A wireless multi-sensor subglacial probe for investigating the deforming glacier bed

Abstract

The mechanisms that control glacier dynamics and fast ice stream flow are still poorly understood, largely due to the inaccessibility of the subglacial environment. One of the ways to improve on current investigations is to broaden the methods used to monitor the subglacial environment. An autonomous multi-sensor wireless probe was developed for use within an Environmental Sensor Network at Briksdalsbreen, Norway. Probes were deployed at the base of boreholes, measuring temperature, pressure, resistance (a proxy for conductivity), case strain, and tilt, six times a day. The probes used radio communications to transfer data to a base station at the glacier surface. The data was then sent to a reference station, which uploaded it onto the internet on a daily basis. In excess of a year's worth of data was obtained from several probes, deployed between 2004-2006. Readings of case strain and tilt indicated when the probes had become incorporated into the deforming bed. A reduction in sensor activity was displayed as the glacier changed from a summer to a winter regime. In the following spring, the sensors showed an increase in activity, particularly in water pressure. An Emergent Spring Event was recorded in February, followed by the actual Spring Event. This marked a transition to summer conditions within the glacier's hydrological system. Sliding was the dominant mechanism of basal motion during this time. In the summer, the probes began to rotate as the dominant form of basal motion transferred from sliding to till deformation. The movement of the probes was in keeping with the Taylor (1923) mechanism of clast rotation. Sedimentological analyses under the scanning electron microscope presented a relatively simple model for grain erosion within the deforming bed. Erosion occurs within a continuum whereby processes of both fracture and abrasion combine, generating intermediate grain forms. The style of deformation experienced is controlled by the nature of the bedrock and the overall strain imposed on the till. At Briksdalsbreen there was good evidence for erosion, but little evidence for the production of a significant silt component. This investigation demonstrates the potential for the use of ESNs within glacial environments. The probes provide a successful means by which the properties of and processes operating within the subglacial environment can be assessed over broad temporal and spatial scales. For the first time, activity was recorded within the subglacial environment much earlier than previously expected (January) and the spring event within unconnected regions of the bed was monitored and characterised. The records produced highlighted the complexity of hydro-mechanical interactions and the need for further study to constrain these processes. The probes also provided the first natural records of in situ clast rotation from within the deforming bed. This movement may have implications for models of clast rotation, the identification and classification of sediments, and models of grain interactions on the micro-scale.

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