GSA Annual Meeting in Seattle, Washington, USA - 2017

Paper No. 378-1
Presentation Time: 9:00 AM-6:30 PM


KING PHILLIPS, Ezekiel J.1, COHEN, Phoebe1, JUNIUM, Christopher K.2 and UVEGES, Benjamin Thornton Iglar3, (1)Geosciences, Williams College, 203 Clark Hall, Williamstown, MA 01267, (2)Department of Earth Sciences, Syracuse University, 204 Heroy Geology Laboratory, Syracuse, NY 13244, (3)Department of Earth Sciences, Syracuse University, Syracuse, NY 13244,

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 δ13Corg 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, Δacritarch or Δa, varies between sections, with the largest Δa 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.