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Deciphering clues to prehistoric climate changes locked in cave deposits

Posted by on Wednesday, May 27, 2015 in News, Research.

Watch the VU Inside on Jessica Oster here.

Read the Vanderbilt News story here.

When the conversation turns to the weather and the climate, most people’s thoughts naturally drift upward toward the clouds, but Jessica Oster’s sink down into the subterranean world of stalactites and stalagmites.

That is because the assistant professor of earth and environmental sciences at Vanderbilt University is a member of a small group of earth scientists who are pioneering in the use of mineral cave deposits, collectively known as speleothems, as proxies for the prehistoric climate.

Above, photo of the small stalagmite in the Mawmluh cave before it was collected. Below, a grayscale image of a slab of the stalagmite after it was prepared for analysis. The red lines show where the layers were counted. The green lines show where the material was dated and the numbers are the dates with the uncertainties of the measurements. (Courtesy of Jessica Oster)

Oster is investigating a cave in Tennessee to determine if it can be used as a proxy that can provide information about how the climate in the past has varied. It turns out that the steady dripping of water deep underground can reveal a surprising amount of information about the constantly changing cycles of heat and cold, precipitation and drought in the turbulent atmosphere above.

As water seeps down through the ground it picks up minerals, most commonly calcium carbonate. When this mineral-rich water drips into caves, it leaves mineral deposits behind that form layers which grow during wet periods and form dusty skins when the water dries up.

Today, scientists can date these layers with extreme precision based on the radioactive decay of uranium into its daughter product thorium. Variations in the thickness of the layers is determined by a combination of the amount of water seeping into the cave and the concentration of carbon dioxide in the cave’s atmosphere so, when conditions are right, they can provide a measure of how the amount of precipitation above the cave varies over time. By analyzing the ratios of heavy to light isotopes of oxygen present in the layers, the researchers can track changes in the temperature at which the water originally condensed into droplets in the atmosphere changes and whether the rainfall’s point of origin was local or if traveled a long way before falling to the ground.

The value of this information is illustrated by the results of a study published May 19 in the journal Geophysical Research Letters by Oster’s group, working with colleagues from the Berkeley Geochronology Center, the Smithsonian Institution National Museum of Natural History and the University of Cambridge titled “Northeast Indian stalagmite records Pacific decadal climate change: Implications for moisture transport and drought in India.”

Read the full article here.

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