Ocean pH Adjustment

The first step in the ocean solubility pump is the reaction between CO2 and water (Eq. 9.2). Water and CO2 combine to form a weak acid known as carbonic acid (H2CO3). Depending on the local pH at the ocean surface, some fraction of the carbonic acid molecules dissociates to form bicarbonate ion and hydrogen ion. Bicarbonate can then further dissociate to form carbonate and another hydrogen ion. This series of reactions is responsible for the increasing acidification of the world's oceans.

CO2(aq) + H2O <—> H2CO3 <—> HCO3- + H+ <—> CO32- + 2H+ (9.2)

Mitigation Option: Broadcast spreading of alkali compounds, such as limestone or soda ash, to neutralize the ocean surface, and amplify carbonate production [45, 46]. A substantial fraction of the carbonate that is formed in this manner is expected to deposit in deep ocean sediments that, according to geological observations, tend to remain stable for thousands of years.

Feasibility: Estimates of the quantity of CO2 that can be sequestered using this approach must account for the variability in ocean surface temperature, wind speeds and phytoplankton, along with the uncertainties associated with quantifying the role of biology and ocean dynamics in carbon sequestration. The equilibrium distribution of CO2 between the ocean and the atmosphere is highly sensitive to the temperature, alkalinity and salinity of surface waters [42]. CO2 concentrations vary widely from season to season, by as much as 60% above and below the mean atmospheric concentration. Wind speed also substantially alters surface CO2 concentrations [43]. Increasing the temperature of the ocean surface drives CO2 out of solution and back into the atmosphere.

The biological pump competes with chemistry in the sequestration of CO2. Whether CO2 reacts and precipitates as carbonate, is utilized for coralline algae and corals for constructing shells, or is converted to organic carbon through photosynthesis, sequestration in ocean sediments depends upon the physical processes driving vertical, downward transport to the ocean floor, including both sedimentation and ocean currents.

Each of the factors described will influence the estimated mass of alkali mineral required for effective CO2 sequestration. The costs associated with locating sufficient supplies of the mineral, mining, and transporting and then distributing it to the designated ocean regions cannot be estimated without first knowing the resource quantity required.

Co-benefits and undesirable consequences: Large-scale manipulation of ocean surface chemistry can be expected to affect the ocean food web, but the potential impacts have not been investigated. Depending upon local conditions, the response by the resident ocean organisms may be positive or negative.

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