DECOUPLED SHELF-OCEAN PHYTOPLANKTON PRODUCTIVITY RESPONSES ACROSS THE PALEOCENE-EOCENE THERMAL MAXIMUM

The Paleocene-Eocene Thermal Maximum (PETM, ~55 Ma) saw dramatic reorganization within both plankton and benthic communities coincident with one of the most abrupt global warming episodes in Earth history. Global ocean temperatures increased by approximately 5-8 degrees corresponding with a perturbation in the carbon cycle. These phenomena are currently best explained by the massive input of dissociated methane hydrate to the ocean-atmosphere system. At several open ocean sites, a series of assemblage shifts within the calcareous nannoplankton community have been recognized across the PETM as part of a rapid biotic turnover among the protists. These biotic changes are thought to reflect a shift from colder, more productive waters, to warmer, more oligotrophic conditions. However, geochemical and faunal data at some of the same oceanic sites have been used to propose global increased phytoplankton productivity and CO₂ draw down as one of the negative feedback mechanisms responsible for the rapid recovery of temperatures after the peak of the PETM. This scenario is supported in shelf settings by dinocyst assemblage changes indicative of increased phytoplankton productivity.

Here we present integrated biotic and geochemical data from open ocean and shelf settings in an attempt to address these contradictions. We compare whole assemblage nannofossil abundances across the PETM from shelf deposits at Wilson Lake, New Jersey, with deep marine sections of Ocean Drilling Program Site 1209 on Shatsky Rise in the Pacific. Nannofossil assemblages at both sites display a pattern of continuous reorganization during the PETM. Assemblage shifts at Shatsky Rise demonstrate an interval of short-lived extremely high or low productivity coincident with the onset of the carbon isotope excursion, reflected by peaks in relative abundance of taxa characteristic of highly-stressed environments. In contrast, stable isotope and dinocyst records for Wilson Lake suggest that surface waters become seasonally more brackish on the New Jersey shelf across the PETM, and nannofossil assemblages confirm a transient shift to more eutrophic conditions associated with increased run-off and well-mixed surface waters. This suggests strong decoupling between the response of neritic and oceanic phytoplankton communities across the PETM.