Reacting CO2 and limestone to alkalize the ocean and sequester Anthropogenic CO2

The ocean-atmosphere system's natural response to high levels of atmospheric CO₂ is to wait for ocean circulation to present highly undersaturated sea water to sediments rich in CaCO₃. Dissolution of deep ocean sediments provides a natural buffer for atmospheric CO₂, but the timescales of ocean circulation, dissolution, and bioturbation are such that this system operates over 10's of millennia. We have documented how carbonate dissolution rates in seawater are catalyzed by 10-1000x in the presence of carbonic anhydrase (CA). This combination of mildly undersaturated water, CaCO₃ mineral, and the presence of CA is likely responsible for much of the carbonate dissolution and alkalinity production that is seen in the upper- and mid-water column. Rather than waiting for this natural buffering system, the chemistry can be scaled up in a variety of engineered systems to potentially match the pace of anthropogenic CO₂ emissions. We present a series of calculations for how atmospheric CO₂ levels could be mitigated by the reaction of undersaturated fresh/seawater + CA + CaCO₃. The amount of freshwater needed to keep pace with global CO₂ emissions is roughly as large as global river runoff if the DIC content is increased by 10 mM over ambient. While not feasible for the whole problem, there are local combinations of power plant CO₂ and existing waste water infrastructure that could provide cities with large quantities of negative emissions. Ultimately, the conversion of CO₂ to bicarbonate in solution must be done in seawater to significantly aid in this the global problem. We also model CaCO₃ dissolution in surface seawater that is driven to undersaturation by the addition of CO₂ either previously captured and stored or emitted by ships. The bicarbonate added to surface seawater during this process, if 1 Gt CO₂ per year were neutralized, is 5x10¹³ equivalents of Alkalinity. While this is 10 times the annual Alk input from the Amazon River, it represents only 1/5x10⁹ of the amount of Alk in the upper 100 m of the N. Atlantic Ocean. The effluent from this process, Ca and bicarbonate, may influence ocean ecosystem structure, a problem we investigate with the ECCO-Darwin ocean biogeochemistry model along existing major shipping routes.