Collaborative Research: Using seismic tremor to constrain seasonal variations in subglacial hydrology at the bed of the Greenland ice sheet

The rate of mass loss from the Greenland ice sheet is accelerating due to a combination of increased surface melt and changes in ice sheet dynamics. Our understanding of the link between increased melt and increased ice velocity is limited by the difficulty in characterizing where basal meltwater (primarily sourced from surface runoff) goes in time and space and how it affects the coupling of the ice sheet to the bedrock. Observations of seasonal variations in ice velocity and hydrology from temperate alpine glaciers have led to a conceptual model for the evolution of the basal drainage system over a melt season, which involves efficient meltwater channelization during periods of peak summer melt production. However, an important caveat when applying subglacial hydrology models derived from alpine glaciers to ice sheets, is that these models are poorly constrained by direct observations of basal fluid flow due to the difficulty of making hydrologic measurements through thick ( 1 km) ice. Thus, a key challenge in linking ice sheet dynamics to variations in surface melt is independently characterizing the subglacial hydrologic system and how it evolves through time. Researchers will perform pilot study to use seismic tremor to investigate seasonal changes in the subglacial hydrologic system and investigate the relation between tremor signals and ice surface velocities on the Greenland ice sheet. Researchers will deploy co-located seismic and GPS arrays to simultaneously monitor subglacial meltwater flow and ice surface velocities at a well-studied site on the western margin of the Greenland ice sheet. This site was chosen because of its strong seasonal velocity variability, experiencing dramatic acceleration in the early summer (>100% greater than winter velocities) and pronounced slowdown in the late summer. Using these co-located measurements of subglacial fluid flow and surface velocities researchers will address the following research questions: (1) Can analysis of seismic tremor, similar to that used to estimate terrestrial and sub-glacial discharge, be extended to measure basal meltwater flow in regions of thick (=1 km) ice? (2) If and when during the melt season does channelization of sub-glacial flow occur and how do the characteristics of the subglacial channels (e.g., density, spacing) vary with time? (3) Do ice surface velocities correlate with the timing of sub-glacial channelization as is hypothesized based on existing models for the characteristic annual ice velocity curve? If successful, the use of seismic tremor will provide a powerful new technique to study subglacial meltwater flow in regions that are difficult to access through other methodologies. Further, the data collected through this study can be used to inform large-scale numerical ice sheet models linking subglacial fluid flow to ice sheet dynamics allowing these results to be applied more generally. The Greenland ice sheet’s contribution to sea level rise over the next century is largely unknown; estimates range from a few centimeters to over one meter. Planning within such a large range of uncertainty has extremely high economic and social costs for governments, communities and businesses worldwide. Thus, the investigation aims at removing uncertainties that limit estimates of future ice-sheet contributions to sea level is of broad societal, as well as scientific, interest. One full-time BC masters student and one WHOI postdoc will be supported through this project, and researchers will pursue opportunities for additional student projects related to this work. The student and postdoc will be involved in all aspects of the project research and fieldwork. Reporter Dr. Heather Goldstone will accompany the science team to the study site and will produce a range of media for general audiences including a daily radio diary to air on National Public Radio stations, as well as other resources and outlets. In particular, the seismic data generated by this project presents a unique opportunity for the public to experience raw data in a sensory way. By transforming seismic data to audible frequencies and condensing it, researchers will produce a brief soundtrack of sub-glacial flow paired with visual imagery from the field, to create video resources ideal for disseminating via the web and social media.