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

The Paleocene‐Eocene thermal maximum (PETM, 56 Ma) is an ancient global warming event closely coupled to the release of massive amounts of 13C‐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 δ13C: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 δ13C 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. Plain Language Summary: The Paleocene‐Eocene thermal maximum (PETM, about 56 million years ago) is a global warming event that is widely regarded as an ancient analogue for climate change being driven by the current rise in atmospheric carbon dioxide (CO2) levels. Accurate measurements of PETM warmth in the tropical oceans are crucial to validating climate model simulations and gauging the effect of global warming on oceanic ecosystems. However, chemical analyses of marine microfossils (foraminifera) typically yield tropical sea surface temperatures (SSTs) for the PETM that are cooler than those computed by climate models. Primary reasons for this discrepancy are poor preservation of the foraminifer shells and displacement of shells from the cooler pre‐PETM interval into overlying PETM sediments via sediment mixing processes. Here, we use in situ microanalytical techniques to measure both the carbon isotope composition (δ13C) and Mg/Ca ratio within the same individual shells. The δ13C values of shells were used to identify displaced pre‐PETM specimens with higher, background δ13C ratios and exclude them from our Mg/Ca‐based temperature record. Our new "isotopically filtered" Mg/Ca‐based temperature record suggests ∼6°C of warming in the tropical Pacific, with SSTs (33–34°C) likely exceeding the maximum temperature tolerances of many calcifying plankton during the PETM. Key Points: Microanalytical techniques used to measure paired δ13C and Mg/Ca ratios in individual foraminifer shells from a pelagic Paleocene‐Eocene thermal maximum (PETM) recordδ13C values of individual foraminifers used to identify and exclude reworked non‐PETM specimens from Mg/Ca‐based sea‐surface temperature (SST) recordUnmixed Mg/Ca‐based temperature record indicates tropical SSTs increased by ∼6°C in central Pacific Ocean during PETM [ABSTRACT FROM AUTHOR]