Single-shell Stable Isotope Signatures as a Taphonomic Filter: Deconvolving the Effects of Sediment Mixing on Tropical Planktic Foraminiferal Assemblages from Paleocene-Eocene Thermal Maximum

The Paleocene-Eocene thermal maximum (PETM; 55.8 Ma) is an ancient global warming event associated with the input of massive quantities of δ¹³C-depleted carbon to the ocean-atmosphere system, making it an informative analog for constraining future outcomes of unabated greenhouse gas emissions. Deep-sea sedimentary records, and the marine microfossils (planktic foraminifera) they preserve, are important archives of ocean-climate change during the PETM. Previous studies have used point-counting methods to document changes in species diversity among planktic foraminifer communities that, in turn, are used to gauge the impact of PETM conditions on the health of pelagic ecosystems. However, we posit that this standard practice underestimates the true biotic response of planktic foraminifers as the stratigraphies of these records are distorted by such taphonomic processes as sediment mixing. Here we use δ¹³C signatures of individual planktic foraminifer shells to deconvolve the taphonomic and biotic signals encoded in the fossil record of a PETM section recovered from Ocean Drilling Program (ODP) Site 865 in the tropical Pacific Ocean. The negative carbon isotope excursion (CIE) marking the PETM imparts a bimodal distribution to planktic foraminifer δ¹³C values across the excursion onset, with CIE shells registering anomalously low δ¹³C values and displaced pre-CIE contaminants returning high, background δ¹³C values. We use such single-shell δ¹³C data to estimate proportions of CIE (in situ) shells versus pre-CIE (reworked) shells for several key taxa within the CIE interval. These estimates are a quantitative basis for correcting species-level abundances initially tabulated by point counting. Our results show that the point-counted and corrected assemblages differ significantly, with some major taxa (Subbotina spp.) within the CIE interval being represented entirely by reworked shells whereas other taxa (Morozovella allisonensis) are entirely in situ. This on-going study provides a more accurate record of how tropical planktic foraminifers responded to paleoenvironmental change fostered by what is arguably the best analog for future climate change, the PETM.