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.
William Davis Rush et al.
Status: open (until 12 Apr 2023)
- RC1: 'Comment on egusphere-2023-58', Anonymous Referee #1, 13 Mar 2023 reply
William Davis Rush et al.
William Davis Rush et al.
Viewed (geographical distribution)
The manuscript provides new data of the ETM2 and H2 early Eocene Hyperthermal events in the Salisbury Embayment (Mid Atlantic coastal plain). This area has been studied in some sequences in the last years with particular interest on the PETM and the hyperthermal events of the lower Eocene , however the data presented by the authors are interesting as they show short - term changes and long - term trends which are often not consistent with data from pelagic, hemipelagic and coastal successions.
The first part of the manuscript is very robust and characterizes the stratigraphic framework of the succession which allows to correlate the investigated succession with the hyperthermal events ETM2 and H2.
I suggest the authors not to use informal terms in their biostratigraphic schemes and in any case to present them in detail in the methods. Specifically, the use of lower/upper before the biozone cannot be added in the biostratigraphy column of the figures. The authors can instead add events in the figure in order to highlight the possible informal subdivision of the biozone. However, these events should be detailed in the text (more details in the text).
The second part is dedicated to the changes that occurred in this depositional setting and to their substantial inconsistency with the available data both in terms of temporal relationships and actual modifications of the environment during these perturbations.
In the pdf file, I have included many comments and suggestions that the authors can use to implement their manuscript. In particular the authors must make sure not to mix results and discussion of them throughout the ms (details are provided in the edited file).
As for the discussion, there are some points, especially in the part that offers scenarios useful to explain the peculiar results of this study, which are either superficially presented or are inconsistent with the available literature (e.g. , the AMOC hypothesis, more details on specific points are provided in the edited file).
Another point that could potentially be improved is the comparison with all the available data and in any case with those of the Spanish sections which, in addition to having played a important role in the development of fundamental hypotheses related to mechanisms active during the hyperthermals (e.g., weathering and clay mineralogy)also represent a suitable continuous hemipelagic-continental transect ideal for a comparison with the record of the Mid Atlantic coastal plain.
Finally, I provide there are some suggestions to improve the figures, These are related to both the lack of units and the use of informal nomenclature ( more details in the text)