The new maps of the ocean floor in Pine Island Bay, which is predominantly 800 to 1,000 metres deep, reveal a previously unmapped submarine ridge and two mountains, the peaks of which reach up to a water depth of 370 metres. The Pine Island Glacier’s more than 400-metre thick ice shelf must have been grounded on the ridge for several decades, as the satellite images of the glacier gathered by the researchers – dating back to 2002 – confirm. In the older images, rises in the ice’s surface can be identified at precisely those points where the peaks of the ridge are directly under the ice shelf. “But after 2006, these landmarks are nowhere to be seen. By that time, the ice shelf must have melted so extensively from below that it was either too light to produce any impression on the surface of the ice, or the ice sheet must have already lost contact with the mountains beneath it,” says co-author Dr Karsten Gohl from the AWI.
When an ice shelf loses contact with such obstacles (known as “pinning points”), the ice stream reacts as if someone had suddenly released a giant brake. With nothing left to stop them, the ice masses rapidly flow out to sea – at least that’s the theory. Using the time series of satellite images for the Pine Island Glacier, the researchers were now able to test this thesis step by step. Much to their surprise, in the process they determined that submarine highs not only stabilise ice shelves like giant brakes; in some cases, these mountains can also set off calving events – for instance, when the calving front advances, causing it to crash into a mountain with full force.
That must be what happened during a calving event in 2007. As the satellite images show, back then the edge of the Pine Island Glacier’s ice shelf collided with one of the newly discovered mountains, hitting it so hard that rifts formed in the surface of the ice. When one of the rifts finally became too large, the entire face of the ice shelf broke off. (see Video 1)
The story was similar, albeit less dramatic, with the iceberg that calved in 2015, breaking into several pieces just a few weeks later. The largest piece became caught on the submarine ridge for nearly a year, turning clockwise over and over again until the combination of ocean currents, wind and melting broke it loose. The researchers surmise that, just as in 2007, the ice shelf’s repeated contact with the ridge is what ultimately led to the calving event. (see Video 2)