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

多温冰川基底热转变附近沉积物迁移的现场和理论研究 爱荷华州立大学的尼尔·艾弗森和丹尼斯·科恩摘要 1) 项目描述 将在瑞典软层冰川 Storglaciaren 的冰冻边缘附近研究冰流和沉积物迁移。目标是充分描述夹带基底沉积物并将其携带到冰川表面的过程；这些过程导致了丘状的末碛和其他冰上发育的地貌，这些地貌是大陆冰川作用的象征。来自 Storglaciaren 河床的碎片在基底热转变 (BTT) 过程中积聚在冰芯冰碛下冰川中：从融化的基底冰到冷的基底冰的转变。 我们的假设是，基础运动阻力在 BTT 处突然增加，导致冰中产生大的应力和速度梯度。 床上正应力的相关增强通过再凝胶渗透夹带沉积物。 集中在 BTT 附近的重新定向应力导致富含碎片的冰被抬离河床上并可能沿着局部剪切应变区域（即逆冲断层）被带到冰川表面。测量将集中在横跨 BTT 的两个区域。 将测量表面速度，以及冰温的钻孔测量、冰的剪切变形、深度平均垂直应变、基础滑动速度、床上的有效法向应力以及富含碎片的冰带附近和内部的应变局部化。对碎片带中的冰进行氧和氚同位素分析，可以将再胶凝引起的碎片夹带与基础水冻结引起的碎片夹带区分开来。为了确定远场冰速度，将在上游横跨冰川宽度测量冰变形和表面速度。这些测量结果与测得的滑动速度一起，将为最低消融区域稳定冰流的 3D 有限元模型提供上游和基础边界条件。 该模型将包括由变化的冰温和富含碎片的冰带引起的流变异质性。模型结果将通过 BTT 附近的表面速度、钻孔变形、垂直应变和床上法向应力的测量进行测试。 2) 重要性的非技术解释 前冰盖输送并沉积了大量碎片，这些碎片主导了前冰川地区（如新英格兰和中西部上部）的景观和近地表地质。 除了影响景观形态外，这些沉积物还极大地影响农业、地下水文学和工程设计。 这项研究将有助于解释从冰盖床上提起碎片并将其带到冰川表面的过程，这些碎片在冰川表面融化并最终在冰川消耗期间沉积在陆地表面。 这些沉积物的工程和水文特性直接反映了这些过程。通过对一个经过充分研究的小型冰川进行测量，并利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，这项工作将展示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化对冰川流速的影响；这一目标在未来几十年将变得越来越重要，因为冰川冰因大气变暖而变暖，从而加速冰川消耗，对海平面和气候产生影响。该项目还将支持博士学位。研究生的研究并将涉及本科生，其目标是刺激地球科学领域的研究生职业。

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.