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Sedimentary pyrite δ^(34)S differs from porewater sulfide in Santa Barbara Basin: proposed role of organic sulfur

Raven, Morgan Reed and Sessions, Alex L. and Fischer, Woodward W. and Adkins, Jess F. (2016) Sedimentary pyrite δ^(34)S differs from porewater sulfide in Santa Barbara Basin: proposed role of organic sulfur. Geochimica et Cosmochimica Acta, 186 . pp. 120-134. ISSN 0016-7037.

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Santa Barbara Basin sediments host a complex network of abiotic and metabolic chemical reactions that knit together the carbon, sulfur, and iron cycles. From a 2.1-m sediment core collected in the center of the basin, we present high-resolution profiles of the concentrations and isotopic compositions of all the major species in this system: sulfate, sulfide (∑H_2S), elemental sulfur (S^0), pyrite, extractable organic sulfur (OS), proto-kerogen S, total organic and dissolved inorganic carbon, and total and reducible iron. Below 10 cm depth, the core is characterized by low apparent sulfate reduction rates (<0.01 mM/yr) except near the sulfate-methane transition zone. Surprisingly, pyrite forming in shallow sediments is ∼30‰ more ^(34)S-depleted than coexisting ∑H_2S in porewater. S^0 has the same strongly ^(34)S-depleted composition as pyrite where it forms near the sediment–water interface, though not at depth. This pattern is not easily explained by conventional hypotheses in which sedimentary pyrite derives from abiotic reactions with porewater ∑H_2S or from the products of S^0 disproportionation. Instead, we propose that pyrite formation in this environment occurs within sulfate reducing microbial aggregates or biofilms, where it reflects the isotopic composition of the immediate products of bacterial sulfate reduction. Porewater ∑H_2S in Santa Barbara Basin may be more ^(34)S-enriched than pyrite due to equilibration with relatively ^(34)S-enriched OS. The difference between OS and pyrite δ^(34)S values would then reflect the balance between microbial sulfide formation and the abundance of exchangeable OS. Both OS and pyrite δ34S records thus have the potential to provide valuable information about biogeochemical cycles and redox structure in sedimentary paleoenvironments.

Item Type:Article
Related URLs:
URLURL TypeDescription
Sessions, Alex L.0000-0001-6120-2763
Fischer, Woodward W.0000-0002-8836-3054
Adkins, Jess F.0000-0002-3174-5190
Additional Information:© 2016 Elsevier Ltd. Received 10 June 2015, Accepted 19 April 2016, Available online 29 April 2016. We gratefully acknowledge the science team and crew of R/V Atlantis cruise AT26-06, especially the efforts of Katherine S Dawson (Caltech), David L. Valentine, Karin Lemkau, and Alex Phillips (UC Santa Barbara). This work benefitted from helpful discussions with Victoria J. Orphan and Guillaume Paris, analytical support from David Lyons (UC Riverside) and Fenfang Wu (Caltech), and assistance with sediment extractions from Emilia S. Hernandez. Funding was provided by the National Science Foundation award OCE1436566 to A.L.S. and the Gordon and Betty Moore Foundation through Grant GBMF#3306 to A.L.S.. This manuscript was much improved by insightful reviews from David Fike (Washington U. in St. Louis), Maya Gomes (Harvard), and Matt Hurtgen (Northwestern U.) and the careful editorial handling of Claire Rollion-Bard.
Funding AgencyGrant Number
Gordon and Betty Moore Foundation3306
Subject Keywords:Pyrite; Sulfur biogeochemistry; Organic sulfur; Porewater profiles; Sulfur isotopes
Record Number:CaltechAUTHORS:20160510-131537800
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Official Citation:Morgan Reed Raven, Alex L. Sessions, Woodward W. Fischer, Jess F. Adkins, Sedimentary pyrite δ34S differs from porewater sulfide in Santa Barbara Basin: Proposed role of organic sulfur, Geochimica et Cosmochimica Acta, Volume 186, 1 August 2016, Pages 120-134, ISSN 0016-7037, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:66907
Deposited By: Ruth Sustaita
Deposited On:11 May 2016 00:29
Last Modified:24 Feb 2020 10:30

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