The sheet broke away last week from the Ward Hunt Ice Shelf off the north coast of Ellesmere Island in Canada's far north. (See map.)
Large ice segments—including this 1.5-square-mile (4-square-kilometer) chunk, above—were seen drifting near the ice shelf.
Derek Mueller, a researcher at Trent University in Ontario, was careful not to blame global warming.
But he said the event was consistent with the theory that the current Arctic climate isn't rebuilding ice sheets.
"We're in a different climate now," Mueller said. "It's not conducive to regrowing them. It's a one-way process."
A crack in the shelf was first spotted in 2002 and a survey this spring found a network of fissures.
The 170-square-mile (440-square-kilometer) Ward Hunt Ice Shelf has been steadily declining since the 1930s, he added.
Gary Stern, co-leader of an international research program on sea ice, said it's the same story all around the Arctic.
Speaking from the Coast Guard icebreaker Amundsen in Canada's north, Stern said he hadn't seen any ice in weeks.
"Nobody on the ship is surprised anymore," Stern said. "We've been trying to get the word out for the longest time now that things are happening fast and they're going to continue to happen fast."