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Friday, August 31, 2012

"Potential methane reservoirs beneath Antarctica" by J. L. Wadham et al., Nature 488 (2012); doi: 10.1038/nature11374

Nature, 488 (30 August 2012) 633-637; doi: 10.1038/nature11374

Potential methane reservoirs beneath Antarctica

Once thought to be devoid of life, the ice-covered parts of Antarctica are now known to be a reservoir of metabolically active microbial cells and organic carbon1. The potential for methanogenic archaea to support the degradation of organic carbon to methane beneath the ice, however, has not yet been evaluated. Large sedimentary basins containing marine sequences up to 14kilometres thick2 and an estimated 21,000 petagrams (1Pg equals 1015g) of organic carbon are buried beneath the Antarctic Ice Sheet. No data exist for rates of methanogenesis in sub-Antarctic marine sediments. Here we present experimental data from other subglacial environments that demonstrate the potential for overridden organic matter beneath glacial systems to produce methane. We also numerically simulate the accumulation of methane in Antarctic sedimentary basins using an established one-dimensional hydrate model3 and show that pressure/temperature conditions favour methane hydrate formation down to sediment depths of about 300metres in West Antarctica and 700metres in East Antarctica. Our results demonstrate the potential for methane hydrate accumulation in Antarctic sedimentary basins, where the total inventory depends on rates of organic carbon degradation and conditions at the ice-sheet bed. We calculate that the sub-Antarctic hydrate inventory could be of the same order of magnitude as that of recent estimates made for Arctic permafrost. Our findings suggest that the Antarctic Ice Sheet may be a neglected but important component of the global methane budget, with the potential to act as a positive feedback on climate warming during ice-sheet wastage.
The production of methane (CH4) by methanogenic archaea is common across many sub-surface environments, including the deep ocean45678, permafrost9 and lake sediments101112 and is promoted by the presence of a suitable organic carbon (OC) substrate and an absence of higher-energy-yielding electron acceptors (for example, O2 and SO42−) with which to degrade organic matter. The release of this biogenic methane to the atmosphere is important in driving changes in global climate on geological, millennial and centenary timescales13. ‘Geological’ methane, produced largely via thermogenic processes in the deep sub-surface, supplements the biogenic component13. This methane may be generated via the thermal breakdown of organic matter and by inorganic synthesis and outgassing from the mantle. The recent discovery that sub-ice-sheet environments are likely to be anoxic14, are host to micro-organisms15 and may contain significant reservoirs of OC identifies them as favourable sites for methanogenesis. Research has so far focused upon the potential biological conversion of overridden OC to methane beneath the Northern Hemisphere Pleistocene ice sheets1416, where high-pressure and low-temperature conditions permit methane to accumulate as hydrate. Methane accumulation beneath the Antarctic Ice Sheet has not yet been evaluated, despite the presence of extensive and deep sedimentary basins containing viable microbial populations15 and OC available for conversion to methane.
Several factors suggest that there should be methane present beneath the Antarctic Ice Sheet. Geophysical data indicate extensive Antarctic sedimentary basins (ASBs) beneath the West and East Antarctic Ice Sheets (WAIS and EAIS), containing sedimentary drapes of up to 14km in thickness (Supplementary Table 1). Many of these basins are located around the Antarctic periphery, but penetrate several 100–1,000km into the Antarctic interior and are associated with the onset of accelerated motion in ice streams and their tributaries (Supplementary Information 1). The inferred origin of these sediments is marine, glaci-marine and crustal sedimentary sources (Supplementary Table 1). Melting conditions beneath about half of the ice sheet mean that sediments contain liquid water beneath the ice cover1718. The widespread presence of either dissolved methane or geochemical evidence for methanogenesis in marine cores, rock cores and seeps around the Antarctic margin indicates that OC is commonly cycled to methane in ocean margin basins (Supplementary Information 1). It is reasonable to expect similar processes to prevail beneath the ice. We therefore evaluated the potential for methane generation and release from sedimentary basins buried beneath the Antarctic Ice Sheet.

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