ISOTOPIC AND MOLECULAR EVIDENCE FOR A MICROBIAL RESPONSE TO THE TRIASSIC-JURASSIC MASS EXTINCTION

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