Award: OCE-1220615

Intellectual Merit: Over 40% of the CO2 emitted by combustion of fossil fuels dissolves in the ocean. While this helps to slow the rate of C accumulation in the atmosphere, the reaction of CO2 with water produces a weak acid, thus lowering the pH of seawater. At projected rates of anthropogenic carbon emissions, the pH of the surface ocean is expected to decline by 0.3 pH units by the end of this century, and 0.7 pH units by 2300. Such levels would be harmful to many marine calcifiers including plankton and corals. The short and long-term response of ocean pH to future C emissions has been estimated using sophisticated models of the carbon cycle. Indeed, this study was motivated in large part by a need to better understand the rate at which C emissions are balanced by sequestration by natural processes. To this end, we used a specific event from Earth?s past, the Paleocene-Eocene Thermal Maximum (PETM), as a natural experiment to test theory. To achieve this goal, we first needed to establish the changes in ocean pH during this event, specifically the pH of the surface ocean. Previous studies quantified the scale of deep sea acidification by establishing changes in the dissolution of carbonate sediments on the seafloor as reconstructed from deep-sea cores. This data was then used to estimate the total mass of C released (thousands of petagrams (Pg) of C) at rates comparable to the anthropogenic fossil fuel emissions today (1 to 10 Pg/yr). This study advanced a step further by quantifying the change in the pH of the surface ocean by reconstructing changes in seawater borate (B) concentration and isotope composition (δ11B) as recorded in planktonic foraminifer shells. Samples were collected from multiple deep-sea cores in the Pacific and Atlantic ocean. The primary record was generated for a shell collected from a sediment core recovered from the central Pacific (Ocean Drilling Program Site 1209). We found that both the B concentration and δ11B of the plankton shells systematically decreased during the onset of the event in less than 5 ky, and then slowly recover over 150 ky, coeval with drop and recovery in the %carbonate of the sediment. Data from other cores in the Pacific and Atlantic showed the same pattern demonstrating that the trends in B are global surface ocean signals, consistent with ocean acidification by massive C emission. Based on calibrations of modern foraminifera we estimate a ?pH of -0.35. This value falls within values of ?pH estimated by models using independent geochemical constraints on the mass of carbon released during the PETM confirming both theory and previous field observations. These estimates of the pH estimate have also been used to estimate the rise in atmospheric pCO2 across the boundary, and are consistent with the release of 5 to 8 thousand PgC to the atmosphere over a period of 5 ky. This would also fully explain the magnitude of global warming (5°C) assuming an earth climate sensitivity of 2.5 to 4.0°C. Broader Impacts: A primary societal implication of this work is that the current and future emission of C to the atmosphere will acidify the ocean in line with predictions of the current generation of carbon cycle models. Moreover, once C emissions have slowed significantly or ceased, it will take roughly 100,000 years for the ocean carbonate system to fully recover through natural sequestration processes (i.e., the chemical weathering of silicate rocks and organic carbon burial). Last Modified: 10/13/2017 Submitted by: James C Zachos