Hidden traces of Green Arabia

In the interior of the arid Arabian desert, 800 m above sea level atop the Arabian Shield, the early morning air is chill. It will still take a while for the sun to rise, filling the clear skies with light and beginning to heat the ground to more than 30 °C.
Hidden traces of Green Arabia

Most of the fieldwork – surveying, sampling, mapping and coring – is done in the morning hours until the early afternoon. At the height of the scorching heat, the rest of the day is taken for documentation, data organization and preparational lab work inside the cool compound of the Tayma Museum of Archaeology and Ethnography. Working in the remote oasis town of Tayma is no doubt challenging. However, participating in the large joint-archaeological excavation of the Saudi Commission for Tourism and National Heritage and the Orient Department of the German Archaeological Institute as a geoscientist is foremost – a truly rewarding experience.

Taking sediment cores inside the sabkha of Tayma. The floor is characterized by evaporitic minerals, in this case a gypsum-dominated buckled crust, with some teepee structures in the middleground.  The major escarpments of Palaeozoic sandstone appear in the background

The most rewarding experience was certainly the opportunity to explore the highest resolved sedimentary archive of the entire northern Arabian Peninsula, reflecting the substantial landscape changes of the Holocene Humid Period with annual precision. I was a graduate student in 2006 at the University of Marburg during our very first fieldwork season in northern Saudi Arabia. Back then, I had little idea when setting foot onto the sabkha (salt flat) north of the oasis town for the very first time, that I was walking over a former lake that had once covered the entire low-lying basin of c. 20 km² in a grass-dominated, steppe-type landscape, where today, barren hamada (stone pavement) surfaces prevail.

During the warmer phases of the Quaternary (the past 2.6 M years), the Sahara and the Arabian Peninsula experienced increased rainfall owing to an intensification and northward migration of summer monsoonal belts. These humid phases led to a greening of the Saharo-Arabian desert and facilitated prehistoric human migrations out of East Africa into Asia and Europe. The most recent of these humid periods occurred approximately between 11,000 and 5500 years ago. These humid phases have been reconstructed based on climate archives, such as lake sediments or speleothems, the fossil content or chemical composition of which accurately reflects past environmental changes. However, precise data for the northern Arabian Peninsula were so far widely absent.

One of the many outcrops of shoreline deposits identified along the margin of the sabkha basin indicating the extent of the palaeolake of Tayma during the Holocene Humid Period.
The shoreline deposits almost entirely consist of gastropod and barnacle shells.

At Tayma, multiple sediment cores were taken, since 2015 in the framework of the interdisciplinary CLEAR research project funded by Deutsche Forschungsgemeinschaft (DFG), with PIs from the universities of Heidelberg (then Cologne) and Jena, and the Hemholtz Centre Potsdam - GFZ German Research Centre for Geosciences. We identified annually laminated sediments in the sediment cores from the sabkha basin and thick shoreline deposits of a former lake along its margins. The laminated sediments inside the sabkha were sampled using sediment cores, and a chronology was derived using error-resistant radiocarbon data from concentrated pollen extracted from the entire stratigraphy of c. 6 m. By utilising a range of different geochemical proxies from the laminated sediments, such as stable isotopes (oxygen, carbon), n-alkanes from leaf waxes or total organic carbon, all of which sensitively respond to environmental factors such as evaporation, bioproductivity and rainfall, we found that the lake had been very short-lived and, thus, locally, the Holocene Humid Period only lasted less than 1000 years – much shorter than in the Sahara or the southern Arabian Peninsula. The sediment geochemistry also indicated that the most humid conditions at Tayma occurred c. 8200 years ago, at a time when climate archives from adjacent regions show a short-term dry spell during the longer humid phase, as reported in our paper published in Communications Earth & Environment. This regional diversity in climate conditions provided favourable conditions for Neolithic human populations to migrate into the interior of the Arabian Peninsula, e.g. from the Levant.

Sampling the annually laminated sediments inside the sabkha of Tayma.

Regional variability of the Holocene Humid Period is associated with complex patterns and changes of the atmospheric circulation. The reduced intensity of monsoonal rains, triggered by a cold phase in the North Atlantic (also known as the 8.2 ka event), only marginally affected Tayma, as the region was only at the northern fringes of the northward shifted monsoonal belt. Moreover, although rare in the present day, we assume that Tayma received additional rainfall during this period through intensified tropical plumes which have spatially small-scale effects. Tropical plumes are synoptical weather patterns also referred to as high atmospheric rivers that convey moist air from the tropics to the subtropics resulting in rainfall events which can last for several days. Our research demonstrates that an interaction of different atmospheric sources of rainfall has to be considered for the Saharo-Arabian Holocene Humid Period, and that this humid phase strongly varied in its timing and intensity at a regional scale. These findings have to be considered for both future research on prehistoric human migration at the crossroads between Africa, Asia and Europe as well as for (palaeo-)climate modelling.

Some moderate greening of interdunal depressions inside the Nefud sand sea after a rainfall event.

Even though we now know this Green Arabia probably only lasted for a very short time in the nothern part of the peninsula, having witnessed some degree of desert greening in the Nefud after a rainfall event in 2013, the image of the fundamentally transformed steppe-type landscape of Green Arabia during the Holocene Humid Period really became tangible in my mind’s eye. In addition to these truly intriguing findings, what resonates, is the experience gained from many seasons of fieldwork in a fascinating arid landscape which is characterized by spectacular eroding scarplands and the stunning beauty of the Great Nefud sand sea.