In a new report published this week, researchers assess the most environmentally viable time periods for coastal migration of early people dispersing from Beringia into North America by evaluating changing ocean climate conditions in the Northeast Pacific between ~30,000 – 10,000 years ago. By combining insights from paleoclimate records and climate models with existing literature on genomics and archaeological data, this research helps to home in on the possible time periods in which early human migration may have occurred. While it does not provide evidence of such a migration, it may serve to help archaeologists refine their search for earlier coastal occupation along ancient shorelines that date to the time periods identified. 

Growing evidence for a human presence in the Americas prior to 15,000 years ago—when ice sheets blocked the continental interior—imply a Pacific Coast route was more likely for dispersals from Beringia into North America between ~26,000 and 14,000 years ago. Paleo sea-ice reconstructions indicate that winter sea ice was present along the Alaskan coast during the glacial period and was still present 15,000 years ago, subsiding more recently. Given the archaeological evidence for human presence in North America prior to 15,000 years ago, and this new paleo sea-ice and climate reconstruction, the researchers hypothesize a “sea-ice highway” may have helped to facilitate movement along the coast, rather than hinder it.  

Researchers from the U.S. Geological Survey, Woods Hole Oceanographic Institution, Oregon State University, and University of Oregon used paleoclimate records and climate models to identify the most environmentally favorable time windows. “We compiled as much paleoclimate data as possible, which included new and previously published records of sea ice, sea surface temperature, salinity, and ice-rafted debris, to figure out what the oceanic conditions could have been like along the Alaskan coast,” said lead author Summer Praetorius, a USGS research scientist. “We wanted to see whether regional climate change affected the viability of the coastal route at different times. For example, understanding where and when sea ice formed in the Gulf of Alaska has implications for how people could move along the coastline—whether by foot or boats.” 

They also used climate models to see how different climate states—such as glacial conditions with strong winds and lower sea levels—would have affected the strength of ocean currents. Ocean currents in the Gulf of Alaska flow against the direction of presumed migration from Beringia, so stronger currents could have made boat travel more difficult. It had been previously hypothesized that ocean currents would have been weaker during the last glacial period due to lower sea levels, and therefore made boat travel along this route easier during peak glacial conditions, around 20,000 years ago. However, the new modeling work presented in this paper shows the opposite effect, with currents getting stronger during the glacial period due to stronger winds.  

“The fact that ocean currents strengthen along the coast of Alaska in our model at a time when humans are trying to migrate south raises some really interesting questions about how difficult boat travel would have been. Was the combination of faster flowing currents in an ocean full of sea ice and icebergs a formidable barrier that prevented migration until conditions became more benign?” said Alan Condron, a Woods Hole Oceanographic Institution scientist who led the high-resolution modeling work.  

The combination of paleoclimate records and modeling work also identified certain times that would have been especially difficult for a coastal journey and may have effectively blocked passage altogether. These were times when glaciers along the Alaskan coast reached all the way to the ocean, which would have made passage and habitability extremely difficult, as there was little land area available. These periods were marked by extensive amounts of icebergs, and the modeling work showed that the addition of freshwater into the ocean would have also increased the strength of the ocean currents. Navigating crevassed glaciers, strong currents, and especially frigid climate conditions during these periods seem to make them the harshest periods for a coastal transit, the authors conclude.  

“Sediment cores we recovered from the northeast Pacific are showing amazing things about the history of ice in the region,” said co-author Alan Mix, an oceanographer and paleoclimatologist at Oregon State University. “We are finding that the ice moved back and forth quite a bit, so land areas opened up along the coast at various times during the ice age, and we wondered if those offered windows of opportunity for people to move south along the coast even though the ice age climate was still pretty cold.” 

The article “Ice and ocean constraints on early human migrations into North America along the Pacific coast” was published today in the Proceedings of the National Academy of Sciences.