Geometry, hydrology and environmental controls on a 'stacked' firn aquifer on a temperate high-accumulation ice field

Mountain glaciers are undergoing rapid change, with many predicted to disappear in the next 50-200 years. These glaciers are vital summer water sources for many human and ecological communities. Additionally, many of the processes observed in mountain glaciers are observed on the margins of ice sheets, and may proliferate as ice sheets warm. Understanding the processes controlling the flow of water across and through glaciers is vital for predicting changes in water resources and sea-level. We report in situ observations of snowmelt, ice flow, and firn aquifer geometry and evolution, from the ice divide of the Juneau Icefield, Alaska, a largely temperate, high accumulation ice field that is rapidly melting. During two melt seasons, autonomous phase-sensitive radio echo sounders (ApRES) were deployed: (2018) in a 3-by-2.5 km grid to measure vertical englacial strain rates and (2019) along an ice flow line in an attempt to track meltwater percolation through the subsurface. ApRES surveys were accompanied by shallow ice cores 5-21m deep, a 400 MHz shallow radar grid, and measurements of surface melt. The 2019 field season investigated a potentially-new structure for mountain glacier aquifers, where a thick ice lens underlies an aquifer containing each summer's meltwater, creating a vertically 'stacked' aquifer configuration. To examine the environmental conditions likely to permit this configuration, we apply a continuum model of water flow and refreezing in compacting firn developed by Meyer and Hewitt. This work, as well as being one of the first uses of the ApRES system in temperate ice, can help us to understand near-surface density structure of temperate, high accumulation glaciers, and how its response will vary from glaciers in other climate regimes.