Measuring Iceberg Calving Fluxes with Underwater Noise

Accurate estimates of iceberg calving fluxes are required to understand the dynamics of tidewater glaciers and quantify their contribution to the ocean's freshwater budget and sea level rise. Here we investigate the use of underwater noise produced by iceberg-water impact to measure calving flux at Hans Glacier, Svalbard. A relationship between the acoustic energy and ice mass loss associated with 169 subaerial calving events is explored through the combination of ambient noise cryology and time-lapse photography. The analysis includes an error budget for all major factors affecting the impact noise: (i) variability of the thermohaline structure in the glacial bay, (ii) varying bathymetry along the propagation path, and (iii) the contribution of underwater sound reflected from the glacier terminus. A robust correlation of 0.75 is found between the (log-transformed) impact noise energy and kinetic energy of the falling ice blocks. The multiplication factor and exponent of the power law relationship are found to be 8 x 10^-7 and 0.92, respectively. A simple model based on this correlation can be used to quantify solid ice discharge, if supplemented by a site-specific average drop height. The accuracy of the acoustic technique improves with number of calving events analyzed: 25% for 25 ice blocks and 10% for 135 impacts. Further improvements to the model will require a better understanding of the source mechanisms of underwater noise generated by calving icebergs.

[work funded by the Ministry of Science and Higher Education of Poland under 'Mobility Plus' program, grant 1621/MOB/V/2017, and US National Science Foundation, grant OPP-1748265]