| 英文摘要: | Submarine permafrost thaw in the Arctic has been suggested as a trigger for the release of large quantities of methane to the water column, and subsequently the atmosphere — with important implications for global warming. Now research shows that microbial oxidation of methane at the thaw front can effectively prevent its release. Methane gas has a high global warming potential on sub-century timescales, and release of currently trapped methane could drive rapid climate change1. Thus the possibility of substantial amounts of this greenhouse gas reaching the atmosphere has attracted attention — both in scientific and policy discussions. A potentially important source could be methane trapped within permafrost, including subsea sources that may be extensive and unstable. Staggering amounts of methane are thought to exist below the Russian Arctic shelf seas, mostly associated with shallow gas hydrates (solid ice-like structures encapsulating gas molecules) beneath and within the permafrost. Writing in the Journal of Geophysical Research: Biogeosciences, Pier Paul Overduin and colleagues report on a sediment core retrieved from beneath the shallow waters (4 m depth) of the southern Laptev Sea, a location inundated only 540 years ago, thus offering insight into sedimentation, thawing, and other processes that affect the inner shelf since sea levels began to rise at the end of the last glaciation. A suite of biogeochemical data directly related to methane dynamics in this setting is presented for the first time, showing methane gas is consumed by microbes before it can reach the overlying ocean2. The effects of the thawing of long-submerged permafrost on marine methane are manifold. Freeze-locked organic carbon becomes available to microbes as permafrost thaws, and the resulting greenhouse gases may be released to overlying sediments. The permafrost itself may act as a low-permeability physical barrier to upward migration of gases from deeper sources. Alternatively, methane may be frozen into the permafrost as gas hydrates and is released at the moment of permafrost thaw. (For the study considered here2, pressures at the depth limit of the core (52 m) are too low for gas hydrate stability, although they could exist at greater depths3). In recent years, interest has focused on the wide, shallow Siberian continental shelf seas, which were inundated after the end of the Last Glacial Maximum resulting in preservation of relict terrestrial permafrost under portions of these shelves4. Shallow areas such as those investigated by Overduin et al. are a prime location for emissions of methane from the seafloor to the atmosphere: shallow waters allow easier diffusive or bubble transfer5, and large amounts of organic-rich sediments provide a ready substrate for methane-producing microbes. Indeed, high summertime concentrations of methane have been reported in these waters6. Understanding the sources of this gas and the dynamics of its exchange is vital to knowing if this carbon, mobilized as methane, has the potential to substantially contribute to atmospheric greenhouse warming. The high-permeability sandy sediments in the borehole described by Overduin et al. give rise to a high water content, which allows for the development of ice-bonded permafrost — the top of which was identified about 25 m below the seafloor (Fig. 1). The entire core has warmed approximately 10 °C since marine inundation resulting in permafrost thaw in the warmer, upper section. This permitted efficient intrusion of seawater into sediment pore space, reaching as deep as the thaw front. Because of the high sulphate concentrations in the intruding seawater, released methane was mostly destroyed by anaerobic microbial oxidation in the overlying sediments. The effectiveness of this process in reducing sediment methane emissions to the ocean at the boundary layer is widely known7, but was thought to be negligible further below in the sediments due to limited sulphate availability. Anaerobic oxidation of methane requires an alternate electron acceptor other than oxygen, and in marine systems this is usually sulphate8. The reaction produces sulphide and the methane is oxidized to CO2, which contributes to the formation of carbonates in the sediments near methane seeps9. Overduin and colleagues show the seawater intrusion and supply of sulphate is sufficient to keep pace with the release of methane from the thawing permafrost below, fuelling an efficient microbial biofilter that prevents it from reaching the atmosphere (Fig. 1).
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