Temporal Changes in Spatial Distribution of Basal Melting and Freezing in the Catchment Areas of Whillans Ice Stream and Ice Stream C, West Antarctica: Interplay of Climatic Changes and Ice Dynamics

Basal thermal regimen of West Antarctic Ice Sheet (WAIS) plays the key role in determining the dynamics and stability of this ice sheet. Basal melt water lubricates the ice base allowing fast ice streaming while basal freeze-on increases basal resistance to ice flow. Within WAIS, basal melting is dominant in the interior, where geothermal heat is trapped underneath ~2-to-4-km-thick layer of ice. Basal freeze-on is dominant beneath the slow moving, ~1-km-thick interstream ridges separating fast-moving ice streams. There, conductive heat escape through exceeds the geothermal flux and basal frictional heating is low. Using a time-dependent basal energy balance model (Vogel et al., in press) we examined spatial and temporal distribution of basal melting and freezing in the catchment areas of Whillans Ice Stream and Ice Stream C since the Last Glacial Maximum (LGM, ~20,000 years ago). Model results indicate that basal melting peaked despite lower surface temperatures during late LGM (~15,000), due to a thickened ice sheet (Steig et al., 2001). This widespread and abundant basal lubrication may have initiated the retreat and thinning of the ice sheet that continued through the Holocene. However, the ice-sheet thinning itself caused gradually a general decrease in basal melting rates in spite of higher Holocene surface temperatures. This reduction in basal water production may be responsible for the recent stoppage of Ice Stream C and slow down of the Whillans Ice Stream. Our modeling results indicate that WAIS is still adjusting to the significant climate warming that marked the end of the LGM and the beginning of Holocene. Only the thinnest portions of the Whillans Ice Stream and Ice Stream C (<1 km) might have adjusted enough to cause locally significant basal freeze-on and to, at least temporarily, slow the ice sheet decay (Joughin and Tulaczyk, 2002). Basal thermal regimen of the rest of WAIS is changing in such a way as to favor increased basal melting, and presumably further ice-sheet decay, in the near future (e.g., Engelhardt, pers. communication). Evaluation of near-future contribution of WAIS to sea-level changes has to take into account the delayed response of the ice sheet to the large global warming that took place at the LGM-Holocene boundary.