We report detailed chemical and isotopic data from a subglacial siliceous deposit on andesitic bedrock recently exposed by glacier retreat. Whereas a single, <1 μm, Si-rich layer covers the highly polished bedrock on the up-glacier (stoss) surfaces, distinct, lithified deposits commonly occur at the lee of small bedrock protuberances, on a scale <0.1 meter. The deposit is millimeters in thickness and consists of laminae tens to hundreds microns thick that differ from one another in color, rock-fragment abundance and chemical composition. Ca-rich laminae that are sufficiently enriched in uranium (~2–50 ppm) to permit U-series isotopic analysis suggest that the subglacial deposit formed 10–20 ka, much earlier than previously assumed. We conclude that (1) the siliceous deposit persisted for at least 10 000 years despite the intervening erosion and weathering, (2) distinct episodes of formation due to significant changes in hydrology and water chemistry are recorded in the deposit, and (3) a siliceous slurry may have existed at the ice-rock interface and influenced the local friction. This work reinforces earlier findings that subglacial chemical deposits can form and persist on geologic time scales and may have implications for the role of the cryosphere in the Earth's geochemical cycles and climate system.

中文翻译：

---

硅质冰下沉积物：末次盛冰期冰下过程的档案

我们报告了最近因冰川退缩而暴露在安山岩基岩上的冰下硅质沉积物的详细化学和同位素数据。单一的、<1 μm 的富硅层覆盖了上冰川（stoss）表面上高度抛光的基岩，而独特的岩化沉积物通常出现在小基岩突起的背风面，规模 <0.1 米。该矿床厚度为毫米，由数十至数百微米厚的纹层组成，其颜色、岩石碎片丰度和化学成分各不相同。富含铀（~2-50 ppm）以允许 U 系列同位素分析的富含 Ca 的薄片表明冰下沉积物形成 10-20 ka，比以前假设的要早得多。我们得出结论：（1）尽管有侵蚀和风化作用，硅质沉积物至少持续了 10 000 年，（2）沉积物中记录了由于水文和水化学的显着变化而形成的不同时期，以及（3）硅质泥浆可能存在于冰岩界面并影响了局部摩擦。这项工作强化了早期的发现，即冰下化学沉积物可以在地质时间尺度上形成并持续存在，并可能对冰冻圈在地球地球化学循环和气候系统中的作用产生影响。