Characteristic of annual laminae gray level variations in a stalagmite from Shihua Cave, Beijing and its climatic significance (ll)

The annual laminae gray level variations in the stalagmite TS9501 of Shihua Cave, Beijing are studied in detail. The environmental factors influencing the laminae gray level are also analyzed. The following conditions may be necessary to the study on the lamina gray level. A) The seasonal differences of climate in the studied area are strong. B) The cave has a rapid and simple hydrological connection with the surface, such that the gray level variation is great; therefore, climatic changes can be more clearly recorded in a stalagmite. C) No water from other sources due to lateral flow adds to the seepage over the cave. D) There are more organic impurities than inorganic ones, whose content changes distinctly with time in the sample. By comparison with the modern instrumental climate records, it was found that the gray level of laminae is mainly related to the air temperature, especially the summer mean temperature. Therefore, the gray level can be used as a proxy of the air temperature. The variation of the lamina gray level also represents the oscillation of Indian summer monsoon as identified in the modern climate records. The variability of the temperature in Beijing area over the last 1 ka is reconstructed. The results show that there are several cold periods corresponding to historical records. An important phenomenon is noticed that the climatic pattern before about 1400AD is different from that after about 1400AD. In Beijing area, before about 1400AD, low amplitude and high frequency temperature oscillations dominated the signal. The climate pattern is warm-dry and cold-wet. After about 1400AD, both the temperature and rainfall varied synchronously, temperature oscillated strongly and the Little Ice Age occurred. The climate pattern is cold-dry and warm-wet. This transition of the climate pattern is also observed in other worldwide paleoclimatic records, demonstrating that there was a global climate event at about 1400AD.