关键词: 古气候, 古环境分析, 稳定同位素, 北京

Abstract: Unlike low resolution (>1000 years) δ ¹⁸O record which mostly indicates temperature change in the past,variations of high-resolution (<100 years) δ¹⁸O record of stalgmite mainly reflect changes in rainfall and the isotopic compition of moisture source in the study area. The δ ¹³C value of speleothem is mainly controlled by isotope composition of soil CO₂that consists of isotopic composition of C₃and C₄plants. The C₃/C₄ratio is strongly correlated with climatic change. When the δ ¹⁸O and δ ¹³C shift to more negative values,the ratio of C₃/C₄indicates wet climate increased precipitation,development of forest. All those climatic conditions may indicate summer monsoon strengthening in the study area. This thoutht provides a new approach to reconstruct high resolution paleomonsoon records. We have first demonstrated this new idea throughout a series studies in Shihua Cave,Beijing. A 12.5cm stalagmite was subsampled at 1mm interval (～25 years resolution) and 109 samples were collected for δ ¹⁸O and δ ¹³C analyses. Using Hendy rule,cave temperature and δ ¹⁸O H₂O ,we have illustrated the isotopic equilibrium between precipitated calcite and its parent solution and the reliability of our measurement. The δ ¹⁸O and δ ¹³C records provide climatic information during the past 3000 years in Beijing. Our results show that there are six apparently climatic cycles during the last 3000 years in Beijing. The wet periods were peaked at 2860,2350,1850,1480,790 and 150 yr B.P.,respectively,with a time step about 500 years. After 600 yr B.P. ,all of the δ ¹³C values show much heaiver than 3ka-mean values. We suspect that this change in C₃/C₄ratio (de-forest) was influenced by human activity also. There is a good agreement in the comparisons of the paleoclimate interpreted from the speeothem δ ¹⁸O and δ ¹³C records with historical records of temperature and precipitation in Beijing,and precipitation in North Central China reconstructed from tree-ring study in Huashan Mountain. Climate in Beijing during the last 500 years,was strongly influenced by the Little Ice Age event which had been found as a global climatic event. In Beijing the climate was relativly arid during the early period of the Little Ice Age. The climate became wetter after 400 yr B.P. and reached a maximum moisture for the 3000 yr period at ～150 yr B.P. Since 100 yr B.P.,the climate has become arid again.

Key words: Palaeoclimate, Paleoenvironmental analysis, Stable isotope, Beijing