Environmental expression of the Shuram-Wonoka carbon isotope excursion in North America

Daniel Segessenman

Abstract: Carbon isotope excursions recorded in marine carbonates have been identified as proxies signifying major geologic, atmospheric, and geochemical change in ancient Earth systems. The Shuram-Wonoka anomaly of the Ediacaran geologic period (635-541 Ma) is the greatest magnitude carbon isotope excursion measured in the stratigraphic record, with a positive excursion that climbs to a maximum of +12‰ and a negative excursion that falls to a minimum of -12‰. In addition to the Shuram-Wonoka excursion, Ediacaran age rocks record the deglaciation of Cryogenian Snowball Earth glaciations, the final stages of Rodinia rifting, contain the earliest known occurrences of animal fossils, and mark a shift from deposition of sediments predominantly on short-term recycled oceanic crust to long-term storage on continental crust. The Shuram-Wonoka anomaly’s relationship to the aforementioned critical transitions in the Ediacaran and the overall driving mechanism behind the expression of the Shuram-Wonoka anomaly remain a matter of debate. One potential factor that could contribute to the observed Shuram-Wonoka isotopic excursion is if there was a shift in the primary depositional environment of marine carbonates during the Ediacaran. Using a compilation of δ¹³C and δ¹⁸O measurements from Ediacaran carbonates set in a chronostratigraphic framework of Ediacaran rocks on North America, we show that there was a shift in primary carbonate depositional environment from continental shelf to primarily slope/basin settings. An expansion of carbonate depositional area coincides with the initial positive excursion of the Shuram-Wonoka, but the pronounced negative excursion does not occur until an expansion of slope/basin depositional settings for marine carbonates in the Ediacaran. Our results are in keeping with existing hypotheses of global sea level rise that coincided with the occurrence of the Shuram-Wonoka anomaly (~580-565 Ma) and global geochemical change in Ediacaran oceans.

Time: April 22nd (14:15)

Advisor: Shanan Peters

Co-Authors: Shanan E. Peters

Stream: Zoom

Email: segessenman@wisc.edu