Investigating hydrologic controls on glacier velocity using 222Rn as a proxy for variable subglacial pressure

Each summer, meltwater forms on the surface of the Greenland Ice Sheet and travels through cracks and moulins to the ice-bed. There, hydraulic pressure in cavities and channels controls glacial sliding; coincident with the highest hydraulic pressures are the fastest annual glacial speeds. Meltwater pathways at the ice-bed undergo a seasonal evolution from high-pressure, inefficient linked-cavity systems at the onset of spring melt to low-pressure, high-capacity channelized systems by midsummer. Radon-222 (t_1/2 = 3.8 days) is a promising new tool for glaciology (Bhatia et al., 2011) as it is injected into meltwater during interaction with sediment and rock through the radioactive decay of naturally occurring ²²⁶Ra. Therefore in proglacial rivers, ²²²Rn can be assumed to trace fluxes of subglacial groundwater or meltwater transiently stored at the ice-bed. Radon-222 was quantified in the proglacial river of Leverett Glacier, a large outlet glacier of the Greenland Ice Sheet, during the summers of 2011 (May 8^th - August 10^th) and 2012 (May 12^th - August 1^st). Continuous (hourly) measurements were made using a RAD-7 (Durridge Inc.) with gas-permeable tubing in place of the air-water equilibrator. We estimated englacial meltwater storage as the difference between proglacial river discharge and meltwater inputs, calculated from a positive degree-day melt model based on temperature sensors on the ice surface and MODIS satellite imagery to determine the timing and size of supraglacial lake drainage events. Periods of high glacial velocity displayed strong subdiurnal covariations with ²²²Rn. We hypothesize that this is the result of increasing englacial meltwater storage, channel pressurization and ²²²Rn tracing groundwater fluxes. When pressure is rising in channels, meltwater is driven distally into adjacent linked cavity networks where it is temporarily stored while channel pressures are centrifugal. During these periods, meltwater traveling though channels likely has minimal interaction with the bed and therefore accumulates negligible amounts of ²²²Rn. Instead, during periods of channel pressurization ²²²Rn likely traces the flux of groundwater. As groundwater discharge to the ice margin is controlled by pressure at the ice-bed, periods of increasing channel pressurization and englacial storage will result in groundwater fluxes proportional to hydraulic pressure and, consequently, ice velocity. In support of this hypothesis, we observed the largest fluxes of ²²²Rn in the proglacial river during multi-day periods of low glacial velocity. These observations are consistent with channel depressurization and drainage of meltwater stored in linked cavity systems. Bhatia, M.P., Das, S.B., Kujawinski, E.B., Henderson, P., Burke, A., Charette, M.A., 2011. Seasonal evolution of water contributions to discharge from a Greenland outlet glacier: insight from a new isotope-mixing model. J Glaciol 57, 929-941.