Last of the Big Thwaites Bergs – Iceberg B22A Modulates Fast Ice and Ice Front Stability, as it Departs the Amundsen Sea Embayment (2017-2023)

Abstract

Using multiple remote sensing assets allows a fuller understanding of the long-delayed departure of Iceberg B-22 from the Amundsen Sea Embayment (ASE). Originally calved from the then more extensive Thwaites Glacier Ice Tongue, the originally ~4000 km² iceberg rapidly shed ice from its eastern margin and became B-22A (iceberg names from the US National Ice Center).For the next ~15 years, the iceberg, ~400 m thick, moved slowly, if it all, across the ASE’s shallows that once held the much larger Iceberg B-10 early in the Landsat era. By 2017 the berg had thinned enough or been forced by storms and tides into a more western part of the shallows near the end of the Bear Peninsula. The episodic motion and periods of stasis suggested by MODIS and Landsat visible imagery are detailed by 12-day repeats from Sentinel-1A coverage. The continuous imaging documents a strong interaction between the berg, shore-fast sea ice, and the stability of the fronts of ice shelves and ice margins in this area of the embayment. While B-22A was stationary, a mélange of sea ice and small bergs remained intact between it and the front of the Crosson Ice Shelf and the ice margin to the south towards Thwaites Glacier. Sea Ice loss, motion of B22A, and disaggregation of the mélange in this sector occur several times over the record, suggesting strong interactions between these components. By early 2023, the berg had been reduced to ~3000 km² due to the calving of peripheral ice. With its initial westward motion impeded, the iceberg rotated fairly rapidly to the north and was carried into deeper waters by the Antarctic Coastal Current. The mélange once protected between the berg and the West Antarctic coast was either carried away by the motion of the large berg or was subsequently impacted by increased exposure to the Amundsen Sea’s forces. The loss of peripheral ice has been considerable in front of the Crosson Ice Shelf. The berg now lies about 285 km NNW of the Bear Peninsula. With no large ice tongue and eventual replacement berg of this size forming, it is likely that conditions in front of Thwaites and Crosson will be significantly more open, and perhaps more dynamic, in coming decades.