UC Santa Cruz

Environmental Changes on the North American Mid-Atlantic Shelf During the Paleocene Eocene Thermal Maximum

Edward A. Ballaron

The Paleocene-Eocene Thermal Maximum (PETM), a large hyperthermal during the Cenozoic (~56 Ma) is characterized by a massive injection of depleted carbon into global reservoirs as indicated by a large negative carbon isotope excursion (CIE) of 3‰-5‰ in marine and terrestrial sedimentary records. Previously studied open ocean (i.e., pelagic) cores have provided evidence for abrupt sea surface warming (5-10°C) and bottom water warming (4-5°C) as well as ocean acidification at the onset of the CIE. However, due to low sedimentation rates and truncation of pelagic sections via dissolution at the CIE, pelagic cores appear condensed or incomplete. Sites on the North American mid-Atlantic margin, provide a unique opportunity to examine both marine and terrestrial responses to a large magnitude climatic perturbation. Rapid and abrupt increases in the flux of siliciclastics including kaolinite to the shelf at the onset of the PETM, indicative of an enhanced hydrologic cycle, result in expanded PETM intervals allowing for higher resolution isotopic sampling. Stable isotopic and trace metal data from New Jersey sections (Millville, Ancora, Bass River and Wilson Lake) have also documented rapid warming, coastal ocean acidification and a freshening event at the onset of the CIE in agreement with evidence for an enhanced hydrologic cycle during the PETM. Maryland sections to the south (South Dover Bridge and Cambridge-Dorchester) studied here, are more proximal to the main drainage system in the Salisbury Embayment and therefore should have experienced environmental changes (salinity) to a greater degree than New Jersey sites. Here we present coupled stable isotopic (carbon and oxygen) and trace metal data (Mg/Ca) to constrain both changes in temperature as well as salinity on the Maryland mid-Atlantic shelf during the PETM. Data presented here shows a temperature increase at SDB and Cam-Dor of ~4-5°C for planktonic foraminifera and ~5-9°C for benthic foraminifera with oxygen derived temperatures being consistently larger than Mg/Ca derived temperatures. This indicates some salinity contribution to δ18O, artificially lowering oxygen derived temperatures. This is further supported by salinity calculations which display a freshening event at the onset of the CIE due to increased runoff. Also observed in this data set is a slight weakening in δ13C and δ18O depth gradients on the shelf. Changes in δ18O gradients can perhaps be attributed to greater warming at depth and a destabilization of the thermal stratification of the water column, similar to observations from New Jersey sites Millivlle and Ancora. The breakdown in vertical δ13C gradients was perhaps due to the vertical migration of mixed layer foraminifera taxa seeking refuge from warming surface waters. Another possible explanation for the change in vertical δ13C gradients is a change in upwelling patterns driven by hyperpycnal flow or changes in wind patterns driving coastal upwelling and bringing depleted δ13C to the surface.