Dynamic sulfur and carbon cycling through the end-Ordovician extinction revealed by paired sulfate-pyrite δ34S

Geochemical records of the end-Ordovician Hirnantian Stage show parallel positive excursions in the stable isotope compositions of sedimentary pyrite sulfur (delta S-34(pyr)), organic carbon (delta C-13(org)), and carbonate carbon (delta C-13(carb)); these isotope excursions coincide with the end Ordovician glaciation and mass extinction. A relative increase in pyrite burial (f(pyr)) attributed to marine anoxia has been invoked to explain the sulfur isotope excursion and link oceanic redox conditions to the extinction of marine fauna. An increase in f(pyr) would necessarily generate a parallel excursion of equal magnitude in the isotopic composition of coeval marine sulfate (delta S-34(SO4)). Here we present new high-resolution paired sulfur isotope data from carbonate-associated sulfate (delta S-34(CAS)) and pyrite from the Hirnantian Stage of western Anticosti Island (Quebec, Canada). These data document a positive 20 parts per thousand enrichment in delta S-34(pyr) (comparable in magnitude to previous reports), but no parallel excursion in delta S-34(CAS). This pattern provides new constraints on the origin of the delta S-34(pyr) excursion and the nature of carbon-sulfur coupling through Hirnantian time. Specifically, these observations preclude enhanced pyrite burial as the cause of the Hirnantian delta S-34(pyr) excursion and suggest the possible role of anoxia in the mass extinction may need to be reevaluated. Rather, the global delta S-34(pyr) excursion is best explained by a transient reduction in the isotopic fractionations expressed during microbial sulfur cycling (epsilon(pyr)). The epsilon(pyr) record shows a strong inverse correlation with delta C-13, suggesting a mechanistic link between carbon cycling and processes controlling epsilon(pyr) during the Hirnantian. Changes in sea level or marine redox state associated with glaciation could further impact the expression of the biological fractionation (e.g., through syndepositional sediment reworking and/or chemocline migration and resultant restricted exchange of porewater sulfate). The magnitude of isotopic fractionation during microbial sulfate reduction is partially controlled by metabolic rates, which are sensitive to the abundance, type, and lability of metabolically relevant substrates. Environmental change associated with the end Ordovician glaciation may have elevated the flux of organic material to marine sediments or caused an increase in physical reworking of sediments, leading to increased microbial sulfate reduction rates and reduced epsilon(pyr). As such, the Hirnantian delta S-34(pyr) excursion may be viewed as a dynamic biological response to global climate change, highlighting the connections between the carbon and sulfur biogeochemical cycles. (C) 2012 Elsevier B.V. All rights reserved.