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

爱荷华州立大学Neal Iverson和Denis Cohen对该项目的描述将在瑞典软层冰川Storglciaren的冰冻边缘附近进行冰流和泥沙输送的研究。目标是充分描述携带基础沉积物并将其带到冰川表面的过程；这些过程导致隆起的末端冰川和其他冰川以上发育的地貌，这些地貌是大陆冰川作用的象征。来自Storglciaren冰床的碎片正在冰芯冰原下冰川中积累，这是基热转变(BTT)：从融化的基冰向冷基冰的转变。我们的假设是，在BTT处，对底部运动的阻力增加得足够突然，从而在冰层中产生了很大的应力和速度梯度。河床上法向应力的联合增强通过回生入渗来携带泥沙。重新定位的应力集中在BTT附近，导致富含碎屑的冰被抬离冰床并被带到冰川表面，可能是沿着局部剪切应变区(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，以及钻孔测量的冰温度、冰的剪切变形、深度平均垂直应变、基本滑动速度、床上的有效法向应力以及富含碎屑的冰带附近和内部的应变局部化。碎片带中冰的氧和氚同位素分析将使重新凝聚的碎片卷吸与冻结基础水造成的碎片卷吸区别开来。为了确定远场冰的速度，将在冰川宽度的上游测量冰的变形和表面速度。这些测量和测量的滑动速度将为最低消融区稳定冰流的三维有限元模型提供上游和基本边界条件。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性不均匀。模型结果将通过BTT附近的地面速度、井眼变形、垂直应变和床层上的法向应力的测量进行验证。2)对重要性的非技术解释形成的冰盖运输和沉积了大量的碎屑，这些碎屑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上段。除了影响景观的形式外，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起碎屑并将其带到冰川表面的过程，在冰川消融期间，碎屑在那里融化并最终沉积在陆地表面。这些沉积物的工程和水文特性直接反映了这些过程。通过对一个研究特别充分的小型冰川进行测量，并利用所得到的数据来驱动和测试冰流和沉积物输送的数学模型，该项目将有助于澄清冰川是如何移动和沉积沉积物的。特别是，这项工作将展示空间可变的冰川温度的作用。这项工作的一个重要辅助目标是阐明冰温变化对冰川流动速度的影响；这一目标在未来几十年将变得越来越重要，因为冰川的冰因大气变暖而变暖，从而加速了冰川的消融，对海平面和气候产生了影响。该项目还将支持一名研究生的博士研究，并将吸引本科生参与，目标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.