Geology Seminar Series - Dr John Husson, University of Victoria - A close look at Earth’s largest carbon isotope excursion

Speaker: Dr. Jon Husson, University of Victoria

Title: A close look at Earth’s largest carbon isotope excursion

Abstract: Understanding the extreme carbon isotope excursions found in the Ediacaran Period (635-541 Ma), where the carbon isotopic composition of marine carbonates reach their minimum for Earth history, is one of the most vexing problems in Precambrian geology. Known as the Shuram excursion, many workers have interpreted this δ13C record as an unprecedented perturbation to the global carbon cycle, leading to speculation about a causal connection to the broadly contemporaneous rise of animal life. Others have interpreted the δ13C signal as a product of diagenesis, thereby minimizing its relevance for understanding the evolution of metazoans. Here we test these various hypotheses with integrated stratigraphic and bulk / in-situ isotopic studies of the Wonoka Formation of South Australia. Together, we find little evidence for the hypothesis that a late diagenetic overprint has generated the Shuram excursion in the Wonoka. These data, which indicate a mineralogical control on δ13C value and large spatial gradients in δ13C, also challenge the interpretation that the Shuram is a global carbon cycle perturbation, as classically defined. Marine environments in the Ediacaran seem to have been predisposed to creating carbonates with low δ13C values, but the exact processes that generated these unusual signals remain unclear.