North Africa with the Sahara and the Arabian Peninsula have repeatedly turned into green landscapes during past warm periods, most recently during the early to middle Holocene period about 11,000 to 5,500 years ago.
The reason for this was increased and northward shifted monsoon rains. Such climatically favourable phases probably made it easier for people to spread and migrate from East Africa to Asia and Europe. Information about the exact temporal course and intensity of this early Holocene wet phase in northern Saudi Arabia is now being provided for the first time by comprehensive studies of sediments from the today dried-up Lake Tayma. These were carried out within the framework of the DFG-funded project “CLEAR – Holocene CLimatic Events in Northern ARabia” by a multidisciplinary team of researchers from the German Research Centre for Geosciences Potsdam (GFZ) and the Universities of Heidelberg, Cologne, Jena, Berlin, Braunschweig and the MPI for Biogeochemistry in Jena and close a crucial regional data gap. Their results have now been published in the journal Communications Earth & Environment.
Significance of climatic humid periods for human dispersal from Africa
Periods of humidity lasting thousands of years are thought to have favoured the spread of humans out of Africa by creating “green corridors” through what is now the arid desert belt of the Sahara and Arabia. Research on the interaction between humans and climate in the Arabian Peninsula has intensified recently, because of the region’s high ecological sensitivity to climatic changes and its status as the geographical crossroads between Africa and Asia.
The ancient oasis settlement of Tayma lies on the fringe of the Nefud Desert and was already a central trading and communication point in the northwest of the Arabian Peninsula in the past. It is one of the best-studied archaeological sites in Saudi Arabia and is being intensively investigated by the Saudi Commission for Tourism and National Heritage (SCTH) and the Orient Department of the German Archaeological Institute (DAI).
Sediments of a dried-up desert lake as a unique climate archive
An important access to the climatic history of this region is now provided by the sediments of a lake that existed north of Tayma about 8000 years ago. These form a unique climate archive for this region, which has been made accessible in recent years. It is the best-dated and highest-resolved record in northern Arabia and provides detailed insights into the temporal course and intensity of various climatic phases. “While for the Sahara and the south of the Arabian Peninsula multiple evidences prove that there was a humid phase in the Holocene, 11,000 to 5,500 years ago, we had no knowledge so far about how long the humid phase lasted in the north and what climatic conditions Neolithic people encountered in this region. The lake deposits in the north of Tayma therefore fill a crucial gap,” says Ina Neugebauer, the lead author of the now published study. “Due to the immediate proximity to the archaeological sites in Tayma, there are unique opportunities for interdisciplinary research, which allow us to link the discovered environmental changes with the results on human settlement history.”
The publication results from the DFG-funded project “CLEAR – Holocene CLimatic Events in Northern ARabia”, in which a multidisciplinary team led by Max Engel (University of Heidelberg), Birgit Plessen (GFZ) and Peter Frenzel (University of Jena) investigated the lake sediments north of the Tayma Oasis (NW Saudi Arabia) using sedimentological, geochronological, geochemical and palaeontological methods. A pilot study from 2012 had already shown the excellent suitability of the deposits as a palaeoclimate archive, as the early Holocene sediments (approx. 10,000-8,000 years before today) even show annual lamination, so-called varves.
Diverse analysis of the cores from the lake sediments
Six-metre-long profiles of sediment cores were drilled from the palaeolake in 2011 and 2013 and were analysed in the sub-project on biogeochemistry and sedimentology led by GFZ using the latest sedimentological and geochemical methods. Numerous parameters relevant to climate, landscape and settlement were considered. The finely layered varve structure was subjected to a so-called microfacies analysis: Thin sections were viewed under the microscope and the varve sub-layers were counted and analysed in terms of thickness and composition. Minerals and algae, for example, can indicate the season in which they were built or washed in; calcium carbonate, in particular, precipitates during strong evaporation. From the thin light-dark lamination, annual rhythms can be read – as with tree rings – and information gained about how long different lake phases lasted. The analysis of pollen provides information about the vegetation history and dating of pollen using the radiocarbon method gives precise ages of the deposits. Humid phases with higher rainfall and lower evaporation can be identified by analysing hydrogen isotopes on leaf waxes of plant remains and by determining oxygen isotopes from carbonates.
Length and characteristics of the humid phase in the northern Arabian desert
In this way, the team was able for the first time to precisely determine the length and characteristics of the early Holocene humid phase in the northern Arabian desert. In particular, the researchers found that the humid phase was much shorter here than in the Sahara, at less than 1000 years. They also found that the recorded wettest conditions in Tayma around 8200 years ago contrast with the contemporaneous short-lived collapse of the humid phase in adjacent regions. “This resulted in particularly favourable conditions for Neolithic people to open up northern Arabia as a habitat,” explains Max Engel (Universität Heidelberg).
Explanation of the differences in the Sahara and Arabia by small-scale weather phenomena
The researchers explain the fact that this phase of humidity in the Saharan-Arabian desert varies so much from region to region with complex changes in atmospheric circulation. The briefly less heavy monsoon rains, triggered by a cold phase in the North Atlantic, the so-called 8.2 ka event (an event 8,200 years before today), may have led to the increased occurrence of tropical plumes in the north of Arabia. These plumes are rather rare today, they transport humid tropical air high in the atmosphere to certain subtropical regions, and might have compensated for the dry phase in northern Arabia. “This illustrates that small-scale weather patterns should be taken into account in (paleo-)climate modelling in order to be able to represent regional differences as well,” says Neugebauer.
- “The unexpectedly short Holocene Humid Period in Northern Arabia” Communications Earth & Environment 3, 47. DOI: 10.1038/s43247-022-00368-y.