AN OCEAN ACIDIFICATION PULSE IN THE PRE-ONSET CARBON ISOTOPE EXCURSION PRECEDING THE PALEOCENE-EOCENE THERMAL MAXIMUM?

The Paleocene-Eocene Thermal Maximum (PETM; ~55.96 Ma) is recognized globally in marine sediments by a carbonate dissolution zone, the extinction or turnover of benthic taxa, and a radiation of planktic excursion taxa, all accompanied by a rapid-onset, negative carbon isotope excursion (CIE). The cause and nature of the massive carbon release leading to this extreme climate event remains under debate. Regardless of cause, the environmental and ecosystem changes centered on the PETM are the subject of much study because they provide an analog to modern deteriorating conditions associated with the ongoing rise in atmospheric carbon dioxide.

We present evidence from sediments of the South Dover Bridge core, deposited on the U.S. mid-Atlantic shelf, for a dissolution level in the latest Paleocene that coincides with a relatively small (-2‰) negative CIE that precedes the larger (-4‰) Paleocene-Eocene CIE onset. Planktic foraminifers within this pre-onset event (POE) show post-deposition dissolution, potentially related to surface-water acidification events, in which the coarsely cancellate and muricate wall textures, characteristic of many late Paleocene species, have been partly dissolved away, leaving smooth, thin-walled specimens often with collapsed chambers. In addition, we document transient biotic responses in benthic, planktic, and terrestrial communities to the POE. These include shifts in foraminifer and pollen assemblages and adaptations in calcareous nannofossil species in response to regional environmental perturbations.

An environmental recovery is evident between the POE and CIE in both the carbon isotopic signal and in the biotic response, providing additional evidence not only for a pulsed carbon release, but also for a more rapid rate of carbon release than is suggested by a single pulse over a longer period of time. The timing, nature and magnitude of ecological changes during the less extreme POE shallow-water ‘acidification’ event may help to define an ecological tipping point of shallow marine ecosystems.