The sea today separates Asia and North America in the Bering Strait, but a land bridge connected the two continents during the last Ice Age. Now, new analysis shows that this land bridge emerged from the sea tens of thousands of years later than previously thought. Apparently, the passage only became passable around 35,700 years ago - around the time when, according to genetic studies, the ancestors of the Native Americans separated from Asian populations. Contrary to what was previously thought, these first settlers of America could have made their way to the New World shortly after the land bridge had dried up.
During the last ice age, huge glaciers covered large parts of the northern hemisphere, these ice masses bound large amounts of water and caused sea levels to drop. As a result, during the glacial maximum around 26,000 to 19,000 years ago, sea level was around 130 meters lower than it is today. Because the sea floor of today's Bering Strait is only a good 50 meters deep, it emerged from the sea and created a land bridge between Asia and North America until the end of the Ice Age around 12,000 years ago. According to recent sediment samples, this land connection between the continents, which is about 1,600 kilometers wide and almost 5,000 kilometers long, could have been a fertile landscape populated by many animals at this time. One theory has it that this may have motivated Native American ancestors to migrate to this area and then further east towards North America.
Nitrogen isotopes as indicators
So far, however, it was unclear when the land bridge in the Bering Strait went dry. Because the estimates of sea level derived from various indirect data in the period 50,000 to 30,000 years ago differ widely: they range from 25 to 105 meters less than today. As a result, it also remained controversial whether the Bering Strait was under water before the glacial maximum or not. To answer this question, Princeton University's Jesse Farmer and his colleagues used another new method to investigate this question: They analyzed the nitrogen isotope ratio of four sediment samples taken just north of what is now the Bering Strait and at a reference site away from the strait . The underlying approach: the water of the Pacific contains a higher proportion of the nitrogen isotope 15N than that of the Arctic Ocean. As long as the Bering Strait is open, Pacific waters are free to flow northward, enriching the western Arctic Ocean with this isotope. However, when the strait is blocked by the Bering Land Bridge, it stops this water exchange and the Arctic Ocean remains low.
With the help of this data and a supplementary model, the research team was able to reconstruct the history of the Bering Strait and thus also that of the sea level in this area for the period from 46,000 years ago to the present day. "What's exciting to me is that this gives us independent information about global sea levels during this period," explains co-author Tamara Pico of the University of California at Santa Cruz. "Some of the previously proposed values are quite different, and we have now been able to see which agree with the nitrogen data and which don't."
Flooded until 35,700 years ago
The analyzes showed that 46,000 years ago the Bering Strait was still underwater and there was a free exchange of water between the Pacific and Arctic Oceans. According to this, the sea level at that time was still significantly higher than some studies had previously suggested, and accordingly the land bridge formed much later than widely assumed. As Farmer and his colleagues explain, this also provides important information about how strong and fast the ice sheets grew during this early phase of the Ice Age. "Our data suggest that there was a substantial delay in ice sheet evolution after temperatures dropped," says Pico. "They suggest that more than half of the ice volume from the glacial maximum was formed after 46,000 years." Farmer adds: "This also means that the ice sheets can change faster than previously thought."
According to the new data, the land bridge between Asia and North America was formed around 35,700 years ago - only around 10,000 years before the glacial maximum. It was only at this time that the sea level dropped sufficiently to uncover the area that is now a good 50 meters under water. This also has implications for our understanding of the migration of ancient humans from Asia to North America: "Previously, the Land Breaches were thought to have been open for a while before humans crossed them," explains co-author Daniel Sigman of Princeton University. "But our new data suggests that people headed to North America as soon as the land bridge dried up." This raises the question of what prompted the ancestors of the Native Americans to migrate to what was then probably still rather barren, unknown territory and when exactly they began to do so.
Source: Jesse Farmer (Princeton University) et al., Proceedings of the National Academy of Sciences, doi: 10.1073/pnas.2206742119