Summary
Reconstructing and modeling extent and dynamics of ice sheets is crucial in understanding and predicting the evolution of ice sheets. Key to ice sheet dynamics are ice streams, which drain most of the ice from the ice sheet interior to the ice shelves, where most of the ice is lost. Satellite and ground observations provide detailed information on modern ice stream locations and drainage patterns. In contrast, locations of paleo-ice streams can be deduced from sea floor morphological features on the continental shelves and slope, including mega-scale glacial lineations, cross-shelf trough systems and sediment deposits. Recent compilations of bathymetry data from the Amundsen Sea sector of West Antarctica revealed two major cross-shelf trough systems, the Pine-Island Thwaites Trough System and the Getz-Dotson Trough System. Several studies show that both trough systems were occupied by grounded ice streams during the last glacial period. Analysis of the trough geometries shows that both systems are of similar in cross-section and depth, with a maximum depth over 1500 m. However, the two trough systems differ in other aspects. One system connects to the Pine Island, Thwaites, and Smith Glaciers and their major ice streams, which drain about 25% of the West Antarctic Ice Sheet. The location of individual branches of that trough system indicates that the present ice streams simply extended farther onto the shelf and then merged during times of more extended ice sheet. On the other hand, the Getz-Dotson Trough System does not connect to major ice streams but only to smaller glaciers today, although the islands that border the Getz ice shelf today may have acted to steer the ice flow in the glacial period. If ice streams of similar magnitude were necessary to form these troughs over successive glacial cycles, their similar features would suggest a different drainage pattern in the past, with larger ice streams flowing through the Getz-Dotson troughs. These ice streams may have drained a larger proportion of the WAIS than today, which would require a significant change of the present ice drainage pattern. This might have occurred if greater advance of the LGM grounding line in the Pine Island Glacier and Thwaites Glacier sectors led to changes in the large scale ice sheet geometry, or if ice flowed across the present ice divide in Marie Byrd Land. Alternatively, ice accumulation rates in the more coastal Marie Byrd Land region may have been sufficient to produce and maintain a fast ice stream that filled the deep Getz-Dotson trough during times of lower sea level. The present ice sheet above the Marie Byrd Land mountain range is also significantly thinner than the ice sheet in the catchments of the Pine Island and Thwaites Glaciers. Therefore, basal temperatures, melt conditions, hydrology and resulting ice flow velocities in the Getz-Dotson catchment might be more sensitive to changes of ice thickness. As a result, thinning of the ice sheet since the Last Glacial Maximum could have caused a slow down of flow into the ice stream. Well-preserved glacial features on the seafloor and the absence of grounding line wedges in the Getz-Dotson trough system suggest a fast retreat after the last glacial maximum. Such studies, which compare paleo-ice flow features with present ice flow pattern, provide an important tool for verifying and calibrating reconstructions and models of long-term ice sheet evolution, involving past ice accumulation and ice sheet dynamics.