Radar evidence for stability of the Ross/Amundsen ice flow divide of the West Antarctic ice sheet.

The marine-based West Antarctic ice sheet is likely the dominant contributor to past and possibly future rapid sea level changes. West Antarctic ice primarily flows westward to the Ross Sea, northward to the Amundsen Sea, and eastward to the Weddell Sea. Observations of dramatic thinning at the grounding lines of the Pine Island and Thwaites glaciers suggest that presently the Amundsen Sea catchment may be well out of balance. The Ross Sea catchment is currently in balance to within measurement uncertainty, although its grounding line has retreated by approximately 1000 km since the last glacial maximum, approximately 15,000 years ago. Such large-scale changes of the ice sheet margin configuration may also be expressed by movement of the divide that separates the flow of ice into these two catchments. Recovering a history of any such divide migration through the Holocene may shed light on the Ross-side retreat history, or alternatively, a recently activated migration may provide information about the current Amundsen Sea catchment imbalance. Ross/Amundsen flow divide migration is of particular interest since this is the site chosen for the second WAISCORES deep ice core. During 1999/2000, we conducted a series of airborne radar surveys of the Ross/Amundsen flow divide region with the objective of detecting stratigraphy patterns indicative of flow divide migration. The surveys consisted of continuous radar profiles along 12 curvilinear paths that follow local ice flow direction while maintaining constant terrain clearance. Such along-flow surveys facilitate flow-band modeling studies for quantitative determination of divide stability and or migration. Each of these approximately 120 km long lines were surveyed twice, once with equipment parameters set to optimize maximum ice penetration (for deep layer and bed detection) and once with settings to optimize near-surface resolution of stratigraphy (for enhanced temporal resolution). Imaged stratigraphy in the upper half of the ice column gives a record of the flow divide position over the past approximately 4 ka. We will discuss this evidence for relative stability of the divide position and its implications for post-glacial retreat of the West Antarctic ice sheet.