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Abstract. Eocene transient global warming events (hyperthermals) can provide insight into a future warmer world. While much research has focused on the Paleocene–Eocene Thermal Maximum (PETM), hyperthermals of a smaller magnitude can be used to characterize climatic responses over different magnitudes of forcing. This study identifies two events, namely the Eocene Thermal Maximum 2 (ETM2 and H2), in shallow marine sediments of the Eocene-aged Salisbury Embayment of Maryland, based on magnetostratigraphy, calcareous nannofossil, and dinocyst biostratigraphy, as well as the recognition of negative stable carbon isotope excursions (CIEs) in biogenic calcite. We assess local environmental change in the Salisbury Embayment, utilizing clay mineralogy, marine palynology, δ18O of biogenic calcite, and biomarker paleothermometry (TEX86). Paleotemperature proxies show broad agreement between surface water and bottom water temperature changes. However, the timing of the warming does not correspond to the CIE of the ETM2 as expected from other records, and the highest values are observed during H2, suggesting factors in addition to pCO2 forcing have influenced temperature changes in the region. The ETM2 interval exhibits a shift in clay mineralogy from smectite-dominated facies to illite-rich facies, suggesting hydroclimatic changes but with a rather dampened weathering response relative to that of the PETM in the same region. Organic walled dinoflagellate cyst assemblages show large fluctuations throughout the studied section, none of which seem systematically related to CIE warming. These observations are contrary to the typical tight correspondence between climate change and assemblages across the PETM, regionally and globally, and ETM2 in the Arctic Ocean. The data do indicate very warm and (seasonally) stratified conditions, likely salinity-driven, across H2. The absence of evidence for strong perturbations in local hydrology and nutrient supply during ETM2 and H2, compared to the PETM, is consistent with the less extreme forcing and the warmer pre-event baseline, as well as the non-linear response in hydroclimates to greenhouse forcing.
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Simulation of oxygen isotopes and circulation in a late Carboniferous epicontinental sea with implications for proxy records
Reconstructions of ancient ocean chemistry are largely based on geochemical proxies obtained from epicontinental seas. Mounting evidence suggests that these shallow inland seas were chemically distinct from the nearby open ocean, decoupling epicontinental records from broader ocean conditions. Here we use the isotope-enabled Community Earth System Model to evaluate the extent to which the oxygen isotopic composition of the late Carboniferous epicontinental sea, the North American Midcontinent Sea (NAMS), reflects the chemistry of its open-ocean sources and connect epicontinental isotope variability in the sea to large-scale ocean-atmosphere processes. Model results support estuarine-like circulation patterns demonstrated by past empirical studies and suggest that orographic runoff produced decreases in surface seawater δ18O (δ18Ow) of up to ∼3between the NAMS and the bordering ocean. Simulated sea surface temperatures are relatively constant across the sea and broadly reproduced from proxy-based δ18O paleotemperatures for which model-based values of epicontinental δ18Oware used, indicating that offshore-onshore variability in surface proxy δ18O is primarily influenced by seawater freshening. Simulated bottom water temperatures in the NAMS are also reproduced from biogenic calcite δ18O using model-based values of epicontinental δ18Ow, suggesting that benthic marine fossil δ18O is also influenced by seawater freshening and coastal upwelling. In addition, glacial-interglacial variations in nearshore seawater freshening counteract the effects of temperature on marine biogenic δ18O values, suggesting that salinity effects should be considered in δ18O-based estimates of glacioeustatic sea level change from nearshore regions of the NAMS. Our results emphasize the importance of constraining epicontinental dynamics for interpretations of marine biogenic δ18O as proxies of paleotemperature, salinity, and glacioeustasy.
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- Award ID(s):
- 1729882
- PAR ID:
- 10506569
- Publisher / Repository:
- Elsevier
- Date Published:
- Journal Name:
- Earth and Planetary Science Letters
- Volume:
- 559
- Issue:
- C
- ISSN:
- 0012-821X
- Page Range / eLocation ID:
- 116770
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1016/j.epsl.2021.116770
