Page 17 - WINTER2019

Figure 6. A sequence of images showing a bubble exploding out of melting glacier ice. Image scale is roughly 1 cm on a side. The interframe time interval between the first 3 frames is 500 μs.
production by the impact of a small ice block dropped into a pool. The first contact between the block and water surface generates a short-duration, high-frequency impulse followed by the creation of an air cavity. The moment of cavity pinch-off from the water surface cavity pinch-off is marked by the onset of breathing mode oscillations of the newly created air bubble.
Calving events do not always originate above the water. Buoy- ancy forces combined with ice fracture can lead to blocks of ice detaching from the submerged glacier terminus, an event known as submarine calving. The frequency of occurrence of submarine calving and its contribution to the overall loss of ice from tidewater glaciers are poorly understood. Icebergs from submarine calving events have no airborne detachment noise. Instead, they emerge unexpectedly on the surface a few hundred meters from the glacier terminus, presenting a sig- nificant hazard for any boats too close to the ice cliff. However, submarine calving events generate underwater noise and are easily detected with hydrophones. As with subaerial calving, there are distinct stages of underwater noise production: a series of cracks announcing the separation of the ice block from the underwater part of the terminus followed by emer- gence noise as the iceberg breeches the surface.
Can the underwater noise of calving be used to quantify calv- ing ice flux? Perhaps, if impact noise can be directly related to the volume and mass of falling ice blocks. Glowacki et al. (2015) analyzed 10 subaerial calving events from Hans Gla- cier, Svalbard, that had been observed with a digital camera and a hydrophone to test the idea. The kinetic energies of impacting icebergs were estimated from time-lapse images of the glacier terminus and then correlated with the resulting acoustic emissions recorded at frequencies below 200 Hz. A model assuming a simple power law relationship between impact energy and underwater noise production explained
93% of the variability seen in the dataset. These results from a single glacier demonstrated that hydroacoustic monitoring of iceberg calving fluxes might be possible in the future.
The Sounds of Melting Glacier Ice
Melting glacier ice sounds a bit like bacon frying (or snap- ping shrimp, if you have ever heard them in the ocean; see, e.g., bit.ly/2RtTKEj). This is because the explosive release of gas from a pressurized bubble makes a loud and impulsive popping noise. Urick (1971) appears to have published the first measurements of noise from melting glacier ice, and attributed the sound produced to “...the explosion of tiny air bubbles entrapped in the ice under pressure and released as melting occurs.”
A typical sequence of events for the explosive release of a bubble from a block of glacier ice melting in the laboratory is shown in Figure 6 as a series of high-speed photographic images. The scene is backlit, and the bubbles appear as dark, roughly circular regions within the ice. A bubble approxi- mately 4 mm in diameter can be seen emerging from the ice from left to right in the bottom half of the 4 right-hand images. The timescale of the main part of the release event is less than a frame in duration (see the blurred, emerging bubble in the second image from the left), which is 500 μs (see, e.g., youtu.be/0Bilzdsi42E; youtu.be/6EHaD_169eU).
Bubble release events like the one shown in Figure 6 and the videos can create peak pressures of over 100 Pa and an exponentially decaying sinusoidal waveform associated with the natural oscillations of an acoustically excited bubble. The superposition of many such events from a melting glacier ter- minus creates a random pressure signal with a broad peak in the frequency range of 1-3 kHz that can be heard underwater several kilometers from the ice cliff.
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