2007 GSA Denver Annual Meeting (2831 October 2007)
Paper No. 5-10
Presentation Time: 11:20 AM-11:40 AM


WILLIFORD, Kenneth H.1, FORIEL, Julien2, WARD, Peter D.2, and SCHWAB, Valerie F.3, (1) Jet Propulsion Laboratory, 4800 Oak Grove Drive, Pasadena, CA 91109, Kenneth.H.Williford@jpl.nasa.gov, (2) Department of Earth and Space Sciences, University of Washington, Box 351310, Seattle, WA 98195, (3) School of Oceanography, University of Washington, 518 Ocean Sciences Building, Seattle, WA 98195

New pyrite and organic sulfur isotope data reveal a dramatic perturbation to the sulfur cycle in the earliest Jurassic coincident with a perturbation to the carbon cycle documented by Williford et al. (2007). Sulfur isotope ratios shift from values consistent with well-developed and microbially mediated sulfur cycling (-20 to -40 vcdt) to values similar to seawater sulfate (20 vcdt) over the same interval as a 5 positive excursion in stable organic carbon isotopes. This suggests a decline in seawater sulfate concentrations below 1 mmol, a value below which sulfate becomes limiting to sulfate reducing bacteria and isotopic fractionation associated with bacterial sulfate reduction ceases. A reduction in seawater sulfate concentration could be explained by the emplacement of evaporites associated with rifting in the nascent Atlantic basin and a short-term increase in bacterial sulfate reduction in the immediate wake of the Triassic-Jurassic extinctions. A survey of lipid biomarkers across this interval shows that the total lipid extract (TLE) in this section is in general immature but highly biodegraded, with a high concentration of hopanes relative to n-alkanes. In the samples with the heaviest carbon and sulfur isotope values, however, biodegradation is low and organic material (TLE) is dominated by n-alkanes rather than hopanes. A decline in bacterial organic remineralization in the water column would have resulted in a diminished export of isotopically light carbon and sulfur to surface waters, contributing to the observed change in bulk isotope values. In the samples with the heaviest carbon isotope values, a relatively high concentration of long chain n-alkanes with an odd over even preference is consistent with a higher plant source. An influx of recalcitrant terrestrial particulate organic matter may have occurred at this time due to a decline in sea level or a plant die-off associated with the T-J extinctions.

Williford, K.H., Ward, P.D., Garrison, G.H., Buick, R., 2007. An extended stable organic carbon isotope record across the Triassic-Jurassic boundary in the Queen Charlotte Islands, British Columbia, Canada. Palaeogeography, Palaeoclimatology, Palaeoecology 244(1-4): 290-296

2007 GSA Denver Annual Meeting (2831 October 2007)
General Information for this Meeting
Session No. 5
Oxygen, Evolution, and Extinction
Colorado Convention Center: 605/607
8:00 AM-12:00 PM, Sunday, 28 October 2007

Geological Society of America Abstracts with Programs, Vol. 39, No. 6, p. 25

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