Seasonal glacier motion variations and underlying hydro-mechanical processes at Glacier d'Argentière, French Alps

Abstract

Subglacial hydrology controls basal sliding of hard-bedded glaciers by modulating basal drag through changes in ice–bed separation. Yet, the underlying mechanisms that control ice–bed separation and its links with basal friction remain poorly understood. In this study, we contribute to a better understanding of this problem by evaluating spatial and temporal changes in bed separation in relation to changes in glacier horizontal velocity using three years of continuous and dense GPS records from Glacier d'Argentière (French Alps). We confirm a previous study showing that spatial and temporal variations in glacier vertical motion mainly reflect changes in ice–bed separation, as they cannot be explained by variations in internal strain rates. We find that the rate of uplift is in anti-phase with subglacial water discharge, being positive in winter in the absence of surface melt and negative during summer melt. We suggest that this behavior results from basal cavities being weakly connected in winter, allowing them to fill slowly under low water input from englacial storage release or basal melt, and then rapidly transitioning to a connected state in summer, enabling efficient drainage of surface meltwater and reduced cavity sizes. A key finding is that changes in horizontal velocity are well correlated, both in time and space, with changes in ice–bed separation. This results in an increase in horizontal velocity in winter that can be quantitatively compared with modeled values related to subsequent variations in basal cavity size. This contrasts strongly with previous observations in steeper parts of Glacier d'Argentière, where velocities were found to decrease consistently in winter and where it was argued that seasonal motion was primarily controlled by cavities being connected year-round. We discuss the potential mechanisms underlying these discrepancies and how they may also explain observations of seasonal glacier dynamics in Greenland.