Tech Digest

Ancient stalagmite reveals information for U researchers about China's history

AN ANCIENT STALAGMITE taken from a cave in China has given two University researchers 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. Their findings were recently published in Science.

Lawrence Edwards, professor of geology and geophysics, and Hai Cheng, researcher in the Department of Geology and Geophysics, display stalagmites. Similar to growth rings in a tree, stalagmites are often composed of concentric layers that represent annual growth periods, and can help researchers tell which years were wet and which were dry, based on different types of oxygen found in the rock.

The work, conducted by Hai Cheng, University researcher in the Department of Geology and Geophysics, and Lawrence Edwards, professor of geology and geophysics, rests on climate records preserved in the layers of stone in a 118-millimeter- long stalagmite found in Wanxiang Cave in Gansu Province, China. The stalagmite started forming some 1,810 years ago, and also recorded information about the weather and the frequencies of monsoon events during the past almost two millennia.

When split in half length-wise, stalagmites expose wavy lines similar to growth rings in a tree. They can be considered nature’s time capsule, showing the ebb and flow of climate on the earth. By measuring amounts of uranium and thorium throughout the stalagmite, the researchers determined the date each layer was formed. They also were able to determine which years were wet and which were dry, based on different types of oxygen found in the rock.

The researchers discovered that periods of weak summer monsoons coincided with the last years of the Tang, Yuan, and Ming dynasties, which are known to have been times of popular unrest. Conversely, the researchers found that a strong summer monsoon prevailed during one of China’s “golden ages,” the Northern Song Dynasty.

“The waxing and waning of summer monsoon rains are just one piece of the puzzle of changing climate and culture around the world,” Edwards said.

Currently, China is also facing a descending monsoon strength trend that started in the 1960s. Some say that global warming is to be blamed, others point at carbon emissions, while a third group claims that man-made soot is responsible.