PTEROPODS FROM THE PALEOCENE-EOCENE THERMAL MAXIMUM (PETM) OF THE ATLANTIC COASTAL PLAIN, USA

While the response of many organisms to the climatic and environmental changes that occurred during the Paleocene-Eocene Thermal Maximum (PETM; ~56 Ma) has been documented, that of marine mollusks is unknown. We describe the first PETM assemblage of pteropods (planktic gastropods), consisting of six species representing three genera (Altaspiratella, Heliconoides and Limacina). Four species could be identified to species level, one of which is new. Only Heliconoides was previously known from pre-Eocene and early Eocene sediments, with a single Campanian specimen and one latest Paleocene species. Specimens are preserved as pyritized internal molds, except for two specimens where shell remains.

Pteropods were recovered from the Marlboro Fm. of the US Atlantic Coastal Plain, in sediments deposited from inner to middle-outer shelf depths. The Atlantic Coastal Plain contains some of the most complete records of shallow marine Paleocene-Eocene boundary deposition worldwide. The biostratigraphy, paleoecology and geochemistry of several of the cores used in this study have been previously analyzed at high resolution. At the Bass River site, the pH of surface waters (as recorded in B/Ca measured on planktic foraminifera) may have declined by 0.3-0.4 units during the PETM (Babila et al., 2016). Apparently, the drop in surface water pH was insufficient to dissolve living or dead pteropods before they reached the seafloor, possibly due to rapid buffering in coastal waters. Most living pteropods inhabit water depths of 200 m or more, and their occurrence in these shallow environments could reflect transport from deeper waters by coastal currents. Pteropods have thin, not easily fossilized shells, and so the apparently sudden appearance of three genera in Atlantic Coastal Plain sediments may indicate better preservational conditions, because the underlying Paleocene Vincentown Fm. shows poor preservation of calcareous microfossils. Specifically, rapid burial, triggered by increased sediment supply due to high rainfall, may explain the preservation of these PETM pteropods. However, because potentially existing ancestors have not been found anywhere else, we argue that the rapid environmental changes during the PETM may have triggered pteropod diversification.

Babila, T. et al. 2016. Geology 44:75-278.