Mathematics – Logic
Scientific paper
Dec 2010
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2010agufm.p13b1390s&link_type=abstract
American Geophysical Union, Fall Meeting 2010, abstract #P13B-1390
Mathematics
Logic
[1041] Geochemistry / Stable Isotope Geochemistry, [1060] Geochemistry / Planetary Geochemistry, [5405] Planetary Sciences: Solid Surface Planets / Atmospheres, [6225] Planetary Sciences: Solar System Objects / Mars
Scientific paper
If confirmed, the recently-discovered methane (CH4) plume on Mars, in the Northern Summer of 2003, would reflect an emission of ~ 19 x103 tonnes y-1 and possibly even ~ 57 x104 tonnes y-1. Serpentinization in ophiolitic rocks is one of the main processes that are inferred for the origin of methane on Mars. Ophiolites or, hydrated mineral-bearing rocks in general on Earth could serve as analogs. So far, however, most of these “analog” studies focused on mineralogical and microbiological processes associated to ophiolitic environments. Analog studies specifically dealing with methane emissions to the Earth’s surface are missing. One of the observations of Mars methane is the transient release of large amounts of methane in a relatively short period, probably a few months. This would imply the existence of a mechanism of gas accumulations in the subsurface and episodic release to the surface. Such release mechanisms may be similar to certain weak and intermittent gas seeps or small mud volcanoes on Earth, rather than to steady, continuous degassing of methane from mineral reactions. Currently, it is not clear whether low-temperature serpentinization can be an abiogenic methane “kitchen” where methane might be generated fast enough to sustain vigorous and long-lasting seeps. In cases of fluxes of the order of several tonnes per year, a pressurized accumulation must exist. In case of lower fluxes, probably gas accumulations are not necessary and low temperature serpentinization can be fast enough to charge episodic seeps. These concepts are fundamental to our understanding of potential sources for the martian methane, and they need to be studied with the support of analog seepage data on Earth. Two examples are presented: (1) a case of terrestrial abiogenic CH4 seepage from ophiolitic rocks at the “eternal fires of Chimaera” in Turkey. Estimated flux data from the abiogenic gas seep of Chimaera in Turkey (>20 tonnes of CH4 per year) suggest a great potential of gas production and subsurface accumulation from low-temperature serpentinization. (2) Not related to serpentinization, we recently-discovered an extreme deuterium enrichment in methane from a gas seep in Romania, with delDCH4 value up to +124 ‰, that far exceed those reported for any terrestrial gas. This extraordinary value was interpreted as a result of abiogenic oxidation of biotic, thermogenic methane, generated from fossil organic matter. This finding shows that extremely positive delDCH4 values are still compatible with biotic sources and that the use of deuterium in CH4 as an indicator of abiogenic methanogenesis is invalidated. Although the D/H ratio is considered a fundamental biomarker, since biological systems prefer hydrogen (H) to deuterium (2H), our results indicate that this preference can be masked by abiogenic oxidation, occurring in the absence of oxygen. This is important in the context of identifying extraterrestrial biomarkers. Similar abiogenic alterations could, for example, occur on Mars, and this shall be taken into account when CH4 isotopic data will be hopefully available from the next Martian missions.
Etiope Giuseppe
Schoell Martin
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