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stalagmites

Cave holds clues to stormy weather 8,200 years ago

California had a 150-year stretch of unusually wet and stormy weather about 8,200 years ago, a study of stalagmites in White Moon Cave in the Santa Cruz Mountains suggests.

The findings offer hints to what would happen if global warming reaches a point where glaciers in Greenland and other parts of the globe melt rapidly enough to dump large amounts of fresh water into the ocean.

The wet weather appears to have included particularly intense winter storms and coincides with a climate anomaly in Greenland ice cores first detected in 1997. Before this “8.2 ka event” was discovered, scientists thought the world’s climate had been unusually stable during the Holocene, the geological epoch that covers the last 11,700 years of Earth’s history.

Since then researchers have associated the distinctive, 3.3-degree Celsius temperature dip in the Greenland ice cores with a catastrophic event: The drainage of two giant glacial lakes (Lake Ojibway and Lake Agassiz) located in northeastern North America caused by the collapse of massive ice sheet that covered much of the continent during the last ice age.

In short order, the two lakes dumped enough melt water into the North Atlantic to disrupt the world’s oceanic and atmospheric circulation patterns and raise the sea level by somewhere between two to 10 feet. The tremendous freshwater flood has been associated with an extended cold snap in Europe, increased drought in Africa, weakened monsoons in Asia, and strengthened monsoons in South America.

“This is the first high-resolution evidence of the response of the coastal California climate to the most distinctive event in the Holocene. Although the effects appear to have been less severe than in other parts of the world, it provides us with new information about the nature of this global climate event,” says Jessica Oster, assistant professor of earth and environmental sciences at Vanderbilt University.

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In her research, Oster uses mineral deposits in caves as proxies for the prehistoric climate. Cave formations, including stalagmites and stalactites, can provide valuable information about the climate for the last 600,000 years by using a built-in clock: Mineral deposits contain radioactive uranium-234 that decays into thorium-230 at a constant rate so the ratio of the two isotopes is determined by the date the mineral deposit formed.

Seasonal variations in water seepage produce layers that can be dated with considerable precision. The ratios of other isotopes in the minerals including oxygen and carbon provide information about the temperature and nature of the vegetation in the region at the time the layers formed. Concentrations of trace elements like magnesium, strontium, and phosphorus provide information about how wet the environment was.

“Events like this are particularly difficult to study because they are so brief,” Oster says. “Fast-growing stalagmites are particularly good for this purpose because they have very high temporal resolution.”

Oster is analyzing stalagmites from two California caves in order to shed new light on the factors that produced megadroughts in the region during the late Pleistocene and early Holocene. During her studies, she discovered a stalagmite that was growing rapidly just before, during and after the 8.2 ka event. An analysis of the oxygen and carbon isotope ratios and the concentrations of the trace elements phosphorus and magnesium in the mineral layers formed from 6,900 to 8,600 years ago, clarified what was going on in the prehistoric California atmosphere.

According to the paper published in Nature Scientific Reports, “…the new record suggests that the 8.2 ka event was associated with a brief period of wetter conditions, potentially arising from increased storminess, and demonstrates a near synchronous climate response to this event on both sides of the Pacific.”

Coauthors are from Vanderbilt; the Berkeley Geochronology Center; the Western Cave Conservancy; and Santa Clara University. The National Science Foundation funded the work.

Source: Vanderbilt University

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