Composition and formation age of amorphous silica coating glacially polished surfaces

Abstract

Recent micrographs of smooth, glacially abraded silicic bedrock reveal an amorphous coating layer adhering to the bedrock, with structures that tie its formation to glacial abrasion. What remains unclear is whether this coating is formed by the physical comminution of bedrock, resulting in amorphous material with a bedrock composition, or by chemical dissolution of silicate minerals followed by precipitation of an amorphous layer enriched in silica and depleted in cations relative to the bedrock. Here, we report the composition and formation age of the amorphous coatings in Yosemite National Park, California, USA. The coatings are depleted in base cations (50%-90%) and enriched in silica (10%-50%) as well as trace Fe and U (4- to 100-fold) relative to the bedrock, reflecting dissolution by and precipitation from subglacial waters. The 234U/238U activity ratio of the amorphous layer is 200%-600% above secular equilibrium, reflecting a surficial U source enriched by α-recoil processes and consistent with the 234U enrichment observed in subglacial waters. The 230Th/238U activity ratio is 30%-100% below secular equilibrium and records Th-U fractionation in subglacial waters at 30-10 ka, consistent with coating formation during the Last Glacial Maximum (LGM). These amorphous coatings are subglacial precipitates that record the chemical weathering of silicates beneath glaciers during the LGM. Collectively, these observations link silicate dissolution and amorphous silica production to physical processes at the glacier bed, a result that may have significant implications for the global Si and CO2 budgets on glacial-interglacial time scales.