STABLE ISOTOPIC RESPONSE OF PLANKTONIC AND BENTHIC FORAMINIFERA DURING THE PALEOCENE-EOCENE THERMAL MAXIMUM, NEW JERSEY PALEOSHELF

The Paleocene-Eocene thermal maximum (PETM) was an abrupt warming event, characterized by a global temperature increase of ~5-8 °C and an injection of light carbon into ocean-atmosphere reservoirs resulting in a negative carbon isotope excursion (CIE) observed in both marine and terrestrial environments. Continental shelf PETM sections recovered from New Jersey are stratigraphically more complete in regard to the CIE onset and “core” (up to 11 m) than open ocean sites and yield well preserved foraminifera, which are advantageous for detailed geochemical reconstructions. Here, we present new δ¹³C and δ¹⁸O records of benthic foraminifera (Gavelinella, Anomalinoides, Cibicidoides) and previously reported isotopic records of surface (Morozovella and Acarinina) and thermocline dwelling (Subbotina) planktonic foraminifera during the PETM from Millville core site located along the New Jersey coastal plain (ODP Leg 174AX). Our new stable isotope data combined with existing data from three other sites located along a paleoshelf transect show that during the PETM surface planktonic foraminifera recorded a larger decrease in δ¹³C values (4.0-4.5 ‰) versus thermocline and benthic species (3.5 ‰); whereas thermocline dwellers and benthic species exhibit a larger change in δ¹⁸O (1.7 ‰) relative to the surface planktonic foraminiferal taxa (1.0 ‰). We propose two hypotheses that explain these isotopic trends in foraminifera: 1) a change in the water column structure during the PETM; and 2) a change in habitat of the surface dwellers due to environmental stress (e.g., warming, ocean acidification). In the first scenario, persistent warming during the PETM would have propagated heat into deeper layers and created a more homogenous water column with a thicker warm mixed layer and deeper, more gradual thermocline. In the second scenario, we assume that environmental change was greater in the mixed layer forcing surface dwellers to descend into thermocline waters as a refuge. Although both scenarios are possible, the first is predicted by polar amplification that would have caused more warming in thermocline waters globally.