Evolving subglacial water systems in East Antarctica from airborne radar sounding
The cold, lightless, and high pressure aquatic environment at the base of the East Antarctic Ice Sheet is of interest to a wide range of disciplines. Stable subglacial lakes and their connecting channels remain perennially liquid three kilometers below some of the coldest places on Earth. The presence of subglacial water impacts flow of the overlying ice and provides clues to the geologic properties of the bedrock below, and may harbor unique life forms which have evolved out of contact with the atmosphere for millions of years. Periodic release of water from this system may impact ocean circulation at the margins of the ice sheet. This research uses airborne radar sounding, with its unique ability to infer properties within and at the base of the ice sheet over large spatial scales, to locate and characterize this unique environment. Subglacial lakes, the primary storage mechanism for subglacial water, have been located and classified into four categories on the basis of the radar reflection properties from the sub-ice interface: Definite lakes are brighter than their surroundings by at least two decibels (relatively bright), and are both consistently reflective (specular) and have a reflection coefficient greater than --10 decibels (absolutely bright). Dim lakes are relatively bright and specular but not absolutely bright, possibly indicating non-steady dynamics in the overlying ice. Fuzzy lakes are both relatively and absolutely bright, but not specular, and may indicate saturated sediments or high frequency spatially heterogeneous distributions of sediment and liquid water (i.e. a braided steam). Indistinct lakes are absolutely bright and specular but no brighter than their surroundings. Lakes themselves and the different classes of lakes are not arranged randomly throughout Antarctica but are clustered around ice divides, ice stream onsets and prominent bedrock troughs, with each cluster demonstrating a different characteristic lake classification distribution. In the bedrock trough of Adventure Subglacial Trench, analysis of satellite altimetry is combined with radar sounding data to calculate a mass budget and infer a flow mechanism for a two cubic kilometer discharge reported to have traveled between two lakes in the region from 1996 -1998. The volume released from the source lake exceeded the volume received by the destination lakes by one and a tenth cubic kilometers, indicating that some water must have escaped downstream from the lowest destination lake over the course of the event. Release of water from the source lake preceded arrival of the water at the destination lakes, 260 kilometers away, by about three months. Water continued draining from the destination lakes for several years after surface subsidence at the source lake had ceased. By 2003, a total of one and a half cubic km or nearly 75% of the water released by the source lake had traveled downstream from the destination lakes. Hydraulic modeling work indicates that the initial release of water from the source lake could have been accommodated by a self-enlarging semicircular channel. Subsequent evolution of the discharge and the three-month delay between release of water from the source lake and arrival of that water at the destination lakes indicates that a shallower and broader distributed water system is responsible for the transport of subglacial water in this region. Such a system would be more stable for the given icebedrock geometry and may explain the observations of intermittent flat bright bedrock reflections in radar data acquired upstream from the destination lake in 2000. For the purpose of better understanding the long-term water budget of the Dome C region, an area upstream of Adventure Trench, eleven dated isochronal internal layers within the ice penetrating radar data were tracked. An age-depth relationship, derived from the European ice core through Dome C is used to calculate strain, estimate melt, model ice temperature, and determine absolute basal reflectivity for the entire region which covers over 28,000 square kilometers. The two largest subglacial lakes within the survey, Concordia and Vincennes, are both associated with enhanced basal melting on their upstream shores at rates locally greater than two millimeters per year. Widely distributed melt rates in the major topographic valleys upstream of these lakes are generally less than one millimeter per year throughout the region with slightly higher melts in the basin draining into Vincennes Subglacial Lake. Although published estimates for geothermal flux are capable of explaining the behavior of ice and water in most of the area, an additional source of basal heat is required to explain melt anomalies and subglacial lakes along the Concordia Ridge. Lake Concordia is expected to discharge water on a similar scale and duration as that observed in Adventure Trench, with a repeat cycle of a few hundred years.
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