While exploring a cave in Southeast Alaska a few years ago, Tim Heaton found the leg bones of a large brown bear and a black bear. He expected the bones to be like others in the area - fossils from animals that lived about 10,000 years ago - but these bears were different. Radiocarbon dating showed they lived about 40,000 years ago.
Heaton's discovery, made with his caving buddy Kevin Allred, has changed a few people's views of Alaska's character during the last ice age. Did glaciers and sea ice bury the coast of ancient Alaska, or did Southeast feature areas with enough land exposed to support bears, caribou and foxes? The discovery of ice-age bear, seal, fox and caribou bones suggests the latter.
Heaton, a paleontologist at the University of South Dakota in Vermillion, has explored caves since he was a teen-ager. As an adult, his spelunking skills have allowed him to wriggle into dark passages where the clues of ancient history rest. Such a place is On Your Knees Cave, on Prince of Wales Island.
Surrounded by walls of limestone, On Your Knees Cave consists of two tunnels small enough to make crawling the only way to advance or retreat. Heaton's friend Kevin Allred, who lives in Haines, found animal bones when first exploring the cave and later guided Heaton to its entrance. Inside the cave, the pair found a wealth of bones buried in layers of sediment, including the bones of brown and black bear, arctic and red fox, caribou, ringed seals, marmots, lemmings, voles and countless birds and fish. Heaton and his crew have made the cave a bit less of a crawl since the early 1990s by removing 5,000 bags of sediment. After sifting through that sediment, they have collected 25,000 bone fragments.
Among those samples are the bones of bears that lived 40,000 years ago; those bones make On Your Knees Cave different from other explored limestone caves in Southeast. Other caves, such as Bumper Cave and El Capitan Cave on Prince of Wales Island, contain the bones of animals that were alive from 12,000 years ago to the present. Some of the samples from On Your Knees Cave are so old they exceed the limits of radiocarbon dating, which can detect ages up to about 40,000 years.
Continuous samples of the bones from 40,000 years to the present are proof that animals used the cave during the last ice age, which means the cave could not have been beneath a glacier for very long, if at all. The presence of seal bones in the cave - probably dragged in by bears or foxes - suggests that at least part of Southeast must have looked something like Barrow. Seals need sea ice to thrive, and caribou bones hint at a tundra ecosystem.
"Even during the coldest period, there was a community of animals there similar to that around the Arctic Ocean today," Heaton said from his office in South Dakota.
Southeast Alaska may have featured a few ice-free zones, known as "refugia," during the last ice age, which lasted from about 25,000 to 13,000 years ago. A study by Gerald Shields, a researcher at the University of Alaska Fairbanks' Institute of Arctic Biology, supports the notion that large mammals may have survived the encroachment of sea ice and glaciers. Shields and then-graduate student Sandra Talbot studied the genetic history of brown bears in Southeast and found them more closely related to polar bears than to the brown bears that populate the rest of Alaska. The Southeast brown bears were somehow isolated, perhaps on large islands of tundra surrounded by ice.
The discoveries in On Your Knees Cave also enliven the debate on how the first people populated the Americas after crossing the Bering Land Bridge. Most anthropologists have argued for an inland, ice-free corridor, but now more people are looking at the coast of Alaska as a possible route south.
"If bears and caribou can survive (in Southeast), it's not hard to imagine people setting up camps," Heaton said.
Ned Rozell is a science writer at the Geophysical Institute, University of Alaska Fairbanks.