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Stalagmites, a Supervolcano, & 100K Years of Climate Data

Kim Cobb doesn't particularly enjoy spelunking. It's damp, dirty, often uncomfortable, and there's guano, lots of guano. Despite that fact, the Georgia Tech paleoclimatologist recently found herself studying stalagmites in the caves of northern Borneo.

Luckily, these caves weren't as dingy as others she's visited, and to top off the not-so-dreary experience, her team's work ended up being a complete success! By analyzing isotopes of oxygen stored in stalagmites, Cobb and her colleagues pieced together 100,000 years worth of past climate data from the Tropical Pacific. Their work has just been published in Science Express.

For the researchers, unraveling past climate secrets begins with rainfall. In Gunung Mulu and Gunung Buda National Parks in Borneo, raindrops seep down through cracks in the ground and eventually wind up dripping from cave roofs. Each splatter deposits a fractional amount of calcium carbonate on the subterranean floors, and in places where water drips repeatedly over many thousands of years, stalagmites slowly sprout, growing at a sloth-like pace of about one centimeter per millennium.

shutterstock_39866341.jpgEach stalagmite isn't just a repository of calcium carbonate, but also of information. The compound contains isotopes of oxygen, which presumably come directly from previous rainfall. By measuring the ratio of these isotopes, scientists can discern regional rainfall levels from the ancient past.

After compiling their models, the researchers found that their climate estimations synced up well with previously known periods of abrupt global climate fluctuations called Heinrich events. Six of these events occurred during the past 60,000 years, each separated by approximately 10,000 years. Fascinatingly, the team also found evidence of an abrupt and harsh drought about 74,000 years ago, around the same time that the Toba Supervolcano is thought to have erupted in Indonesia.

The Toba eruption was the largest in the past 2.5 million years, about five thousand times more powerful than the Mount Saint Helens eruption in 1980. In the first two years after its eruption, average regional temperatures dropped by as much as 22 degrees Fahrenheit! The study provides further evidence that the eruption caused a dramatic shift in climate patterns in southeast Asia and the Indo-Pacific, a change which lasted about 1,000 years. Many scientists believe that this shift may have reduced the then fragile human population to a mere 3,000 to 10,000 individuals, creating a genetic bottleneck, though this theory has come under fire of late.

As with any lengthy climate model of the ancient past, Cobb's isn't perfect. At a presentation at the University of Minnesota, she noted that small amounts of thorium polluted the samples, inhibiting absolutely precise dating of the stalagmites.

"This is a complicated record from a complicated place," Cobb said. "But it is very important in showing how you can use stalagmites to model climate mechanisms."

(Image: Mulu Caves via Shutterstock)