A BIOMARKER APPROACH TO INTERPRETING PETM ORGANIC CARBON RECORDS (Invited Presentation)

Hyperthermal events, such as the Paleocene-Eocene Thermal Maximum (PETM), offer unique opportunities to explore how the Earth system responded to natural perturbations to the global carbon cycle in the geologic past and provide valuable insight into future impacts of anthropogenic climate change. Carbon isotope excursions (CIE) mark episodes of global warming in the geologic record, and the shape and stratigraphic thickness of these CIEs are used to correlate terrestrial and marine sections globally. However, the magnitude and shape of the PETM CIE vary widely among different carbon archives. Marine and terrestrial organic carbon (TOC) isotope records (δ¹³C_org) display similar mean CIE magnitudes (-3.5‰) but are thought to be attenuated relative to the atmospheric CO₂CIE (≈ -5.6‰).

Terrestrial organic matter records are particularly noisy and subject to local factors such as extensive carbon loss, selective preservation, and refractory carbon input, in addition to landscape-scale heterogeneity in biomass production, transport, and deposition. Records from the Bighorn Basin, Wyoming suggest that PETM δ¹³C_org records were modified by both microbial degradation, which doubled, and inputs of fossil carbon, which increased by about half. A compilation of the 12 published terrestrial δ¹³C_org records reveals that terrestrial organic carbon decreased by ~10% during the PETM, suggesting a global increase in microbial degradation. The doubling of soil respiration during the PETM was probably an important positive feedback on warming, and the increase in reworked organic matter suggests increased erosion and redeposition in response to climate change.

Conversely, marine organic carbon records indicate a 2-to-3-fold increase in organic carbon during the PETM (n=20). New data from cores collected along the eastern coastal US will be used to investigate the increase in marine TOC and attenuated marine CIE. Hypotheses include, but are not limited to, increased productivity, terrestrial carbon input, and/or microbial biomass. An examination of marine TOC and δ¹³C_org records, in conjunction with recent advancements in our understanding of PETM soil carbon dynamics, may offer further insight into patterns of PETM carbon release and sequestration.