GREEN SAHARA

Rapid end of the Green Sahara 8000 years ago 

9,000 years ago most of the Sahara was not the ultra-arid desert as we know it today. Due to higher precipitation it was covered by large lakes and savannah that were populated by herds of wild game. Scientists from the GEOMAR Helmholtz Centre for Ocean Research Kiel and the Royal Netherlands Institute for Sea Research (NIOZ) now have discovered that vegetation at the end of the Green Sahara disappeared much faster than previously assumed. This probably forced the Neolithic populations to start farming. The study has been published in the international open-access journal PLOS ONE. 

The Sahara is one of the most arid regions in the world. 9000 years ago, it was a green  savannah. Details on the transition could now be reconstructed from samples obtained  off the the mouth of the river Nile in the Mediterranean (yellow dot) [Credit: GEBCO world map 2014] 

Reconstructing the climate of the past is an important tool for scientists to better understand and predict future climate changes. This also applies to the relationships between human activities and environmental changes. In that respect, the Sahara Desert and the Nile valley are key areas since they underwent drastic environmental modification during the course of the last 10,000 years. In the same period, called Holocene by scientists, they were as well the scene of crucial steps of human evolution. At the beginning of the Holocene precipitation in northern Africa was much higher than today. The Sahara was a green savannah where large herds of wild game lived. But in the following millennia the region became one of the most arid areas on Earth. Now researchers from the GEOMAR Helmholtz Centre for Ocean Research Kiel and the Royal Netherlands Institute for Sea Research (NIOZ) have discovered that the vegetation in the Sahara changed much faster than previously assumed. “Though the changes in temperatures and precipitation needed millennia, the vegetation changed dramatically in the course of centuries or even decades”, says Dr. Cecile Blanchet. She is lead author of the study. The initial forcing of the climatic changes during the Holocene was a southward migration of the African monsoon and its associated rain belt. “But there are large regional discrepancies in timing and intensity of the changes, the underlying mechanisms of which remain equivocal”, explains Professor Martin Frank, palaeo-oceanographer at GEOMAR and co-author of the study. During the same epoch, Neolithic human populations also underwent important evolution, such as the initiation of food production and the sedentariness of previously mobile hunter-gatherer populations. 

Approximately 8,000 year-old rock carvings of two giraffes from Dabous, Niger,  documenting that the Sahara was a green fertile Savannah at that time [Credit: Albert Backer/WikiCommons] 

“Providing a more precise timeframe for the wet-dry transition at the end of the African Humid Period is therefore crucial to understand early human-environment relationships”, adds Professor Stefan Schouten from NIOZ, also co-author. For the study Blanchet, Frank and Schouten analyzed a marine sediment core collected off the mouth of the Nile River in 700 metres depth by the German research vessel POSEIDON. Using an innovative combination of geochemical methods they were able to detect traces of the vegetation and eroded soil which had been transported by the river down to the Mediterranean. This allowed them to precisely reconstruct the changes in vegetation, erosion and river runoff in the Nile watershed and their driving mechanisms during the past 9500 years. This high-resolution reconstruction shows that a rapid degradation of the vegetation occurred about 8000 years ago while the overall river runoff only decreased gradually. These asynchronous changes highlight the role of threshold mechanisms in controlling the timing of environmental changes. Furthermore, the rapid shifts in vegetation and sediment source occurred earlier than in other regional records and may have exerted a significant pressure on the Neolithic population in northern Africa. “Maybe these fast changes forced people to use domesticated cattle as a sustainable food resource instead of hunting and gathering”, says Dr. Blanchet. Finally the aridification may have forced most of the population to move to the fertile Nile valley, which ultimately gave rise to the development of the high cultures of the Egyptian Pharaonic Kingdoms. "Of course, further research is needed to confirm this relationship between climate and human evolution. But with our results, we have strong evidence that even a slow climate change can cause rapid and dramatic changes in the environment, which would surely impact human societies" says Dr. Blanchet. 

Source: GEOMAR Helmholtz Centre for ocean research [February 10, 2015] 

Migrazioni umane e sistemi fluviali

In fin dei conti, la logica già invitava  in quella direzione ormai da anni. Ma la ricerca sul campo di archeologi e geologi insieme (lavoro interdisciplinare!) dà i suoi risultati pratici: applicando nuove metodiche alle induzioni della logica, si sono trovati almeno tre fiumi che - circa 100.000 anni fa - servirono come 'vie verdi' iniziali, per attraversare il Sahara e per la prima fase della migrazione dell'Uomo fuori dall'Africa.

Hydrological models 

locate 

ancient human migration routes

Archaeologists and geologists have long hypothesized that major river systems flowed north through the Sahara Desert about 100,000 years ago.

Simulated probability of surface water during the last interglacial in North Africa
[Credit: PLoS ONE 8(9): e74834. doi:10.1371/journal.pone.0074834]

These rivers would have provided a sort of network of “green corridors” across the Sahara that early humans could have traversed as they migrated out of Africa. 

Ancient lake records, fossil river systems, and radioisotope data have offered evidence for the existence of flowing water in the region.

Now, a team of researchers has developed a novel paleohydrogeological and hydraulic modeling approach to help confirm the existence and location of these ancient rivers. Their research complements existing evidence that all three of these rivers could have provided the necessary water and food sources for migrating humans.

Tom Coulthard and Mike Rogerson of the University of Hull in England and their colleagues started their model by simulating the yearly rainfall over the Trans-Saharan Mountains about 100,000 years ago. Then they looked at the mountainous topography at the time and simulated where monsoon rains would have struck and drained toward the Mediterranean. These watersheds are buried by sand dunes today.

The simulation, Rogerson says, relied on basic hydrological representations of the flow and spread of water. The physical limitations are governed by a balance between the rate of water flux to the coast relative to the rate of the lateral spread of water and subsequent loss to evaporation.

The simulation is also extremely conservative, Rogerson says, in the sense that the team overestimated the parameters to obtain more robust probabilities for determining locations that were more humid and temperate.

The model provides a geographic distribution for regions within the Sahara coupled with degrees of certainty for the presence of water, he says. Even at the most conservative rainfall amounts, the team’s simulation consistently generated three river channels, the largest of which, during peak flow, is roughly the size of the Nile during low flow periods, the team reported in PLOS One.

Modeling paleoenvironments like this could more precisely pinpoint a location for archaeological field study, says Roger Blench, a developmental anthropologist with the Kay Williamson Educational Foundation in England who wasn’t involved with the new study.

However, Blench says, a modeling approach does not test hypotheses developed from in-field observations and is thus limited by its own parameters. Existing hypotheses should be backed up by satellite imagery coupled with traditional hands-on, in-field archaeological methods, he says.

Author: Darren Cheah | Source: Earth Magazine [November 01, 2013]