The Bering Bog Bridge

The Bering Bog Bridge
by Madeline Reinsel
Fairbanks AK (SPX) Dec 10, 2024

During the last Ice Age, the land masses that make up modern-day Siberia and Alaska were connected by a broad swath of land, now submerged in the ocean. For decades, scientists have assumed that 36,000 to 11,000 years ago, the Bering Land Bridge resembled Ice Age Alaska and Siberia - an arid steppe grassland. But new research based on a first-of-its kind field effort suggests the Bering Land Bridge was more likely a boggy ecosystem traversed by meandering rivers.

"We were looking for several large lakes," said Sarah Fowell, a paleogeologist at the University of Alaska Fairbanks. "What we actually found was evidence of lots of small lakes and river channels."

Fowell and her colleagues will present their work on Tuesday, 10 December 2024 and Friday, 13 December 2024 at the AGU Annual Meeting in Washington, D.C., where more than 28,000 scientists will gather to discuss the latest Earth and space science research.

Secrets of the sediment

During the last Ice Age, global sea levels were approximately 122 meters (400 feet) lower than they are today. That means that much of what is now the Bering Sea was land, and that many species could have migrated freely between Alaska and Siberia.

Until now, scientists have assumed the Bering Land Bridge was simply an extension of Alaska and Siberia's Ice Age steppe ecosystems. But the region more likely resembled the modern-day Yukon-Kuskokwim Delta in western Alaska, a boggy riverine ecosystem.

"We've been looking on land to try to reconstruct what is underwater," said Jenna Hill, a geologist with the U.S. Geological Survey, who will also present her work at AGU24. "But that doesn't really tell you what was on land that is now submerged between Alaska and Siberia."

In the summer of 2023, Fowell, Hill and colleagues set out on the R/V Sikuliaq to plumb the depths of the Bering Sea. They based their route on previous work that had mapped low spots on the sea floor, which may have been lakes when the region was above water.

The sediment cores and sonar readings that they took on that research trip could radically change scientists' understanding of what the Bering Land Bridge looked like, and what animals and plants were there when it was above water.

The cores reveal a sharp transition from freshwater to marine sediment at the end of the last Ice Age, when the land bridge became the Bering Strait. The cores that they collected from 36 sampling sites contained sediment from freshwater lakes, as well as macrofossils, organic matter later used for radiocarbon dating, pollen and sedimentary ancient DNA. Pollen samples reveal that there were woody trees on the Bering Land Bridge, while Daphnia egg cases, moss leaves and other macrofossils show widespread freshwater in the region.

The results could help answer an old question: Why did some animals cross the Bering Land Bridge, while others stayed behind? Although the land bridge was likely dominated by freshwater rivers and bogs, which may have appealed to animals like birds, some higher elevation, drier areas must have let larger animals like horses, mammoths and bison make the crossing. The researchers even detected ancient mammoth DNA at one site.

"It may have been marshy, but we are still seeing evidence of mammoths," Fowell said. "Even if it was mostly floodplains and ponds, the grazers were around, just uphill following higher, drier areas."

But the wet landscape could have stopped some animals, like woolly rhinos, American camels and short faced bears, from moving from one continent to the other.

"The watery, wet landscape could have been a barrier for some species, or a pathway for species that actually travel by water," Hill said. "That's how this fits into the bigger picture."