One of the hottest debates in archaeology is how and when humans first arrived in North America.
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Archaeologists have traditionally argued that people walked through an ice-free corridor that briefly opened between ice sheets an estimated 13,000 years ago.
But a growing number of archaeological and genetic finds—including human footprints in New Mexico dated to around 23,000 years old—suggests that people made their way onto the continent much earlier.
These early Americans likely traveled along the Pacific coastline from Beringia, the land bridge between Asia and North America that emerged during the last glacial maximum when ice sheets bound up large amounts of water causing sea levels to fall.
Now, in research to be presented Friday, 15 December at the American Geophysical Union Annual Meeting (AGU23) in San Franciso, paleoclimate reconstructions of the Pacific Northwest hint that sea ice may have been one way for people to move farther south.
The idea that early Americans may have traveled along the Pacific Coast isn't new. People were likely south of the massive ice sheets that once covered much of the continent at least 16,000 years ago.
Given that the ice-free corridor wouldn't be open for thousands of years before these early arrivals, scientists instead proposed that people may have moved along a "kelp highway."
This theory holds that early Americans slowly traveled down into North America in boats, following the bountiful goods found in coastal waters.
Archaeologists have found evidence of coastal settlements in western Canada dating from as early as 14,000 years ago
But in 2020, researchers noted that freshwater from melting glaciers at the time may have created a strong current that would make it difficult for people to travel along the coast.
To get a fuller picture of ocean conditions during these crucial windows of human migration, Summer Praetorius of the US Geological Survey and her colleagues looked at climate proxies in ocean sediment from the coast.
Most of the data came from tiny, fossilized plankton. The abundance and chemistry of these organisms help reconstruct ocean temperatures, salinity, and sea ice cover.
Arctic people today travel along sea ice on dog sleds and snow mobiles.
Early Americans may also have used the 'sea ice highway' to get around and hunt marine mammals, slowly making their way into North America in the process, Praetorius said.
The climate data suggest conditions along the coastal route may have been conducive to migration between 24,500 and 22,000 years ago and 16,400-14,800 years ago, possibly aided by the presence of winter sea ice. ■