VARIATIONS IN THE URANIUM ISOTOPIC COMPOSITION OF SUBGLACIAL SUSPENDED SEDIMENTS

Suspended sediments have been largely overlooked when considering the impact of increased volumes of glacial meltwater on sedimentary fluxes from glacial watersheds. We present uranium isotope data from the sediments of three geographically, geologically, and climatologically distinct glaciers over two field seasons, the Athabasca and Saskatchewan Glacier, Canada (AG, SG, Precambrian Sedimentary bedrock) and the Lemon Creek Glacier, USA (LCG, Southeast Alaska Metamorphic Belt), in order to more accurately understand the interaction of suspended sediments in glacial meltwater. In undisturbed rocks and minerals older than ~1 Ma, the activity of the parents and the daughters should be equal: [²³⁸U]λ_238U = [²³⁴U]λ_234U (where brackets indicate concentration and λ refers to the decay constant). Departure from equilibrium can occur during water-rock interaction via the preferential release of daughter products from the solid by recoil associated with the high-energy alpha decay of the parent nuclides. Solids become depleted in daughter products and liquids become enriched. Because radioactive decay is a function of time, the activity ratio (²³⁴U/²³⁸U) will depend on the communition age of the sediment, and in the case of subglacial suspended sediments, the storage or residence time within or beneath the ice. Activity ratios of sediments from LCG, a wet-based glacier, were all found to be within error of equilibrium (1.000 +/- 0.005), suggesting a relatively short residence time. Samples from AG and SG, polythermal glaciers, had activity ratios as low as 0.976 and 0.987, a result of a longer residence time in the subglacial environment.