RADIOGENIC SR AS AN INDICATOR OF SUBGLACIAL WEATHERING PROCESSES: SEASONAL SHIFTS AT AN ALPINE GLACIER

Meltwater radiogenic strontium (⁸⁷Sr/⁸⁶Sr) measurements reflect the dominant chemical weathering processes occurring in a subglacial system and the evolving state of the subglacial drainage network morphology. During the 2011 melt season, filtered subglacial meltwater samples were collected daily during the onset of melt (May) and peak melt (July) at the Athabasca Glacier, located in Jasper National Park, Alberta, Canada. These samples were analyzed for ⁸⁷Sr/⁸⁶Sr isotopic composition to infer the evolution of subglacial weathering processes. Both the underlying bedrock and till composition and the subglacial water-rock interaction times are the primary influences on meltwater ⁸⁷Sr/⁸⁶Sr. composition The Athabasca Glacier is located on Middle Cambrian carbonate bedrock that also contains Sr-bearing silicate minerals such as muscovite. Previous studies at the Athabasca Glacier have placed constraints on average subglacial water residence times as the melt season progresses: in May, the average subglacial water residence time is approximately 30 years, which decreases to approximately one year in July. Accordingly, the meltwater ⁸⁷Sr/⁸⁶Sr composition follows a decreasing trend from a high of 0.7159 in May to a low of 0.7124 in July. Over the course of the melt season, increasing trends in Ca/K and Ca/Mg correspond to overall decreasing trends in ⁸⁷Sr/⁸⁶Sr, which indicate a shift in weathering processes from the presence of silicate weathering to primarily carbonate weathering.

As the melt season progresses and the subglacial drainage network develops into a more efficient system, decreasing the subglacial water residence time, meltwater compositions become dilute with fresh meltwater and move away from over saturation. Because carbonate minerals are preferentially weathered in unsaturated waters, there is a progressive shift from silicate weathering during times where meltwater is saturated with resect to carbonate minerals back toward primarily carbonate weathering with the incorporation of fresh meltwater. Overall, the Sr flux derived from the Athabasca Glacier reflects the Sr signals of the local carbonate bedrock as the continual contribution of new meltwater permits the carbonate weathering signal to dominate, and thus silicate weathering has minimal influence on overall meltwater Sr flux.