A Field and Theoretical Study of Sediment Transport near the Basal Thermal Transition of a Polythermal Glacier

摘要/ abstract摘要：本文以瑞典软层冰川Storglaciaren冻结边缘附近为研究对象，研究了沉积流和沉积物输运过程。目标是充分描述携带基底沉积物并将其带到冰川表面的过程；这些过程导致了丘状终碛和其他冰川上发育的地貌，这些地貌是大陆冰川作用的象征。从基底热转变（BTT），即从融化到寒冷的基底冰的转变中，来自冰川床的碎屑在冰川下的冰芯冰碛中积累。我们的假设是，对基底运动的阻力在BTT处急剧增加，从而在冰中造成较大的应力和速度梯度。河床上相应的正应力增强通过沉降渗透夹带沉积物。集中在BTT附近的重定向应力导致富含碎屑的冰从床上被抬升并被带到冰川表面，可能沿着局部剪切应变带（即逆冲断层）。测量将集中在横跨BTT的两个区域。将测量地表速度，以及钻孔测量冰温、冰的剪切变形、深度平均垂直应变、基本滑动速度、床上的有效正应力，以及在富含碎屑的冰带附近和内部的应变局部化。对碎片带中冰的氧和氚同位素分析将使碎片带带与基底水冻结引起的碎片带带区分开来。为了确定远场冰的速度，将在上游的冰川宽度处测量冰的变形和表面速度。这些测量结果，连同测量到的滑动速度，将为最底部消融区稳定冰流的三维有限元模型提供上游和基础边界条件。该模型将包括由变冰温度和富含碎屑的冰带引起的流变非均质性。模型结果将通过在BTT附近测量地表速度、钻孔变形、垂直应变和床上的正应力来测试。2)重要性的非技术解释以前的冰盖运输和沉积了大量的碎片，这些碎片主导了以前冰川地区的景观和近地表地质，如新英格兰和中西部北部。除了影响景观形态外，这些沉积物还极大地影响了农业、地下水文和工程设计。这项研究将有助于解释从冰盖床上抬升碎片并将其带到冰川表面的过程，在冰川流失期间，这些碎片在冰川表面融化并最终沉积在陆地表面。这些沉积物的工程和水文性质直接反映了这些过程。通过对一个小冰川的测量，特别是经过充分研究的冰川，并使用所得数据来驱动和测试冰流和沉积物运输的数学模型，该项目将有助于阐明冰川如何移动和沉积沉积物。特别是，这项工作将证明空间变化的冰川温度的作用。这项工作的一个重要辅助目标是阐明冰温变率对冰川流速的影响；这一目标在未来几十年将变得越来越重要，因为随着大气变暖，冰川的冰变暖，从而加速冰川的流失，对海平面和气候产生影响。该项目还将支持一名研究生的博士研究，并将涉及本科生，目的是刺激地球科学研究生的职业生涯。

A FIELD AND THEORETICAL STUDY OF SEDIMENT TRANSPORT NEAR THE BASAL THERMAL TRANSITION OF A POLYTHERMAL GLACIERNeal Iverson and Denis Cohen, Iowa State UniversityABSRACT1) Description of the projectIce flow and sediment transport will be studied near the frozen margin of Storglaciaren, a soft-bedded glacier in Sweden. The goal is to fully characterize the processes that entrain basal sediment and carry it the glacier surface; these processes result in hummocky end moraines and other supraglacially-developed landforms of that are emblematic of continental glaciation. Debris derived from the bed of Storglaciaren is accumulating in an ice-cored moraine down-glacier from the basal thermal transition (BTT): the transition from melting to cold basal ice. Our hypothesis is that resistance to basal movement increases sufficiently abruptly at the BTT to cause large stress and velocity gradients in the ice. Associated enhancement of normal stress on the bed entrains sediment by regelation infiltration. Reoriented stresses concentrated near the BTT cause debris-rich ice to be lifted off the bed and carried to the glacier surface, perhaps along zones of localized shear strain (i.e., thrust faults). Measurements will be focused in two regions that straddle the BTT. Surface velocity will be measured, together with borehole measurements of ice temperature, shear deformation of ice, depth-averaged vertical strain, basal sliding speed, effective normal stress on the bed, and strain localization near and within debris-rich ice bands. Oxygen and tritium isotopic analyses of ice in debris bands will allow debris entrainment by regelation to be distinguished from that due to freeze-on of basal water. To determine the far-field ice velocity, ice deformation and surface velocity will be measured upstream, across the glacier width. These measurements, together with the measured sliding speed, will provide upstream and basal boundary conditions for a 3-D finite-element model of steady ice flow in the lowermost ablation area. The model will include rheological heterogeneity caused by variable ice temperature and debris-rich ice bands. Model results will be tested with measurements near the BTT of surface velocity, borehole deformation, vertical strain, and normal stress on the bed. 2) Non-technical explanation of importanceFormer ice sheets transported and deposited huge volumes of debris that dominate the landscapes and near-surface geology of formerly glaciated regions, such as New England and the Upper Midwest. In addition to affecting the form of the landscape, these sediments greatly impact agriculture, subsurface hydrology, and engineering design. This research will help explain the processes that lift debris from the beds of ice sheets and carry that debris to the glacier surface, where it melts out and is eventually deposited on the land surface during glacier wastage. The engineering and hydrological properties of these sediments directly reflect these processes. Through measurements on a small, particularly well-studied glacier and use of the resultant data to drive and test mathematical models of ice flow and sediment transport, this project will help clarify how sediment is moved and deposited by glaciers. In particular, the work will demonstrate the role of spatially-variable glacier temperature. An important ancillary objective of this work is to illuminate the effects of ice-temperature variability on glacier-flow velocity; this objective will be increasingly important in the coming decades as the ice of glaciers warms in response to atmospheric warming, thereby accelerating glacier wastage with impacts on sea level and climate. This project will also support the Ph.D. research of a graduate student and will involve undergraduates with the goal of stimulating post-graduate careers in Earth science.