Recurrent humid phases interrupt an overall aridity trend in Arabia over the past 8 million years creating windows of opportunity for biogeographic dispersals

The Saharo-Arabian desert is part of the largest near-continuous chain of drylands stretching from north-western Africa to the northern China. This harsh and often hyper-arid belt acts as a transition zone separating major biogeographic realms, including the Palearctic, Afrotropics and Indomalayan. This aridity is thought responsible for the creation of unique geographic endemism between Africa and Eurasia. However, there are no direct hydroclimate records from the Arabian hyper-arid interior before the mid-Pleistocene, leaving the terrestrial hydroclimate and the role of Arabia as a biogeographic crossroads or barrier largely unknown.

We use desert speleothems preserved from the northern Arabian interior to identify past humid phases over the last 8 million years. These are particularly useful terrestrial climate archives as they act as underground rain gauges, requiring a minimum of ~300 mm a^-1 precipitation, pedogenesis and vegetation cover to form. Moreover, they can be accurately and precisely dated and are subsequently a valuable tool in identifying past large-scale hydrological and vegetation changes in ancient drylands. Our data reveal evidence of multiple ‘windows of opportunity’ of climate amelioration, allowing biogeographic exchange and dispersals to occur across the Arabian hyper-arid zone. Further, the novel analyses of the isotopic composition (d¹⁸O and d²H) of speleothem fluid inclusion waters, representing ‘fossil rainwater’, reveal the diminishing influence of tropical rain-belt precipitation in Arabia across Earth’s transition from a largely ‘ice-free’ northern hemisphere to an ‘ice-age’ world. The extent of Arabian aridity may thus be important in controlling biogeographic dispersals through the Arabian corridor, becoming increasingly less favourable through time. This is supported by fossil evidence which suggest that exchange between biogeographic regions across the Old World Savannah Biome were favoured in the Late Miocene, but became increasingly latitudinally fragmented from the Pliocene onwards. These results have significant implications for understanding the drivers of dryland aridity in non-polar deserts globally.