PP23D-07
Insights into Methane Formation Temperatures, Biogenic Methanogenesis, and Natural Methane Emissions from Clumped Isotopes

Tuesday, 15 December 2015: 15:10
2003 (Moscone West)
Peter M Douglas1, Daniel A Stolper2, Katey M Walter Anthony3, Scott Dallimore4, Charles K Paull5, Martin Wik6, Patrick M Crill6, Mattias Winterdahl7, Derek A Smith1, Andrew J Luhmann8, Kang Ding9, William E Seyfried Jr9, John M Eiler10, Camilo Ponton11 and Alex L Sessions1, (1)California Institute of Technology, Pasadena, CA, United States, (2)Princeton University, Princeton, NJ, United States, (3)University of Alaska Fairbanks, Water and Environmental Research Center, Fairbanks, AK, United States, (4)Geological Survey of Canada Pacific, Vancouver, BC, Canada, (5)Monterey Bay Aquarium Research Institute, Watsonville, CA, United States, (6)Stockholm University, Dept. of Geological Sciences, Stockholm, Sweden, (7)Stockholm University, Stockholm, Sweden, (8)University of Minnesota, Minneapolis, MN, United States, (9)University of Minnesota Twin Cities, Minneapolis, MN, United States, (10)Caltech, Pasadena, CA, United States, (11)University of Southern California, Los Angeles, CA, United States
Abstract:
Multiply substituted isotopologues of methane are a valuable new tool for characterizing and understanding the source of methane in different Earth environments. Here we present methane clumped isotope results from natural gas wells, hydrothermal vents, marine and lacustrine methane seeps, and culture experiments.

We observe a wide range of formation temperatures for thermogenic methane. Methane samples from low-maturity reservoirs indicate formation temperatures between 102-144° C, high-maturity conventional and shale gasses indicate temperatures between 158-246 °C, and thermogenic coal gases indicate temperatures between 174-267 °C. Methane formation temperatures generally correlate positively with δ13C, and negatively with gas wetness indices. Methane samples from a set of marine hydrothermal vents indicate a formation temperature of 290-350 °C.

Methane sampled from subsurface and marine biogenic sources typically indicate temperatures consistent with the formation environment (0-64° C). In contrast, freshwater biogenic methane samples, and cultures of hydrogenotrophic and methylotrophic methanogens, express low levels of isotopic clumping inconsistent with their formation temperature. These data and complementary models suggest that kinetic isotope effects, likely modulated by rates and pathways of methanogenesis, affect biogenic methane in cultures and freshwater environments. Alternatively, non-equilibrium signatures may result from mixing of methane with widely differing δD and δ13C values.

Analyses of biogenic methane emissions from lakes indicate a correlation between methane flux and non-equilibrium clumped isotope fractionations in a given lake. Results from large methane seeps in Alaskan lakes confirm that some seeps emit thermogenic methane, but also indicate that other seeps emit subsurface biogenic methane or variable mixtures of biogenic and thermogenic methane. These results point to diverse sources for large Arctic methane seeps.

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