Epochs of changing atmospheric CO₂ and seawater CO₂-carbonic acid system chemistry and acidification have occurred during the Phanerozoic at various time scales. On the longer geologic time scale, as sea level rose and fell and continental free board decreased and increased, respectively, the riverine fluxes of Ca, Mg, DIC, and total alkalinity to the coastal ocean varied and helped regulate the C chemistry of seawater, but nevertheless there were major epochs of ocean acidification (OA). On the shorter glacial-interglacial time scale from the Last Glacial Maximum (LGM) to late preindustrial time, riverine fluxes of DIC, total alkalinity, and N and P nutrients increased and along with rising sea level, atmospheric PCO₂ and temperature led, among other changes, to a slightly deceasing pH of coastal and open ocean waters, and to increasing net ecosystem calcification and decreasing net heterotrophy in coastal ocean waters. From late the preindustrial time to the present and projected into the 21^st century, human activities, such as fossil fuel and land use emissions of CO₂ to the atmosphere, increasing application of N and P nutrient subsidies and combustion N to the landscape, and sewage discharges of C, N, P have led, and will continue to lead, to significant modifications of coastal ocean waters. The changes include a rapid decline in pH and carbonate saturation state (modern problem of ocean acidification), a shift toward dissolution of carbonate substrates exceeding production, potentially leading to the “demise” of the coral reefs, reversal of the direction of the sea to air flux of CO₂ and enhanced biological production and burial of organic C, a small sink of anthropogenic CO₂, accompanied by a continuous trend toward increasing autotrophy in coastal waters.

Mackenzie F. T., Andersson A. J., Arvidson R. S., Guidry M. W., & Lerman A., in press. Land-sea Carbon and Nutrient Fluxes and Coastal Ocean CO2 Exchange and Acidification: Past, Present, and Future. Applied Geochemistry doi:10.1016/j.apgeochem.2011.03.087. Article (subscription required).