USING NITROGEN ISOTOPES AS A PALEOREDOX PROXY IN THE LATE DEVONIAN PAXTON MEMBER, MICHIGAN BASIN

The Late Devonian Frasnian-Famennian (FF) extinction event was one of the larger mass extinction events in the Phanerozoic. The cause of the extinction has yet to be determined, but it has been proposed that during the FF extinction the marine environment evolved into anoxic and ultimately euxinic conditions. In order to evaluate this hypothesis, nitrogen isotope analysis can be used to determine water column paleoredox conditions during this event. Using bulk δ¹⁵N measured from sedimentary samples, one can characterize the depositional environments as either oxic, anoxic or suboxic.

The Antrim Shale Formation in the Michigan Basin was deposited during the Late Devonian and contains a sedimentary record of the changes that occurred during the FF extinction event. The Michigan Basin was situated in the Eastern Interior seaway that changed from a carbonate platform to a deep basin during the Taghanic Onlap. The FF boundary is associated with the Upper Kellwasser Event (UKE) and is correlated to the Paxton-Lachine Member boundary in this basin. The Paxton Member was deposited during a time of marine regression, but what remains unclear is how the redox conditions evolved through this interval.

Nitrogen isotope profiles were measured through the Paxton Member from two separate cores: the State Chester #18 (Ostego County, Michigan, U.S.A.) and the Consumers Power #1-10 (Oscoda County, Michigan, U.S.A.). The State Chester #18 was situated on the paleo-basin margin, and the Consumers Power #1-10 was in the central basin. In the State Chester #18 core, the positive δ¹⁵N values oscillate from 0.0 to 0.8‰, and show no definite trend with depth. In contrast, the Consumers Power #1-10 shows δ¹⁵N values ranging from -1.3 to -3.3‰, with a decreasing trend in the data moving up section through the Paxton. Along with other geochemical analyses, including TOC and sequential iron extractions, the data indicate that oxic to suboxic depositional conditions were maintained through the regression at the basin margin core (State Chester #18), while the deep basin site (Consumers Power #1-10) shows evidence of increasing anoxia through the interval to the FF boundary. These results indicate that δ¹⁵N can be used to evaluate the evolution of paleoredox states in the Michigan Basin during the FF extinction event.