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Isotopic exchange of carbon-bound hydrogen over geologic timescales

Sessions, Alex L. and Sylva, Sean P. and Summons, Roger E. and Hayes, John M. (2004) Isotopic exchange of carbon-bound hydrogen over geologic timescales. Geochimica et Cosmochimica Acta, 68 (7). pp. 1545-1559. ISSN 0016-7037. doi:10.1016/j.gca.2003.06.004.

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The increasing popularity of compound-specific hydrogen isotope (D/H) analyses for investigating sedimentary organic matter raises numerous questions about the exchange of carbon-bound hydrogen over geologic timescales. Important questions include the rates of isotopic exchange, methods for diagnosing exchange in ancient samples, and the isotopic consequences of that exchange. This article provides a review of relevant literature data along with new data from several pilot studies to investigate such issues. Published experimental estimates of exchange rates between organic hydrogen and water indicate that at warm temperatures (50–100°C) exchange likely occurs on timescales of 10^4 to 10^8 yr. Incubation experiments using organic compounds and D-enriched water, combined with compound-specific D/H analyses, provide a new and highly sensitive method for measuring exchange at low temperatures. Comparison of δD values for isoprenoid and n-alkyl carbon skeletons in sedimentary organic matter provides no evidence for exchange in young (<1 Ma), cool sediments, but strong evidence for exchange in ancient (>350 Ma) rocks. Specific rates of exchange are probably influenced by the nature and abundance of organic matter, pore-water chemistry, the presence of catalytic mineral surfaces, and perhaps even enzymatic activity. Estimates of equilibrium fractionation factors between organic H and water indicate that typical lipids will be depleted in D relative to water by ∼75 to 140‰ at equilibrium (30°C). Thus large differences in δD between organic molecules and water cannot be unambiguously interpreted as evidence against hydrogen exchange. A better approach may be to use changes in stereochemistry as a proxy for hydrogen exchange. For example, estimated rates of H exchange in pristane are similar to predicted rates for stereochemical inversion in steranes and hopanes. The isotopic consequences of this exchange remain in question. Incubations of cholestene with D_2O indicate that the number of D atoms incorporated during structural rearrangements can be far less than the number of C-H bonds that are broken. Sample calculations indicate that, for steranes in immature sediments, the D/H ratio imparted by biosynthesis may be largely preserved in spite of significant structural changes.

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Sessions, Alex L.0000-0001-6120-2763
Summons, Roger E.0000-0002-7144-8537
Additional Information:© 2004 Elsevier Ltd. Received June 2, 2003; accepted in revised form June 29, 2003. We wish to thank Daniel Montluçon and the NOSAMS prep lab for assistance in preparing and analyzing incubation experiments, Simon Brassell for help identifying sterane products, Janet Hope for preparing petroleum silicalite fractions, and Arndt Schimmelmann for analyzing and providing hydrogen-isotopic reference materials. Timothy Eglinton provided sediments from the Gulf of Mexico. Martin Schoell, Bob Kagi, and Arndt Schimmelmann contributed through numerous discussions. Two anonymous reviewers provided helpful comments. This work was supported by grants from the NASA Exobiology (NAG5-9422) and Astrobiology (NCC2-1053) programs to JMH.
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Issue or Number:7
Record Number:CaltechAUTHORS:20130626-103329702
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Official Citation:Alex L. Sessions, Sean P. Sylva, Roger E. Summons, John M. Hayes, Isotopic exchange of carbon-bound hydrogen over geologic timescales, Geochimica et Cosmochimica Acta, Volume 68, Issue 7, 1 April 2004, Pages 1545-1559, ISSN 0016-7037,
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:39103
Deposited By: Tony Diaz
Deposited On:02 Jul 2013 22:50
Last Modified:09 Nov 2021 23:42

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