Bering Land Bridge formed surprisingly late during last ice age
Princeton scientists found that the Bering Land Bridge was flooded until 35,700 years ago, with its full emergence occurring only shortly before the migration of humans into the Americas.
Peer-Reviewed PublicationIMAGE: THE SAMPLES WERE COLLECTED ON THE U.S. COAST GUARD ICEBREAKER POLAR SEA ON AN AMBITIOUS TRANS-ARCTIC EXPEDITION IN 1994. view more
CREDIT: US COAST GUARD
A new study shows that the Bering Land Bridge, the strip of land that once connected Asia to Alaska, emerged far later during the last ice age than previously thought.
The unexpected findings shorten the window of time that humans could have first migrated from Asia to the Americas across the Bering Land Bridge.
The findings also indicate that there may be a less direct relationship between climate and global ice volume than scientists had thought, casting into doubt some explanations for the chain of events that causes ice age cycles. The study was published on December 27 in the Proceedings of the National Academy of Sciences.
“This result came totally out of left field,” said Jesse Farmer, postdoctoral researcher at Princeton University and co-lead author on the study. “As it turns out, our research into sediments from the bottom of the Arctic Ocean told us not only about past climate change but also one of the great migrations in human history."
Insight into ice age cycles
During the periodic ice ages over Earth’s history, global sea levels drop as more and more of Earth’s water becomes locked up in massive ice sheets. At the end of each ice age, as temperatures increase, ice sheets melt and sea levels rise. These ice age cycles repeat throughout the last 3 million years of Earth’s history, but their causes have been hard to pin down.
By reconstructing the history of the Arctic Ocean over the last 50,000 years, the researchers revealed that the growth of the ice sheets — and the resulting drop in sea level — occurred surprisingly quickly and much later in the last glacial cycle than previous studies had suggested.
“One implication is that ice sheets can change more rapidly than previously thought,” Farmer said.
During the last ice age’s peak of the last ice age, known as the Last Glacial Maximum, the low sea levels exposed a vast land area that extended between Siberia and Alaska known as Beringia, which included the Bering Land Bridge. In its place today is a passage of water known as the Bering Strait, which connects the Pacific and Arctic Oceans.
Based on records of estimated global temperature and sea level, scientists thought the Bering Land Bridge emerged around 70,000 years ago, long before the Last Glacial Maximum.
But the new data show that sea levels became low enough for the land bridge to appear only 35,700 years ago. This finding was particularly surprising because global temperatures were relatively stable at the time of the fall in sea level, raising questions about the correlation between temperature, sea level and ice volume.
“Remarkably, the data suggest that the ice sheets can change in response to more than just global climate,” Farmer said. For example, the change in ice volume may have been the direct result of changes in the intensity of sunlight that struck the ice surface over the summer.
“These findings appear to poke a hole in our current understanding of how past ice sheets interacted with the rest of the climate system, including the greenhouse effect,” said Daniel Sigman, Dusenbury Professor of Geological and Geophysical Sciences at Princeton University and Farmer’s postdoctoral advisor. “Our next goal is to extend this record further back in time to see if the same tendencies apply to other major ice sheet changes. The scientific community will be hungry for confirmation.”
New context for human migration
The timing of human migration into North America from Asia remains unresolved, but genetic studies tell us that ancestral Native American populations diverged from Asian populations about 36,000 years ago, the same time that Farmer and colleagues found that the Bering Land Bridge emerged.
“It’s generally believed that the land bridge was open for a while, and then humans crossed it at some point,” Sigman said. “But our new data suggest that the land bridge was not open, and as soon as it opened up, human populations made their way into North America.”
The finding raises questions about why humans decided to migrate as soon as the land bridge opened, and how humans made their way across the land bridge with no previous knowledge of the landscape.
The researchers noted that they need to be cautious when considering these implications, as the interpretation requires combining very different types of information, including the new data and the information of human geneticists and paleoanthropologists. They look forward to seeing how their results are built upon by these other scientific communities.
A window to the past
To reconstruct the history of the Bering Strait, Farmer and Sigman sought an ocean chemical fingerprint.
Pacific waters carry high concentrations of nitrogen molecules that have a distinct chemical composition, known as an isotope ratio. Today, waters from the Pacific Ocean travel northwards across the Bering Strait into the Arctic Ocean, carrying a traceable nitrogen isotope ratio.
By measuring nitrogen isotopes in sediments at the bottom of the Arctic Ocean, Farmer found that the fingerprint of Pacific Ocean nitrogen disappeared when the Bering Strait was closed during the peak of the last ice age, as expected.
But when Farmer continued his analyses further back in time – to about 50,000 years ago – he found that the Pacific nitrogen fingerprint returned far more recently than researchers had thought possible.
“When Jesse showed me his data, he didn't need to explain to me what had happened,” Sigman said. “It was too large of a change to be anything other than a previous opening of the Bering Strait.”
To understand the implications for global sea level, Farmer and Sigman collaborated with Tamara Pico, a sea level expert and professor of Earth and Planetary Sciences at UC Santa Cruz, Princeton undergraduate Class of 2014, and co-lead author on the paper. Pico compared Farmer’s results with sea level models based on different scenarios for the growth of the ice sheets.
“When Jesse contacted me I was so excited,” Pico said. “A large part of my PhD thesis was focused on how fast global ice sheets grew leading into the Last Glacial Maximum, and much of my work suggests that they might have grown faster than previously thought.”
Farmer’s nitrogen analyses provided a new set of evidence to back up Pico’s research about sea levels during the last ice age.
“The exciting thing to me is that this provides a completely independent constraint on global sea level during this time period,” Pico said. “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.”
“This study brought together experts in the Arctic Ocean, nitrogen cycling and global sea level. And the outcome has consequences not only for climate and sea level but also for human prehistory,” Farmer said. “One of the thrilling aspects of paleoclimate research is the opportunity to collaborate across such a broad range of subjects.”
“The Bering Strait was flooded 10,000 years before the Last Glacial Maximum,” by Jesse R. Farmer, Tamara Pico, Ona M. Underwood, Rebecca Cleveland Stout, Julie Granger, Thomas M. Cronin, François Fripiat, Alfredo Martínez-García, Gerald H. Haug, and Daniel M. Sigman appears in the current issue of Proceedings of the National Academy of Sciences (DOI: 10.1073/pnas.2206742119). The research was supported by the U.S. National Science Foundation (OCE-2054780 and OCE-2054757), the Tuttle and Phillips Funds of the Department of Geosciences, the Max Planck Society, and the USGS Climate Research and Development Program.
JOURNAL
Proceedings of the National Academy of Sciences
ARTICLE TITLE
The Bering Strait was flooded 10,000 years before the Last Glacial Maximum
ARTICLE PUBLICATION DATE
27-Dec-2022
Bering Land Bridge formed surprisingly late during last ice age, study finds
By reconstructing the sea level history of the Bering Strait, scientists found that the strait remained flooded until around 35,700 years ago, not long before humans began migrating into the Americas
Peer-Reviewed PublicationA new study that reconstructs the history of sea level at the Bering Strait shows that the Bering Land Bridge connecting Asia to North America did not emerge until around 35,700 years ago, less than 10,000 years before the height of the last ice age (known as the Last Glacial Maximum).
The new findings, published the week of December 26 in Proceedings of the National Academy of Sciences, indicate that the growth of the ice sheets—and the resulting drop in sea level—occurred surprisingly quickly and much later in the glacial cycle than previous studies had suggested.
“It means that more than 50 percent of the global ice volume at the Last Glacial Maximum grew after 46,000 years ago,” said Tamara Pico, assistant professor of Earth and planetary sciences at UC Santa Cruz and a corresponding author of the paper. “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.”
Global sea levels drop during ice ages as more and more of Earth’s water gets locked up in massive ice sheets, but the timing of these processes has been hard to pin down. During the Last Glacial Maximum, which lasted from about 26,500 to 19,000 years ago, ice sheets covered large areas of North America. Dramatically lower sea levels uncovered a vast land area known as Beringia that extended from Siberia to Alaska and supported herds of horses, mammoths, and other Pleistocene fauna. As the ice sheets melted, the Bering Strait became flooded again around 13,000 to 11,000 years ago.
The new findings are interesting in relation to human migration because they shorten the time between the opening of the land bridge and the arrival of humans in the Americas. The timing of human migration into North America remains unresolved, but some studies suggest people may have lived in Beringia throughout the height of the ice age.
“People may have started going across as soon as the land bridge formed,” Pico said.
The new study used an analysis of nitrogen isotopes in seafloor sediments to determine when the Bering Strait was flooded during the past 46,000 years, allowing Pacific Ocean water to flow into the Arctic Ocean. First author Jesse Farmer at Princeton University led the isotope analysis, measuring nitrogen isotope ratios in the remains of marine plankton preserved in sediment cores collected from the seafloor at three locations in the western Arctic Ocean. Because of differences in the nitrogen composition of Pacific and Arctic waters, Farmer was able to identify a nitrogen isotope signature indicating when Pacific water flowed into the Arctic.
Pico, whose expertise is in sea level modeling, then compared Farmer’s results with sea level models based on different scenarios for the growth of the ice sheets.
“The exciting thing to me is that this provides a completely independent constraint on global sea level during this time period,” Pico said. “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 results support recent studies indicating that global sea levels were much higher prior to the Last Glacial Maximum than previous estimates had suggested, she said. Average global sea level during the Last Glacial Maximum was about 130 meters (425 feet) lower than today. The actual sea level at a particular site such as the Bering Strait, however, depends on factors such as the deformation of the Earth’s crust by the weight of the ice sheets.
“It’s like punching down on bread dough—the crust sinks under the ice and rises up around the edges,” Pico said. “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 findings imply a complicated relationship between climate and global ice volume and suggest new avenues for investigating the mechanisms underlying glacial cycles.
In addition to Pico and Farmer, the coauthors include Ona Underwood and Daniel Sigman at Princeton University; Rebecca Cleveland-Stout at the University of Washington; Julie Granger at the University of Connecticut; Thomas Cronin at the U.S. Geological Survey; and François Fripiat, Alfredo Martinez-Garcia, and Gerald Haug at the Max Planck Institute for Chemistry in Germany. This work was supported by the National Science Foundation.
JOURNAL
Proceedings of the National Academy of Sciences
ARTICLE TITLE
The Bering Strait was flooded 10,000 years before the Last Glacial Maximum
ARTICLE PUBLICATION DATE
26-Dec-2022
COI STATEMENT
The authors declare no competing interests.
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