Persistent, stable, sub-flotation-pressure conduits beneath the inland Greenland Ice Sheet

Abstract:

The specific impacts of subglacial hydrology on ice sheets are sensitively dependent on hydrologic drainage structure, which changes over short time periods and short length scales between efficient subglacial conduits and inefficient till or linked cavities. Here, we report seismic observations and other geophysical data from the Paakitsoq region of the Greenland Ice Sheet with which we infer the structure and pressure of subglacial conduits. We interpret temporal change in tremor amplitude as reflecting temporal change in subglacial water discharge, and spatial differences in tremor amplitude as reflecting different distances from turbulently-flowing subglacial conduits.

Our linear array of 6 surface-installed seismometers, spaced 900 m apart at approximately 930 m elevation within the ice sheet ablation area, serves as a monitoring "fence" sensitive to subglacial discharge at the ice sheet bed. Data from these sensors supports our inference that subglacial conduit flow begins abruptly early in June, in immediate response to the first multi-day period of above-freezing weather in 2018. This interpretation is at odds with a more gradual transition from inefficient to efficient drainage. Additionally, we find that subglacial tremor amplitudes are strongest near the center of our linear array, and decrease with distance from this central location in a manner consistent with the propagation of seismic surface waves. The inferred conduit location is at local low points in both ice surface and bed elevations, but is not along the path of a subglacial conduit modeled assuming a hydraulic grade surface at the ice flotation level. Instead, the stable, efficient, flow path we identify requires local, conduit water pressures at less-than-flotation levels. The remote mapping of subglacial conduits reported here provides first-of-its-kind mapping of melt-season-long, stably located conduits, at high elevations beneath the Greenland Ice Sheet, as well as water flow at sub-flotation water pressures.