Today the sea separates Asia and North America in the Bering Strait, but a land bridge connected the two continents during the last ice age. Now, a 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 accepted around 35,700 years ago—around the time, according to genetic studies, that the ancestors of the Native Americans separated from the Asian population. Contrary to what was previously thought, these early settlers in America could have made their way to the New World soon after the land bridge had dried up.
During the last ice age, huge glaciers covered large parts of the northern hemisphere, entrapping these glaciers with large amounts of water and causing sea levels to drop. As a result, during the Maximum Ice Age around 26,000 to 19,000 years ago, sea level was about 130 meters lower than it is today. Since the seabed of today’s Bering Strait is only 50 meters deep, it emerged from the sea and formed a land bridge between Asia and North America until the end of the Ice Age about 12,000 years ago. According to recent sediment samples, this land link between the continents, which is about 1,600 km wide and about 5,000 km long, could be a fertile area inhabited by many animals at this time. One theory is that this may have motivated the ancestors of the Native Americans to migrate into this region and then east towards North America.
Nitrogen isotopes as indicators
However, it is not yet clear when the Bering Strait land bridge dried up. Because sea level estimates derived from various indirect data for the period 50,000 to 30,000 years ago vary greatly: it ranges from 25 to 105 meters lower than today. As a result, it also remained controversial whether or not the Bering Strait was flooded prior to the ice cap. To answer this question, Jesse Farmer of Princeton University and his colleagues used another new method to investigate this question: They analyzed the nitrogen isotope ratio of four sediment samples taken north of what is now the Bering Strait and at a reference site farther from the strait. . Basic approach: Pacific waters contain a higher proportion of the isotope N15 than those in the Arctic Ocean. As long as the Bering Strait is open, Pacific waters are free to flow north, enriching the 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 complementary model, the research team was able to reconstruct the history of the Bering Strait and thus also the history of sea level in this region for the period from 46,000 years ago to the present day. “What’s exciting to me is that this gives us independent information on global sea levels over this period,” explains co-author Tamara Biko of the University of California, Santa Cruz. “Some of the previously proposed values are very different, and we have now been able to see which ones are consistent with the nitrogen data and which are not.”
It flooded until 35,700 years ago
Analyzes showed that 46,000 years ago the Bering Strait was still under water and there was a free exchange of water between the Pacific Ocean and the Arctic Ocean. Accordingly, the sea level at that time was still much higher than some studies had previously suggested, and thus the land bridge formed much later than was widely assumed. As Farmer and colleagues show, this also provides important information about how strong and fast the ice sheets grew during this early Ice Age phase. “Our data indicate a significant delay in ice sheet evolution after lower temperatures,” says Biko. “They suggest that more than half of the ice volume of the ice cap formed after 46,000 years.” “It also means that the ice sheets could be changing faster than previously thought,” Farmer adds.
According to the new data, the land bridge between Asia and North America was formed about 35,700 years ago — only about 10,000 years before the ice cap. It was only at this time that the sea level dropped enough to reveal the area that is now 50 meters below the water’s surface. This also has implications for our understanding of the migration of ancient humans from Asia to North America: “Previously, it was thought that land breaches were open for some time before humans crossed them,” explains co-author Daniel Sigman of Princeton University. “But our new data suggests that people headed to North America once the land bridge dried up.” This raises the question of what prompted the ancestors of the Native Americans to migrate to what was likely at the time still rather barren, unknown lands and when exactly did they begin to do so.
Source: Jesse Farmer (Princeton University) et al., Proceedings of the National Academy of Sciences, Available here. doi: 10.1073/pnas.2206742119
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