A Bridge to Another Land

After looking at how changing sea levels are affecting humanity today, I now want to look at how these sea levels fluctuations affected past humans, allowing us to mould and alter the ecosystems unlocked to us.

During the Quaternary climate fluctuated between warm interglacial and cool glacial periods, during these glacial events water was locked up to various amounts in global ice sheets. This caused varying proportions of water allocated to storage in oceans. When glaciers expanded, ocean shorelines retreated, and in areas of low ocean depth the sea floor was exposed, in some cases creating land bridges between major continents and islands. These land bridges played a large part in the migration of early humans and our population across all major continents of the globe.

The Bering Land Bridge was an ancient land bridge approximately 1000 miles at its widest, which connected Asia to North America. It was in existence at various times during The Pleistocene ice ages, the series of glacial events during the Quaternary from 2.58 Ma to present. At these times sufficient water was locked up in permanent and fluctuating global ice sheets that sea level dropped low enough to expose the sea floor. Sea levels worldwide were thought to have at some times been lowered by as much as 120 meters.

Specifically it is thought the Bering Land Bridge was exposed during Oxygen Isotope Stage 3 (OIS3), between 60,000 and 25,000 cal BP, and cut off during Oxygen Isotope Stage 2 (OIS2).



Animation showing an approximation of the changing coastline of Beringia from 21,000 Cal years BP to present.
Source: NCDC

Many other land bridges were exposed in the same way during The Quaternary, such as the connection between Australia, New Guinea and Tanzania, as well as the dry beds of the English Channel and North Sea between The British Isles and mainland Europe.

The Bering land bridge, known as Beringia, is of particular interest as it is the suspected route of migration to The Americas from Asia approximately 21,000 Cal years BP. It was previously thought people simply left Siberia, crossed the Bering Land Bridge and passed onto the Canadian land mass through an ice free corridor. However recent investigations indicate that this ice free corridor was blocked between 30,000 and 11,500 Cal years BP. Archeological findings in Northern America and Beringia suggest that migrants may have lived on the Bering Land Bridge for millennia whilst their path was blocked by advancing ice sheets in Siberia and Canada.

Pollen studies used to reconstruct the climate of Beringia have suggested that between 29,500 and 13,300 Cal years BP it was cool and arid and dominated by a herb-grass-willow tundra.

This land bridge was completely inundated by rising sea levels, caused by melting ice sheets, sometime between 10,000 and 11,000 Cal years BP, as the climate began to give way to the warmer interglacial period we are experiencing today. Its current depth was reached approximately 7,000 Cal years BP.

Most evidence of Beringia has long since been wiped from the surface of the Earth, and whilst we now no longer require such land bridges to cross between continents, it is clear that in the past these strips of land played a defining role in shaping humanity's current presence across the globe as well as the major effects we've has shaping its surface.


 
References
 
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.
 
D.M. Hopkins, et al. (1982). "Paleoecology of Beringia", New York: Academic Press.
 
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.