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US Droughts May Last for Centuries, Cave Find Shows

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A 7.9-inch stalagmite from a West Virginia cave suggests that eastern North America experienced several century-long droughts over the past 7,000 years.

Ohio University researchers who examined the stalagmite found evidence of at least seven major droughts, according to an article published online in the journal Geophysical Research Letters.

It is the most detailed geological record to date on climate cycles of the region. And it confirms a theory that fluctuations in the sun’s activity every 1,500 years causes the cooling of the North Atlantic Ocean, which in turn impacts rainfall over North America.

“This really nails down the idea of solar influence on continental drought,” said Gregory Springer, the research team leader and an assistant professor of geological sciences at Ohio University.

The climate record suggests that North America could face a major drought event again in 500 to 1,000 years, the researchers said. However, manmade global warming could offset the cycle, they added.

Photo courtesy Greg Springer, Ohio University

Marine Geologist Gerard Bond first suggested that every 1,500 years weak solar activity caused by fluctuations in the sun’s magnetic fields creates more icebergs and ice melting, cooling the North Atlantic Ocean.

Results of the stalagmite study confirm the Bond events, the Ohio University team said, but also show that this climate cycle triggered droughts, including some that were particularly pronounced about 6,300 to 4,200 years ago. These droughts lasted for decades or even entire centuries.

Though modern records show that a cooling North Atlantic Ocean actually increases moisture and precipitation, the historic climate events were different, Springer said. In the past, the tropical regions of the Atlantic Ocean also grew colder, creating a drier climate and prompting the series of droughts.

“Global warming will leave things like this in the dust.” Springer said. “The natural oscillations here are nothing like what we would expect to see with global warming.”

Stalagmites are conical mounds formed on the floor of caves by mineral-rich water dripping from the roof. Stalagmites such as this one found in Buckeye Creek Cave provide an excellent record of climate cycles, Springer said, because West Virginia is affected by the jet streams and moisture from the Gulf of Mexico and the Pacific Ocean.

Other studies have gleaned climate cycle data from lakes, but fish and other critters tend to churn the sediment, muddying the geological record there, said study co-author Harold Rowe, an assistant professor of geological sciences at the University of Texas at Arlington.

“(The caves) haven’t been disturbed by anything. We can see what happened on the scale of a few decades. In lakes of the Appalachian region, you’re looking more at the scale of a millennium,” Rowe said.

In the recent study, the scientists cut and polished the stalagmite, examined the growth layers and then drilled out 200 samples along the growth axis. They weighed and analyzed the metals and isotopes to determine their concentrations over time.

Strontium occurs naturally in the soil, and rain washes the element through the limestone. During dry periods, it is concentrated in stalagmites, making them good markers of drought, Rowe said.

Carbon isotopes also record drought, Springer said, because drier soils slow biological activity. This causes the soil to “breathe less, changing the mix of light and heavy carbon atoms in it.”