Extensive saline groundwater beneath Whillans Ice Stream, West Antarctica

Abstract:

Antarctic subglacial hydrologic systems modulate ice sheet dynamics, influence grounding line stability, host microbial communities, and deliver freshwater and nutrients to the Southern Ocean. Most subglacial hydrologic studies focus on water systems at the base of the ice sheet or within the shallow till layer. However, groundwater extending 100s meters below the ice base may also exist and be actively interacting with till and basal water systems. In the McMurdo Dry Valleys, briny groundwater systems within the upper few 100 ms of the subglacial environment have been identified beneath relatively thin ice (< 500 m) via airborne electromagnetic (EM) surveying. Ground-based EM methods are required for imaging groundwater systems beneath thicker ice (> 500 m). Here we use the magnetotelluric method, a passive EM geophysical technique, to image the electrical resistivity structure of the upper few kilometers beneath the Whillans Ice Stream, where ice thickness is ~ 800 m, at two locations: 1) Whillans Subglacial Lake (SLW), an active subglacial lake on lower Whillans Ice Stream, West Antarctica, that undergoes filling and draining cycles with a 2-4 year periodicity, and 2) the grounding zone where SLW is predicted to drain into the sub-ice-shelf cavity beneath Ross Ice Shelf. Our survey builds on previous geophysical and direct subglacial access studies from the WISSARD project. We present 2D electrical resistivity models of the upper 4 km of the subglacial environment, which we interpret in terms groundwater salinity and depth to bedrock. At both survey sites, we find that porous subglacial sediments are 0.5 to 1.4 km thick and contain groundwater with increasing salinity with depth. At the bottom of the sediment package, salinity values exceed that of seawater. Our calculated salinities agree well with in situ pore water measurements from the top 38 cm of sediment beneath SLW. These saline groundwater systems are laterally continuous at both locations, suggesting that Whillans Ice Stream may be underlain by significant quantities of deep saline groundwater, influencing subglacial heat fluxes, ice streaming, and deep biogeochemical processes. Furthermore, our observed groundwater distributions may be used as constraints in paleo-hydrologic model studies to determine the timing of past marine incursions.