Seasonal progression of uranium series isotopes in subglacial meltwater: Implications for subglacial storage time

The residence time of subglacial meltwater impacts aquifer recharge, nutrient production, and chemical signals that reflect underlying bedrock/substrate, but is inaccessible to direct observation. Here we report the seasonal evolution of subglacial meltwater chemistry from the 2011 melt season at the terminus of the Athabasca Glacier, Canada. We measured major and trace analytes and U-series isotopes for twenty-nine bulk meltwater samples collected over the duration of the melt season. This dataset, which is the longest time-series record of (234U/238U) isotopes in a glacial meltwater system, provides insight into the hydrologic evolution of the subglacial system during active melting. Meltwater samples, measured from the outflow, were analyzed for (238U), (222Rn) and (234U/238U)activity, conductivity, alkalinity, pH and major cations. Subglacial meltwater varied in [238U] and (222Rn) from 23 to 832 ppt and 9 to 171 pCi/L, respectively. Activity ratios of (234U/238U) ranged from 1.003 to 1.040, with the highest (238U), (222Rn) and (234U/238U)activity values occurring in early May when delayed-flow basal meltwater composed a significant portion of the bulk melt. From the chemical evolution of the meltwater, we posit that the relative subglacial water residence times decrease over the course of the melt season. This decrease in qualitative residence time during active melt is consistent with prior field studies and model-predicted channel switching from a delayed, distributed network to a fast, channelized network flow. As such, our study provides support for linking U-series isotopes to storage lengths of meltwater beneath glacial systems as subglacial hydrologic networks evolve with increased melting and channel network efficiency.