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Friday, March 4, 2011

Ward Hunt Ice Shelf breaks off from Ellesmere Island and reveals the remains of driftwood and narwhals that radiocarbon date to 3,000 to 6,800 years ago

Another Ice Island Breaks Off Arctic Glacier

Date: 27 August 2010

Acquired on Aug. 18, 2010, this satellite image shows the fractures on the ice shelf immediately south and east of Ward Hunt Island, and a large chunk of ice appears to be floating free from the shelf immediately east-northeast of the island. CREDIT: NASA Earth Observatory image created by Jesse Allen.

Sometime earlier this month, a Bermuda-sized ice island broke free from the Ward Hunt Ice Shelf along the northern coast of Canada's Ellesmere Island.

The breakup on this ice shelf continued a years-long pattern of retreat on the Ward Hunt Ice Shelf, and a decades-long pattern of retreat of the ice shelves along the Ellesmere coast in the high Arctic.

NASA's Aqua satellite detected fractures on the shelf on Aug. 18. Compared to images of the ice shelf from eight years earlier, the fractures show the dramatic change to the coast.

The fractures on the shelf in 2010 are immediately south and east of Ward Hunt Island, and a large chunk of ice appears to be floating free from the shelf immediately east-northeast of the island. Although the ice shelf appears larger west-northwest of Ward Hunt Island in 2010, the larger area of ice is due to sea ice temporarily attached and frozen to the ice shelf. South of the ice shelf, fjords that are covered with ice in 2002 are largely free of ice in 2010.

At roughly 21 square miles (54 square kilometers), the ice chunk that broke free from the Ward Hunt was much smaller than the ice island that calved from Greenland's Petermann Glacier a few weeks earlier, but in some ways, the Ward Hunt fracture was more significant.

Fed by the Greenland Ice Sheet, the Petermann Glacier is a floating ice tongue that periodically calves large icebergs, and is replenished by new ice upstream. In contrast, the Ward Hunt Ice Shelf formed from compressed sea ice that was gradually replaced by accumulated snow. No glacier feeds the Ward Hunt and advances it seaward. The calving cycle on the Ward Hunt is slow, and to recover from the recent retreat would require centuries.

Driftwood and narwhal remains found along the Ellesmere coast have radiocarbon dates from roughly 3,000 to 6,800 years ago, implying that the ice has been intact since those remains were deposited. Breakup along the Ward Hunt indicates a change in the conditions that previously allowed this ice shelf to persist for millennia.

At the dawn of the 20th century, glaciologists estimate, the northern coast of Ellesmere Island was lined by a continuous ice shelf covering roughly almost 3,500 square miles (8,900 square kilometers). A century later, the once-uninterrupted shelf had been reduced to a cluster of much smaller ice shelves, including the Ward Hunt. The Ward Hunt cracked in 2002, draining a freshwater lake that floated on top of denser seawater, and wiping out a unique microbial ecosystem. In 2008, another rapid breakup occurred.

Despite the fracturing on the Ward Hunt that occurred in the summer of 2002, that year actually experienced cooler conditions than 2010. When the Ward Hunt fractured in July 2002, nearby sea ice held the fragments in place, like giant pieces of a jigsaw puzzle. Later sea ice retreat in the Arctic gave the ice islands room to roam. North of the ice shelf, sea ice appears more abundant in 2010 than in 2002, but warmer conditions and a slight retreat of sea ice from the Ellesmere coast gave the ice that fractured in 2010 similar space to migrate.

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