Hot Tropical Temperatures During the Paleocene‐Eocene Thermal Maximum Revealed by Paired In Situ δ13C and Mg/Ca Measurements on Individual Planktic Foraminifer Shells

Abstract The Paleocene‐Eocene thermal maximum (PETM, 56 Ma) is an ancient global warming event closely coupled to the release of massive amounts of 13 C‐depleted carbon into the ocean‐atmosphere system, making it an informative analogue for future climate change. However, uncertainty still exists regarding tropical sea‐surface temperatures (SSTs) in open ocean settings during the PETM. Here, we present the first paired δ 13 C:Mg/Ca record derived in situ from relatively well‐preserved subdomains inside individual planktic foraminifer shells taken from a PETM record recovered in the central Pacific Ocean at Ocean Drilling Program Site 865. The δ 13 C signature of each individual shell was used to confirm calcification during the PETM, thereby reducing the unwanted effects of sediment mixing that secondarily smooth paleoclimate signals constructed with fossil planktic foraminifer shells. This method of “isotopic screening” reveals that shells calcified during the PETM have elevated Mg/Ca ratios reflecting exceptionally warm tropical SSTs (∼33–34°C). The increase in Mg/Ca ratios suggests ∼6°C of warming, which is more congruent with SST estimates derived from organic biomarkers in PETM records at other tropical sites. These extremely warm SSTs exceed the maximum temperature tolerances of modern planktic foraminifers. Important corollaries to the findings of this study are (a) the global signature of PETM warmth was uniformly distributed across different latitudes, (b) our Mg/Ca‐based SST record may not capture peak PETM warming at tropical Site 865 due to the thermally‐induced ecological exclusion of planktic foraminifers, and (c) the record of such transitory ecological exclusion has been obfuscated by post‐depositional sediment mixing at Site 865.