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

多温冰川底部热转变附近沉积物输运的野外和理论研究Neal Iverson和Denis Cohen，爱荷华州州立大学ABSRACT 1）项目描述将研究瑞典软层冰川Storglaciaren冻结边缘附近的冰流和沉积物输运。其目标是充分表征夹带基底沉积物并将其携带到冰川表面的过程;这些过程导致丘状端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。来自Storglaciaren床的碎片正在积累在冰芯冰碛下冰川的基底热转换（BTT）：从融化到冷的基底冰的过渡。 我们的假设是，底部运动的阻力增加足够突然在BTT造成大的应力和速度梯度的冰。 床面上正应力的相应增加通过再循环入渗来携带泥沙。 集中在BTT附近的重定向应力导致富含碎屑的冰被抬离河床并被带到冰川表面，可能是沿着局部剪切应变的沿着（即，逆冲断层）。测量将集中在跨越BTT的两个区域。 将测量表面速度，同时钻孔测量冰的温度、冰的剪切变形、深度平均垂直应变、基底滑动速度、河床上的有效正应力以及富含碎屑的冰带附近和内部的应变局部化。冰的氧和氚同位素分析碎片带将允许碎片夹带regulatory区分，由于冻结的基础水。为了确定远场冰的速度，冰的变形和表面速度将被测量上游，横跨冰川的宽度。这些测量，连同所测得的滑动速度，将提供上游和基础边界条件的3-D有限元模型的稳定冰流在最低消融区。 该模型将包括由变化的冰温和富含碎屑的冰带引起的流变学不均匀性。模型结果将通过测量BTT附近的地表速度、钻孔变形、垂直应变和床面正应力进行检验。2)对重要性的非技术性解释前冰盖运输和沉积了大量的碎片，这些碎片主导了前冰川地区的景观和近地表地质，如新英格兰和上中西部。 除了影响景观的形成，这些沉积物还极大地影响农业、地下水文和工程设计。 这项研究将有助于解释从冰盖床中抬起碎片并将碎片带到冰川表面的过程，在那里它融化并最终在冰川流失期间沉积在陆地表面。 这些沉积物的工程和水文性质直接反映了这些过程。通过对一个小型、特别是经过充分研究的冰川进行测量，并利用所得数据来驱动和测试冰流和沉积物迁移的数学模型，该项目将有助于澄清沉积物是如何移动和沉积在冰川上的。特别是，这项工作将展示空间可变冰川温度的作用。 这项工作的一个重要的辅助目标是阐明冰温变化对冰川流速的影响;在未来几十年里，随着冰川的冰因大气变暖而变暖，这一目标将变得越来越重要，从而加速冰川的损耗，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，并将涉及本科生与刺激地球科学研究生的职业目标。

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.