LAUSR

Antarctica’s fast-flowing ice streams drain the ice sheet, with their velocity modulated by subglacial water systems. Current knowledge of these water systems is limited to the shallow portions near the ice-bed interface, but hypothesized deeper groundwater could also influence ice streaming. Here, we use magnetotelluric and passive seismic data from Whillans Ice Stream, West Antarctica, to provide the first observations of deep sub–ice stream groundwater. Our data reveal a volume of groundwater within a >1-kilometer-thick sedimentary basin that is more than an order of magnitude larger than the known subglacial system. A vertical salinity gradient indicates exchange between paleo seawater at depth and contemporary basal meltwater above. Our results provide new constraints for subglacial water systems that affect ice streaming and subglacial biogeochemical processes. Description The deeper story Shallow, dynamic subglacial water systems provide lubrication that facilitates the movement of overlying ice. But are these thin layers the whole story? Gustafson et al. show that the subglacial sediments beneath Whillans Ice Stream in West Antarctica are saturated with a mixture of fossil seawater and freshwater from the glacier (see the Perspective by Chu). This groundwater, extending downward for more than a kilometer, contains more than 10 times as much fluid volume as the shallow hydrologic system above and actively exchanges with it. Therefore, it has the potential to modulate ice streaming and subglacial biogeochemical reactions. —HJS Fossil seawater mixes with basal melt in sediments underlying the Antarctic ice sheet.