The 2016 Nansen Ice Shelf Calving Event: Hydroacoustic and Meteorological Observations of Ice Shelf Fracture and Iceberg Formation

In December 2015, three hydrophones were moored at the east side of Drygalski Ice Tongue (DIT), seaward of the Nansen Ice Shelf (NIS). On 7 April 2016, the front of the NIS calved into two medium-sized icebergs, presenting a rare opportunity to measure the temporal variation of acoustic signals produced by a large scale ice shelf fracturing event. The acoustic data indicates there are hundreds of short duration, broadband (10–400 Hz) signals throughout the 4-month hydrophone record. We interpret these signals as cryogenic, caused by the cracking of the ice shelves and impacts of nearby sea-ice. The majority of these ice-cracking signals occur during the 21 January to 21 March time frame, two months to two weeks prior to the shelf fracturing observed by satellite on 7 April. Meteorological records of barometric pressure, air temperature, and wind speed show that the day the two icebergs drifted from the NIS coincided with the largest low pressure storm system recorded in the previous 7 months. Our interpretation is the NIS leading edge broke free during the January–March high acoustic energy time period, but the iceberg remained grounded on the seafloor until the low pressure event. The combination of low atmospheric pressure, high winds and high ocean tide, freed the iceberg keel from the seafloor. Strong bio-acoustic signals were also observed on the hydrophones, where the vocalizations of blue whales and leopard seals dominate certain time periods and frequency bands of the long-term acoustic record.