At the time, A68a, which is roughly 100 miles long by 30 miles wide, was centered about 50 miles southeast of the area we were sailing in, according to data from the National Ice Center. Could it have been blocking ice from the area?

According to Jonathan R. Green, an adventure tour expedition leader, that’s precisely what A68a was doing. “We certainly saw it was having a damming effect,” said Mr. Green, who was aboard a ship, the G Expedition, that was in the same area a few days before ours.

He and the ship’s captain had been studying satellite images and saw that there was an opportunity to get into the Weddell — and in his ship’s case, actually sail to the iceberg.

“There was a two- or three-week window with little ice,” he said.

Mr. Green said he believed the G Expedition was the first ship ever to see the iceberg. “It was indescribable,” he said. “From horizon to horizon all you could see was the line of this iceberg disappearing in both directions,” he said. The ship put out an inflatable boat and Mr. Green and others got within 100 yards of A68a, which, while 100 miles long, is only a few hundred yards thick.

The low-ice conditions ended later in December, when, according to Mr. Green, high winds drove A68a farther north, freeing up the ice jam. A passenger on a cruise ship that tried to get into the Weddell during the last week of December reported that the route was impenetrable because of ice.

Adrian Luckman, a glaciologist at Swansea University in Wales who tracks A68a, said the iceberg probably affected sea ice movement in the Weddell. “But the ice cover is quite variable from year to year, so it is not the most important factor,” he said.

Since December, A68a has continued moving north, and is approaching the edge of the perennial Antarctic ice pack, Dr. Luckman said. That should make it more susceptible to being broken up by wave action in the Southern Ocean.