The oceanic uptake of excess atmospheric carbon dioxide alters the chemistry of seawater, which may impact a wide range of marine organisms from plankton to coral reefs (Doney et al., 2009a,b; NRC, 2010) (see also Section 6.3). Ocean acidification is in fact a series of interlinked and wellknown changes in acid-base chemistry and carbonate chemistry due to the net flux of CO₂ into surface waters (Figure 4.26). The chemical shifts include increases in the partial pressure of carbon dioxide (pCO₂), the concentration of aqueous CO₂, and the hydrogen ion (H⁺) concentration and decreases in pH (pH = –log10[H⁺]). The increase in hydrogen ion concentration acts to lower the concentration of carbonate ions (CO₃^2–) through the reaction H⁺ + CO₃^2– => HCO₃^–, even though the total amount of dissolved inorganic carbon (DIC) goes up (DIC = [CO₂] + [HCO₃^–] + [CO₃^2–]). Declining CO₃^2– in turn lowers calcium carbonate (CaCO₃) mineral saturation state, Ω = [Ca²⁺][CO₃^2–]/K_sp, where K_sp is the thermodynamic solubility product that varies with temperature, pressure, and mineral form. Ocean surface waters

FIGURE 4.26 Schematic indicating the effects on seawater carbonate chemistry due to the uptake of excess carbon dioxide (CO₂) from the atmosphere. Ocean acidification causes increases in some chemical species (red) and decreases in other species (blue). Ocean acidification also causes a reduction in pH (pH = –log₁₀[H⁺]) and the saturation states, Ω, of calcium carbonate minerals in shells and skeletons of planktonic and benthic organisms and in carbonate sediments. On millennial and longer time scales, ocean pH perturbations are buffered by external inputs of alkalinity, denoted by calcium ions (Ca²⁺) and changes in the net burial rate of carbonate sediments.