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.
More information: Lefeuvre P.-M., T. Zwinger, M. Jackson, O. Gagliardini, G. Lappegard and J.O. Hagen, 2018. Stress Redistribution Explains Anti-correlated Subglacial Pressure Variations. Front. Earth Sci. 5:110. doi:10.3389/feart.2017.00110
The variability – temporal as well as spatial - in basal friction for Kronebreen, Svalbard, a fast-flowing tidewater glacier is evaluated. This is done by inverting surface velocity data over a period of 3 years (2013–15). Due to the excellent data coverage, this is achieved at a high temporal resolution of about 11 days. Results clearly show that sliding behaviour of Kronebreen seasonally is strongly influenced by changes in water input patterns as well as a strong inter-annual variability. Results lead to the conclusion that a physical description of the sliding of a tidewater glacier needs to exceed the complexity of a simple fixed parameter description. Basal sliding may not only be governed by local processes such as basal topography or summer melt, but also be mediated by factors that vary over a larger distance and over a longer time period such as subglacial hydrology organisation, ice-thickness changes or changes in calving front geometry.
Read more: Vallot, D., R. Pettersson, A. Luckman, D. Benn, T. Zwinger, W.J.J. van Pelt, J. Kohler, M. Schäfer, B. Claremar and N.R.J. Hulton, 2017. Basal dynamics of Kronebreen, a fast-flowing tidewater glacier in Svalbard: Non-local spatio-temporal response to water input, Journal of Glaciology, 1-13, doi:doi:10.1017/jog.2017.69.
Basal slip accounts for a large part of the flow of ice streams draining ice from Antarctica and Greenland into the ocean. Therefore, an appropriate representation of basal slip in ice flow models is a prerequisite for accurate sea level rise projections. Various friction laws have been proposed to describe basal slip in models. Here, we compare the influence on grounding line (GL) dynamics of four friction laws: the traditional Weertman law and three effective pressure-dependent laws, namely the Schoof, Tsai and Budd laws. It turns out that, even when they are tuned to a common initial reference state, the Weertman, Budd and Schoof laws lead to thoroughly different steady-state positions, although the Schoof and Tsai laws lead to much the same result. In particular, under certain circumstances, it is possible to obtain a steady GL located on a reverse slope area using the Weertman law. Furthermore, the predicted transient evolution of the GL as well as the projected contributions to sea level rise over a 100-year time horizon vary significantly depending on the friction law. We conclude on the importance of choosing an appropriate law for reliable sea level rise projections and emphasise the need for a coupling between ice flow models and physically based subglacial hydrological models.
Read more: Brondex, J., O. Gagliardini, F. Gillet-Chaulet and G. Durand, 2017. Sensitivity of grounding line dynamics to the choice of the friction law, Journal of Glaciology, 63(241), 854-866, doi:10.1017/jog.2017.51.