Ocean Ice Interactions At Breidamerkurjokull Glacier Southeast Iceland PDF Download

Abstract: The purpose of this study is to determine how glacier motion and stresses vary spatially and temporally in order to clarify weaknesses in current understanding of soft-bedded glacier motion using data collected from Breiðamerkurjökull, Iceland. The dynamics of ice motion are the most substantial source of uncertainty in current models of future ice sheet mass-loss and resulting sea level rise. Currently, there is a general lack of quantitative understanding of how glacial basal conditions, such as the hydrology and till rheology at the bed, control ice motion. This study focuses on the examination of high spatial and temporal resolution surface velocities retrieved from a 12-station GPS grid in the melt seasons of 2009 and 2010 to evaluate the variation of glacial motion and strain rates over time on Breiðamerkurjökull. The first specific objective is to identify any short-term velocity variations. The second is to use the surface motion data to calculate strain rates and other components of the force budget. The third objective is to explain the variations in velocity and force budget components while taking into account glaciomorphic features of the bed. Results reveal five distinct periods of increased surface motion, termed sliding events, corresponding to periods of rainfall and/or increased temperatures during the 2009 and 2010 melt seasons. Along-flow strain rates show extension upglacier and compression downglacier during sliding events. The force budget solution indicates that upglacier, basal drag decreases substantially during speed-up events and cannot resist the local driving stress, most likely indicating pressurization of a distributed subglacial drainage system. The excess driving stress is then transferred downglacier, through gradients in longitudinal stress, to a more efficiently draining terminus where water pressures are lower and basal drag is sufficient to support the excess stress. The results demonstrate that the till at the terminus accommodates the excess stress, possibly through extensive grain bridging and dilatant hardening or by a relocation of stress to bedrock bumps during sliding events. This buttressing role of the till-bedded margin in resisting increased upglacier sliding, likely over bedrock, is novel and counter to the prevailing view of soft beds, with implications for simulating the evolution of past and current ice masses.