A HISTORICAL EVENT IN A STALAGMITE FROM YANGZIPO CAVE, GUIZHOU, CHINA
贵州羊子坡石笋记录的历史事件*

This article reports on the study of an active stalagmite 18.6cm long and ca.5cm wide that was recovered 180m into Yangzipo Cave, about 30km southwest of Guiyang City, the capital of Guizhou Province in Southern China in December 1999. The stalagmite was cut into two halves along the central growth axis with a band saw, and four large (51mm×76mm) thin sections were cut so as to minimize the loss of petrographic information. The pertographic study was conducted under a Leitz Laborlux 12 Pol Petrographic Microscope in the University of Georgia Sedimentary Petrography Lab and the Laboratory for Speleothem Studies. Thin section analysis revealed 125 annual layers in the upper 4.1cm of the stalagmite. The layers could be identified by detrital grains at the top of each layer. Because of the change in petrography and the convergence of the very thin layers, layer counting was difficult to continue further down. Thin section studies also showed that the basal one-fourth of the formation is predominantly aragonite while the upper, darker three-fourth section is largely calcite. The presence of aragonite is the result of deposition in relatively dry or evaporative conditions at higher saturation states, and calcite is considered as the result of precipitation in wetter conditions at lower precipitation rates and saturation states. After polishing and wetting to enhance color differences, one exposed surface was photographed. This photo was scanned at 600 dpi, and gray-scale brightness was measured along the central growth axis in 8-bit format (bright white = 255; black = zero) using software for image analysis.Several dating methods including 210 Pb dating of 4 samples at the Skidaway Institute of Oceanography, University of Georgia, and thermal ionization mass spectrometry (TIMS) U-series dating of 14 samples at the Department of Geology and Geophysics, University of Minnesota, USA, and annul layers counting were applied to obtain a reliable and accurate age for it. 210 Pb analysis and annual layer counts in the upper part of the stalagmite indicate deposition at about 0.32mm/a during about the last 125 years. Interestingly, the growth of the stalagmite about 130 years ago corresponds well with a farmer rebellion around 1869A.D. The variation on grey scale intensity, δ13 C, and petrography was too great to be explained by the change in climate alone. Close thin section studies with additional field trips to Yangzipo Cave area revealed that the sharp change was at least partially influenced by the clearance of trees and a layer of wood detritus created by a historical event taken placed around the cave at that time. During this event, the trees above the cave were cut down by the rebels to set up a fire to kill many local villagers hiding inside. Electronic scanning microscopy study indicated that there was a clear carbon detritus layer recorded in the stalagmite at that specific time period, further confirmed our claim. This study of a stalagmite from Gu