Cave mineral study yields mega-drought evidence

FRESNO — As Californians worry about the three-year drought dragging on another season, researchers say climate change could create much longer dry spells, lasting decades or even centuries.

Scientists from the University of California at Davis this month announced the first proof of such mega-droughts after studying mineral formations in caves along the Sierra Nevada.

The big droughts seem connected to rapid warming, especially in the Arctic. The dry spells occurred during a climate warm-up that started about 15,000 years ago after the last Ice Age, said Davis geochemist Isabel Montanez.

Scientists fear climate warming this century might similarly trigger mega-droughts that would devastate agriculture, increase wildfires and cut water supply in the nation's most populous state.

"We don't know why warming in the polar region creates drought here," Montanez said. "But it does happen. And some estimates show the warming will make Arctic sea ice disappear during summer months by 2020."

The three-year study on cave minerals by Montanez and doctoral student Jessica Oster was published Nov. 5 in the journal Earth and Planetary Science Letters.

Researchers took microscopic core samples at more than a dozen caves, including Moaning Cavern in Calaveras County and Bear Den Cave in Sequoia National Park. Analysis of changes in trace minerals established the age of stalactites and stalagmites and the lengths of droughts.

The idea of longer droughts is not far-fetched, said climatologist Mike Anderson in the state Department of Water Resources, which did not participate in the study. California is known for sporadic droughts throughout the past century.

"There's evidence of a 140-year drought around the year 1100," he said. "So, it actually does sound possible for mega-droughts to happen again."

Such a lengthy drought would herald drastic changes in the Sierra Nevada, one of the state's biggest natural assets, with millions of forested acres. Some animals and vegetation would move to higher elevations. Some might not survive.

The Sierra probably would be more prone to large, intense fires, said Anthony Westerling, a University of California at Merced researcher who has studied wildfire in the Sierra.

"There could be a sudden and dramatic shift," he said. "Areas that will burn in the Sierra would expand."

Montanez and Oster found proof of past mega-droughts in the mineral formations that grew over thousands of years after the last Ice Age. The formations were created by water seeping into caves during storms.

The water leaves trace chemicals such as uranium that it picks up as it passes through the air and soil. The caves are chemically suited to capture these chemicals in a mineral record because they occur in layers of marble — fossilized tropical reefs from perhaps 300 million years ago in the Pacific Ocean.

In such formations, a mineral, such as calcium carbonate, is drawn out of the marble as water drips from the cave ceiling and hits the floor. As the water dries, the mineral hardens into a stalactite, a cone hanging from the ceiling.

When the drips of water hit the cave floor, the remainder of the mineral begins to form rounded or conelike stalagmites.

In the stalactites and stalagmites, the trace minerals from the rain are deposited in distinct microscopic layers, representing up to a decade.

Generally, layers with higher levels of trace chemicals signify dry times; it means there was less rainwater entering the cave at the time.

"It's almost like tree rings," Montanez said. "We can't quantify the amount of precipitation, but we can see the shifts from wet to dry. Some dry times lasted for centuries."