Dynamic distributed drainage implied by the flow evolution of the 1996-1998 Adventure Trench subglacial lake discharge

摘要

The transport of subglacial water beneath the East Antarctic Ice Sheet is an enigmatic and difficult to observe process which may affect the flow of the overlying ice and mixing of the oceans in the sub ice shelf cavities, and ultimately global climate. Periodic outbursts are a critical mechanism in this process. Recent analysis of satellite data has inferred a subglacial hydraulic discharge totaling 2 km(3) traveling some 260 km along the ice-bed interface of the Adventure Subglacial Trench between 1996 and 1998 (Wingham et al., 2006. Rapid discharge connects Antarctic subglacial lakes. Nature 440,1033-1036). Using radar echo sounding data from the Adventure Subglacial Trench region in conjunction with the previously reported satellite observations, along with some basic modeling, we calculate a mass budget and infer a flow mechanism for the 1996-1998 event. The volume released from the source lake exceeded the volume received by the destination lakes by similar to 1.1 km(3). This discrepancy indicates that some water must have escaped downstream from the lowest destination lake from 1997 onward. The downstream release of water from the destination lakes continued until at least 2003, several years after the 1998 cessation of surface subsidence at the source lake. By 2003 a total of 1.5 km(3) or nearly 75% of the water released by the source lake had traveled downstream from the destination lakes. The temporal evolution of discharge from the outlet can be simulated with the classic ice-walled semicircular channel model, if and only if the retreat of the source lake shoreline is taken into account. Further downstream, the ice bedrock geometry along the inferred flow path downstream includes many sections where thermal erosion of the overlying ice would not be sustainable. Along these reaches mechanical lifting of the ice roof and/or erosion of a sedimentary substrate by a broad shallow water system would be most effective means of sustaining the discharge. A distributed system is also consistent with the 3-month delay between water release at the source lake and water arrival at the destination lake. Observations of intermittent flat bright bed reflections in radar data acquired along the flow path are consistent with the presence of a broad shallow water system. Ultimately the presence of large subglacial lakes along the flow path of the 1996-1998 Adventure Subglacial Trench flow path delayed the arrival of water to points downstream by approximately 12 months.