南京汤山洞穴石笋稳定同位素指示的气候与环境意义

Palaeoclimatic and Environmental Significance as Indicated by the Stable Isotopic Composition of Cave Stalagmite in Tangshan, Nanjing

对南京汤山葫芦洞南壁钙板上一支长82 mm石笋进行了高精度热电离质谱(TIMS)定年及氧碳同位素分析。结果表明,该支短石笋生长时段长达300 ka左右(距今463～159ka),其底部年龄进一步证实南京直立人生存时代早于450 kaBP。δ^(18)O值变化反映了研究区域冰期与间冰期年均温差可达10℃以上,代表了中国东部冬夏季风环流强度显著增强时期。δ^(13)C值在MIS6/7界线上的显著漂移揭示了一次土壤侵蚀事件,结束了洞穴文化层的堆积历史。

Stalagmite, secondary calcite deposits formed in caves, has long been used to infer climate histories based on the variation of the stable isotopic compositions. Here the discussion focuses on a long-term regional climate change and local cave environment evolutions based on the stable isotope records of a 82 mm-long stalagmite from Hulu Cave at Tangshan town, Nanjing city. Using the thermal ionization mass spectrometer techniques, five subsamples taken from the stalagmite were dated. The dated results with high precise suggested that the stalagmite developed from 463 -159 ka BP, and that the fossils of Nanjing Homo erectus beneath the dated sample in the cave stratigraphy should be older than 450 ka BP. Oxygen isotope variation of the stalagmite, representing regional palaeoclimate changes above the cave, showed four cycles of glacial and inter-glacial periods matching the marine oxygen stages 13-6. The stalagmite-based climate signal gives a long-term cooling tendency from 463-159 ka BP. The authors estimated differences at mean annual temperatures between glacial and interglacial periods from the oxygen isotopic composition of stalagmite calcites, provided that the isotopic composition of the meteoric waters has not been changed since the deposition of the stalagmite. The dramatic difference at the average annual temperature with the maximum value of 10.2℃ consequently confirms that the East Asian monsoon circulation in eastern China was strengthened during a geological period of 0. 6-0. 1 Ma B. P. Measurement of δ13C in the stalagmite suggests that changes of plant cover above the cave took place at a slow rate of transition of glacial/interglacial stages. The dramatic shift ofthe δ13C value at the boundary of MIS 6/7, from -13‰- 9‰ to - 4‰-6‰, could notbe explained with the mechanism of vegetation change. We believed that the shift of δ13C was indicative of an event that the soil cover above the cave has been eroded. Such conditions would favor a direct exchange of oxygen and carbon isotopic compositions between meteoric waters and wall rocks (marine limestones), producing heavily 13C-enriched value in the cave stalagmite.