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Calculation of hydrogen isotopic fractionations in biogeochemical systems

Sessions, Alex L. and Hayes, John M. (2005) Calculation of hydrogen isotopic fractionations in biogeochemical systems. Geochimica et Cosmochimica Acta, 69 (3). pp. 593-597. ISSN 0016-7037. doi:10.1016/j.gca.2004.08.005.

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Hydrogen-isotopic data are often interpreted using mathematical approximations originally intended for other isotopes. One of the most common, apparent in literature over the last several decades, assumes that delta values of reactants and products are separated by a constant fractionation factor: δ_p = δ_r + ε_(p/r). Because of the large fractionations that affect hydrogen isotopes, such approximations can lead to substantial errors. Here we review and develop general equations for isotopic mass balances that include the differential fractionation of each component in a mixture and discuss their use in three geochemical applications. For the fractionation of a single component, the reactant and product are related by δ_p = α_(p/r)δ_r + ε_(p/r), where α and ε refer to the same fractionation. Regression of δ_p on δ_r should give equivalent fractionations based on the intercept and slope, but this has not generally been recognized in studies of D/H fractionation. In a mixture of two components, each of which is fractionated during mixing, there is no unique solution for the three unknown variables (two fractionation factors and the elemental mixing ratio of the two hydrogen sources). The flow of H from CH_4 and H_2O to bacterial lipids in the metabolism of Methylococcus capsulatus provides an example of such a case. Data and conclusions from an earlier study of that system (Sessions et al., 2002) are reexamined here. Several constraints on the variables are available based on plausible ranges for fractionation factors. A possible refinement to current experimental procedures is the measurement of three different isotopes, which would allow unique determination of all variables.

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Sessions, Alex L.0000-0001-6120-2763
Additional Information:© 2005 Elsevier Ltd. Received March 10, 2004; accepted in revised form August 5, 2004. The authors gratefully acknowledge the helpful advice and comments of Tapio Schneider. Associate editor Jeffrey Seewald, reviewer David Valentine, and two anonymous reviewers provided many helpful suggestions. A.L.S. is supported by NSF EAR-0311824, and J.M.H. is supported by the NASA Astrobiology Institute.
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Issue or Number:3
Record Number:CaltechAUTHORS:20130723-112638988
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Official Citation:Alex L. Sessions, John M. Hayes, Calculation of hydrogen isotopic fractionations in biogeochemical systems, Geochimica et Cosmochimica Acta, Volume 69, Issue 3, 1 February 2005, Pages 593-597, ISSN 0016-7037,
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:39523
Deposited By: Tony Diaz
Deposited On:19 Aug 2013 22:15
Last Modified:09 Nov 2021 23:45

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