Evolution of the firn pack of Kaskawulsh Glacier, Yukon: meltwater effects, densification, and the development of a perennial firn aquifer

摘要

In spring?2018, two firn cores (21 and 36?m in length) were extracted from the accumulation zone of Kaskawulsh Glacier, St. Elias Mountains, Yukon. The cores were analyzed for ice layer stratigraphy and density and compared against historical measurements made in 1964 and 2006. Deep meltwater percolation and refreezing events were evident in the cores, with a total ice content of 2.33±0.26 ?m in the 36?m core and liquid water discovered below a depth of 34.5?m. Together with the observed ice content, surface energy balance and firn modelling indicate that Kaskawulsh Glacier firn retained about 86?% of its meltwater in the years 2005–2017. For an average surface ablation of 0.38?m?w.e.?yr ?1 over this period, an estimated 0.28?m?w.e.?yr ?1 refroze in the firn, 0.05?m?w.e.?yr ?1 was retained as liquid water, and 0.05?m?w.e.?yr ?1 drained or ran off. The refrozen meltwater is associated with a surface lowering of 0.73±0.23 ?m between 2005 and 2017 (i.e., surface drawdown that has no associated mass loss). The firn has become denser and more ice-rich since the 1960s and contains a perennial firn aquifer (PFA), which may have developed over the past decade. This illustrates how firn may be evolving in response to climate change in the St.?Elias Mountains, provides firn density information required for geodetic mass balance calculations, and is the first documented PFA in the Yukon–Alaska region.