Carbon dioxide removal (CDR) is a term used to describe anthropogenic activities that directly or indirectly remove carbon dioxide (CO2) from the atmosphere and durably store it in geological, terrestrial, or ocean reservoirs, or in products. Marine carbon dioxide removal (mCDR) is a subset of CDR approaches that leverage the ocean to remove CO2 and/or store captured CO2 in ocean reservoirs.  

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.

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.

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.

Chemical

• 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).

Biological

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

Chemical

• 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)

Physical

• 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.

Biological

• 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).

It is difficult to assess the range of potential impacts to society from OAE activities due in large part to the hefty list of unknowns around the technical and scientific aspects of these techniques. As this field progresses, it will be critically important that work to assess social impacts progresses in turn (Nawaz et al., 2023). While some impacts such as increased mining activities for alkaline materials pose a host of known potential risks, other impacts remain unclear.

 Social Risks

• 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.

It is difficult to assess the range of potential impacts to society from OAE activities due in large part to the hefty list of unknowns around the technical and scientific aspects of these techniques. As this field progresses, it will be critically important that work to assess social impacts progresses in turn. While some impacts such as increased mining activities for alkaline materials pose a host of known potential risks, other impacts remain unclear.

Social Co-Benefits

• 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)