Measurements of shear and stress at Bowdoin Glacier, Northwest Greenland

Englacial stress, the elusive variable governing glacier motion, has rarely been measured in situ. Instead, our empirical understanding of ice dynamics largely relies on laboratory flow-law experiments, but field measurements of stress-induced glacier surface velocity and englacial tilt indicate that crystal orientation, molten ice fraction and impurities may complicate the application of laboratory-derived laws in nature. Here, we present a three-year record of englacial deformation and near-vertical stress from sensors frozen 123 to 265 metres deep into the Bowdoin tidewater glacier in Northwest Greenland.

Inclinometers show that the glacier movement is largely dominated by sliding, as horizontal shear deformation of 16 to 19 metres accounts for 4 to 5 percent of independently observed surface displacement. During seasonal speed-up events, englacial tilt rates increase proportionally to surface velocities derived from geopositioning, automated cameras and satellite remote sensing. Daily and tidal components are also present in the tilt rates record but are yet to be isolated from the sampling noise before phase correlation with other signals.

Piezometers were initially intended to locate instruments in hotwater-drilled boreholes, but they continued to record pressure changes after the complete refreezing of the boreholes and the stabilisation of ice temperatures well below the melting point. All sensors recorded in-phase stress variations with 12-hour, 24-hour and 14-day periodicities, revealing a tidal signal in winter, disturbed during independently documented speed-up events in summer. The signal shows amplitudes of one to four kilopascals, only an order of magnitude weaker than the two metres tidal amplitude measured at sea. However, stress measurements are anticorrelated with the tide, and show a delay of one to two hours, so that maximum stresses occur a little after low tide. While detailed interpretations are hampered by the lack of calibration, our data indicate that direct stress measurements in glaciers are feasible.