Hydraulics & sediment deformation beneath an ice stream: a multi-component geophysical AVO investigation

快速流动的冰流（冰流）在格陵兰岛和南极洲等大冰原的运动中起着重要作用。虽然它们只覆盖了整个冰盖面积的一小部分，但它们排放的冰量占流入周围海洋的冰量的四分之三以上。如果冰盖在全球变暖的条件下迅速衰减，那么大部分的净质量损失以及由此导致的海平面上升将通过冰流的加速而发生。地质证据表明，冰流在末次冰期覆盖北美和欧亚大陆大部分地区的冰原中扮演了类似的角色。冰流流得快是因为它们流过的表面摩擦力小。最近的研究表明，在许多（可能是大多数）被研究的案例中，这是因为床的大片区域被容易变形的沉积物覆盖，形成了一个快速变形、低摩擦的地毯，减缓了冰流的移动。为了在相当厚的冰下变形，沉积物需要解冻，内部水压高，几乎可以平衡上覆冰造成的压力。这些水来自冰盖底部的融化，有时来自冰盖表面渗透的水。如果要使冰盖不因底部积聚的水而变得浮力和不稳定，水必须通过冰盖系统排出，并从其边缘排出。冰川学尚未解决的一个核心问题是确定沿这条排水路径的水压梯度，这将决定当地的水压，从而决定冰/床界面的局部摩擦和流量。最近在南极西部冰盖的拉特福德冰流上的工作揭示了一个研究这个问题的绝佳地点，在那里，利用地震初级(P)波能够识别冰下沉积物是否正在变形，从而确定了变形的冰下沉积物地毯。我们建议通过使用P和S（剪切）地震波来扩展这项技术，这将使我们能够增加解决变形层厚度的微小差异的能力，并使用更复杂的称为AVO的调查技术和最近发展的孔隙弹性理论来推断更大范围的冰下条件。其中，最重要的将是冰下沉积物中的水压，这将使我们能够重建冰下“地下水位”的形式，最终控制沉积物变形和基底摩擦。我们将使用推断的水力模式来检验冰下排水理论。在拉特福德的另一个重要发现是“鼓点”（沿冰流方向流线型的长脊）的存在，它也被证明是由变形的沉积物构成的，并沿着冰流的方向移动。鼓状冰在以前的冰川地区分布广泛，尤其密集地聚集在以前的冰流沿线。如果鼓点是变形的冰川床的特征，正如已经提出的那样，正如拉特福德观测所暗示的那样，那么通过研究在欧亚大陆和北美很容易观察到的冰河时期的冰盖床，它们将提供一种强有力的手段来理解冰盖床的大规模动力学是如何运作的。因此，我们研究的一个重要部分将是使用雷达系统对冰河床的形式和鼓点的分布进行成像，将这些与水力条件和变形床过程联系起来，并通过两个实地季节的研究（加上来自同一地区现有的早期数据）来监测水力和变形床系统如何随时间变化。我们还希望估计通过冰流系统的水和沉积物的通量，以便更好地了解它在南极洲这一地区的地貌和沉积变化速率中所起的作用

Streams of fast-flowing ice (ice streams) play a major role in the movement of large ice sheets such as those of Greenland and Antarctica. Although they cover only a small part of the total ice sheet area, they discharge over 3/4 of the ice mass that flows into surrounding oceans. If the ice sheets were to decay rapidly under conditions of global warming, much of the net mass loss, and the resultant rise in sea level, would take place through the speeding up of ice streams. Geological evidence shows ice streams to have played similar roles in the ice sheets that covered much of North America and Eurasia during the last glacial period. Ice streams flow quickly because of the low friction surface over which they flow. Recent research has shown in many, possibly the majority, of cases studied that this is because extensive areas of the bed are covered by sediment which deforms readily, and provides a rapidly-deforming, low friction carpet that eases ice stream movement. In order to deform under considerable ice thicknesses, the sediment needs to be unfrozen, with high internal water pressures that almost balance the pressure due to overlying ice. This water is derived from melting at the base of the ice sheet, and sometimes from water from the surface that percolates through the ice sheet. If the ice sheet is not to become buoyant and unstable due to water build up at its base, the water must drain through the ice sheet system and be discharged from its margin. A central unsolved problem in glaciology is to determine the water pressure gradient along this drainage pathway, which will determine the local water pressures and therefore the local friction and flow at the ice/bed interface. Recent work on the Rutford Ice Stream of the West Antarctic Ice Sheet has revealed an excellent site for the study of this problem, where a deforming subglacial sediment carpet has been identified using seismic primary (P) waves that are able to recognize whether or not a subglacial sediment is deforming. We propose to extend this technique by using both P and S (shear) seismic waves that will permit us to increase the power to resolve small differences in deforming layer thickness, and to use a more sophisticated survey technique called AVO and a recent development of poro-elasticity theory to deduce a much wider range of subglacial conditions. Amongst these, the most important will be water pressure in the subglacial sediment, which will permit us to reconstruct the form of the subglacial 'water table' which ultimately controls sediment deformation and basal friction. We will use the inferred hydraulic patterns to test theories of subglacial drainage. Another important discovery at the Rutford has been the existence of 'drumlins' (long ridges, streamlined in the direction of ice flow), which have also been shown to be made of deforming sediment, and which move in the direction of ice flow. Drumlins are widespread in areas of former glaciation, and particularly densely clustered along former ice streams. If drumlins are characteristics of a deforming glacier bed as has been suggested, and as is implied by the Rutford observations, they will provide a powerful means of understanding how the large scale dynamics of the bed of an ice sheet operates by studying the beds of Ice Age ice sheets that can be so readily observed over Eurasia and North America. An important part of our research therefore will be to use a radar system to image the form of the ice stream bed and the distribution of drumlins, to relate these to hydraulic conditions and deforming bed processes, and by studies in two field seasons (added to existing, earlier data from the same area), to monitor how the hydraulic and deforming bed system changes through time. We also hope to estimate the flux of water and sediment through the ice stream system, in order better to understand the role it plays in the rate of geomorphological and sedimentary change in this part of Antarctica