文章标题：Greenland melt drives continuous export of methane from the ice-sheet bed
文章作者：Guillaume Lamarche-Gagnon, Jemma L. Wadham, Barbara Sherwood Lollar, Sandra Arndt, Peer Fietzek, Alexander D. Beaton, Andrew J. Tedstone, Jon Telling, Elizabeth A. Bagshaw, Jon R. Hawkings, Tyler J. Kohler, Jakub D. Zarsky, Matthew C. Mowlem, Alexandre M. Anesio & Marek Stibal
DOI:https://doi.org/10.1038/s41586-018-0800-0
原文链接：https://www.nature.com/articles/s41586-018-0800-0
内容提要：在全球甲烷预算中，冰盖目前被忽略了。尽管有人提出，冰盖中蕴藏着大量甲烷，如果这些气体在冰层快速消退期间释放，可能会导致大气甲烷浓度上升，但目前还没有关于冰盖甲烷足迹的数据。本研究发现格陵兰冰盖冰川下集水区的有效排水系统迅速地将冰川下产生的甲烷驱动到冰川边缘。他们报告了在融化季节，甲烷—过饱和水（CH4(aq)）持续从冰盖中的连续输出。高CH4(aq)浓度的脉冲与冰上强迫的冰下冲刷事件相吻合，证实了冰下来源并突出了融化对甲烷输出的影响。研究估计了冰下甲烷储量，从冰盖床横向运输CH4(aq)大约有6.3吨（排放加权平均值；范围从2.4~11吨）。稳定同位素分析揭示了甲烷的微生物来源可能是埋在冰下的无机和古代有机碳的混合物。研究指出，冰盖覆盖着广泛的、生物活性强的产甲烷湿地，冰川下水文学对于控制来自冰盖的甲烷通量至关重要，此类环境因素以前被低估了，应该在地球的甲烷预算中加以考虑。
论文摘要：Ice sheets are currently ignored in global methane budgets1,2. Although ice sheets have been proposed to contain large reserves of methane that may contribute to a rise in atmospheric methane concentration if released during periods of rapid ice retreat3,4, no data exist on the current methane footprint of ice sheets. Here we find that subglacially produced methane is rapidly driven to the ice margin by the efficient drainage system of a subglacial catchment of the Greenland ice sheet. We report the continuous export of methane-supersaturated waters (CH4(aq)) from the ice-sheet bed during the melt season. Pulses of high CH4(aq) concentration coincide with supraglacially forced subglacial flushing events, confirming a subglacial source and highlighting the influence of melt on methane export. Sustained methane fluxes over the melt season are indicative of subglacial methane reserves that exceed methane export, with an estimated 6.3 tonnes (discharge-weighted mean; range from 2.4 to 11 tonnes) of CH4(aq) transported laterally from the ice-sheet bed. Stable-isotope analyses reveal a microbial origin for methane, probably from a mixture of inorganic and ancient organic carbon buried beneath the ice. We show that subglacial hydrology is crucial for controlling methane fluxes from the ice sheet, with efficient drainage limiting the extent of methane oxidation5 to about 17 per cent of methane exported. Atmospheric evasion is the main methane sink once runoff reaches the ice margin, with estimated diffusive fluxes (4.4 to 28 millimoles of CH4 per square metre per day) rivalling that of major world rivers6. Overall, our results indicate that ice sheets overlie extensive, biologically active methanogenic wetlands and that high rates of methane export to the atmosphere can occur via efficient subglacial drainage pathways. Our findings suggest that such environments have been previously underappreciated and should be considered in Earth’s methane budget.
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