LONDON: Two major surprises greeted researchers in the Department of Geography and Environmental Sciences at Northumbria University when they returned to the UK from Saudi Arabia with samples of ancient stalagmites collected in caves northeast of Riyadh.

The plan was to use the samples to identify specific humid phases in central Arabia over the past million years or so which would have temporarily “greened” the more usually arid region, transforming the so-called Saharo-Arabian Desert Barrier into a land bridge across which many mammalian species ­—­ including early humans — would have made the journey out of Africa.

The initial surprise was that the first, commonly employed technique they used to date such samples did not work.

Two different radiometric dating methods are used to date the calcium carbonate laid down in stalagmites — uranium-thorium (U-Th) and uranium-lead (U-Pb) — which both measure the extent to which radioactive isotopes in the material have decayed.

“At first we suspected the stalagmites might be quite a lot younger, so we used the uranium-thorium method,” said Monika Markowska, a senior research fellow in the Geography and Environmental Sciences Department at Northumbria University in Newcastle, England.

“This method is good only for about 600,000 years, and we kept finding we couldn’t date anything.”

Then came the second surprise, revealed in a paper published in the journal Nature this month.

“So, we switched to the uranium-lead method and all of a sudden it revealed just how old these stalagmites were,” she said.

“There was this sudden realization in the lab: ‘Oh my goodness, these are 8 million years old.’ It was quite a moment.”

At a stroke, the story of life in central Arabia had been pushed back more than 7 million years.

Stalagmites, stalactites and “flowstones,” known collectively as speleothems, are created on the floors, roofs and walls of caves by rainwater dripping through from the surface.

As water passes through the soil or rock above a cave, it collects minerals, including calcium carbonate. In the case of stalagmites, over many millennia, these minerals form conical towers, slowly growing up from the floor of the cave.

The age and rate at which these stalagmites have grown can be accurately measured using various methods of radiometric dating, which calculates a material’s age by measuring the presence of radioactive elements.

Until recently, evidence of environmental conditions in Arabia extended back only 450,000 years. But in 2020, a paper published in the International Journal of Earth, Climate and Life Interactions reported that analysis of stalagmites collected from caves in southern Arabia — Yemen and northern Oman — had pushed the record back 1.1 million years.

In that time, analysis of the stalagmites revealed there had been no fewer than 21 “pluvial periods” — periods of increased rainfall, creating “numerous opportunities” for early humans to migrate out of Africa and into Eurasia, unimpeded by the more typical periods of arid weather and desert conditions that otherwise acted as barriers to early human dispersal.

But, as the authors of the latest research report in Nature said, “although numerous humid phases occurred in southern Arabia during the past 1.1 million years, little is known about Arabia’s paleoclimate before this time.”

The new research has now pushed back the region’s hydroclimate record a further 7 million years, in the process revealing much earlier “recurrent humid periods characterized by increased water availability and vegetation cover.”

The caves selected for the study, located in the Al-Sulb Plateau, northeast of Riyadh, had been identified by the Saudi Geological Survey.

“What drew us to this site was the fact that the modern landscape there is hyper arid, yet in the caves we see lots of evidence of stalactite and stalagmite growth,” said Markowska. “This means that it was wetter in the past because these just don’t form under the modern conditions.”

For “conservation reasons,” the paper gives only the general locations of the caves. However, the precise coordinates are available to some professional organizations and researchers from the Saudi Heritage Commission and the Saudi Geological Survey.

The story told by the 22 speleothems collected from the seven cave systems, all found within a 10 km radius, is an extraordinary one. Analysis of the samples has revealed a series of humid episodes over four distinct periods — between 0.86 and 1.37 million years ago (Ma), 2.01 and 2.29 Ma, 3.16 and 4.10 Ma, and 6.25 and 7.44 Ma.

This last humid episode occurred during what geologists classify as the Late Miocene. This is the period from which numerous fossils found further south, in the UAE, date — the remains of animals such as elephants, hippopotamuses, crocodiles, antelopes, hyenas and large carnivores, including saber-toothed cats, which have been unearthed in the Western Region of the emirate of Abu Dhabi.

These animals flourished there, according to a summary of decades of research published in 2012, thanks to “a very large river system flowing slowly through the area, along which was flourishing vegetation, including large trees.”

That river system may well have flowed from the interior of Arabia — and the fresh evidence from the caves northeast of Riyadh suggests the possibility that it was rainfall here that fed it.

“There is evidence in Arabia of rivers in the past, and certainly in the UAE fossils have been found where a river channel once ran,” said Markowska.

“But rivers can have very distant origins and the big thing about our paper is that for the first time we were able to determine that there was rainfall specifically in the center of the desert — evidence of actual rainfall in that area, rather than humidity from rivers that were sourced from distant locations.

“Before now, we knew a little bit about what was going on in the south of Arabia, that there were numerous humid phases going back over the past 1.1 million years. But we’ve been able to push that back to 8 million years, and this is for the center of the desert, where there was really no information before.”

The importance of this discovery is highlighted by the vast well of international specialist knowledge upon which the study drew. This is reflected in the list of the paper’s 31 co-authors, who include researchers and specialists from universities in the UK, Germany, Switzerland, Australia and the US.

It also includes the College of Tourism and Archaeology at King Saud University in Riyadh, the Kingdom’s Heritage Commission, Geotourism Department and the Saudi Geological Survey.

The paper thanks Prince Badr bin Abdullah bin Farhan Al-Saud, the Saudi minister of culture, for giving his permission for the research to be carried out, and acknowledges the vital role of the Heritage Commission in providing logistical and technical field support.

So far, as the paper makes clear, the “vertebrate fossil record of Arabia is scant,” with remains found only in two principal locations: The sandstone and mudstone Baynunah Formation in the UAE and the Nefud Desert in northwestern Saudi Arabia.

The fossils in the Nefud are “only” about 500,000 years old, and 6 million years younger than those from the Baynunah Formation, currently the only site where animal remains dating from the Late Miocene have been found.

But, said Markowska, the lack of fossil evidence from the same period in central Arabia “doesn’t necessarily mean there wasn’t fauna there at that time, but only that we haven’t found them yet.”

And, as the paper concludes, “considering our findings, it seems probable that movements between Africa and Eurasia would have taken place through the Arabian Peninsula, which, during humid phases, was likely characterized by well-watered grasslands and woodlands.”

Many of the animal species identified from fossilized remains in the UAE “would have been well suited for life in (central) Arabia during the wet episodes identified in our speleothem record. The scant fossil record aside, we suggest that Arabia probably acted as a hitherto unrecognized but important crossroad for biogeographic exchange between Africa and Eurasia over the past 8 million years.”

The researchers add that “the nature of these exchanges, and the exact role of Arabia in these, may only be elucidated with an improved fossil record.”

For now, a more pressing piece of research is underway, for the stalagmites of Saudi Arabia have more secrets to divulge.

Finding out exactly what plants and trees thrived during the wet periods over the past 8 million years is key to understanding what species of herbivorous mammals — and, therefore, the meat-eaters that preyed upon them — might have existed in the once greener, lusher Saudi Arabia.

“This vegetation issue is our big next research question,” said Markowska.

“We have been able to identify these humid periods, and to identify what the dominant mechanisms for causing them were — basically, more moisture coming up from the south, during monsoon seasons. But we’re also very interested in this vegetation story.”

As speleothems grow, “they trap tiny amounts of pollen within their structure, and now we’re going to see if we can extract some ancient pollen from the speleothems, which is a technique we specialize in at Northumbria.”

This is a painstaking effort, “which involves a lot of microscope work. You extract the material, put it on a microscope slide and then count and identify the different pollen grains you find. I work with other colleagues who are experts on (ancient) pollen, and together I think we should be able to unwrap this other side of the story.”