Abstract

Carbonate-associated sulfate (CAS) has the potential to generate high-resolution records of seawater sulfate (δ34SSW) that improve upon existing evaporite-based records. With improved resolution, however, significant isotopic offsets have become apparent between the evaporite and CAS proxies. Here we present high-resolution δ34S measurements of both these proxies from the Ara Group, Sultanate of Oman, a series of six carbonate-evaporite sequences deposited ca. 547–540 Ma. The δ34SCAS from Ara carbonates show little scatter and provide our estimate of δ34SSW. Repeated enrichments (as much as 4‰ relative to δ34SCAS) were observed in floor and roof anhydrite units (δ34SEVAP) bounding the Ara carbonates. These enrichments cannot be explained by secular variation or from isotopic fractionation during evaporite deposition and require the existence of an additional 34S-depleted sink, which we attribute to H2S production via ongoing bacterial sulfate reduction during evaporite deposition. To preserve this isotope signature, the resulting sulfide must be sequestered as pyrite. The magnitude of the resulting δ34SEVAP offset (relative to CAS) is a function of local pyrite burial (fpyr). In periods of low pyrite burial, it is possible for δ34SEVAP to be depleted relative to δ34SSW, whereas during episodes of substantial pyrite burial (fpyr > 0.05), δ34SEVAP can be strongly enriched relative to δ34SSW. Our data suggest that local fpyr of ∼0.13 is consistent with the observed δ34SEVAP enrichments found in the Ara Group anhydrites. Such elevated pyrite burial in evaporitic settings requires a substantial iron source, supporting possible ferruginous ocean conditions across the Ediacaran-Cambrian boundary. Isotopic offsets between paired δ34SCAS34Sanhydrite data may thus serve as an independent proxy for marine redox through time, in addition to quantifying the importance of microbial activity in a setting where direct evidence (e.g., total organic carbon or biomarkers) may be scarce and physical processes are thought to dominate.

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