Grounding Zone Heterogeneity in West Antarctica

Grounding zones of ice sheets are critical to understanding marine ice sheet dynamics as processes here determine the mass flux from grounded to floating ice, and thus eventually to the ocean. Furthermore, ocean-driven basal melt at grounding lines can initiate grounding line retreat and possibly trigger eventual deglaciation through the marine ice-sheet instability. Despite their importance to ice sheet dynamics and potential to influence sea level, direct basal observations of grounding zones remain sparse. Here we present geophysical data (ice-penetrating radar, active-source seismic, laser altimetry, and GPS data) and modeling results from three grounding zones in West Antarctica (two on Whillans Ice Stream (WIS) and one on Thwaites Glacier (TG)). These data show that grounding zones that have significantly different surface expressions (in the form of either differing surface slopes, recent grounding line behavior, or grounding zone width) also have significant differences in basal features and processes which are important to capture in ice flow models. On WIS, we compare the grounding zone of a subglacial embayment (an area where subglacial water from several subglacial lakes is suspected to drain to the ocean) to that of a subglacial promontory (characterized by steep surface slopes). The embayment is characterized by less dramatic surface and basal slopes, and less basal reflectivity contrast across the grounding zone. This suggests that there is less of a barrier to seawater intrusion into, and possibly, upstream, of the low-tide grounding line. In contrast, data collected over the promontory depict steep surface slopes, dramatic ice thinning across the grounding line, and a strong contrast in basal reflectivity. This indicates that the grounding zone in this promontory is likely a strong barrier to seawater intrusion and thus to grounding zone retreat. The grounding zone of TG has significantly steeper surface slopes than those on WIS and high basal reflectivity extends several kilometers inland of the grounding line in some areas. As on WIS, we interpret this as an indicator of possible seawater intrusion upstream of the grounding-line which may cause basal melt. Including this in our modeling gives results that indicate that if the width of the grounding zone, over which we partially extend basal melt, is greater than the width of underlying bedrock or sedimentary stabilizing structures, the grounding line is prone to retreat over these stabilizing features. These results highlight the need to more accurately incorporate grounding-zone processes and detailed subglacial topography into ice-sheet models in order to prognostically simulate future ice sheet behavior.