贵州羊子坡石笋记录的历史事件*

文章报道了贵州羊子坡洞一支长约18.6cm、宽约5cm石笋研究的结果。 对石笋分别作了²¹⁰ Pb、铀系质谱测年和显微镜下数沉积年轮方法定年;石笋的灰度和碳同位素结果可相互印证,表明其记录是连续可靠的。测年结果显示此石笋保存了从8629年前至今(1999年) 的气候变化,而发生在大约130年前(1869A.D.)各代用指标数值的大幅度变化尤其引人注目,其石笋的灰度、 同位素和岩性的变化尺度之大以至于不可能单纯从气候变化方面来解释。 进一步的深入研究和野外实地调查揭示出,这些急剧变化是由发生在当时一历史事件所产生的植被破坏及树木灰烬微粒被带入洞中而造成的。能谱扫描电镜的结果也证实了此层树木灰烬微粒的存在。此研究表明洞穴石笋具有记录人类活动的能力。

A HISTORICAL EVENT IN A STALAGMITE FROM YANGZIPO CAVE, GUIZHOU, CHINA

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 Guizhou Province in China suggests that stalagmites from this region may provide a high-resolution record of climate change and at the same time may also preserve information about land use/land cover therefore human activities above the cave.