Paper No. 6
Presentation Time: 8:00 AM-6:00 PM
AN INVESTIGATION THE SULFUR CYCLE DURING CAMBRIAN SPICE EVENT THROUGH THE USE OF MULTIPLE SULFUR ISOTOPE STRATIGRAPHY
GILL, Benjamin, Department of Earth and Planetary Sciences, Harvard University, Cambridge, MA 02138, LYONS, Timothy W., Department of Earth Sciences, University of California, Riverside, CA 92521, GILHOOLY III, William, Dept. of Earth Sciences, University of California, Riverside, CA 92521 and FARQUHAR, James, Earth System Science Interdisciplinary Center and Department of Geology, University of Maryland, College Park, MD 20742, bgill@fas.harvard.edu
The Paleozoic carbon isotope record is punctuated by several excursions that are thought to represent significant perturbations to the global carbon cycle. The later Cambrian Steptoean Positive Carbon Isotope Excursion, or
SPICE, is one such excursion. Not only does the SPICE suggest a large perturbation to the global carbon cycle, but our recent work has also documented a large-magnitude, global sulfur isotope excursion —recorded in both sulfate and pyrite and in phase with the carbon data. The coupled carbon and sulfur isotope relationship along with simple geochemical box modeling of the carbon and sulfur cycles points to parallel burial of significant quantities of organic carbon and pyrite sulfur within the marine system under widespread anoxic conditions. This relationship taken with the observation that the initiation of the SPICE coincides with a well-known extinction of Laurentian trilobites and increased biological turnover on other paleocontinents indicates that this interval was a time of severe environmental instability.
Here we present multiple sulfur isotope (32S, 33S, 34S and 36S) data to further investigate the environmental changes and the internal dynamics of the sulfur cycle during the SPICE. Specifically, we interpret these data as recording a scenario where the removal of sulfur via pyrite burial transiently exceeded the sources of sulfur to the ocean. Further, comparison of the Δ33S and δ34S from both sulfate and pyrite indicate “closed-system” isotope behavior indicating almost quantitative drawdown of the marine sulfate reservoir over the SPICE, likely a response to the transient increase in pyrite burial. This depletion of sulfate likely had severe consequences for the budgets of other chemical species in the ocean — notably oxygen, phosphorus and molybdenum — because the sulfur cycle plays a key role in the regulation of these element cycles. Thus, negative feedbacks driven by the drawdown of the marine sulfate reservoir must have contributed significantly to the demise of anoxic conditions during the SPICE.
