Modulation of Whillans Ice Stream Stick Slip Cycle: Loading Rate, Subglacial Hydrology and Slowdown

The Whillans Ice Stream (WIS) is one of the fast flowing glaciers that discharges ice from West Antarctica into the Ross Ice Shelf. WIS moves by a tidally modulated stick-slip motion where for 6 to 24 hours the glacier slides slowly (~200 m/yr) followed by short periods (30 minutes) of fast sliding (slip event) in excess of ~3000 m/yr equivalent. Slip events are thought to nucleate at a consistent geographic region beneath the glacier when basal stress exceeds the yield strength of the bed. Data from continuous GPS station deployed on the surface of WIS between December 2007 and May 2013 observed surface displacement of the ice stream including more than 3100 stick-slip cycles. In 2008, WIS experienced a 6 month speed up of 2% during subglacial lake activity. WIS has been slowing since at least the 1960';s with current deceleration at the greatest recorded magnitude (10 m/yr2). Changes in annual ice discharge from velocity variability are between 6% and 14% of the catchments estimated +5 Gt/yr annual mass budget imbalance. Recent changes in ice velocity over both short time scales, subglacial lake drainage, and long term, multi-year deceleration, result from modulation of the stick-slip cycle by changing basal resistive strength and the rate of basal stress accumulation at slip nucleation sites. Although the basal resistance is changing on a regional scale, modeling of stress accumulation showed no change in the magnitude of the stress threshold necessary to initiate slip events over the duration of the observations. Suggesting that slip nucleation sites are not included in the regions of change or unaffected by basal hydrological changes. Instead, slip frequency and slow sliding speeds changed in response to basal resistive stress. Subglacial Lake Whillans filled and drained over a year period in 2008 and 2009 and increased basal water pressures across a region of WIS bed. Increased slip frequency during subglacial lake activity accounts for 90 to 100% of the extra displacement during the 2008 speed up. Basal resistance in regions with elevated basal water pressure is reduced and may have been redistributed towards slip nucleation sites increasing the rate of stress accumulation. Greater rates of stress accumulation result in more frequently occurring slip events. Once subglacial hydrologic activity ceases in early 2009, WIS resumes a trend of deceleration through the end of the observational period. The decrease in velocity is a combination of the reduction of both the slip frequency (-30 slips/yr) and slow sliding rates (-8 m/yr2). Increasing basal resistance over some portion of the bed (20 to 40 Pa/yr), possibility through till strengthening by draining of pore water, decreases slow sliding rates and prevents some stress from accumulating at slip nucleation sites. An increasing magnitude of basal resistance supported over a broad region of the bed continues to lower the rate of stress accumulation at slip nucleation sites and decreases slip frequency through time. Basal resistive stress change, in response to hydrological activity, is the most significant cause of current observed changes of WIS surface velocity and reinforces the importance of internal thermo-dynamic feedbacks to the sensitivity of glacier motion.