Direct observations of evolving subglacial drainage beneath the Greenland Ice Sheet

摘要

Seasonal acceleration of the Greenland Ice Sheet is influenced by the dynamic response of the subglacial hydrologic system to variability in meltwater delivery to the bed via crevasses and moulins (vertical conduits connecting supraglacial water to the bed of the ice sheet). As the melt season progresses, the subglacial hydrologic system drains supraglacial meltwater more efficiently, decreasing basal water pressure and moderating the ice velocity response to surface melting. However, limited direct observations of subglacial water pressure mean that the spatiotemporal evolution of the subglacial hydrologic system remains poorly understood. Here we show that ice velocity is well correlated with moulin hydraulic head but is out of phase with that of nearby (0.3-2 kilometres away) boreholes, indicating that moulins connect to an efficient, channelized component of the subglacial hydrologic system, which exerts the primary control on diurnal and multi-day changes in ice velocity. Our simultaneous measurements of moulin and borehole hydraulic head and ice velocity in the Paakitsoq region of western Greenland show that decreasing trends in ice velocity during the latter part of the melt season cannot be explained by changes in the ability of moulin-connected channels to convey supraglacial melt. Instead, these observations suggest that decreasing late-season ice velocity may be caused by changes in connectivity in unchannelized regions of the subglacial hydrologic system. Understanding this spatiotemporal variability in subglacial pressures is increasingly important because melt-season dynamics affect ice velocity beyond the conclusion of the melt season.%向格陵兰冰层底下的融水输送量的增加，将会提高冰层速度，加快其向海洋中的急速流动（这是不可避免的）和随后的海平面上升。真会是这样吗？关于这个话题的辩论处在冰冻圈研究的前沿，但一直受阻于对冰川锅穴（将水输送到冰层底部的垂直冰柱)和钻孔（用来监测底水压力）内的水头同时所做的观测结果的缺乏。现在，Lauren Andrews及同事提供了来自西格陵兰一个小区域的这些观测结果，它们显示：冰川锅穴向渠道化底流(basal flow)内的水输送的确与冰层流速的短期波动有关。然而，季末冰层流速的降低似乎是由非渠道化流动的变化控制的，而不是由冰川锅穴系统中的任何变化控制的。