A number of noteworthy studies have recently highlighted the importance of what's going on at the bottom of glaciers that flow into the ocean. The topography beneath the glacier—as well as the “grounding line” beyond which a glacier becomes thin enough to float in the water rather than rest on the seafloor—have a lot to do with its stability.
In 2002, the Larsen B ice shelf on the Antarctic Peninsula abruptly collapsed, scattering 3,200 square kilometers (yes, approximately one standard Rhode Island unit of area) of 200 meter thick ice into the waves. But why? Did warming water beneath the ice shelf loosen it from the grounding line and destabilize the ice shelf in front? Or can we pin the blame on the warming temperatures of the region?
With the ice shelf gone, researchers looking for answers have been able to look at the seafloor that once sat beneath it. In 2006, a research vessel spent some time at the site of the collapse, looking for clues. The findings of that team, led by the Italian National Institute of Oceanography and Experimental Geophysics’ Michele Rebesco and the University of South Florida’s Eugene Domack, have now been published in the journal Science.
Mapping the seafloor, they identified what looked like a large grounding line ridge more than 10 kilometers out to sea relative to a smaller ridge that probably represents the grounding line of the glacier during the 2002 collapse. The seafloor dipped heading up the fjord that holds Crane Glacier—one of several that had fed into the ice shelf. This is why, once the grounding line failed, the ice retreated rapidly.
To see if this testified to a rapidly retreating grounding line leading to the demise of the ice shelf, the team collected shallow sediment cores from several locations. Out on the larger ridge, they found the chaotic sediment typical of glacial deposits, but it was covered by more orderly muds characteristic of the region beneath an ice shelf. Dating some organic matter at the boundary between the two, they discovered that the glacier retreated from this grounding line around 12,000 years ago—much too long ago to explain its recent behavior.
While digging down a little over a meter gets you to 12,000-year-old sediment there, the sediment core nearest the recent grounding line looks very different. More than 2.5 meters of sediment has been deposited there since 2002. That’s partly due to landslides of sediment from the sides of the fjord (now free from the influence of the ice shelf) and partly due to the accelerated flow that came with the retreat of the Crane Glacier after 2002. Ice shelves act like buttressing supporting the rest of the glacier—remove one and the ice comes spilling out.
All this indicates that a rapid retreat of the grounding line pre-2002 wasn’t the cause of the Larsen B ice shelf’s collapse. Instead, it adds more support to studies that have focused on the rapid atmospheric warming on the Antarctic Peninsula—nearly 3°C over the last half century. The ponds of meltwater that formed on the surface of the ice shelf, for example, clearly played a key role in its disintegration.
Understanding exactly how various glaciers have responded to warming air or water and changing precipitation enhances our ability to predict how the cryosphere will fare in the future. The collapse of the Larsen B was a surprise—and we want to minimize surprises.