Methane plume observations on Mars: Difference between revisions
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One way to distinguish between biogenic and abiogenic sources of methane might be to measure the carbon-12 to carbon-14 ratio. Methanogens produce a gas which is much richer in the lighter carbon-12 than the products of serpentization.<ref name=Baucom/>
Here carbon 12 is the light stable isotope of carbon which gets taken up preferentially by biological processes through {{Wikipedia|Kinetic fractionation|kinetic fractionation}}. The energy costs are lower if the carbon in the organism uses the lighter isotope. Carbon 13 is also stable but not so much favoured by biology. (Techy note, this is not to be confused with carbon 14 dating - carbon 14 is radioactive and unstable. Carbon 12 and 13 are both stable and don't decay at all.)
Plants have values of from -10% or so down to -30% or less, clustering at around -13% and -28%<ref>O'Leary, Marion H. [http://sethnewsome.org/sethnewsome/EE_files/O%27Leary%201988.pdf "Carbon isotopes in photosynthesis."] Bioscience 38, no. 5 (1988): 328-336.</ref>. Algae have a similar range of values, from higher than -10% down to -30% or less. Coal and marine petroleum typically has values around -25%, terrestrial petroleum around -30%, and land plants average around -25%, but with a fair bit of variation around those figures. See figure 1 in this article.Park, Roderic, and Samuel Epstein. [https://authors.library.caltech.edu/62406/1/4259733.pdf "Metabolic fractionation of C13 & C12 in plants."] Plant Physiology 36, no. 2 (1961): 133.
However, sometimes abiotic methane can have carbon 13 depleted to as low as -50%, for instance in hydrothermal vents. <ref>
McDermott, Jill M., Jeffrey S. Seewald, Christopher R. German, and Sean P. Sylva. [http://www.pnas.org/content/pnas/112/25/7668.full.pdf "Pathways for abiotic organic synthesis at submarine hydrothermal fields."] Proceedings of the National Academy of Sciences 112, no. 25 (2015): 7668-7672.</ref>
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