The green past of the Saharo-Arabian Desert

• The Saharo-Arabian Desert experienced repeated wetter periods over the past eight million years.
• Wetter conditions favoured the exchange of mammals between Africa and Eurasia.
• Fossilised rain water reveals monsoon rains reached Arabia in such wet periods.

The Saharo-Arabian Desert is one of the largest biogeographic barriers on Earth, hindering the dispersal of animals between Africa and Eurasia, and is at least eleven million years old. How did water-dependent mammals, including our early ancestors, manage to cross this inhospitable desert in the past?

A new study published in the journal Nature shows that Arabia repeatedly experienced time periods of higher precipitation during the last eight million years and was presumably vegetated. According to the study, these wetter periods probably supported migrations of water-dependent animals, including our ancestors. Wetter conditions were likely sustained by monsoonal precipitation, coming from the South, a source of rainfall which gradually weakened over millions of years.

This was revealed by an international team of researchers led by the Max Planck Institute for Chemistry in Mainz, with the support of the Saudi Heritage Commission and the Saudi Ministry of Culture, and with the participation of the Johannes Gutenberg University in Mainz and the Goethe University in Frankfurt.

On the basis of isotope analyses of stalactites and stalagmites (speleothems) from seven Saudi Arabian caves, the team identified multiple past intervals during which the climate of Arabia repeatedly was much wetter than it is today. Such wetter phases may have spanned thousands of years each, and transformed the Arabian landscape from a dry desert to habitable landscape.

Short humid intervals favoured the exchange of mammals between Africa and Eurasia

Hubert Vonhof, group leader at the Max Planck Institute in Mainz and co-author on the new study, said: “The repetitive recurrence of wetter conditions on the Arabian peninsula is not only of climatological importance. As the aridification of the Saharo-Arabian desert intensified over the past eight million years, these short intervals of wetter conditions became increasingly important for enabling mammalian exchange between Africa and Eurasia, likely including dispersals of our human ancestors.”

Faisal al-Jibrin, lead Saudi archaeologist of the Heritage Commission, said “Arabia has traditionally been overlooked in Africa-Eurasia dispersals, but studies like ours increasingly reveal its central place in mammalian and hominin migrations.”

“Although it was clear already from fossil finds that water-dependent animals like crocodiles and hippos lived on the Arabian Peninsula in the past, longer Saudi Arabian paleoclimate records such as speleothems were simply not available until now. We were able to study the hydroclimate of the Arabian Peninsula more comprehensively than ever before and found that during the last eight million years, a southward displacement of Monsoon rains gradually decreased rainfall during the wetter intervals. As a whole, the Arabian peninsula became increasingly drier,” says Monika Markowska. The first author of the paper worked as Postdoc at the Max Planck Institute for Chemistry and is now a Senior Research Fellow of the Royal Society at the University of Northumbria in England. According to the geochemist, the loss of monsoonal rains over Arabia was ultimately caused by cooling of the Northern hemisphere, which displaced the monsoonal rain belt to the South.

BACKGROUND

Speleothems document temperature and precipitation of the past

Limestone caves are excellent climate archives because the chemical composition of the calcium carbonate in the speleothems changes with climate above the cave. By analysing the deposits, researchers can directly determine past climate with temperature and precipitation patterns at the time the speleothem was formed. Speleothems only form when sufficient rainwater permeates the soil, dissolving calcium carbonate from limestone rock. This water then enters the underlying cave, where calcium carbonate crystallizes again, depositing layer by layer on the ceiling or floor.

In collaboration with specialists from the Johannes Gutenberg University in Mainz and the Goethe University in Frankfurt, the researchers successfully determined the age of the limestones through radiometric dating. This technique relies on the radioactive decay of naturally occurring uranium isotopes that were carried into the caves by water and subsequently deposited in the limestones. By identifying both the original and decay isotopes, the age of the calcification can be established.

Fossilised water indicates monsoon rainfall

Tiny rainwater inclusions within the speleothems enabled palaeoclimate researchers to ascertain that the rainfall likely originated from the monsoon, specifically from the south. The isotopic composition of oxygen and hydrogen in the water reveals the geographical region of its origin.