Measuring tidewater glacier melt rates with underwater noise

Marine tidewater glaciers are undergoing a global retreat, contributing to sea level rise and freshening the oceans in polar regions. Predicting their future behavior requires accurate models of how glacier termini disintegrate through the twin processes of melting and iceberg calving. However, direct measurement of the melt rates of the termini of glaciers and ice sheets remains a difficult problem. The explosive release of high-pressure gas bubbles as glacier ice melts generates loud and impulsive sounds in the spectral band 1 - 3 kHz, providing an opportunity for remote observation of melt rates using underwater sound. Here, we describe the results of an experiment undertaken to explore the connection between bubble size distributions, ice densities and mean gas pressures and the sounds produced as the ice melts. Studies of four glaciers around Hornsund Fjord were staged from the Polish Polar Station in southwestern Spitsbergen. Measurements of noise vertical directivity made directly in front of termini and laboratory observations of samples of ice melted under controlled conditions were made. Patterns of noise directionality were similar for the four termini and show that the underwater noise is mostly generated by ice within 20 m of the waterline. The ice melting experiments support the hypothesis that the loudest bubble release events are associated with the highest gas pressures and bubble cap ice fracture. The fraction of bubbles released by ice fracture depends on multiple variables, including water depth and the time interval between ice sample collection and processing. These preliminary results suggest that the remote monitoring method may work if noise from the glacier terminus and the sounds of any nearby melting icebergs can be reliably separated. [Work supported by US Grants OPP-1748265 and ONR N00014-17-1-2633, Institute of Geophysics, Polish Academy of Sciences Grant 2a/IGF PAN/2017 and the National Research Foundation, Prime Minister's Office, Singapore under its Marine Science Research and Development Programme (MSRDP-P42)]