The geological record of ocean acidification

Marine sediments contain information on a variety of past climate and carbon cycle perturbation events that may provide insights into the consequences of ongoing ocean acidification and potential future biotic changes. A variety of candidate events have been proposed, for which we assess the evidence for elevated CO2, global warming, and seawater carbonate chemistry changes, as well as having been primarily driven by massive carbon release. However, in order to be able to draw valid parallels with anthropogenic ocean acidification, we must distinguish between long-term changes and quasi steady states of marine carbon cycling, and transient changes, as only events shorter than ~10,000 years will exhibit the coupled reductions in ocean pH and carbonate saturation state that characterize today's (and the future) ocean. Quantifying the magnitude and rate of carbon cycle perturbations is therefore key to identifying and interpreting past intervals of ocean acidification. However, much evidence for paleocean acidification has been inferred from biotic changes, which could alternatively have been caused by other environmental changes such as changes in temperature, nutrient supply and oxygenation. Independent geochemical proxy evidence for paleo-seawater pH and carbonate saturation is therefore critical to confirm that ocean acidification did indeed take place. As a case study, we focus on the Paleocene/Eocene Thermal Maximum (56 Ma), an extreme climate event that displays close similarities with worst-case scenarios for the future, in terms of massive carbon release (~4,500 Pg C), associated warming (5-9°C), and its worldwide extent. We have applied the boron isotope/paleo-pH proxy to quantify the extent of ocean acidification associated with this event, and find a consistent pH decrease in the surface and deep ocean over the extent of the carbon isotope excursion, both in the Atlantic and Pacific Ocean basins, confirming that ocean acidification did take place.