Where deserts characterize the landscape today, it was once green in North Africa and Saudi Arabia. Examination of sediments from a dried-up lake now provides more precise insights into the climatic development in the contact area of these two regions, which is important for the spread of humans. Accordingly, there was only a short friendly phase in the north-west of the Arabian Peninsula as part of the large-scale Saharan Arabian wet period 11,000 to 5500 years ago. According to the researchers, the area could have served Stone Age people as an important green corridor between North Africa and Asia, especially during the approximately 200-year peak phase of these favorable climatic conditions.
In addition to geological traces, rock drawings by prehistoric people make it clear: where the sun burns down on the bare ground today in the Sahara and on the Arabian Peninsula, there were once bodies of water and green landscapes. Increased monsoon rains that have shifted northwards are considered to be the reason for the repeated wet climate phases in the greater Saharan-Arabic region. The last time such a wet period occurred was in the early to middle Holocene, around 11,000 to 5,500 years ago. It falls into an important phase of human development. It can be assumed that during this time green corridors offered people opportunities to expand in what is now a dry desert belt. But so far there has been a lack of climate history data from a region that probably played a particularly important role: the north-west of the Arabian Peninsula, which lies in the bridge area between Egypt in north-east Africa and Asia.
Desert lake as a climate archive
In order to gain insights into the exact course over time and the intensity of the wet phase in the northwest of the Arabian Peninsula, the researchers led by Ina Neugebauer from the Helmholtz Center Potsdam – German Research Center for Geosciences examined sediment cores from a dried up lake in the region. About 8,000 years ago, the body of water was north of the ancient oasis settlement of Tayma, which is considered one of Saudi Arabia’s best-studied archaeological sites. According to finds, the settlement played a central role as a trading and communication point in the region early on.
The six meter long drill cores from the former lake bed were examined using the latest sedimentological and geochemical methods in order to obtain information on climate development. Structures reminiscent of tree rings are emerging in the layers, say the researchers. The analysis of pollen grains was able to provide information about the former vegetation history in the area of the lake and radiocarbon dating enabled the chronological classification. The wet phases with higher precipitation and lower evaporation were also reflected in the analysis results of hydrogen isotopes of the leaf waxes of plant residues in the sediment.
Special conditions in a node region
As the team reports, the results show that there were strong regional differences during the large-scale Saharan-Arabic wet phase: A wet climate shaped the area of the lake only 8800 to 7900 years ago and was therefore much shorter than the generally defined one Holocene wet period. “While there is a wide range of evidence for the Sahara and the south of the Arabian Peninsula that there was a wet phase in the Holocene from 11,000 to 5500 years, we had no knowledge of how long the wet phase lasted in the north and what climatic conditions Neolithic people in have found in this region. The lake deposits in the north of Tayma are therefore now closing this crucial gap,” says Neugebauer.
In detail, the researchers also identified a two-century-long peak moisture in the area of the lake, which, interestingly, occurred at a time when a hundred-year-old dry anomaly interrupted the Holocene wet period in the adjacent regions about 8,200 years ago. “For the people of the Neolithic Age, there were probably particularly favorable conditions during this time to develop northern Arabia as a habitat,” says co-author Max Engel from Heidelberg University.
The fact that this phase was regionally so different in the greater area of the Sahara-Arabian desert apparently had to do with complex changes in the atmospheric circulation. As the researchers explain, 8,200 years ago a cold phase in the North Atlantic shaped the weather in the greater region. It could also have led to a special regional effect: The tropical cloud plumes, which are rather rare today and transport humid tropical air high in the atmosphere to certain subtropical regions, could have reached northern Arabia, the researchers explain. “This makes it clear that small-scale weather conditions should be taken into account in paleo-climate modeling in order to also be able to represent regional differences,” concludes Neugebauer.
Source: Helmholtz Center Potsdam – GFZ German Research Center for Geosciences, specialist article: Communications Earth & Environment, doi: 10.1038/s43247-022-00368-y