Quantifying iceberg calving fluxes with underwater noise

Abstract

Abstract. Accurate estimates of calving fluxes are essential to understand small-scale glacier dynamics and quantify the contribution of marine-terminating glaciers to both eustatic sea level rise and the freshwater budget of polar regions. Here we investigate the application of ambient noise oceanography to measure calving flux using the underwater sounds of iceberg-water impact. A combination of time-lapse photography and passive acoustics is used to determine the relationship between the mass and impact noise of 169 icebergs generated by subaerial calving events from Hans Glacier, Svalbard. The analysis includes three major factors affecting the observed noise: 1. fluctuation of the thermohaline structure, 2. variability of the ocean depth along the waveguide, and 3. reflection of impact noise from the glacier terminus. A correlation of 0.76 is found between the (log-transformed) kinetic energy of the falling iceberg and the corresponding acoustic energy. An error-in-variables linear regression is applied to estimate the coefficients of this relationship. Energy conversion coefficients for non-transformed variables are 8 × 10−7 and 0.92, respectively for the multiplication factor and exponent of the power law. As we demonstrate, this simple model can be used to measure solid ice discharge from Hans Glacier. Uncertainty in the estimate is a function of the number of calving events observed; 50 % is expected for 8 blocks dropping to 20 % and 10 %, respectively, for 40 and 135 calving events. It may be possible to lower these errors if the influence of different calving styles on the received noise spectra can be determined.