Planet Earth pulled out all the stops, it seems, to enable the first humans to reach North America. When a glacial period lowered sea levels and turned parts of the Bering Strait into a land bridge, a warm ocean current shielded that region from the worst cold, turning it into a refuge where ancestors of the first Americans found shelter for thousands of years.
Evidence of that current, which doesn’t exist today, was published recently in Science Advances.
That there was a “Beringian Standstill” in the trek between the continents is suggested by DNA differences between Native Americans and Asians. These differences indicate that it’s been about 25,000 years since the two populations diverged. But scientists know humans spread out across the Americas only about 15,000 years ago.
One possible explanation for this discrepancy is that the migration from Asia to the Americas stalled in Beringia, a region now submerged by the Bering Sea. The new study, led by James Rae of the University of St Andrews in the United Kingdom, suggests why ancient migrants may have found Beringia an attractive place to settle. “Rather than being this harsher version of modern Kamchatka and Siberia, it might actually have been more like Scotland,” Rae said.
Proposing a PMOC
Rae’s analysis of sediment cores and climate model calculations suggests that during the Last Glacial Maximum (LGM), a Pacific meridional overturning circulation (PMOC) pattern was active. The proposed PMOC functioned much like the Atlantic version, the AMOC, that today gives western Europe its temperate climate. The AMOC is driven by cold and salty, and hence dense, water sinking near the Arctic, pulling in warm surface water from the south while itself streaming southward underneath.
Oceanographers have wondered why today there isn’t such a conveyor belt for heat in the North Pacific. In fact, even today’s climate models sometimes switch Earth into a mode where there’s a PMOC instead of an AMOC. In such cases, models are normally nudged to represent Earth as we know it by increasing the net amount of fresh water the Pacific receives as rain, thanks to water vapor being blown west across the isthmus of Panama. This phenomenon makes Pacific water less salty and thus less dense.
For his study, Rae did the reverse in a number of different climate models, until a PMOC arose. He compared the results with what was known about past conditions in the ocean, culled from sediment cores containing the remains of tiny shelled organisms, foraminifera.
Such cores are relatively hard to come by because the Pacific is “not surrounded by as many oceanographic institutes as the North Atlantic,” Rae said. The Pacific is also deeper and more acidic—and both pressure and acidity promote the dissolving of the carbonate shells of foraminifera. “Most of the ocean floor is just red clay, barren of any foram shells,” Rae said.
Rae thinks his results confirm the existence of a PMOC. Running the climate models with a PMOC showed that the ocean’s surface layer would become depleted in nutrients because it would consist of water from the subtropics, where year-round stratification and unfavorable wind stress restrict nutrient supply from below. And indeed, cores taken from shallower depths, from the Asian and American continental slopes and from the flanks of sea mounts, had so far shown that during the LGM the surface water of the North Pacific contained fewer nutrients than it does today.
Beringian Standstill and Human Migration
Rae happened to read an article about the Beringian Standstill in Science and applied his results to the concept. “I became aware of this idea that people may have lived in Beringia in the peak of the last Ice Age. And there is some indication from pollen and things like fossil beetles that the climate there might have been surprisingly mild.”
Rae’s models showed that the PMOC would give Beringia a regional climate that was vastly more welcoming than anything to the east or west. Conditions were even milder than John Hoffecker, one of the authors of the Science article and a researcher at the University of Colorado Boulder, expected.
Hoffecker cautioned, however, that although Rae’s results support the idea of a northern refugium for people in Beringia, it doesn’t clinch the case. “We still can’t confirm that people were actually present in Beringia during the LGM,” he wrote Eos in an email. “And, even if we can eventually confirm a human presence, we will have to confirm that they represented the Native American founder population.”
—Bas den Hond ([email protected]), Science Writer