Prehistoric Climate Changes Hidden Inside Cave Deposits

Prehistoric climate changes that took place thousands of years ago are apparently hidden inside Indian cave deposits, and may hold clues to better predicting future climate in the region, according to new research.

When we think of the climate, often our thoughts focus on the clouds and atmosphere, but scientists from Vanderbilt University are taking a different approach by looking at the world of stalactites and stalagmites. This team is pioneering a unique method that involves studying mineral cave deposits - collectively known as speleothems - to paint a more accurate picture of the prehistoric climate.

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, as well as precipitation and drought in the atmosphere. As water seeps down through the ground it picks up minerals - most commonly calcium carbonate - which leave behind layers of deposits that can later be studied.

Today, scientists can date these layers with extreme precision based on the radioactive decay of the elements uranium and thorium. Variations in the thickness of the layers are determined by a combination of the amount of water seeping into the cave and the concentration of carbon dioxide in the cave's atmosphere. Remarkably, these factors can provide a measure of how the amount of precipitation above the cave varies over time.

In addition, 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.

Described in the journal Geophysical Research Letters, lead author Jessica Oster and her colleagues made a detailed record of the last 50 years of growth of a stalagmite that formed in Mawmluh Cave in the East Khasi Hills district in the northeastern Indian state of Meghalaya.

Their results were consistent with the historical record, which suggest that reduced monsoon rainfall in central India has occurred when the sea surface temperatures in specific regions of the Pacific Ocean were warmer than normal. These naturally occurring "anomalies" in sea surface temperature are known as the El Niño Modoki.

"Now that we have shown that the Mawmluh cave record agrees with the instrumental record for the last 50 years, we hope to use it to investigate relationships between the Indian monsoon and El Niño during prehistoric times such as the Holocene," Oster said in a statement.

The Holocene Climate Optimum was a period of global climate warming that occurred between six to nine thousand years ago. At that time, global average temperatures were somewhere between four to six degrees Celsius higher than they are today - an increase climatologists say is likely in the future if greenhouse gas emissions continue to rise from human activity.

This study may provide important information about the behavior of the monsoon in the decades to come and how it may impact the 600 million people that live on the Indian subcontinent.

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