Tritium in Basal Ice and Debris Bands: Demonstration of Freeze-On from Subglacial Waters

拟议研究的目标是确定冰川下冰块的增殖率，确定冰川碎屑带的成因，并评估冻结机制的一般性。作者最近的研究表明，阿拉斯加中南部马塔努斯卡冰川富含碎屑的基冰是在开放的水文系统中通过基面冻结形成的。证据包括：1)富含氚的基础冰；2)夏季融化季节冰在次冰川供给的高压喷口中的增长；以及3)测量到的水温为零度以下。当压力熔点上升的速度快于温度上升的速度时，流向末端的冰层下的水就会过冷。在大流量期间，过冷发生在床的坡度比冰川表面的坡度更陡峭且方向相反的地方。氚分析提供了自1952年以来形成的冰的“指纹”。底层冰中氚浓度的升高表明1952年后来自大气衍生水域的冻结。对来自马塔努斯卡冰川多个基面的氚、(18(和(D))的高分辨率分析将提供必要的数据，以计算冰下吸积率和估计不同水源对冰下排水系统的相对贡献。该地区1958-1993年降水中氚浓度的记录提供了一个输入函数，可将其与基础冰中的氚分布相关联。此外，在马塔努斯卡冰川跨冰川碎片带获取的样品中的氚、(18(和(D))将提供关于这些普遍存在和有争议的特征的成因的信息。对海因里希事件的解释、对冰川碎屑流量的估计以及冰川滑动的模型，都需要了解或假设冰下条件和碎屑夹带的机制。从这项研究中获得的信息将增加我们对冰下过程的理解，并将为冰川系统中冰川流动和泥石流运移的数值模型提供关键的L约束。

The objectives of the proposed research are to determine the rates of subglacial ice accretion, identify the genesis of englacial debris bands, and assess the generality of freeze-on mechanisms. Recent research by the authors demonstrates that debris-rich basal ice at the Matanuska Glacier, in south-central Alaska, forms by basal freeze-on in an open hydrologic system. Evidence includes: 1) tritium-rich basal ice; 2) summer melt-season ice growth in subglacially-fed, high-pressure discharge vents; and 3) measurements of sub-zero water temperatures. Subglacial water flowing towards the terminus supercools when the pressure-melting point rises faster than the temperature increases. During periods of high discharge, supercooling occurs where the slope of the bed is steeper and in the opposite direction than the slope of the glacier surface. Tritium analysis provides a "fingerprint" of ice which formed since 1952. Elevated concentrations of tritium in basal ice indicate post-1952 freeze-on from atmospherically-derived waters. High-resolution analysis of tritium, (18(, and (D from multiple basal sections at the Matanuska Glacier will provide data necessary to calculate subglacial accretion rates and to estimate the relative contributions of different water sources to the subglacial drainage system. A record of tritium concentrations in precipitation in this region from 1958 to 1993 provides an input function to which the tritium profiles in basal ice can be correlated. In addition, tritium, (18(, and (D in samples obtained across englacial debris bands at the Matanuska Glacier will provide information on the genesis of these ubiquitous and controversial features. Interpretations of Heinrich events, estimates of glacial debris flux, and models of glacier sliding all require knowledge or assumptions of subglacial conditions and mechanisms of debris entrainment. The information gained from this research will increase our understanding of subglacial processes and will provide critica l constraints to numerical models of glacier flow and debris transport in the glacial system.