Weathering of western Greenland: Influences on oceanic fluxes of radiogenic isotopes

This proposal addresses the hypothesis that as the Greenland Ice Sheet (GrIS) retreats, increased exposure and weathering will alter fluxes of radiogenic isotopes to the oceans. Radiogenic strontium (Sr) and lead (Pb) isotopes, and non­radiogenic neodymium (Nd) are preferentially leached during initial weathering of fresh bedrock and sediments in glacial terrains. With increased duration and intensity of weathering, isotopic compositions of runoff approach parent rock values. Therefore, magnitudes of offsets between Sr, Nd and Pb isotope values of water and bedrock provide information about the extent of weathering. Specific hypotheses to be tested are (1) more radiogenic Sr and Pb and less radiogenic Nd discharge in water originating from the GrIS than from deglaciated watersheds without connections to the ice sheet, (2) values of isotope ratios depend on the exposure age of the deglaciated watersheds and the intensity of weathering in each environment, and (3) the total flux of radiogenic isotopes from continental glaciers depends on the relative aerial extents of glaciated and deglaciated watersheds. These hypotheses will be tested in western Greenland near Kangerlussuaq, where retreat of the GrIS since the Last Glacial Maximum has exposed an approximately 175 km wide strip of land. Water, sediment, and moraine material will be sampled from subglacial streams discharging from the outlet portals of the Russell and Leverett Glaciers, in the proglacial watershed of the Watson River, and in four deglaciated watersheds, not connected to the GrIS that drain moraines ranging in age from about 600 to 18,000 years. Water samples will be measured for major and trace element concentrations to model potential mineral dissolution and contributions to isotope fluxes, and for Sr, Nd, and Pb isotope ratios to assess their fluxes from subglacial, proglacial, and deglaciated watersheds. This work will link terrestrial weathering products in a continental glacier setting to oceanic fluxes of radiogenic isotopes, thereby improving the ability to interpret marine records. The work is important because of its location near the formation regions of North Atlantic Deep Water and it should contribute to planning for a proposed ocean drilling expedition off Greenland. The work will impact human resources through incorporation of 4 students from undergraduate to Ph.D. level, all of whom will participate in field work and will work collaboratively to complete laboratory and data analyses. Results of the work will be used in several graduate and undergraduate courses and will be presented at national and international scientific meetings and through publications. Outreach will include lectures at local schools, as invited speakers at colleges and universities, and in the annual open house at the Florida Museum of Natural History.

本提案旨在验证如下假设：随着格陵兰冰盖（Greenland Ice Sheet, GrIS）退缩，裸露面积扩大与风化作用增强将改变放射性同位素（radiogenic isotopes）向海洋的通量。在冰川地形中新鲜基岩与沉积物的初始风化过程中，放射性锶（Sr）、铅（Pb）同位素以及非放射性钕（Nd）同位素会被优先淋滤。随着风化持续时间延长与强度增加，径流的同位素组成将趋近于母岩的同位素值。因此，水体与基岩中锶、钕、铅同位素值的偏移量可反映风化作用的程度。拟验证的具体假设包括：（1）源自格陵兰冰盖的水体中，放射性锶、铅含量更高，而放射性钕含量低于与冰盖无关联的冰退流域（deglaciated watershed）水体；（2）同位素比值取决于冰退流域的暴露年龄（exposure age）及各环境中风化作用的强度；（3）大陆冰川放射性同位素的总通量取决于冰川覆盖与冰退流域的相对面积比例。这些假设将在格陵兰西部康克鲁斯瓦格（Kangerlussuaq）附近进行验证——自末次盛冰期（Last Glacial Maximum）以来，格陵兰冰盖的退缩已在此处暴露了一条宽约175公里的陆地带。研究将采集以下样本：从拉塞尔冰川与莱弗里特冰川出口流出的冰下河流（subglacial streams）中的水体、沉积物与冰碛物（moraine material）；沃森河冰前流域（proglacial watershed）的样本；以及四个与格陵兰冰盖无关联的冰退流域的样本——这些流域的冰碛物年龄介于600至18000年之间。水样将被测定主要与微量元素浓度，以模拟潜在的矿物溶解过程及其对同位素通量的贡献；同时测定锶、钕、铅同位素比值，以评估冰下、冰前及冰退流域的同位素通量。本研究将大陆冰川环境下的陆地风化产物与海洋放射性同位素通量联系起来，从而提升解读海洋记录的能力。该研究具有重要意义，因其位于北大西洋深层水形成区附近，且将为格陵兰近海拟议的海洋钻探考察计划提供支持。研究将通过纳入4名从本科到博士阶段的学生来培养人力资源，所有学生均将参与野外工作，并协作完成实验室分析与数据处理。研究结果将应用于多门研究生与本科课程，并在国内外学术会议上展示及通过出版物发表。科普推广活动包括在当地学校举办讲座、作为特邀讲者在高校发言，以及参与佛罗里达自然历史博物馆的年度开放日活动。