CO₂ Removal by Alkaline Mineral Dissolution

CO₂ Removal by Alkaline Mineral Dissolution can be quantified as:

$CO_2eOAE=Alk_{add}-Sec_{precip}-Carb-Acid_{add}-PMF+POC$

Where:

$Alk_{add}$= Moles of alkalinity added to surface seawater through dissolution of alkaline minerals.

• Conversion of alkalinity into CO₂e can be calculated based on the measured addition of alkaline materials into the ocean and their modeled rate of dissolution prior to sinking, informed by laboratory testing in different conditions.

$Sec_{precip}$= Moles of secondary precipitation of calcium carbonate (CaCO₃) associated with high surface water alkalinity.

• Can be minimized by ensuring rapid dispersal of added alkalinity relative to the rate of alkalinity addition, and potentially assessed by measurements of Total Alkalinity (TA), pH, $[Ca^2+]$, and/or dissolved inorganic carbon (DIC) in the deployment region.

$Carb$= Moles of DIC added to seawater as carbonate equivalents (CO32-) by dissolution of calcium carbonate rather than as bicarbonate.

• Represents the uptake efficiency uncertainty associated with carbonate versus bicarbonate formation, as carbonate formation reduces the marginal storage capacity of DIC (via atmospheric CO₂) in seawater per unit of added alkalinity.

• Aqueous carbonate vs bicarbonate ion formation in a given area is based on environmental conditions (temperature, pressure, salinity) and pH, and must be accounted for to determine the total reallocation of fast cycle carbon (aqueous CO₂ pool) to the bicarbonate pool as a removal. Water sampling during deployments will be completed for a baseline understanding of these environmental conditions.

• Laboratory testing measuring alkalinity, pH, and DIC are conducted on water samples to calculate the distribution of aqueous carbonate species between aqueous CO₂, bicarbonate ions HCO3-, and carbonate ions (CO32-) at a given point in time.

$Acid_{add}$= Any addition of acidity to the ocean that reduces the alkalinity of surface seawater and the associated sequestration of atmospheric CO₂, expressed as the resulting change in moles of alkalinity.

• Terrestrial biomass contains organic compounds with functional groups that, when dissolved in water, may contribute acidity to the surface ocean environment. Leaching experiments in a laboratory setting will quantify the amount of acidity that is generated from organic carbon dissolution, which can be extrapolated to the scope of the project activity.

• This release of acidity would effectively counteract a molar-equivalent portion of alkalinity enhancement associated with alkaline mineral dissolution.

$PMF$= Physical Mixing Factor. The effect of ocean mixing processes on the efficiency of moving CO₂ from the fast to slow cycle – i.e., surface ocean subduction to deeper waters following alkalinity addition and prior to complete re-equilibration.

• A chemical perturbation to the surface ocean, such as alkalinity enhancement, that reallocates dissolved CO₂ to the stable bicarbonate reservoir increases the residence of the dissolved carbon, thereby constituting a transfer from the fast to slow cycle. This reallocation or increase in buffering capacity can lead to a chemically unstable gradient between the partial pressure of CO₂ (pCO₂) in the surface ocean and the atmosphere, drawing additional fast cycle CO₂ into the ocean for conversion to bicarbonate ion. If subduction of surface water due to physical mixing processes occurs prior to complete re-equilibration, the fast-to-slow transfer of carbon from that natural physical pathway may be reduced, and as such should be accounted for with a discount factor.

• The impact on total carbon removed from this factor is only in relation to the difference between the total amount of dissolved carbon mixed into the deep ocean for durable storage in the absence of project activity versus what is mixed into the deep ocean following the addition of alkalinity and the associated addition of DIC to the surface ocean.

• This remains an active area of research and discussion within the scientific community.

$POC$= Additional formation of dissolved inorganic carbon could lead to more Particulate Organic Carbon (POC) in the surface ocean, thereby providing additional CO₂ drawdown via increased photosynthetic activity (i.e., a “fertilization effect”).

• The positive increase in POC in the surface ocean would only lead to additional carbon removed based on the portion of additional POC that made its way to the deep ocean for durable storage via natural fast-to-slow pathways (i.e. biological pump or ocean mixing activity). When deployed in tandem with the macroalgae carbon removal pathway, this effect may be complementary to macroalgae growth and relevant for nutrient competition considerations (see the BAF variable below). This effect remains uncertain and is an area of further research and exploration.

• Quantification of increased primary production due to alkalinity-induced increase in DIC would require remote and/or in-situ measurement of localized chlorophyll-a. concentrations and/or phytoplankton populations.