes include, but are not limited to, shifts in gender balance, increase in non-resident workforces, appropriation of land from local communities, social inequality, and impacts on Indigenous communities (NASEM 2022)
  • Negative health impacts include cancer, respiratory diseases, injuries, and prolonged exposure to chemical agents (Candeias et al., 2011)
  • Environmental implications include impacts to soil and air quality, impacts to ground and surface waters, and erosion.

• Risks associated with the expansion of mining activities for OAE
  • Demographic changes include, but are not limited to, shifts in gender balance, increase in non-resident workforces, appropriation of land from local communities, social inequality, and impacts on Indigenous communities^[3]National Academies of Sciences, Engineering, and Medicine 2022. A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration. Washington, DC: The National Academies Press. https://doi.org/10.17226/26278.
  • Negative health impacts include cancer, respiratory diseases, injuries, and prolonged exposure to chemical agents^[4]Candeias, C., P. Ávila, P. Coelho, and J. P. Teixeira. 2019. Mining activities: Health impacts. Pp. 415-435 in Encyclopedia of Environmental Health, 2nd ed., J. O. Nriagu, ed. Cambridge, MA: Elsevier.
  • Environmental implications include impacts to soil and air quality, impacts to ground and surface waters, and erosion.

• Risks associated with the expansion on mining activities for OAE
  • Demographic changes including, but not limited to, shifts in gender balance, increase in non-resident workforces, appropriation of land from local communities, social inequality, and impacts on Indigenous communities^[3]National Academies of Sciences, Engineering, and Medicine 2022. A Research Strategy for Ocean-based Carbon Dioxide Removal and Sequestration. Washington, DC: The National Academies Press. https://doi.org/10.17226/26278.
  • Negative health impacts including cancer, respiratory diseases, injuries, and prolonged exposure to chemical agents^[4]Candeias, C., P. Ávila, P. Coelho, and J. P. Teixeira. 2019. Mining activities: Health impacts. Pp. 415-435 in Encyclopedia of Environmental Health, 2nd ed., J. O. Nriagu, ed. Cambridge, MA: Elsevier.
  • Environmental implications include impacts to soil and air quality, impacts to ground and surface waters, and erosion.

• Risks associated with the expansion on mining activities for OAE
  • Demographic changes including, but not limited to, shifts in gender balance, increase in non-resident workforces, appropriation of land from local communities, social inequality, and impacts on Indigenous communities
  • Negative health impacts including cancer, respiratory diseases, injuries, and prolonged exposure to chemical agents
  • Environmental implications include impacts to soil and air quality, impacts to ground and surface waters, and erosion.

• Risks associated with the expansion on mining activities for OAE
  • Demographic changes including, but not limited to, shifts in gender balance, increase in non-resident workforces, appropriation of land from local communities, social inequality, and impacts on Indigenous communities (NASEM 2022)
  • Negative health impacts including cancer, respiratory diseases, injuries, and prolonged exposure to chemical agents (Candeias et al., 2018)
  • Environmental implications include impacts to soil and air quality, impacts to ground and surface waters, and erosion.

• Creation of job opportunities from the OAE supply chain
• Amelioration of ocean acidification could increase food security and preserve cultural values (Nawaz et al., 2023)

• Creation of job opportunities