SILICATE CHEMICAL WEATHERING BENEATH GLACIERS RECORDED BY THE COATING LAYER OF GLACIAL POLISH

Chemical weathering by glaciers estimated from the dissolved load in the runoff of modern alpine glaciers suggests that the dissolution of silicate rocks is minor compared to reactions with carbonate and sulfate. If the extent of chemical weathering beneath glaciers is indeed limited to the dissolution of these trace phases, glaciers are a source of CO₂, potentially buffering the net cooling that occurs during glacial intervals. Silicate dissolution, however, could be occurring at the rock-ice interface: recent images of glacially polished silicic rocks reveal an amorphous layer adhering to the host rock with structures that tie it’s formation to glacial action. What remains unclear is whether this amorphous layer is formed by the mechanical comminution of rock or if silicate minerals are being partially dissolved leaving behind a layer of hydrated silica. Although mechanical processes alone can reduce grain sizes, the occurrence of amorphous Si-rich coatings on silicate rock does occur as a result of exposure to fluids in both natural and laboratory settings. Here we present the results of an in situ analytical investigation to determine the major and trace element composition and formation age, using U-series geochronology, of the amorphous layer found within glacially polished surfaces in Yosemite National Park, California. These data show that the amorphous glacial polish is formed by both the mechanical abrasion and chemical dissolution of silicate minerals. Glaciers grind up the rock, increasing the surface area of wear particles produced at the rock-ice interface. Interaction with subglacial fluids removes base cations (Na, Mg, Al, K, Ca), concentrating silica. Minor and trace elements (Zr, Th and U) are reprecipitated within the amorphous layer at concentrations 4-10x the host rock. The ²³⁴U/²³⁸U activity ratio is above secular equilibrium (2-5), pointing to U sourced from sub-glacial fluids. The ²³⁰Th/²³⁸U is below secular equilibrium and define a thorium-uranium fractionation event at ~15-25ka, consistent with the last known glaciation of these samples. Collectively, these observations link amorphous silica production to glacial action, a result that has consequences for global Si and CO₂ budgets on glacial-interglacial timescales.