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 presented at AGU’s annual meeting earlier this month and based on a first-of-its kind field effort suggests the Bering Land Bridge was more likely a boggy ecosystem traversed by meandering rivers. 

In the summer of 2023, the researchers set out on the RV Sikuliaq to plumb the depths of the Bering Sea. They based their route on previous work that had mapped low spots on the seafloor, 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 (water flea) 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 such as birds, some higher elevation, drier areas must have allowed for larger animals such as horses, mammoths, and bison to make the crossing. The researchers even detected ancient mammoth DNA at one site. 

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