Molecular and isotopic composition of hydrate-bound and dissolved gases in the southern basin of Lake Baikal, based on an improved headspace gas method
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Assessments of the molecular and isotopic composition of hydrate-bound and dissolved gases in pore water were conducted during the multi-phase gas hydrate project (MHP-09) cruise VER09-03 to the southern basin of Lake Baikal in September 2009. To avoid changes in gas composition during core sampling and transport, various headspace methods were investigated aimed at preserving the dissolved gases in pore water. When distilled water was added to the sediment samples, the concentrations of carbon dioxide and oxygen decreased because of dissolution into the water and/or microbial consumption. When the headspace was not flushed with inert gases, trace levels of hydrogen and ethylene were detected. The findings suggest that best preparation is achieved by flushing the headspace with helium, and adding a saturated aqueous solution of sodium chloride. This improved headspace method served to examine the molecular and isotopic compositions of gas samples retrieved at several new sites in the southern basin. Methane was the major component, and the proportion of ethane ranged widely from 0.0009 to 1.67 mol% of the total hydrocarbon gases. The proportions of propane and higher hydrocarbons were small or less than their detection limits. The carbon isotope signatures suggest that microbial-sourced methane and ethane were dominant in the Peschanka study area, whereas ethane was of thermogenic origin at all other study sites in the southern basin of Lake Baikal.
KeywordsPore Water Southern Basin Headspace Method Thermogenic Origin Southern Baikal Basin
We greatly appreciate the assistance of the shipboard crews of RV G. Yu. Vereshchagin during the Lake Baikal expeditions. We express our gratitude to Dr. Stephan A. Klapp and an anonymous reviewer for their helpful comments and suggestions. This work was supported by funding agencies in Japan (Grant-in-Aid for Scientific Research 19550077, 21254006, 21360219, 22540485, and 23254008 from the Japan Society for the Promotion of Science), and the Integration Project of RAS SB 27.
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