EVIDENCE FOR SHELF ACIDIFICATION DURING THE ONSET OF THE PALEOCENE-EOCENE THERMAL MAXIMUM

Drillers from the United States Geological Survey have played an instrumental role in recovering high-quality cores from the Atlantic Coastal Plain (ACP). These materials have greatly advanced our understanding of paleoshelf processes during intervals of extreme warmth, including the Paleocene-Eocene Thermal Maximum (PETM). The majority of paleoshelf cores contain an interval of sediment with reduced amounts of carbonate material associated with dissolution at the onset of the PETM that limits investigations of biotas and geochemical proxies. This “dissolution zone” also signifies changes in key processes that took place on the paleoshelf during a critical phase of the event.

We have studied six cores in Maryland and New Jersey covering a transect from the inner to outer paleoshelf that show significant changes in the nature of the dissolution zone with water depth. An outer paleoshelf core at Bass River (New Jersey) contains a thin (20 cm) interval barren of carbonate microfossils. Middle paleoshelf cores contain a much thicker (up to 1.6 m) dissolution zone with samples devoid of carbonate microfossils and those with rare, etched specimens. An inner paleoshelf core at Mattawoman Creek-Billingsley Road (Maryland) is the only location without a clearly defined dissolution zone exclusively tied to the PETM.

A number of processes may have contributed to the absence or paucity of carbonate microfossils. The interval corresponds to a pulse of clastic sedimentation equivalent to the modern Amazon prodelta, but spatial patterns in the decrease in carbonate content are inconsistent with dilution as the sole cause of the decrease in carbonate. The observed carbonate preservation pattern implies a shoaling of the calcite compensation depth (CCD) and lysocline to middle shelf depths. This reduced carbonate preservation interval is observed during the onset of the PETM on other continental margins raising the possibility that extreme shoaling of the CCD and lysocline was a global phenomenon. An alternative scenario is that shoaling along the ACP was enhanced by regional and local factors, such as eutrophication and/or microbial activity associated with warming, which exacerbated the impact of acidification on the shelf. In our presentation we discuss evidence for each of the potential mechanisms for the dissolution zone.