Tomorrow’s issue of Science hosts lots of interesting papers, one of which is titled, “A Test of Climate, Sun, and Culture Relationships from an 1810-Year Chinese Cave Record,” and reports on the analysis of a 1.2-meter-long stalagmite from Wanxiang Cave in northern China. The analysis tells us that the rock holds records of waning Asian monsoon rains around 1,100 years ago. The dry spell was due to weakening of the sun, possibly from a sunspot, and this climate change is thought to have been what brought down the Tang dynasty.

Map of Wanxiang Cave, China

Map of Wanxiang Cave, China

Stalagmites are calcium carbonate mounds which form from dripping groundwater. Chemical analysis of this Wanxiang stalagmite told the researchers that there’s a lot of uranium and exceptionally low clay-borne thorium. That allowed them to do a uranium-thorium radiometric dating of the layered deposits down to an interval of 2.5 years. With such accuracy, the authors were able to calculate precise dates for variations in the stalagmite’s oxygen isotope composition. Oxygen isotope levels reflect variations in rainfall near the cave. The isotope levels match that of drought conditions.

Comparing this result to Chinese historical records of rainfall, the authors matched the chemical analysis to the written record. Furthermore, previously published climate record from a lake on the southern coast of China also confirm this. This 9th-century dry period is also thought to be what also doomed the Mayan civilization. Poor rainfall affected crops and the carrying capacity of each civilization.

    P. Zhang, H. Cheng, R. L. Edwards, F. Chen, Y. Wang, X. Yang, J. Liu, M. Tan, X. Wang, J. Liu, C. An, Z. Dai, J. Zhou, D. Zhang, J. Jia, L. Jin, K. R. Johnson (2008). A Test of Climate, Sun, and Culture Relationships from an 1810-Year Chinese Cave Record Science, 322 (5903), 940-942 DOI: 10.1126/science.1163965