ABSTRACT. Basal pressure has been recorded at the Svartisen Subglacial Laboratory, northern Norway, for 20 years, and is measured by load cells installed at the ice-rock interface under �200 m of glacier ice. Synchronous pressure variations between load cells are investigated as evidence of stress redistribution and hydrological bed connectivity. A running Pearson correlation is used to study the temporal variation in the response of several sensors. By studying the nature of this correlation as well as the correlation between sensor pairs, it is possible to investigate the evolution of the degree of synchronous response, and to some extent basal connectivity, at the glacier bed. Persistent seasonal variations associated with the melt season are observed throughout the measurement period, indicating dependence on surface hydrological forcing. Overlying this pattern, specific years with longer periods of positive and negative correlation of pressure between sensors are presented to show contrasting interannual variability in basal pressure. An anticorrelated connectivity is associated with a local increase in the rate of daily subglacial discharge, and is caused by load transfer or passive cavity opening. Stable weather appears to enhance connectivity of the sensors, which is attributed to the development of a persistent drainage system and stress redistribution.

Abstract. Frontal ablation contributes significantly to the mass balance of tidewater glaciers in Svalbard and can be recovered with high temporal resolution using continuous seismic records. Determination of the relative contribution of dynamic ice loss through calving to frontal ablation requires precise estimates of calving volumes at the same temporal resolution. We combine seismic and hydroacoustic observations close to the calving front of Kronebreen, a marine-terminating glacier in Svalbard, with repeat lidar scanning of the glacier front. Simultaneous time-lapse photography is used to assign volumes measured from lidar scans to seismically detected calving events. Empirical models derived from signal properties such as integrated amplitude are able to replicate volumes of individual calving events and cumulative subaerial ice loss over different lidar scan intervals from seismic and hydroacoustic data alone. This enables quantification of the contribution of calving to frontal ablation, which we estimate for Kronebreen to be about 18 %–30 %, slightly below the subaerially exposed area of the glacier front. We further develop a model calibrated for the permanent seismic Kings Bay station (KBS) at about 15 km distance from the glacier front, where 15 %–60 % of calving events can be detected under variable noise conditions due to reduced signal amplitudes at distance. Between 2007 and 2017, we find a 5 %–30 % contribution of calving ice blocks to frontal ablation, which emphasizes the importance of underwater melting (roughly 4–9 m d−1). This study shows the feasibility to seismically monitor not only frontal ablation rates but also the dynamic ice loss contribution continuously and at high temporal resolution.

We used a finite element model to interpret anti-correlated pressure variations at the base of a glacier to demonstrate the importance of stress redistribution in the basal ice. We first investigated two pairs of load cells installed 20 m apart at the base of the 210 m thick Engabreen glacier in Northern Norway. The load cell data for July 2003 showed that pressurisation of a subglacial channel located over one load cell pair led to anti-correlation in pressure between the two pairs. To investigate the cause of this anti-correlation, we used a full Stokes 3D model of a 210 m thick and 25-200 m wide glacier with a pressurised subglacial channel represented as a pressure boundary condition. The model reproduced the anti-correlated pressure response at the glacier bed and variations in pressure of the same order of magnitude as the load cell observations. The anti-correlation pattern was shown to depend on the bed/surface slope. On a flat bed with laterally constrained cross-section, the resulting bridging effect diverted some of the normal forces acting on the bed to the sides. The anti-correlated pressure variations were then reproduced at a distance >10-20 m from the channel. In contrast, when the bed was inclined, the channel support of the overlying ice was vertical only, causing a reduction of the normal stress on the bed. With a bed slope of 5 degrees, the anti-correlation occurred within 10 m of the channel. The model thus showed that the effect of stress redistribution can lead to an opposite response in pressure at the same distance from the channel and that anti-correlation in pressure is reproduced without invoking cavity expansion caused by sliding.