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Study of thermochemical sulfate reduction mechanism using compound specific sulfur isotope analysis

Meshoulam, Alexander and Ellis, Geoffrey S. and Ahmad, Ward Said and Deev, Andrei and Sessions, Alex L. and Tang, Yongchun and Adkins, Jess F. and Liu, Jinzhong and Gilhooly, William P., III and Aizenshtat, Zeev and Amrani, Alon (2016) Study of thermochemical sulfate reduction mechanism using compound specific sulfur isotope analysis. Geochimica et Cosmochimica Acta, 188 . pp. 73-92. ISSN 0016-7037.

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The sulfur isotopic fractionation associated with the formation of organic sulfur compounds (OSCs) during thermochemical sulfate reduction (TSR) was studied using gold-tube pyrolysis experiments to simulate TSR. The reactants used included n-hexadecane (n-C_(16)) as a model organic compound with sulfate, sulfite, or elemental sulfur as the sulfur source. At the end of each experiment, the S-isotopic composition and concentration of remaining sulfate, H_2S, benzothiophene, dibenzothiophene, and 2-phenylthiophene (PT) were measured. The observed S-isotopic fractionations between sulfate and BT, DBT, and H_2S in experimental simulations of TSR correlate well with a multi-stage model of the overall TSR process. Large kinetic isotope fractionations occur during the first, uncatalyzed stage of TSR, 12.4‰ for H_2S and as much as 22.2‰ for BT. The fractionations decrease as the H_2S concentration increases and the reaction enters the second, catalyzed stage. Once all of the oxidizable hydrocarbons have been consumed, sulfate reduction ceases and equilibrium partitioning then dictates the fractionation between H_2S and sulfate (∼17‰). Experiments involving sparingly soluble CaSO_4 show that during the second catalytic phase of TSR the rate of sulfate reduction exceeds that of sulfate dissolution. In this case, there is no apparent isotopic fractionation between source sulfate and generated H_2S, as all of the available sulfate is effectively reduced at all reaction times. When CaSO_4 is replaced with fully soluble Na_2SO_4, sulfate dissolution is no longer rate limiting and significant S-isotopic fractionation is observed. This supports the notion that CaSO_4 dissolution can lead to the apparent lack of fractionation between H_2S and sulfate produced by TSR in nature. The S-isotopic composition of individual OSCs record information related to geochemical reactions that cannot be discerned from the δ^(34)S values obtained from bulk phases such as H2S, oil, and sulfate minerals, and provide important mechanistic details about the overall TSR process.

Item Type:Article
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Sessions, Alex L.0000-0001-6120-2763
Adkins, Jess F.0000-0002-3174-5190
Additional Information:© 2016 Elsevier Ltd. Received 26 November 2015; accepted in revised form 13 May 2016; available online 26 May 2016. Alexander Meshoulam thanks the Ministry of National Infrastructures Energy and Water Resources of Israel for MSc grant. Alon Amrani thanks the Israeli Science Foundation (ISF) Grant Number 1269/12 for partial support of this study. Liu Jinzhong acknowledge the support by the Chinese National Scientific Foundation number 41173069 and 41321001. We are also thank Gilad Antler (Cambridge University, UK) for bulk S analysis of some of the starting materials for interlaboratory comparison. We also grateful to Li Gao (PEERI), Bob Dias (U. S. Geological Survey) and three anonymous referees for helpful comments on an earlier version of this manuscript. Any use of trade, product, or firm names is for descriptive purposes only and does not imply endorsement by the U.S. Government.
Funding AgencyGrant Number
Ministry of National Infrastructures Energy and Water Resources (Israel)UNSPECIFIED
Israel Science Foundation1269/12
National Natural Science Foundation of China41173069
National Natural Science Foundation of China41321001
Subject Keywords:Thermochemical alteration; Organic matter; Sulfur compounds; Sulfate dissolution; Isotopes
Record Number:CaltechAUTHORS:20160915-075013877
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Official Citation:Alexander Meshoulam, Geoffrey S. Ellis, Ward Said Ahmad, Andrei Deev, Alex L. Sessions, Yongchun Tang, Jess F. Adkins, Jinzhong Liu, William P. Gilhooly III, Zeev Aizenshtat, Alon Amrani, Study of thermochemical sulfate reduction mechanism using compound specific sulfur isotope analysis, Geochimica et Cosmochimica Acta, Volume 188, 1 September 2016, Pages 73-92, ISSN 0016-7037, (
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:70358
Deposited By: Tony Diaz
Deposited On:28 Sep 2016 18:24
Last Modified:24 Feb 2020 10:30

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