We use CryoSat-2 interferometric satellite radar altimetry over the Mercer and Whillans ice streams, West Antarctica, to derive surface elevation changes due to subglacial lake activity at monthly resolution for the period 2010 to 2013. We validate CryoSat-2 elevation measurements, trends, and spatial patterns of change using satellite image differencing and in situ vertical movement from Global Positioning System (GPS) data. Two subglacial lake discharge events occur in the same subglacial-hydrological catchment within a 9 month period. Using GPS measurements that are spanning the gap between the Ice, Cloud, and land Elevation Satellite and Cryosat-2 missions, we cross-calibrate the two missions to establish the efficacy of CryoSat-2 altimetry to measure dynamic changes on the ice sheets.
A clean hot-water drill was used to gain access to Subglacial Lake Whillans (SLW) in lateJanuary 2013 as part of the Whillans Ice Stream Subglacial Access Research Drilling (WISSARD) project.Over 3 days, we deployed an array of scientific tools through the SLW borehole: a downhole camera, aconductivity–temperature–depth (CTD) probe, a Niskin water sampler, an in situ filtration unit, threedifferent sediment corers, a geothermal probe and a geophysical sensor string. Our observations confirmthe existence of a subglacial water reservoir whose presence was previously inferred from satellitealtimetry and surface geophysics. Subglacial water is about two orders of magnitude less saline than seawater (0.37–0.41psu vs 35psu) and two orders of magnitude more saline than pure drill meltwater(<0.002psu). It reaches a minimum temperature of –0.558C, consistent with depression of the freezingpoint by 7.019MPa of water pressure. Subglacial water was turbid and remained turbid followingfiltration through 0.45mm filters. The recovered sediment cores, which sampled down to 0.8m belowthe lake bottom, contained a macroscopically structureless diamicton with shear strength between 2and 6kPa. Our main operational recommendation for future subglacial access through water-filledboreholes is to supply enough heat to the top of the borehole to keep it from freezing.
The subglacial water system of lower Whillans Ice Stream on the Sip le Coast, West Antarctica, contains numerous connected subglacial lakes in three hydrological basins (northern, central and southern). We use Ice, Cloud and land Elevation Satellite (ICESat) data to derive estimates of lake volume change and regional thickness changes. By combining these results with a water budget model, we show that a uniform, localized thickness increase perturbed the hydropotential, resulting in a change in course of a major flow path within the system in 2005. Water originating from upper Whillans and Kamb Ice Streams that previously supplied the southern basin became diverted toward Subglacial Lake Whillans (SLW). This diversion led to a tenfold filling rate increase of SLW. Our observation suggests that water piracy may be common in the Sip le Coast region, where the gentle basal relief makes the basal hydropotential particularly sensitive to small changes in ice thickness. Given the previously inferred connections between water piracy and ice-stream slowdown elsewhere in the region, the subtle and complex nature of this system presents new challenges for numerical models.