The negative organic carbon isotope excursion (CIE) associated with the end-Triassic mass extinction (ETE) is conventionally interpreted as the result of a massive flux of isotopically light carbon from exogenous sources into the atmosphere (e.g., thermogenic methane and/or methane clathrate dissociation linked to the Central Atlantic Magmatic Province [CAMP]). Instead, we demonstrate that at its type locality in the Bristol Channel Basin (UK), the CIE was caused by a marine to nonmarine transition resulting from an abrupt relative sea level drop. Our biomarker and compound-specific carbon isotopic data show that the emergence of microbial mats, influenced by an influx of fresh to brackish water, provided isotopically light carbon to both organic and inorganic carbon pools in centimeter-scale water depths, leading to the negative CIE. Thus, the iconic CIE and the disappearance of marine biota at the type locality are the result of local environmental change and do not mark either the global extinction event or input of exogenous light carbon into the atmosphere. Instead, the main extinction phase occurs slightly later in marine strata, where it is coeval with terrestrial extinctions and ocean acidification driven by CAMP-induced increases in
- Award ID(s):
- 2026877
- NSF-PAR ID:
- 10324386
- Date Published:
- Journal Name:
- Proceedings of the National Academy of Sciences
- Volume:
- 118
- Issue:
- 37
- ISSN:
- 0027-8424
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract Carbon isotope (δ13C) records from marine sediments and sedimentary rocks have been extensively used in Cenozoic chemostratigraphy. The early Paleogene interval in particular has received exceptional attention because negative carbon isotope excursions (CIEs) documented in the sedimentary record, for example, at the Paleocene Eocene Thermal Maximum (PETM), ca ∼56 Ma, are believed to reflect significant global carbon cycle perturbations during the warmest interval of the Cenozoic era. However, while bulk carbonate δ13C values exhibit robust correlations across widely separated marine sedimentary basins, their absolute values and magnitude of CIEs vary spatially, especially over time intervals characterized by major deviations in global carbon cycling. Moreover, bulk carbonates in open‐marine environments are an ensemble of different components, each with a distinct isotope composition. Consequently, a comprehensive interpretation of the bulk‐δ13C record requires an understanding of co‐evolution of these components. In this study, we dissect sediments, from the late Paleocene‐early Eocene interval, at ODP Site 1209 (Shatsky Rise, Pacific Ocean) to investigate how a temporally varying bulk carbonate ensemble influences the overall carbon isotope record. A set of 45 samples were examined for δ13C and δ18O compositions, as bulk and individual size fractions. We find a significant increase in coarse‐fraction abundance across the PETM, driven by a changing community structure of calcifiers, modulating the size of the CIE at Site 1209 and thus making it distinct from those recorded at other open‐marine sites. These results highlight the importance of biogeography in the marine stable isotope record, especially when isotope excursions are driven by climate‐ and/or carbon cycle changes. In addition, community composition changes will alter the interpretation of weight percent coarse fraction as proxy for carbonate dissolution.
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1073/pnas.2014701118
