A δ34SSO4 approach to reconstructing biogenic pyrite burial in carbonate-evaporite basins: An example from the Ara Group, Sultanate of Oman

Abstract

Carbonate-associated sulfate (CAS) has the potential to generate high-resolution records of seawater sulfate (δ^(34)S_(SW)) 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 δ^(34)S 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 δ^(34)S_(CAS) from Ara carbonates show little scatter and provide our estimate of δ^(34)S_(SW). Repeated enrichments (as much as 4‰ relative to δ^(34)S_(CAS)) were observed in floor and roof anhydrite units (δ^(34)S_(EVAP)) 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 ^(34)S-depleted sink, which we attribute to H_(2)S 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 δ^(34)S_(EVAP) offset (relative to CAS) is a function of local pyrite burial (f_(pyr)). In periods of low pyrite burial, it is possible for δ^(34)S_(EVAP) to be depleted relative to δ^(34)S_(SW), whereas during episodes of substantial pyrite burial (f_(pyr) > 0.05), δ^(34)S_(EVAP) can be strongly enriched relative to δ^(34)S_(SW). Our data suggest that local fpyr of ~0.13 is consistent with the observed δ^(34)S_(EVAP) 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 δ^(34)S_(CAS)-δ^(34)S_(anhydrite) 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.