Measuring tidewater glacier melt rates with underwater noise

Direct measurement of the submarine melt rates of tidewater 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 by an international team from the USA, Poland and Singapore 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 four termini. Also, laboratory and field 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 melting ice within 20 m below the waterline. The laboratory observations support the hypothesis that the loudest bubble release events are associated with the highest gas pressures and fracture of the bubble cap ice. The fraction of bubbles released by ice fracture depends on multiple variables, including water depth, the time interval between collection and processing, and thermal history of the samples. 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)]