Sessions, Alex L. (2016) Factors Controlling the Deuterium Contents of Sedimentary Hydrocarbons. Organic Geochemistry, 96 . pp. 43-64. ISSN 0146-6380. http://resolver.caltech.edu/CaltechAUTHORS:20160328-081725938
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The hydrogen isotopic compositions of sedimentary hydrocarbon molecules are now being used to address a range of scientific questions, from paleoclimate to environmental reconstruction to understanding of petroleum systems. Here I review the environmental, biological, and physical/chemical factors that influence the H isotopic compositions of sedimentary hydrocarbons. A hierarchy of four main controls can be recognized: i) the composition of environmental water that serves as ultimate hydrogen source to the biosphere ii) physiologic and metabolic processes in organisms that fix water hydrogen into organic molecules, iii) hydrogen exchange processes that alter D/H ratios slowly over time under geologic conditions, and iv) kinetic fractionations that arise during the thermal conversion of sedimentary organic matter to liquid hydrocarbons. Variations in the terrestrial hydrologic cycle, and in biologic fractionations, create lipids with δD values spanning a huge range, and carrying abundant information about source organisms and environments. This information is gradually lost over geologic timescales due to hydrogen exchange, although the rates of this process appear to vary by orders of magnitude in clastic versus carbonate sediments. The H isotopes of hydrocarbons in many sediments of low to moderate thermal maturity may only be partially exchanged. Finally, additional kinetic fractionations are imposed during thermal cracking to generate mobile hydrocarbons. The δD values of sedimentary hydrocarbons are thus concluded to generally reflect a complicated combination of processes. Although it is tempting to view this as a failure of the isotopic record, there are numerous opportunities for understanding environment, diagenetic, and catagenetic processes if we can learn to quantitatively disentangle the competing fractionations.
|Additional Information:||© 2016 Elsevier Ltd. Received 30 October 2015; Received in revised form 23 February 2016; Accepted 25 February 2016; Available online 17 March 2016. I thank the many colleagues and friends who have helped me contribute to studies of hydrogen isotopes in petroleum systems over the years, including Arndt Schimmelmann, Roger Summons, Olaf Podlaha, John Hayes, Eugenio Santos Neto, Ying Wang, Alexandre Ferreira, and Wanglu Jia. Thanks to Kliti Grice and an anonymous reviewer who provided valuable comments on the manuscript, and to Dariusz Strapoc and Erdem Idiz who handled the editorial duties. Preparation of an earlier version of this manuscript was supported by Royal Dutch Shell; support was also provided by National Science Foundation award #EAR-1529120 to ALS.|
|Subject Keywords:||Deuterium; D/H; Hydrogen isotopes; Fractionation; Hydrocarbons; n-Alkanes|
|Official Citation:||Alex L. Sessions, Factors controlling the deuterium contents of sedimentary hydrocarbons, Organic Geochemistry, Volume 96, June 2016, Pages 43-64, ISSN 0146-6380, http://dx.doi.org/10.1016/j.orggeochem.2016.02.012. (http://www.sciencedirect.com/science/article/pii/S0146638016000516)|
|Usage Policy:||No commercial reproduction, distribution, display or performance rights in this work are provided.|
|Deposited By:||Tony Diaz|
|Deposited On:||30 Mar 2016 19:45|
|Last Modified:||27 May 2016 18:00|
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