During the Previous Ice Age, the Bering Land Bridge Formed Surprisingly Late

Scientists have reconstructed the sea level history of the Bering Strait and discovered that it was flooded up until around 35,700 years ago, just before humans migrated into the Americas.

The Bering Land Bridge, which connects Asia to North America, did not appear until roughly 35,700 years ago, or fewer than 10,000 years before the peak of the last ice age, according to a recent study that reconstructs the history of water level at the Bering Strait (known as the Last Glacial Maximum or LGM). The new research, which was released in Proceedings of the National Academy of Sciences¹ the week of December 26, shows that the growth of the ice sheets—and the subsequent drop in sea level that resulted—took place surprisingly quickly and much later in the glacial cycle than earlier studies had suggested. Co-author and professor Tamara Pico said,

“It means that more than 50 percent of the global ice volume at the Last Glacial Maximum grew after 46,000 years ago. This is important for understanding the feedbacks between climate and ice sheets, because it implies that there was a substantial delay in the development of ice sheets after global temperatures dropped.”

As more and more of the Earth's water is trapped in enormous ice sheets, global sea levels fall during ice ages, although the exact timing of these processes has been difficult to determine. Large portions of North America were buried by ice sheets during the LGM, which occurred between 26,500 and 19,000 years ago. A massive landmass known as Beringia, which stretched from Siberia to Alaska and was home to herds of horses, mammoths, and other Pleistocene animals, was made visible by dramatically lower sea levels. Around 13,000 to 11,000 years ago, the Bering Strait was once more inundated due to the melting of the ice sheets.

Hydrography of the polar Northern Hemisphere and Bering Strait bathymetry. (A) Mixed layer (10 m depth) salinity (7) (blue shading) and subsurface (~50 to 200 m) nitrogen isotopic composition (δ15N) of nitrate (diamonds). From published data sets, sample depths were chosen to capture the nitrate being supplied to the mixed layer in the Arctic and North Atlantic (NA), and the North Pacific (NP) nitrate being transported across the Bering Strait (see data sources and selection criteria in SI Appendix, section S2). The mean salinity and nitrate δ15N values for the western Arctic (WA), NP, and NA are indicated on the color bars. Cyan circles show locations of sediment cores; light blue and orange arrows show schematic circulation of Atlantic- and Pacific-sourced nitrate-rich subsurface waters, respectively (8, 9). (B) Bering Strait bathymetry (1) contoured at 10 m intervals. Red lines indicate principal ocean transport pathways (10).

Because they reduce the amount of time between the opening of the land bridge and the arrival of humans in the Americas, the new discoveries are intriguing in terms of human migration. Although the exact time of human migration into North America is still unclear, certain research indicate that people may have resided in Beringia for the duration of the last ice age.

The new study determined when the Bering Strait was flooded throughout the previous 46,000 years, allowing water from the Pacific Ocean to flow into the Arctic Ocean, by analyzing nitrogen isotopes in bottom sediments. The isotope analysis was carried out by the first author Jesse Farmer, who determined the ratios of nitrogen isotopes in the marine plankton remnants found in sediment cores dug up from the seafloor at three different locations in the western Arctic Ocean. Farmer was able to determine a nitrogen isotope signature showing when Pacific water migrated into the Arctic because of changes in the nitrogen composition of Pacific and Arctic waters. Pico next compared Farmer's findings to sea level models constructed using various ice sheet expansion scenarios. Pico said,

“People may have started going across as soon as the land bridge formed. The exciting thing to me is that this provides a completely independent constraint on global sea level during this time period. Some of the ice sheet histories that have been proposed differ by quite a lot, and we were able to look at what the predicted sea level would be at the Bering Strait and see which ones are consistent with the nitrogen data.”

The findings are consistent with recent research showing that global sea levels were significantly higher than previously thought before the LGM. During the LGM, the average worldwide sea level was around 130 meters (425 feet) lower than it is today. However, variables like the deformation of the Earth's crust caused by the weight of the ice sheets have a role in determining the real sea level at a certain location, such the Bering Strait. Pico said,

“It’s like punching down on bread dough—the crust sinks under the ice and rises up around the edges. Also, the ice sheets are so massive they have gravitational effects on the water. I model those processes to see how sea level would vary around the world and, in this case, to look at the Bering Strait.”

The results show a complex relationship between climate and the volume of ice on Earth and offer new directions for studying the processes that underlie glacial cycles.

1. Farmer, J. R., Pico, T., Underwood, O. M., Cleveland Stout, R., Granger, J., Cronin, T. M., Fripiat, F., Martínez-García, A., Haug, G. H., & Sigman, D. M. (2023). The Bering Strait was flooded 10,000 years before the Last Glacial Maximum. Proceedings of the National Academy of Sciences of the United States of America, 120(1), e2206742119. https://doi.org/10.1073/pnas.2206742119