14 Apr

Geographical Review Preview: Global Choke Points May Link Sea Level And Human Settlement At The Last Glacial Maximum

By: Jerome E. Dobson, University of Kansas; Giorgio Spada and Gaia Galassi, University of Urbino “Carlo Bo.”

What is the main purpose of your study?
To investigate the transportation potential of the world’s global choke points at the Last Glacial Maximum (LGM) – 20,000 years ago – when ice sheets were most extensive and sea level was 125 m. lower than today.

Figure 1. Sea level in the Bering Strait at the Last Glacial Maximum (20,000 years ago) versus today. Note the intricate archipelago that was present 20,000 years ago, but not today.  Its islands (outlined in red) might have served as stepping stones for the first settlers crossing from Asia to North American.


What are the practical, day to day, implications of your study?
We studied nine global choke points – Bering Strait, Isthmus of Panama, Bosporus and Dardanelles, Strait of Gibraltar, Straits of Sicily and Messina, Isthmus of Suez, Bab al Mandab, Strait of Hormuz, and Strait of Malacca – and found stunning revelations for each of them. Changing sea levels dramatically altered seven of nine global choke points and impacted regional and global transportation potentials. At each choke point we found evidence commending submerged sites for future reconnaissance by underwater archaeologists. Our results thus promote enhanced capability across all relevant disciplines to explore the seafloor in strategic and evidence-based manners.

Figure 2. Sea level surrounding the Isthmus of Suez at the Last Glacial Maximum (20,000 years ago) versus today. Note that the Gulf of Suez was dry land 20,000 years ago, making the portage 3.5 times as long as today’s crossing.


How does your study relate to other work on the subject? 
This study is unique; no one has attempted to map global choke points for times when sea level was lower.  We are able to do so because glacial isostatic adjustment (GIA) models – which account for the Earth’s deformation and gravity variations in response to glacial melting – yield greatly improved spatial and temporal resolutions for maps of ancient sea floors.

What are two or three interesting findings that come from your study?
 In the Bering Strait we discovered a new alternative pathway for the first Americans crossing from Asia to Alaska (Figure 1).  Science writer Fen Montaigne, writing in Smithsonian Magazine, calls this crossing, “one of the greatest mysteries of our time . . . when humans made the first bold journey to the Americas.”  Through improved seafloor modeling we found scores of previously undetected islands that likely existed 30,000 to 8,000 years ago and would have served as stepping stones along the way.

On the Isthmus of Suez we found that the portage between the Red Sea and Mediterranean Sea, where the Suez Canal lies today, would have been 3.5 times as long at LGM as it was just prior to construction of the canal (Figure 2).  It likely would have been displaced by a western route from Foul Bay, Egypt to the first cataract of the Nile, thence downriver to the Mediterranean Sea.

The Black Sea was cut off from the world ocean as sea level dropped below the Bosporus and Dardanelles (Figure 3). Rather than today’s saltwater channel of 300 km, there was an overland route of 350 km, one third of which was a deep lake now submerged beneath the Sea of Marmara. In response, now submerged settlements may have existed west of the current mouth of the Dardanelles, offshore near the eastern end of the Gulf of Soros, and beside the eastern and western ends of the Sea of Marmara.

Figure 3. Sea level in the Bosporus and Dardanelles at the Last Glacial Maximum (20,000 years ago) versus today. Note that 20,000 years ago one third of the route from the Black Sea to the Aegean Sea was occupied by a deep lake, which would have interrupted the portage and might have encouraged human settlement at each of its ends.


What might be some of the theoretical implications of this study? 
GIA modeling joins Radar, Sonar, and Lidar as means of mapping and imaging three-dimensional earth surfaces. GIA modeling is ideally suited to the needs of research focusing on aquaterra – all lands inundated and exposed repeatedly during the ice ages.  We anticipate that dramatically new findings like ours will come rapidly and will impact the theoretical cores of many disciplines. An oceanographer’s knowledge and expertise are essential to understand aquaterra’s complex physical systems. A geographer’s knowledge and expertise are essential to understand its complex infrastructures, networks, economies, and cultural systems. An underwater archaeologist’s skills and perspectives are essential to find definitive evidence. A host of diverse fields and disciplines–geologists, biologists (especially evolutionary biologists), microbiologists, nautical engineers, spatial statisticians, and many others–will be essential to address issues ranging from detection to forensics to seafloor access. Collaboration is key.

How does your research help us think about Geography?
Mapping and understanding ancient landscapes are the very essence of geography.  Now oceanographers, underwater archeologists, and anthropologists can envision long lost seacoasts, seaside settlements, and ancient ports.  Ultimately, this new knowledge will help scientists of many disciplines and the public understand the global feature aquaterra and its role in human advancement and history.

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