Assessing Environmental Change Associated with Early Eocene Hyperthermals in the Atlantic Coastal Plain, USA

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 smaller magnitude can be used to characterize climatic responses over different magnitudes of forcing. This study identifies two events, Eocene Thermal Maximum 2 (ETM2 and H2) for the first time in a shallow marine setting along the United States Atlantic margin in the Salisbury Embayment of Maryland, based on magnetostratigraphy, calcareous nannofossil and dinocyst biostratigraphy, and 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). Paleo-temperature proxies show broad agreement between surface water and bottom water temperature changes. However, the timing of warming does not correspond to the CIE of ETM2 as expected from other records, and the highest values are observed during H2, suggesting factors other than pCO₂ forcing 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 ETM2. 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.