Stalagmites tell tale of 100,000 years of climate change

Earth's climate has gone through major changes in the last 100,000 years, and stalagmites found in caves in Borneo are revealing this climatic record for the South Pacific.

Evidence from ice cores taken in Antarctica and Greenland document the way our planet has changed over the centuries in those areas. But, until now, little was known about how climate changes affected the South Pacific region.

Researchers from Georgia Tech discovered that tropical regions of the South Pacific reacted quite differently to climate change than did other parts of the world.

From 1,700 samples of calcium carbonate collected from four stalagmites found in three caves in Borneo, researchers were able to determine that climatic change can be magnified and prolonged in the South Pacific.

They did this by studying oxygen isotopes in the collected samples, which reveal how much rainfall was occurring at different periods in time. Each of the pieces was sliced in half, and 25 samples an inch were drilled from their centers to be analyzed. Every sample drilled differed in age from the one next to it by 60 to 200 years, depending on how quickly the stalagmite formed. The rate of growth was determined by measuring the ratios of isotopes of uranium and thorium in the material.

The ratio of oxygen isotopes in stalagmites is determined by rainfall. As water drips into caves, it dissolves limestone. That material slowly builds up stalagmites at the rate of around one inch every 2,500 years.

"This is a new record from a very important area of the world. This record will provide a new piece of the puzzle from the tropical Pacific showing us how that climate system has responded to forcing events over the past 100,000 years," Kim Cobb, from the Georgia Institute of Technology, said.

One type of climate change process known as Heinrich events, which are associated with the last ice age, showed up well in the records, while another variety, Dansgaard-Oeschger excursions (D-O events), did not. These D-O events are brief warming periods during ice ages. This discovery was slightly surprising since both types of events clearly show in stalagmites found in China.

"To my knowledge, this is the first record that so clearly shows sensitivity to one set of major abrupt climate change events and not another," Cobb said.

The climate record of the stalagmites also show a distinct mark created 74,000 years ago when Toba, a super-volcano near Borneo, erupted.

The National Science Foundation funded the research. Results of the study were published online in Science Express, the electronic version of the journal Science.

Learning more about how various regions of the world respond to climatic change can help climatologists further refine computer models to better predict future changes.

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