Stalagmite is scribe for monsoons, society

Cave formation has recorded monsoon strength in China since the third century

12:58pm, November 6, 2008
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A volleyball-sized stalagmite taken from a cave in northern China has given scientists insight about how the region’s precipitation has varied — and possibly influenced the rise and fall of various dynasties — for the past 1,800 years.

Researchers collected the telltale formation about 1 kilometer inside Wanxiang Cave, which lies along the eastern edge of the Tibetan Plateau. In recent years the area has received an average annual precipitation of 48 centimeters, with about 80 percent of that falling during the May-to-September monsoon season.

But rainfall amounts have varied significantly from year to year in recent centuries, says Larry Edwards, a paleoclimatologist at the University of Minnesota in Minneapolis. He and his colleagues analyzed an 11.8-centimeter-long section of the stalagmite and report their results in the Nov. 7 Science.

As rainfall from Earth’s surface seeps through rocks, it dissolves some minerals. If those waters later enter a cave, those minerals crystallize in a variety of cave formations as the waters evaporate (SN: 4/29/06, p. 266). Fluctuations in the ratio of oxygen isotopes deposited on the surface of a stalagmite as it grows reflect changes in the amount of precipitation the area received, Edwards says.

Lower-than-average proportions of the oxygen-18 isotope denote higher-than-normal amounts of rainfall for the year, says Edwards. Measurements of the radioactive isotope thorium-230 enabled the researchers to estimate the age of each of more than 700 samples extracted from the stalagmite that the team analyzed, Edwards notes.

Data indicate the formation had been growing continuously since 190, so its record of climate from then until the rock was collected in 2003 should be complete.

Extended droughts, possibly with significant social effects, struck the region around the cave several times in the past 1,800 years, the analyses suggest. A lengthy dry spell late in the 9th century often has been blamed for the decline of the Tang Dynasty and disunity in the years that followed, says Edwards.

Droughts in the late 14th and late 16th centuries coincided with the end of the Yuan and Ming dynasties.

On the other hand, a six-decade stretch of abundant rainfall late in the 10th century likely led to increased rice cultivation and a dramatic increase in population.

Droughts, especially extended ones, “can impose strong strains on a society” that subsists on agriculture, says Raymond S. Bradley, a paleoclimatologist at the University of Massachusetts in Amherst. Many archaeologists are hesitant to blame societal failures on environmental problems, often preferring to point the finger at political or societal causes, he notes. “The beauty of this record is that it shows that the environment hasn’t been a constant through time,” he adds.

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