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A δ^(34)S_(SO_4) approach to reconstructing biogenic pyrite burial in carbonate-evaporite basins: An example from the Ara Group, Sultanate of Oman

Fike, D. A. and Grotzinger, J. P. (2010) A δ^(34)S_(SO_4) approach to reconstructing biogenic pyrite burial in carbonate-evaporite basins: An example from the Ara Group, Sultanate of Oman. Geology, 38 (4). pp. 371-374. ISSN 0091-7613.

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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.

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Fike, D. A.0000-0003-2848-0328
Grotzinger, J. P.0000-0001-9324-1257
Additional Information:© 2010 Geological Society of America. Received 12 March 2009; revision received 9 November 2009; accepted 10 November 2009. We thank the Oman Ministry of Oil and Gas for permission to publish this paper. This research was supported by Petroleum Development Oman (PDO) and a grant from the Agouron Institute. Fike was also supported by a National Science Foundation Graduate Research Fellowship and the Massachusetts Institute of Technology Global Habitability Longevity Award. We thank PDO for access to samples and logistical support, L. Pratt for use of laboratory facilities and discussions, S. Studley for laboratory assistance, and T. Lyons for comments.
Funding AgencyGrant Number
Petroleum Development OmanUNSPECIFIED
Agouron InstituteUNSPECIFIED
NSF Graduate Research FellowshipUNSPECIFIED
Massachusetts Institute of Technology (MIT)UNSPECIFIED
Issue or Number:4
Record Number:CaltechAUTHORS:20100415-110343838
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Official Citation:D.A. Fike and J.P. Grotzinger A δ^(34)S_(SO_)4 approach to reconstructing biogenic pyrite burial in carbonate-evaporite basins: An example from the Ara Group, Sultanate of Oman Geology April 2010, v. 38, p. 371-374, doi:10.1130/G30230.1
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:17995
Deposited By: Jason Perez
Deposited On:29 Apr 2010 21:13
Last Modified:13 Dec 2019 02:19

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