Colloquium: Jon Husson

Multi-proxy constraints on the origin of Earth’s deepest δ13C excursion

Jon Husson, Postdoctoral Fellow, University of Wisconsin

AbstractThe global carbon cycle plays a critical role in maintaining an equable climate on Earth; thus, studying its operation in deep time forms a cornerstone in our understanding of the co-evolution of life and the surface environment. This presentation focuses on a time period of profound non-uniformitarian change in the Earth system: the Ediacaran Period (635-541 Ma), when macro-scale animal fossils first appeared in abundance in the rock record. Broadly coeval with this biotic revolution, measurements of δ13C on shallow marine limestones record the most negative carbon isotope anomaly in the geological record. Known colloquially as the ‘Wonoka-Shuram' excursion, the -17‰ change in δ13C strains current paradigms to describe the carbon cycle. As models attempting to explain the excursion as a record of global ocean dissolved inorganic carbon (DIC) ran into increasing difficulty managing Earth's oxidant budget and climate, new ideas linking the δ13C signal to diagenetic processes began emerging. We present a chemostratigraphic study of 12 measured sections of the Ediacaran-aged Wonoka Formation (Fm.) of South Australia to test the various models. Field relationships and development of both traditional (δ13C, δ18O, trace element abundances) and non-traditional ( δ44/40Ca) geochemical datasets document the spatially consistent basinal expression of chemostratigraphic signals. Results further demand that the extraordinary range of δ13C values (-12 to +5‰) be syn-depositional in age and are products of the surface environment, thus requiring any explanatory model to address the unique occurrence of the Wonoka-Shuram excursion in the Ediacaran Period.