The last ice age wasn’t totally icy

From Mexico City, at the 60th annual meeting of the Society of Vertebrate Paleontology

Radiocarbon dating of fossils taken from caves on islands along southeastern Alaska’s coast suggest that at least a portion of the area remained icefree during the last ice age.

Some of the caves, including those on Prince of Wales Island, were natural traps into which the animals fell and couldn’t escape, says Timothy H. Heaton, a paleontologist at the University of South Dakota in Vermillion. Other sites, like On Your Knees Cave—a lengthy tunnel with an extremely low roof throughout—served as dens for carnivores such as bears, foxes, and otters.

Heaton and his colleagues have excavated more than 5,000 bags of sediment from On Your Knees Cave. In the process, they recovered at least 22,000 bones and fragments that represent both predators and their prey.

Carbon-14 dating of 100 of the specimens show that the cave has been almost continuously occupied during the past 40,000 years, which indicates ice didn’t cover the cave’s entrance for any extended period. The changes in species in the cave throughout this period indicate the shifts in local ecology as the climate changed.

Black bears, brown bears, caribou, hoary marmots, and brown lemmings died within or were dragged into On Your Knees Cave in the millennia preceding the last glacial maximum.

Bones dating from the time when the ice was most widespread represent arctic fox and their prey, ringed seals. Because these seals now live in areas where the waters are frozen all or part of the year, Heaton says that Prince of Wales Island must have had similar conditions.

After the glaciers began their general retreat from the rest of the North American continent, the remains of bears and caribou again showed up in the cave, along with new species such as mule deer, Heaton says.

Although the carbon dating now shows a period of vacancy 19,000 to 14,000 years ago in On Your Knees Cave, Heaton says, local geology indicates that the cave wasn’t disturbed by glaciers during that time.