Transient Meltwater in Mullins Valley Glacier, McMurdo Dry Valleys, Antarctica

Abstract:

Mullins Glacier is a cold-based debris-covered glacier feeding into Beacon Valley, at high altitude in the McMurdo Dry Valleys of Antarctica. Ice is exposed at the headwall in Mullins Valley but the majority of the glacier is buried beneath a sublimation till (lag deposit composed of englacial and supraglacial debris). This till is initially ~10 cm thick but gradually thickens to ~60 cm at the glacier terminus (~8 km distant). Mullins Glacier has been postulated to be one of the world’s oldest alpine glaciers: tephrachronology places a minimum age of the overlying sublimation till near the terminus at 7.9 Ma. Our measurements of the complex resistivity (aka spectral induced polarization or dielectric spectroscopy) of massive Mullins Glacier ice reveal two distinct origins. The electrical properties of clean ice or ice with rock fragments are typical of meteoric polar ice (Stillman et al., JGR, 2013). However, "dirty" ice is electrically distinct, indicating soluble impurity content near lattice saturation. This behavior, which we also observed for Lake Vostok accretion ice, is consistent with freezing from saline, draining water. Therefore one hypothesis it that the dirty ice formed by infiltration in former clement environments. However, very efficient segregation is subsequently required, and not all dirty ice is at the top of the ice column. Dirty ice likely samples debris bands, which are more commonly observed in cores where Mullins Glacier has advanced onto the main (Beacon) valley floor and is nearly stagnant. If debris bands are correlated to lattice impurity saturation via the dirty ice, then they may have been transiently at or near melting. This may be a primary feature of the environment during debris accumulation or simply due to the high thermal inertia of debris. Alternatively, debris bands and associated salts may be carried below the annual thermal wave where they experience near-constant, supereutectic temperatures. Elevated temperatures may be electrically imprinted even in cold-based glaciers.