Sediment Transport by Glaciers on Unconsolidated Beds: A Laboratory Study of Entrainment Mechanisms and Bed Lubrication

在某些条件下，含水饱和的沉积物实际上可以润滑冰川床。因此，这些沉积物可能在更新世期间促成了冰川的快速流动和冰盖的不稳定。目前对夹带机制的了解还不够充分，无法确定冰下沉积物作为润滑层持续存在或被侵蚀的条件。同样，冰川在松散地层上的流动机制也不为人所知。虽然水流通常被认为是河床变形的结果，但对一些现代冰川下面河床变形的连续测量表明，在足够低的有效压力下，河床不会变形，相反，运动集中在冰川底部。为了研究沉积物夹带和冰川与松散地层之间的耦合，提出了一系列实验，利用现有的设备，可以在异常良好的控制条件下严格测试假设。沉积物床将以稳定的速度在固定的温带冰块下面滑动。其中一个目标是通过测量研究冰侵入沉积物孔隙空间的情况。在以前的实验中，用该装置成功地研究了冰通过多孔介质的调节，这可能是冰川夹带基底碎屑的主要机制。主要的不确定性是滑动的影响，这可能会阻碍入侵。因此，入侵率可以作为滑动速度的函数来测量。实验后测量的碎屑冰的稳定同位素将与现代冰川的同位素数据进行比较。第二个目标将是发展冰在沉积物上运动的经验滑动规律。特别是，冰对沉积物床施加的剪切应力将作为有效压力和滑动速度的函数进行测量。因此，将阐明滑动比床层变形更有利的条件范围。实验后的观察也将允许在一系列条件下评估犁耕和滑动的相对重要性。

9418671 Iverson Under certain conditions water-saturated sediment may, in effect, lubricate glacier beds. Such sediment, therefore, may have contributed to fast glacier flow and ice sheet instability during the Pleistocene. There is at present insufficient understanding of entrainment mechanisms to define the conditions under which subglacial sediment either persists as a lubricating layer, or is eroded. Similarly, the flow mechanism of glaciers on unconsolidated beds is not well known. Although flow is usually assumed to arise as a result of bed deformation, continuous measurements of bed deformation beneath some modern glaciers suggest that under sufficiently low effective pressures, the bed does not deform, and motion, instead, is focused at the glacier sole. To study both sediment entrainment and the coupling between glaciers and unconsolidated beds, a series of experiments is proposed with an existing apparatus that allows hypotheses to be rigorously tested under unusually well-controlled conditions. A sediment bed will be slid at a steady speed beneath a fixed block of temperate ice. One goal will be to study intrusion of ice into the sediment pore space by regelation. Regelation of ice through porous media has been studied successfully in previous experiments with the apparatus and may be the primary mechanism by which glaciers entrain basal debris. The principal uncertainty is the effect of sliding, which may impede intrusion. Intrusion rates well therefore be measured as a function of sliding speed. Post-experimental measurements of stable isotopes in debris-laden ice will be compared with isotopic data from modern glaciers. A second goal will be to develop an empirical sliding law for ice motion over sediment. In particular, the shear stress that ice exerts on the sediment bed will be measured as a function of effective pressure and sliding speed. The range of conditions under which sliding is favored over bed deformation, therefore, will be elucidated. Post-exper imental observations will also allow the relative importance of ploughing and sliding to be assessed under a range of conditions.