Iceberg Damage to Seafloor Increases

Iceberg W-86, one of the two icebergs in the study, in December 2005 in the Weddell Sea. CREDIT: Henry A. Ruhl

Worms, sea spiders, urchins and other creatures that dwell on the Antarctic seafloor are pounded daily by icebergs scraping up their homes.

Now scientists say these denizens of the deep take more hits as global warming diminishes the layer of sea ice that blocks the icebergs and protects habitats.

Icebergs are large chunks of ice that have broken off from a glacier or ice shelf and float in open water. As they float, pushed around by winds and tides, their bottoms, which can sometimes reach to a depth of 1,600 feet (500 meters), scour the seafloor beneath them.

In the winter, a type of sea ice sometimes called "fast ice" forms along coastlines as the ocean water freezes, locking icebergs in place and temporarily halting the scouring.

But around the West Antarctic Peninsula, winter fast ice has declined dramatically in recent decades, both in terms of the area of ocean it covers and how long it stays around. This is a result of the region's air temperature having risen by nearly 5.4 degrees Fahrenheit (3 degrees Celsius) on average in the last 50 years, an amount several times the global average.

Scientists with the British Antarctic Survey (BAS) studied how this decline in winter fast ice was affecting ice scouring at a shallow water site on the peninsula. They placed a grid of concrete markers on the seabed at three different depths, and for five years, scuba divers returned each January to inspect how much damage icebergs had done to the markers.

They found that more scouring occurred during years when winter sea ice shrank back, removing the icebergs' road block and giving them free reign over the seabed, where 80 percent of all Antarctic life lives.

The results are detailed in the July 18 issue of the journal Science.

"It has been suggested previously that iceberg disturbance rates may be controlled by the formation of winter sea ice, but nobody's been able to go out and measure it before," said study lead author Dan Smale of the BAS. "We were surprised to see how strong the relationship between the two factors is."

As icebergs scrape the sea floor, they change the local habitats. While the damage they leave behind can actually create space for new animals to move in and thereby increase the community's diversity, any increase in scouring could drastically change the type and number of marine creatures found in the area.

Smale and his co-authors caution that because the results come from just one site, it's hard to generalize them to the entire shelf ecosystem, but they say that their findings seem to point to more scouring in a warming world, with potentially large, unknown changes in store for Antarctic marine creatures.

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