2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 15
Presentation Time: 11:45 AM


FIKE, David, Earth, Atmospheric, & Planetary Sciences, MIT, 77 Massachusetts Ave., 54-812, Cambridge, MA 02139, GROTZINGER, John, Department of Earth, Atmospheric, & Planetary Sciences, MIT, 77 Massachusetts Ave., 54-820, Cambridge, MA 02139, PRATT, Lisa, Department of Geological Sciences, Indiana University, 1001 East 10th Street, Bloomington, IN 47405 and SUMMONS, Roger, Earth and Planetary Sciences, Massachusetts Institute of Technology, 44 Carleton St, E34-246, Cambridge, MA 02138, dfike@mit.edu

Carbon and sulfur chemostratigraphy of the Huqf Supergroup [~635 – 540 Ma] indicate progressive oxygenation of the ocean during the terminal Neoproterozoic.  Following the Fiq (Marinoan-equivalent) glaciation, fractionation between sulfate and sulfide d34Ssulfate-sulfide increases from ~ <10‰ to ~28‰, indicating a post-Marinoan increase in seawater sulfate concentrations beyond the threshold for significant microbial fractionation.  Across the same stratigraphic interval, bitumen d13C exhibits a ~2‰ depletion relative to kerogen; however, bitumen is equivalent or enriched in 13C relative to kerogen in older Huqf strata.  Isotopic depletion of bitumen d13C, found in younger Huqf strata and throughout the Phanerozoic, signals a fundamental change in carbon cycling.


The overlying Shuram Formation contains a negative excursion >13‰ in carbonate d13C, which is correlated with a similar excursion in other globally-distributed localities.  Sulfate d34S remains relatively invariant throughout deposition of the Shuram, indicating that significant sulfide oxidation did not accompany the d13C excursion.  Coupled organic carbon and carbonate d13C data reveal non-steady state carbon cycling in which organic d13C is buffered from changes in coexisting carbonate d13C.  Thus, the Shuram d13C excursion may be explained by oxidation of a vast reservoir of dissolved organic carbon in the deep ocean.  Following the Shuram excursion, coeval organic and carbonate d13C exhibit covariance, including across a 7‰ negative excursion in d13C at the Precambrian/Cambrian (PC-C) boundary.


In strata younger than the Shuram d13C excursion, sulfate d34S increases dramatically from ~23‰ to a maximum of ~42‰ at the PC-C boundary.  An increase in pyrite d34S (from ~-5‰ to ~15‰) parallels that found in sulfate d34S.  Consistency in d34Ssulfate-sulfide indicates that this increase in d34S results from an increase in pyrite burial. Buildup of oxidants derived from an increase in burial of pyrite may have been sufficient to oxygenate the deep ocean, as is consistent with a decline in sulfate d34S following the PC-C boundary.  Together these data indicate a stepwise reorganization of the global carbon and sulfur biogeochemical cycles in the terminal Neoproterozoic resulting in the ventilation of the deep ocean.