Prehistoric Transoceanic Navigation into America

Icame across a very interesting paper today, its title is revealing: Modelling pre-historic transoceanic crossings into the Americas (Alvaro Montenegro, Reneé Hetherington, Michael Eby, and Andrew J. Weaver. Quaternary Science Reviews, Vol, 25, Issues 11–12, June 2006, pp. 1323-1338 https://doi.org/10.1016/j.quascirev.2005.11.008, 🔒), and it explores the time it would have taken pre-Hispanic navigators to reach the New World from Europe, Asia, Africa and Oceania.

The abstract summarizes this notion as follows:

"A method for simulating vessel movement in an oceanic setting is described and used to provide temporal and spatial constraints to potential pre-historic transoceanic crossings into the Americas. The technique is based on current and wind output from numerical models and on leeway drift estimates from the United States Coast Guard. Simulations were carried out for present day (PD) and last glacial maximum (LGM) wind and current conditions. The fastest crossing time for the PD no-paddling experiments occurs between Australia and New Zealand in 23 days. The fastest transoceanic crossings occur between Central Europe and Iceland/Greenland in 72 days, Japan and North America in 83 days and northern Africa and South America in 91 days. The crossing with the highest probabilities of occurrence under 180 days occurs between southern Africa and South America (13%). This increases to 18% when paddling is allowed. Crossings from northern Africa to South America, from Australia to New Zealand and from New Zealand to South America have probabilities of occurrence ≥5%. Differences between LGM and PD crossing times and rates of occurrence range between 25% and 30%, indicating that the PD results serve as a rough estimate of LGM crossings. Mid-latitude crossings are of shorter duration and more probable during the LGM."

It is interesting to see that the NZ to South America transpacific crossing has chances higher than 5%, yet, it is lower than the 13% to 18% probability for a South Africa to South America crossing.

Another paper I found (Royer, T.C., and B. Finney. 2020. An oceanographic perspective on early human migrations to the Americas. Oceanography 33(1):32–41, https://doi.org/10.5670/oceanog.2020.102. 🔓) investigates the navigation of the North Pacific during the period that is now recognized as the peopling-window from Asia into NW America. Of course it only contemplates the end of the last Ice Age, and the LGM, but its analyisis is sound from a geoscience point of view, even if I disagree with the timeline it takes from anthropologists and archaeologists.

This paper "on the roles that oceanographic conditions played in initially helping and later hindering these early marine travels to the Americas.", and asks about the currents, boat technology, haul-outs and stopping points, and also something I hadn't heard about before, "forebulging".

Forebulges

I was familiar with isostatic rebound, when the ice of the glaciers melted, the enormous weight of the ice sheet vanished, and the continent rose like a cork floating on the undelying mantle. This was almost immediate following deglaciation, but the process still continues at roughly 1 cm per year uplift (roughly 0.4 in per year).

Forebulges is the reaction of the Earth's crust to an advancing ice sheet. As the weight of the ic pushes down the crust beneath, the crust in front of the ice sheet rises, uplifted and bulging. This could have made the land around the frontal area of the ice sheet rise by more than 100 meters (330 ft.). The seafloor could have risen even more than the lower sea levels caused by the Ice Age (which locked up millions of gallons of water as ice, on top of the continents).

Glacial rebound and forebulge. Source

The paper's remark that "...recognizing that sea level was at least 120 m lower than it is now, combined with a possible uplift of the seafloor, allows that there could have been sufficient areas of dry land available for island-hopping mariners in skin boats to transit the coastline of Alaska and British Columbia at about 16,000 yr BP." This is interesting, because skin boats are flimsy and not suitable for long open sea voyages, and also because these same conditions would have existed in any of the previous Glacial Maximums, and could have been used by sailors to cross from Asia to the West Coast of North America.

The glacial maximums correspond with the even-numbered Marine Isotope Stages or MIS:

• MIS 2: The Last Glacial Maximum (LGM), which peaked around 22,000 years ago
• MIS 4: Started ~71,000 years ago.
• MIS 6: or the Penultimate Glacial Period, started ~191,000 years ago.
• MIS 8: Started ~300,000 years ago, when we believe the first Homo sapiens appeared in Africa. Before this date we only had Neanderthals and possibly Denisovans.
• MIS 10: Started approx. 374,000 years ago.
• MIS 12: Started some 478,000 years ago.
• MIS 14: Started around 563,000 years ago.
• MIS 16: Started ~676,000 years ago.
• MIS 18: Started about 761,000 years ago.
• MIS 20: Started ~814,000 years ago.

The analysis mentions that after 15.000 years ago, it would have been more difficult for those sailing from Asia, to reach America due to rising sea levels, and stronger counter currents (east-to-west). Only after deglaciation did the current coastal islands appear along the coast of Alaska and Canada allowing new arrivals some 11.300 years ago. This is interesting! It could indicate why there were differnt peopling pulses into America, and also the reason for some back migrations into Siberia.

The paper also gives an idea on the rapid changes of that time: "A hiatus in the use of this eastward and southward waterway probably lasted for several thousand years, prohibiting new settlers to the Americas but allowing northward passage back to Beringia. It seems likely that early migrants to the Americas were victims of a severe climate change when sea levels rose at 40–50 mm yr–1 for 340 years, more than an order of magnitude greater than our present-day sea level rise of about 3 mm yr–1."

Again, this would have happened during any of the previous deglaciation events.

Finally, it suggests some actions that could help learn more about this matter:

• Do shell middens, stone fish weirs, and structures exist beneath meters of glacial sediments, particularly within submerged flow channels?
• Can sub-bottom seismic profiling and coring be used as effective tools to investigate for possible prehistoric underwater archaeology features (e.g., Dixon and Monteleone, 2014)?
• Can local sea level curves, elevational models, and archaeological site prediction models be used to locate subaerial coastal sites documenting early human occupation (e.g., McLaren et al., 2019)?

I really like the pragmatic attitude of earth-science scholars, more concrete than the findings of those studying DNA (less algorithms and computer models, and more "real" data).

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