Bering Strait and the Bering Land Bridge

The Earliest Entryway into the New World

Frozen Spring in Tundra, Arctic National Wildlife Refuge
Frozen Spring in Tundra, Arctic National Wildlife Refuge. Madhav Pai

The Bering Strait is a waterway that separates Russia from North America. It lies above the Bering Land Bridge, also called Beringia (sometimes misspelled Beringea), a submerged landmass that once connected the Siberian mainland with North America. While Beringia's shape and size while above water is variously described in publications, most scholars would agree the land mass included the Seward Peninsula, as well as existing land areas of northeast Siberia and western Alaska, between the Verkhoyansk Range in Siberia and the Mackenzie River in Alaska.

As a waterway, the Bering Strait connects the Pacific Ocean to the Arctic Ocean over the polar ice cap, and eventually the Atlantic Ocean.

The climate of the Bering Land Bridge (BLB) when it was above sea level during the Pleistocene was long thought to have been primarily a herbaceous tundra or steppe-tundra. However, recent pollen studies have shown that during the Last Glacial Maximum (say, between 30,000-18,000 calendar years ago, abbreviated as cal BP), the environment was a mosaic of diverse but cold plant and animal habitats.

Living on the BLB

Whether Beringia was habitable or not at a given time is determined by the sea level and presence of surrounding ice: specifically, whenever the sea level drops about 50 meters (~164 feet) below its present position, the land surfaces. The dates when this happened in the past have been difficult to establish, in part because the BLB is currently mostly underwater and difficult to reach.

Ice cores seem to indicate that most of the Bering Land Bridge was exposed during Oxygen Isotope Stage 3 (60,000 to 25,000 years ago), connecting Siberia and North America: and the land mass was above sea level but cut off from east and west land bridges during OIS 2 (25,000 to about 18,500 years BP).

Beringian Standstill Hypothesis

By and large, archaeologists believe that the Bering land bridge was the primary entryway for the original colonists into the Americas. About 30 years ago, scholars were convinced that people simply left Siberia, crossed the BLB and entered down through the mid-continental Canadian ice shield through a so-called "ice-free corridor". However, recent investigations indicate the "ice free corridor" was blocked between about 30,000 and 11,500 cal BP. Since the northwest Pacific coast was deglaciated at least as early as 14,500 years BP, many scholars today believe a Pacific coastal route was the primary route for much of the first American colonization.

One theory gaining strength is the Beringian standstill hypothesis, or Beringian Incubation Model (BIM), the proponents of which argue that instead of moving directly from Siberia across the strait and down the Pacific coast, the migrants lived--in fact were trapped--on the BLB for several millennia during the Last Glacial Maximum. Their entry into North America would have been blocked by ice sheets, and their return to Siberia blocked by the glaciers in the Verkhoyansk mountain range.

The earliest archaeological evidence of human settlement to the west of the Bering Land Bridge east of the Verkhoyansk Range in Siberia is the Yana RHS site, a very unusual 30,000-year-old site located above the arctic circle.

The earliest sites on the east side of the BLB in the Americas are Preclovis in date, with confirmed dates usually no more than 16,000 years cal BP. The Beringian Standstill Hypothesis helps explain that long-standing gap.

Climate Change and the Bering Land Bridge

Although there is a lingering debate, pollen studies suggest that the climate of the BLB between about 29,500 and 13,300 cal BP was an arid, cool climate, with grass-herb-willow tundra. There is also some evidence that near the end of the LGM (~21,000-18,000 cal BP), conditions in Beringia deteriorated sharply. At about 13,300 cal BP, when rising sea levels began to flood the bridge, the climate appears to have been wetter, with deeper winter snows and cooler summers.

Sometime between 18,000 and 15,000 cal BP, the bottleneck to the east was broken, which allowed human entrance into the North American continent along the Pacific coast. The Bering Land Bridge was completely inundated by rising sea levels by 10,000 or 11,000 cal BP, and its current level was reached about 7,000 years ago.

The Bering Strait and Climate Control

A recent computer modeling of the ocean cycles and their effect on abrupt climate transitions called Dansgaard-Oeschger (D/O) cycles, and reported in Hu and colleagues 2012, describes one potential effect of the Bering Strait on global climate. This study suggests that the closing of the Bering Strait during the Pleistocene restricted cross-circulation between the Atlantic and Pacific Oceans, and perhaps led to the numerous abrupt climatic changes experienced between 80,000 and 11,000 years ago.

One of the major fears of coming global climate change is the effect of changes in the salinity and temperature of the North Atlantic current, resulting from glacial ice melt. Changes to the North Atlantic current have been identified as one trigger for significant cooling or warming events in the North Atlantic and surrounding regions, such as that seen during the Pleistocene. What the computer models seem to show is that an open Bering Strait allows ocean circulation between the Atlantic and Pacific, and continued admixing may suppress the effect of the North Atlantic freshwater anomaly.

The researchers suggest that as long as the Bering Strait continues to stay open, the current water flow between our two major oceans will continue unhindered. This is likely, say scholars, to repress or limit any changes in the North Atlantic salinity or temperature, and thus lessen the likelihood of sudden collapse of the global climate.

Researchers caution, however, that since researchers aren't even guaranteeing that fluctuations in the North Atlantic current would create problems, further investigations examining glacial climate boundary conditions and models are needed to support these results.

Climate Similarities between Greenland and Alaska

In related studies, Praetorius and Mix (2014) looked at the oxygen isotopes of two species of fossil plankton, taken from sediment cores off the Alaskan coast, and compared them to similar studies in northern Greenland. Briefly, the balance of isotopes in a fossil being is direct evidence of the kind of plants--arid, temperate, wetland, etc.--which were consumed by the animal during its life. (See Stable Isotopes for Dummies for a somewhat broader explanation.) What Praetorius and Mix discovered was that sometimes Greenland and the coast of Alaska experienced the same kind of climate: and sometimes they did not.

The regions experienced the same general climate conditions between 15,500-11,000 years ago, just before the abrupt climate changes that resulted in our modern climate. That was the onset of the Holocene when temperatures rose sharply, and most of the glaciers melted back to the poles. That may have been a result of the connectivity of the two oceans, regulated by the opening of the Bering Strait; the elevation of ice in North America and/or the routing of freshwater into the North Atlantic or Southern ocean.

After things settled down, the two climates diverged again and the climate has been relatively stable since then. However, they appear to be growing closer. Praetorius and Mix suggest that the simultaneity of climates may presage rapid climate change and that it would be prudent to monitor the changes.

Important Sites

Archaeological sites important to the understanding of the American colonization along the Bering Strait include:

Sources

This glossary entry is part of the About.com Guide to Populating America and the Dictionary of Archaeology. Bibliographic sources for this article are on page two.

Ager TA, and Phillips RL. 2008. Pollen evidence for late Pleistocene Bering land bridge environments from Norton Sound, northeastern Bering Sea, Alaska. Arctic, Antarctic, and Alpine Research 40(3):451–461.

Bever MR. 2001. An Overview of Alaskan Late Pleistocene Archaeology: Historical Themes and Current Perspectives. Journal of World Prehistory 15(2):125-191.

Fagundes NJR, Kanitz R, Eckert R, Valls ACS, Bogo MR, Salzano FM, Smith DG, Silva WA, Zago MA, Ribeiro-dos-Santos AK et al. 2008. Mitochondrial Population Genomics Supports a Single Pre-Clovis Origin with a Coastal Route for the Peopling of the Americas. The American Journal of Human Genetics 82(3):583-592. doi:10.1016/j.ajhg.2007.11.013

Hoffecker JF, and Elias SA. 2003. Environment and archeology in Beringia. Evolutionary Anthropology 12(1):34-49. doi:10.1002/evan.10103

Hoffecker JF, Elias SA, and O'Rourke DH. 2014. Out of Beringia? Science 343:979-980. doi:10.1126/science.1250768

Hu A, Meehl GA, Han W, Timmermann A, Otto-Bliesner B, Liu Z, Washington WM, Large W, Abe-Ouchi A, Kimoto M et al. 2012. Role of the Bering Strait on the hysteresis of the ocean conveyor belt circulation and glacial climate stability. Proceedings of the National Academy of Sciences 109(17):6417-6422. doi: 10.1073/pnas.1116014109

Praetorius SK, and Mix AC. 2014. Synchronization of North Pacific and Greenland climates preceded abrupt deglacial warming. Science 345(6195):444-448.

Tamm E, Kivisild T, Reidla M, Metspalu M, Smith DG, Mulligan CJ, Bravi CM, Rickards O, Martinez-Labarga C, Khusnutdinova EK et al. 2007. Beringian Standstill and Spread of Native American Founders. PLoS ONE 2(9):e829.

Volodko NV, Starikovskaya EB, Mazunin IO, Eltsov NP, Naidenko PV, Wallace DC, and Sukernik RI. 2008. Mitochondrial Genome Diversity in Arctic Siberians, with Particular Reference to the Evolutionary History of Beringia and Pleistocenic Peopling of the Americas. The American Journal of Human Genetics 82(5):1084-1100. doi:10.1016/j.ajhg.2008.03.019