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Ice Adrift From Warming Scrapes Antarctic Seabed Bare

Kimberly Johnson
for National Geographic News
July 17, 2008
 
Rapid warming along the Antarctic Peninsula is causing more skyscraper-sized icebergs to break free, drift, and scour away practically all life along swaths of the seafloor, according to a new study.

Ocean-bottom scrubbings along the West Antarctic Peninsula will increase as temperatures rise, annihilating some animal and plant populations but helping others by clearing the habitat, the study said.

The study establishes for the first time the intimate link between increased scouring and declines in winter sea ice due to climate change, researchers said.

In the past, these icebergs were locked in place by winter sea ice for longer periods and only free to crash into the seabed in summer.

"Our results suggest that as the winter sea ice season shortens, the thousands of icebergs that float around the coastline of the Antarctic Peninsula will be free to move around and collide with the seabed creatures with ever increasing frequency," lead author Daniel Smale of the British Antarctic Survey, said in an e-mail.

A Significant Degree

Antarctica is one of the fastest warming regions in the world, the researchers said.

Sea temperatures around Antarctica tend to be stable, spanning from 28.8 degrees Fahrenheit (-1.8 degrees Celsius) to 35.6 degrees Fahrenheit (2 degrees Celsius).

The Bellingshausen Sea along the western side of the Antarctic Peninsula, however, has seen its temperatures increase by about a degree Celsius in the last 50 years, said marine biologist and study co-author David Barnes, also of the British Antarctic Survey.

"This may not sound like a lot, but it is nearly a third of the total annual variability" for that region of the sea, Barnes said.

The study will be published in tomorrow's issue of the journal Science.

Each winter, a frozen crust called fast ice forms on the sea's surface. It can stretch for thousands of square kilometers but is rarely found more than seven feet (two meters) thick.

Warming is reducing the area and time that seasonal sea ice covers the sea surface, Barnes said.

The winter ice decays as winter temperatures rise, releasing slabs of iceberg to drift in the sea.

These icebergs can vary dramatically in size—from the size of fists to small countries, Barnes said.

Once freed from the fast ice, icebergs are blown by winds and carried by sea currents until they smash into the seabed in shallow depths.

"They not only kill virtually everything underneath on the seabed—mainly animals—but crush rock and reshape the seabed," Barnes said.

Vulnerable animals include Antarctic worms, sea spiders, and urchins, the scientists said.

(Related photos: "Giant, Unknown Animals Found off Antarctica" [March 28, 2008])

An Unfamiliar Look

Researchers studied the ice scouring over a five-year period, regularly monitoring markers placed at varying depths along the seafloor.

Made of molded concrete and plastic, the markers deformed and splintered when they encountered the force of a drifting iceberg.

The variety in iceberg sizes creates a range of damaged areas on the seafloor, Barnes said.

In shallow waters, researchers measured damaged areas a few meters by a meter. At deeper regions, however, they found impacted areas up to 0.62 mile (a kilometer), he said.

Smale added that icebergs can ground out at depths of up to 550 yards (500 meters), which means much of polar seabed could be susceptible to ice scouring.

The impact is dramatic, Barnes said. "Diving on the big ice scours after they have happened is like visiting a completely new area, everything looks unfamiliar," he said.

Life is able to eventually rebound in the deadly wake of a scrubbing, but the process takes a while, Barnes said.

"Antarctic animals grow very slowly—in fact, they do virtually everything very slowly—so re-colonization is not quick," he said.

Part Biodiversity, Part "Black Pools of Death"

By clearing out the seafloor, however, the scouring also enables a wide range of species to live, Barnes said.

"Think of it like a forest," he said. "The weedy species that are normally crowded out and out-competed by the dominant species persist where big trees fall and create a clearing."

The research can help expand awareness about the impact of warming temperatures, said a marine biologist not affiliated with the study.

"It is a nice story about what sea ice can do to marine life and sediment characteristics on the seafloor, a place where the general public would not associate processes happening on the surface with what is happening on the bottom of the ocean," said Kathleen Conlan at the Canadian Museum of Nature in Ottawa.

Conlan, who has conducted research on ice scouring in the eastern Canadian Arctic, agreed that its effect on marine life could be varied.

In some areas, it can promote diversity, where it turned over nutrient-rich sediments and pushed out dominant grazers, such as sea urchins.

In other locations, however, the result could be catastrophic, "producing black pools of death," she said.
 

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