The Arabian Desert, today the largest expanse of windswept sand dunes on Earth, experienced recurring periods of humidity millions of years ago, researchers report April 9 in Nature. The study may explain how mammals at that time survived the trek across what is now a vast and barren landscape.
The findings come from mineral formations deep inside caves beneath the Arabian Peninsula. These speleothems — stalagmites and stalactites, formed by dripping rainwater — provide evidence that the region underwent repeated humid periods stretching back nearly 8 million years. The scientists used uranium dating to precisely determine the ages of speleothem samples, offering one of the oldest climate records for the region.
“You go underground into one of these caves and run into speleothems … which to me are an immediate and clear indicator that the surface was wet,” says Hubert Vonhof, a paleoclimatologist at the Max Planck Institute for Chemistry in Mainz, Germany.
Speleothems need rainwater, vegetation and soil to form, so their existence is evidence of a once-green Arabia, Vonhof and colleagues say. And the formations grow only when rainfall regularly reaches the cave. By examining oxygen isotopes — variations of the element with different numbers of neutrons — in the speleothems, the team could estimate how much rain fell during specific periods.
Vonhof says that these humid periods were probably driven by slow, cyclical changes in the orientation and shape of Earth’s orbit over tens of thousands of years. These changes, in turn, alter the amount of sunlight reaching our planet at any given time.
A warmer North Atlantic would have shifted weather patterns and brought monsoons to Arabia from the south, Vonhof says. Such deluges created lakes, rivers and lush grasslands, inviting mammals and hominids to migrate there over time, says archaeologist Michael Petraglia of Griffith University in Brisbane, Australia.
As evidence, he points to the Baynunah Formation in the United Arab Emirates, where 7-million-year-old fossils reveal the presence of ancient hippos, elephants, giraffes and primates. Previously, it was thought that until a few hundred thousand years ago, the Arabian Peninsula occupied the center of an impenetrable desert barrier across North Africa and the Middle East. But the team’s new research challenges that scenario, Petraglia says, establishing the area as a verdant crossroads between Africa and Asia.
Anthropologist Miriam Belmaker, who was not involved in the research, connects two of the newly identified humid periods to the first appearances of hominids outside of Africa — in Georgia and Romania about 2 million years ago, and in Israel, Europe and China about 1 million years later. Green Arabia wasn’t the only possible route out of Africa at the time though, says Belmaker, of the University of Tulsa in Oklahoma.
The new findings fill a gap in our understanding of Arabia’s climate history, says Madelaine Böhme, a paleoclimatologist at the University of Tübingen in Germany not involved in the study. But she urges more research to get a complete picture of what the landscape looked like back then — and whether it could have functioned as a gateway to the wider world.