Utilizing pyritic δ34S to characterize the depositional conditions of the Neo-Tethys in the Late Permian
Abstract
Understanding changes in the sulfur cycle during the end-Permian is an important step in detangling local from global environmental changes occurring at the time. In this study, pyrite δ34S was measured in shales (Gungri Formation, Late Permian) samples collected at five Neo-Tethys PT sections (Attargo, Lingti 1, Lingti 2, Guling, and Mud) from Spiti Valley, Himichal Pradesh, India. Pyritic sulfur was extracted from samples collected at variable resolution, from 10-cm to 50-cm intervals from the P-T Boundary to the base of exposed outcrop, by chromium reduction. δ34S was measured by EA-IRMS (Costech 4010 Elemental Analyzer and Thermo Delta V+Isotope Ratio Mass Spectrometer). One of the main factors controlling the isotopic composition of pyrite is the isotopic fractionation caused by bacterial sulfate reduction (BSR) since the lighter 32S is preferentially reduced over the heavier 34S. In an open marine system, seawater sulfate is refreshed providing sufficient 32S to be continuously reduced. This renewal of sulfate potentially yields fractionations as large as 40-60‰. In these systems, organic matter is typically the limiting factor in BSR, producing a positive correlation between organic carbon and pyritic sulfur. In a closed marine system, where the sulfate supply is not refreshed, the pyrite δ34S values become enriched as BSR uses the heavier 34S isotope due to a lack of 32S. Complete utilization of sulfate produces pyrite δ34S values equal to or greater than the initial seawater sulfate. Additional factors that can influence pyritic δ34S and enhance the "sulfate reservoir" effect include changes in detrital inputs and burial fluxes of both pyrite and sulfate. Therefore, the δ34S incorporated into pyrite potentially contains important information on its formation environment, particularly when used in conjunction with other C-S-Fe proxies such as carbon-to-sulfur ratios (C/S) and degree of pyritization (DOP). These proxies were combined to characterize changes in the depositional environment of the Neo-Tethys, both temporally and spatially, and suggest that the Neo-Tethys was isolated from the Paleo-Tethys prior to the end-Permian extinction.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2013
- Bibcode:
- 2013AGUFM.V32C..07S
- Keywords:
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- 1041 GEOCHEMISTRY Stable isotope geochemistry;
- 0473 BIOGEOSCIENCES Paleoclimatology and paleoceanography