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The Paleocene‐Eocene thermal maximum (PETM, 56 Ma) is an ancient global warming event closely coupled to the release of massive amounts of d13C‐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 d13C: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 d13C 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.
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Diagenetic Attenuation of Carbon Isotope Excursion Recorded by Planktic Foraminifers during the Paleocene-Eocene Thermal Maximum
Earth surface temperatures warmed by ~5ºC during an ancient (~56 Ma) global warming event referred to as the Paleocene-Eocene thermal maximum (PETM). A hallmark of the PETM is a carbon isotope excursion (CIE) signaling the release of massive amounts of 13C-depleted carbon into the ocean-atmosphere system, but substrate-specific differences in the CIE magnitude are a source of uncertainty for estimating the mass of carbon emitted. Here we report that SIMS-based in situ measurements of d13C in minute (7 um) domains of planktic foraminifer shells (ODP Site 865, central Pacific Ocean) yield a CIE that is ~2‰ larger than that delineated by conventional ‘whole-shell’ d13C values for this same PETM record. SIMS-based measurements on diagenetic crystallites yield d13C values (~2.8‰) that fall between those of pre-CIE and CIE planktic foraminifer shells, indicating that the crystallites are an amalgamated blend of pre-CIE and CIE carbonate. This suggests that diagenesis shifts the whole-shell d13C compositions of pre-CIE and CIE foraminifers found in samples straddling the base of the PETM interval toward the intermediate d13C composition of the crystallites thereby dampening the amplitude of the isotopic excursion. The diagenetic process envisioned would be most consequential for carbonate-rich PETM records that have suffered chemical erosion of pre-CIE carbonate. Given that the domains targeted for SIMS analysis may not be pristinely preserved, we consider the 4.6‰ excursion in our SIMS-based d13C record to be a conservative estimate of the full CIE for surface-ocean dissolved inorganic carbon.
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- Award ID(s):
- 1405224
- PAR ID:
- 10084945
- Date Published:
- Journal Name:
- Paleoceanography and paleoclimatology
- Volume:
- 33
- Issue:
- 4
- ISSN:
- 2572-4525
- Page Range / eLocation ID:
- 367-380
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript
- Journal Article:
- https://doi.org/https://doi.org/10.1002/2017PA003314
