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

液压系统

• 批准号: NE/F015879/1
• 负责人: Andrew Smith
• 金额: $ 16.51万
• 依托单位: British Antarctic Survey
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2009
• 资助国家: 英国
• 起止时间: 2009 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=NE%2FF015879%2F1
• 关键词: Hydraulics; sediment; deformation; beneath; ice

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

快速流动的冰流（冰流）在格陵兰岛和南极洲等大型冰盖的运动中发挥着重要作用。虽然它们只覆盖了冰盖总面积的一小部分，但它们将超过3/4的冰块排入周围的海洋。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加速冰流而发生。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盖中发挥了类似的作用。冰流的流动速度很快，因为它们流过的表面摩擦力很低。最近的研究表明，在许多，可能是大多数的情况下，研究，这是因为床的大面积覆盖的沉积物很容易变形，并提供了一个快速变形，低摩擦地毯，减轻冰流运动。为了在相当厚的冰下变形，沉积物需要解冻，高的内部水压几乎平衡了由于覆盖冰造成的压力。这些水来自于冰盖底部的融化，有时来自于穿过冰盖的地表水。如果冰盖不想因为底部积水而变得有浮力和不稳定，水必须通过冰盖系统排出，并从其边缘排出。冰川学中一个尚未解决的中心问题是确定沿沿着这条排水路径的水压梯度，这将确定局部水压，从而确定冰/床界面处的局部摩擦和流动。最近对西南极冰盖的拉特福德冰流的研究揭示了一个研究这一问题的极好地点，在那里，变形的冰下沉积物毯已经被确定使用地震一次（P）波，能够识别冰下沉积物是否变形。我们建议通过使用P和S（剪切）地震波来扩展这种技术，这将使我们能够增加功率来解决变形层厚度的微小差异，并使用一种更复杂的测量技术，称为AVO和最近发展的孔隙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。我们将使用推断出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存在“鼓”（长脊，在冰流的方向上呈流线型），这也被证明是由变形沉积物组成的，并且沿着冰流的方向移动。鼓鱼广泛分布于前冰川地区，特别是密集地聚集在沿着前冰流。如果鼓形冰丘是变形冰川床的特征，正如已经提出的那样，也正如拉特福德观测所暗示的那样，那么它们将提供一种强有力的手段，通过研究在欧亚大陆和北美上空很容易观察到的冰河时代冰盖床，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流床的形式和鼓的分布，将这些与水力条件和变形床过程相关联，并通过两个实地季节的研究（添加到现有的，来自同一地区的早期数据），监测水力和变形床系统如何随时间变化。我们还希望估算通过冰流系统的水和沉积物通量，以便更好地了解它在南极洲这一部分地貌和沉积物变化速率中所起的作用

项目成果

Towards a common terminology in radioglaciology

寻找放射冰川学的通用术语

• DOI: 10.1017/aog.2023.2
• 发表时间: 2023
• 期刊: Annals of Glaciology
• 影响因子: 2.9
• 作者: Schlegel R