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Researchers link the end of the green Sahara to the mega-scales of Southeast Asia.
Physical evidence found in caves in Laos helps tell a story of a link between the end of the Green Sahara – when once heavily vegetated North Africa became a hyper-arid landscape – and a previously unknown megadrought that crippled l Southeast Asia 4000 to 5000 years ago.
In an article published today in Nature Communications, scientists from the University of California, Irvine, University of Pennsylvania, William Paterson University of New Jersey, and other international institutions explain how this transformation Major climate change has led to a shift in patterns of human settlement in Southeast Asia. , which is now inhabited by over 600 million people.
“In this study, we provide the first evidence of a strong link between the end of the Green Sahara and the failure of the monsoon in Southeast Asia during the Middle to Late Holocene period,” said co-author Kathleen Johnson, UCI Associate Professor of Earth System Science. . “Our high-resolution and well-dated balance sheet suggests a strong link between North Africa and mainland Southeast Asia during this time.”
To set a paleoclimatic record for the study, co-author Kathleen Johnson, UCI Associate Professor of Earth System Science, and other researchers collected stalagmite samples from caves in northern Laos. The specimens hold geochemical evidence of past climate change in the heavily populated Asian monsoon region. Credit: Amy Ellsworth
To create a paleoclimatic record for the study, Johnson and other researchers collected stalagmite samples from caves in northern Laos. In his UCI laboratory, they measured the geochemical properties of oxygen and carbon isotopes, carbon-14 and trace metals found in the samples. This helped them verify the occurrence of drought and extrapolate its impacts on the region.
Johnson said he combined data from the analysis of these stalagmite-derived proxies with a series of idealized climate model simulations – conducted by co-author Francesco Pausata of the University of Quebec at Montreal – in which Saharan vegetation and dust concentrations have been altered in a way. this allowed them to study the ocean-atmosphere feedbacks and teleconnections associated with such a sudden change in precipitation.
Modeling experiments suggest that reduced plant growth in the Sahara led to an increase in airborne dust which acted to cool the Indian Ocean and shift Walker’s circulation pattern eastward. , which caused it to behave similarly to modern El Niño events. This ultimately led to a sharp reduction in the humidity of the monsoon season in Southeast Asia that lasted for over 1,000 years, according to Johnson.
Anthropologists and archaeologists have previously studied the effects of the disappearance of the Green Sahara, also known as the African wet period, on population centers closer to West Asia and North Africa, noting the collapse of the Akkadian Empire of Mesopotamia, the de-urbanization of the Indus civilization (near present-day Pakistan and India) and the spread of pastoralism along the Nile.
But the link to the origin of the mega-drought in Southeast Asia and the lifestyle changes in the region had not been explored before, according to lead author Michael Griffiths, professor of science. environment at William Paterson University in New Jersey.
“Archaeologists and anthropologists have been studying this event for decades now, in terms of adaptations and societal upheavals, but its exact cause has escaped the scientific community,” said Griffiths, postdoctoral researcher supported by the National Oceanic and Atmospheric Administration in Johnson’s lab. and has been collaborating with her on this research topic for over 10 years.
“The results of this work provide a new and compelling explanation of the origin of the mega-drought in Southeast Asia and may help us better understand, to varying degrees, the societal changes observed in many parts of the tropics. and extra-tropics, ”he said.
Researchers suggest that the secular mega-drought corresponds to the “missing millennia” in Southeast Asia between 4,000 and 6,000 years ago, a time characterized by a notable lack of archaeological evidence in interior Southeast Asia by compared to earlier and later parts of the Holocene.
They suggest that the mid-Holocene mega-drought may have been a push for massive population movements and the adoption of new, more resilient livelihood strategies – and that it should now be seen as a possible driver. for the beginning of Neolithic agriculture in mainland Southeast Asia.
“This is exceptional evidence of the kind of climate change that must have affected society, what plants were available, what animals were available,” said co-author Joyce White, assistant professor of anthropology at the University of Pennsylvania. . “All of life has had to adapt to this very different climate. From an archaeological standpoint, this is really a game-changer in how we try to understand or reconstruct the Middle Holocene period.
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Reference: August 21, 2020, Nature’s communications.
DOI: 10.1038 / s41467-020-17927-6
The collection of speleothem samples from Laotian caves has been made possible through Johnson’s collaboration since 2010 with the Middle Mekong Archaeological Project, managed by White, a consultant researcher at the Museum of Archeology and Anthropology at the University of Pennsylvania. This project, which also involved researchers from the University of Oxford and the University of Northumbria in the UK, the Los Alamos National Laboratory and the University of New Mexico, was funded by National Science Foundation, NOAA, Swedish Research Council, and a grant from the Henry Luce Foundation to the Penn Museum.
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