EXAMINING THE MARINE ISOTOPIC GRADIENT THROUGH THE LATE DEVONIAN USING MICROFOSSILS FROM THE KELLWASSER HORIZONS

The Late Devonian is characterized by declining ρCO₂, increased deposition of organic matter, widespread anoxia, and marine extinctions, specifically prevalent in shallow low latitude seas. The biggest pulses of extinction are linked to widespread laminated black shales, known as the Kellwasser horizons, which are correlated with two positive excursions in δ¹³C values of both carbonates and organic matter. In order to better understand the origin of these excursions, we extracted kerogens from black shales of the Kellwasser horizons at Cameron Creek and Eighteenmile Creek in upstate New York. Resulting macerates contained organic material that was predominantly of marine origin and include abundant and diverse organic-walled microfossils (i.e. acritarchs). To learn more about the driver of the δ¹³C excursions and to help resolve unknowns about the paleoecology of the organic microfossils, we measured the δ¹³C of 150 microfossils and kerogen grains using a cryotrapping, capillary-focusing ‘nanoEA’ for trace δ¹³C measurements.

The δ¹³C values of individual kerogen grains were isotopically similar between grains, as well as similar to bulk δ¹³C_org within each section. The δ¹³C values of individual microfossils were variable between fossils and significantly different from coeval kerogens. The difference between the microfossil and kerogen values, Δ_acritarch or Δ_a, varies between sections, with the largest Δ_a values occurring in the bed below the Lower Kellwasser horizon and the bed containing the Upper Kellwasser Horizon. While the paleoecology and taxonomic affinity of these fossils is uncertain, the δ¹³C values suggest that they are consistent with an algal origin. We speculate that the isotopic difference between bulk organic matter and the organic-walled microfossils may be the result of a surface water ecology where DIC is ¹³C-enriched because of a strong biological pump. This work sets the stage for more nuanced analyses of carbon cycle dynamics across the end-Devonian extinction event and for a better understanding of Paleozoic organic-walled microfossil paleoecology.