Massive icebergs scoured Arctic seafloor

by Sara E. Pratt
Monday, October 6, 2014

In August 1990, the R/V Polar­stern departed Tromsø, Norway, to investigate the ocean bottom bathymetry of the Fram Strait between Greenland and Svalbard. More than 20 years later, marine geologist Jan Erik Arndt and his colleagues at the Alfred Wegener Institute Helmholtz Centre for Polar and Marine Research in Bremerhaven, Germany, were reviewing data from the cruise when they discovered something new — the deepest evidence of iceberg scouring ever found.

A reanalysis of the multibeam-echosounder and sub-bottom profiler data revealed five previously unseen 4-kilometer-long scour marks — roughly 15 meters deep by 300 meters wide — oriented north-south along the Hovgaard Ridge in the Fram Strait.

Arndt and his colleagues suspect the scours were left by multiple icebergs — calved from ice sheets that extended much farther into the Arctic Ocean during the last glacial maximum. Given sea level at the time, the icebergs must have had keel depths of at least 1,200 meters, the team reported in Geophysical Research Letters.

The finding gives researchers an upper limit on the size of icebergs at the time. (By contrast, today, the largest icebergs are found in Antarctic waters and reach up to 700 meters thick, about half the size of the massive icebergs that scoured the Hovgaard Ridge.)

“Bearing in mind that only the very deepest icebergs scoured Hovgaard Ridge, there must have been many more icebergs,” Arndt says. “Furthermore, the size of the icebergs is giving us a new estimate for ice-sheet thicknesses” where the ice streams entered the Arctic Ocean.

As the only deepwater connection between the Arctic and North Atlantic oceans, the Fram Strait plays a critical role in the exchange of surface and deepwater masses in the global ocean.

“These marks are showing that freshwater discharge through the Fram Strait to the North Atlantic was heavily influenced by large icebergs and not only by sea ice,” Arndt says.

It is a “very important finding … that fits with many of the latest results we have from the central Arctic Ocean,” says Martin Jakobsson, a professor of marine geology and geophysics at Stockholm University who was not involved with the study. However, he says, “the data are not as crisp and good as from many other mapped areas with scours because the Hovgaard Ridge data were collected with an older swath-bathymetric system.” Nonetheless, “the data leave me with little doubt that Hovgaard Ridge was scoured by icebergs. It’s one more piece in the Arctic Ocean glacial puzzle.”

The discovery was made possible by advances in technology since the 1990 cruise. “In the course of time, the data visualization software, and thus the probability of detecting such features like these iceberg scours, have improved massively,” Arndt says.

The team next plans to reanalyze cores taken on the cruise to better constrain the time frame of the scouring events.