GLOBAL OCEAN ACIDIFICATION DURING THE PETM: EVIDENCE FROM THE SOUTHERN OCEAN AND NEW JERSEY MARGIN

During the Paleocene-Eocene Thermal Maximum (PETM, ~56 Ma), the geologically rapid release of thousands of gigatons of carbon into the ocean-atmosphere system caused a large carbon isotope excursion, global surface warming, severe biotic disruption, and the widespread dissolution of seafloor carbonates. On the basis of a large decrease in surface- and thermocline-dwelling foraminiferal B/Ca ratios and boron isotopes at Site 1209 in the North Pacific, Penman et al. (2014, Paleoceanography) argued for significant (~0.3 pH units) ocean acidification during this event. However, uncertainties regarding the exact chemical controls on foraminiferal B/Ca and the possible impacts of dissolution, diagenesis, foraminiferal ecology, and/or local factors on both boron proxies complicate the robust interpretation of records from a single site. We present supporting records of planktic foraminifer B/Ca from several additional sites which unambiguously confirm the global nature of the Site 1209 records and strongly argue against the dominant control of any of the above complicating factors, which would have affected each site differently due to their varied paleoceanographic settings. Sites 689 and 690 (Maud Rise, Southern Ocean) were situated under significantly colder, more eutrophic surface waters than Site 1209 yet produced nearly identical B/Ca records, so ecological effects (such as symbiont bleaching) are unable to account for the B/Ca decline in both of these regions. Furthermore, the large difference in depth between the two Southern Ocean sites (~1 km) suggests that dissolution is not to blame, which would have affected the deeper site more severely. Finally, supporting records of B/Ca at Bass River (NJ Margin) from shallow-water (~200m depth), high sedimentation rate siliciclastic sediments would be very difficult to reconcile with a diagenetic origin for the large B/Ca decrease observed at all sites. We therefore conclude that the only possible driver for the simultaneous, consistent B/Ca decrease seen in each of these ecologically, paleoceanographically, and sedimentologically different records is the acidification of seawater caused by the rapid injection of carbon dioxide.