Review: Ocean acidification during the Paleocene-Eocene Thermal Maximum

Ocean acidification (OA) is a decrease in seawater pH caused by an increase in the dissolved carbon dioxide concentration in seawater and is an ongoing environmental issue. It is a threat to marine organisms with a calcified skeleton. Decreases in the populations and diversity of these marine organisms may change the carbon and nutrient cycles and cause the depletion of marine biological resources. Predicting the long-term (over a decadal scale) change in seawater pH is difficult. Long-term OA has been identified in geological records. The Paleocene-Eocene thermal maximum (PETM; ~56 Ma) was accompanied by benthic foraminifer and ostracod extinction events and is an ideal target for studying OA over geological time. We review previous studies on OA during the PETM. Many studies report a decrease in the carbonate content of deep-sea sediments, suggesting a shallowing of the carbonate compensation depth (CCD) and lysocline during the PETM. Carbonate ion concentrations on the Pacific seafloor were greater than those in the Atlantic, in contrast to the Holocene. Studies using boron isotopic compositions of planktic foraminifer shells estimate a decrease in sea-surface water pH of 0.2-0.5 within 60 ky of the onset of the PETM. The estimated pH values are associated with measurement and calculation uncertainties. The uptake of CO2 into the ocean and its emission into the atmosphere and ocean circulation affect the geographic pattern of seawater pH and are still debated.