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盖的移动中发挥着重要作用。尽管它们只覆盖了整个冰盖面积的一小部分，但它们排放的冰块超过了流入周围海洋的冰块的四分之三。如果冰盖在全球变暖的情况下迅速退化，大部分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发生。地质证据表明，在上一次冰川时期，冰流在覆盖北美和欧亚大陆大部分地区的冰盖中扮演了类似的角色。冰流的流动速度很快，因为它们流过的表面摩擦力很小。最近的研究表明，在许多(可能是大多数)所研究的案例中，这是因为床层的大片区域被容易变形的沉积物覆盖，并提供了一种快速变形、低摩擦的地毯，使冰流的运动变得容易。为了在相当厚的冰层下变形，沉积物需要解冻，内部水压力高，几乎平衡了上覆冰造成的压力。这些水来自冰盖底部的融化，有时来自表面渗入冰盖的水。如果冰盖不会因为底部积水而变得浮力和不稳定，水必须通过冰盖系统排出，并从边缘排出。冰川学中一个中心悬而未决的问题是确定沿这条排水路径的水压力梯度，这将决定局部水压力，从而确定冰/床界面的局部摩擦和流动。最近对西南极冰盖拉特福德冰流的研究揭示了一个研究这一问题的绝佳地点，在那里利用地震初级(P)波识别出正在变形的冰下沉积物地毯，该波能够识别冰下沉积物是否正在变形。我们建议通过同时使用P波和S(剪切)地震波来扩展这一技术，这将使我们能够增加解决变形层厚度微小差异的能力，并使用一种更复杂的测量技术--AVO和最近发展的孔弹性理论来推断更广泛的冰下条件。其中，最重要的是冰下沉积物中的水压，这将使我们能够重建最终控制沉积物变形和基面摩擦的冰下“地下水位”的形式。我们将使用推断的水力模式来测试冰下排水的理论。在拉特福德的另一项重要发现是存在“隆起”(沿着冰流方向呈流线形的长脊)，这种隆起也被证明是由变形的沉积物组成的，它们沿着冰流的方向移动。鼓声广泛分布在以前的冰川地区，特别是沿着以前的冰川密集聚集。如果隆隆声像已经提出的那样是变形冰川床的特征，并且正如拉特福德的观测所暗示的那样，它们将通过研究在欧亚大陆和北美可以如此容易地观察到的冰川时代冰盖的床来提供一个强大的手段来了解冰盖床的大尺度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来成像冰河床的形状和鼓声的分布，将这些与水力条件和变形床过程联系起来，并通过两个野外季节的研究(加上同一地区现有的更早的数据)，监测水力床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这一地区地貌和沉积变化速度中所起的作用。

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