Shelf ecosystems along the U.S. Atlantic Coastal Plain prior and during the Paleocene-Eocene Thermal Maximum

Projections of anthropogenic global warming predict extreme impacts on ecosystems, including loss in biodiversity, fluctuations in hydrological cycles, and variations in duration and intensity of seasons. Climate prediction, and resulting mitigation needs depend on the accuracy of climate models, which in turn rely on trustworthy climate data from the past. The Paleocene-Eocene Thermal Maximum (PETM) is considered one of the best analogs for the rate of current climate change. The PETM is a short, transient global warming event (~170 kyr), known as a hyperthermal, that took place ~56 Myr ago. It is the most pronounced of a series of hyperthermals that punctuated Earth’s long-term warming trend spanning the late Paleocene to early Eocene. The PETM is marked by negative δ18O and δ13C excursions in the isotopic record, caused by the release of large amounts of light carbon into the ocean-atmosphere system, and a concurrent warming trend of 5–8 °C, traceable from the deep oceans to the terrestrial surface. Additionally, the PETM also marks the beginning of distinct faunal and floral radiation, migration and turnover patterns in terrestrial and marine ecosystems, including a benthic foraminiferal extinction event in the deep sea.

The PETM is an intensely studied event, with an onset duration presumed to be (much?) less than 6kyr. However, while the timing of the isotopic excursions is well defined, the rate of change, and, connected to that, the trigger of the event, has been under debate ever since. Suggested triggers range from instantaneous ones, such as an extraterrestrial impact to more gradual ones, such as melting of methane clathrates or North Atlantic volcanism. During the Paleocene-Eocene transition, the U.S. Atlantic Coastal Plain was part of a clastic paleoshelf, with relatively high sedimentation rates, resulting in a well time-resolved recording of that period. Studies over the last few years, from sites in New Jersey and Maryland suggest a potentially more gradual, or stepwise development prior to the main isotopic excursion. Several reports document a lead-lag relationships between the Carbon Isotope Excursion (CIE) and for instance the temperature increase, or grainsize changes, which are typically associated with the PETM in the area. Additionally, the shallow marine δ13C record of South Dover Bridge (Maryland) shows a smaller δ13C excursion preceding the PETM within a finer grained interval – the so-called pre-onset excursion (POE).

In this work, I focused on the biotic and environmental changes on the U.S. Atlantic paleoshelf, to better understand the marine ecosystem prior to the PETM. I (i) established environmental background conditions and foraminiferal assemblage patterns across a shelf transect under varying river-influence and paleodepths, (ii) investigated the POE, associated environmental changes and biotic responses and (iii) explored the variability in environmental and biogeographic patterns during the Paleocene-Eocene transition.

The benthic foraminiferal assemblage of the shallow to outer neritic Paleocene shelf was heterogeneous and diverse, exhibiting a distinct bathymetric distribution. The shallow areas in the south were strongly influenced by a large river system emptying into the basin. The river influence decreased towards the northeast, where paleodepth increased. The food flux across the shelf was high and constant to the sea floor. The shelf was mainly dominated by currents and storm events, leading to well oxygenated bottom waters.

The POE, so far only described from the δ13Cbulk record, was confirmed by a δ13Cbenthic foraminifera record. Two more sites on the U.S. Atlantic Coastal Plain recorded characteristics associated with the PETM (Howard’s Tract and CamDor). An ocean acidification event is associated with the POE (δ11B excursion), as well as an increase in sea-surface temperatures. The δ13C and pH values recover before the PETM, indicating a rapid carbon release (shorter than ocean mixing time scales), and a recovery potentially driven by deep-sea mixing. As not enough foraminiferal material was available for trustworthy measurements, surface and bottom temperatures cannot be traced for the interval, but immediately after the POE temperatures started to increase towards the PETM (δ18O, Mg/Ca), showing a distinct offset between the δ13C excursion and temperature changes. The POE marks the beginning of the (regional?) benthic foraminiferal assemblage turnover towards the PETM: high-oxygen indicators start to decrease while, opportunistic taxa increase in number, or appear in the record.

The PETM on the U.S. Atlantic Coastal Plain is characterized by thick deposits of silty-clay (Marlboro Clay). An intensified hydrological cycle on land led to increased riverine input, not only transporting the fine grained sediments from the hinterland, but shifting the studied part of the shelf to a river dominated regime. Food supply to the sea floor became pulsed, but remained high. Bottom currents ceased, allowing salinity induced stratification of the water column. Dysoxic conditions developed on the sea floor, particularly in the shallower sites close to the river mouth. The dysoxic conditions were not severe or prolonged enough to cause a full disappearance of the benthic fauna, but a distinct shift to more resilient taxa. The diverse latest Paleocene benthic foraminiferal assemblage is replaced by a more homogeneous, opportunistic assemblage. This assemblage is dominated by few taxa, but still exhibits a poorly developed bathymetric distribution.

The recovery of the PETM took place gradually. Riverine influence, and connected dysoxic conditions, decreased and vanished. Increasing currents brought back stable oxygenated bottom waters. The opportunistic assemblage is slowly replaced by typical late Paleocene taxa, which either reappeared after vanishing from the record, or managed to persist in very small numbers during the CIE main phase.

This work shows that shelf ecosystems are highly sensitive to environmental changes and provide sedimentation rates high enough to record short term lasting developments, making them excellent settings to study climatic events of varying scales (POE, PETM). The POE CIE is associated with shifts in the environment, potentially in connection to the PETM, including a change in benthic foraminiferal assemblages. The effects of the POE on the shelf community indicates environmental effects caused by carbon emissions can outlast the carbon input and recovery intervals. Anthropogenic carbon release might be more similar to the POE than the PETM, but in both cases the greenhouse background conditions differ strongly from current, cooler global temperatures, and modern carbon release rates are still unprecedented.