Cave formations record Black Sea deluges

Noah, better get more arks. Over hundreds of millennia large floods drowned the Black Sea and its environs over and over again, a new study shows.

Using chemical cues stuck in cave stalagmites, an international team of researchers has drawn up the most complete record to date of Black Sea flooding. And the sea’s history is stormy indeed, researchers report online March 13 in Nature Geoscience. The Mediterranean may have rushed over Turkey and into the Black Sea at least 12 times over the past 670,000 years.

“It’s a very dynamic region,” says study coauthor Dominik Fleitmann.

Dynamic, but until recently not well studied. Fossils and mineral samples from deep-sea cores hinted at the Black Sea’s floody past, but available tools couldn’t accurately date those deluges, says Fleitmann, a geologist at the University of Bern in Switzerland. The solution, it seems, lay hundreds of meters into a sandy-brown and drippy Turkish cave called Sofular about 10 kilometers south of the Black Sea.

As water evaporates off the Black Sea, a massive body of water today about two kilometers deep, it forms clouds, leading to rain and puddles in the area nearby. Thousands of years of puddles dripping down into Sofular Cave formed stalagmites, the spiky formations that stick up from cave floors. “Stalagmites are petrified precipitation,” Fleitmann says. Like striped Popsicles, some of these petrified rainstorms contain easy-to-date layers with distinct chemical flavors: Oxygen atoms from evaporated salt water tend to weigh more than those from fresh water. So the stalagmites in Sofular Cave give a record of the saltiness of the neighboring Black Sea going back hundreds of thousands of years, though with gaps of tens of thousands of years in places.

According to this Popsicle-flavor chronology, the Mediterranean Sea frequently overflowed the Bosporus Strait, the narrow channel connecting the Black and Aegean seas near Istanbul, mixing marine and fresh waters. The Caspian Sea, which today sits hundreds of kilometers to the east, also dumped into the Black Sea at least seven times. It all had to do with glaciers, Fleitmann says: “When you start melting the ice, global sea level rises. And the same was true for the Caspian Sea.” Meltwater filled oceans and the Caspian Sea to the brim, and the overflow then gushed into the Black Sea.

As for the coming flood (sea levels across the globe continue to climb), the Black Sea won’t turn fresh any time soon, Fleitmann says. “The Black Sea will remain a brackish sea.”

“These cave deposits are extremely interesting data,” says William Ryan, a marine geologist at Columbia University’s Lamont-Doherty Earth Observatory in Palisades, N.Y. He’s already started using Fleitmann’s timeline in his own Black Sea research.

Flood studies aren’t interesting just for cradle-of-civilization fans. Geologists can infer a lot about ancient Eurasian glaciers from these records. Biologists, too, will be able to explore how the region’s plants and animals responded to the frequent soakings. “The faunal assemblage has changed dramatically between the lake and the marine times,” he says.

Biblical historians may want to take note as well. Ryan and colleagues previously proposed that the most recent Black Sea flood may have been literally biblical, with sea waters surging into Turkey in a relatively short time span. The theory gained a lot of attention because this disaster about 7,000 years ago could have inspired flood legends like Noah’s Earth-drowning deluge. Fleitmann, however, thinks this Mediterranean spillover was more of a trickle. “I think it was a gradual change,” he says.

So Noah would’ve had plenty of time to build his ark.