25-20kaB.P. Travertine Deposition Characteristics And Ancient Climate At The Qilian Shule River

Global climate change has became the focus of international science research. According to the former research at home and abroad, the travertine deposits that distributed in the surface can be used as a good carrier for the ancient climate research. Through the research for ice cores, tree ring, the loess, coral, stalagmites and other paleoclimate environment records, many significant achievements has been made. Climate records of ancient environment, which is used to analyze and reconstruct the paleoclimate, have their own advantages, also problems and deficiency at the same time: The ice cores only distributed in high latitudes and cold area; The tree ring may be incomplete and discontinuous, even appears multi-tree rings; Research for the season resolution of stalagmites has been in process, and false annual layer and multi-layer for many years exists. Travertine is a chiefly freshwater carbonate deposits that outputs on the surface of the earth, with a fast deposition rate, which may reach 0.1 ~ 20 mm/a, 1-100 times to the deposition rate of stalagmites. Previous studies indicate that stalagmites laminated is more accurate as record of the ancient environmental climate, but the present study suggests that, travertine as ancient environmental alternatives have many more advantages than other substitutes. Travertine can provide the paleoclimate change information with higher sensitivity and resolution than stalagmites, becoming the important carrier of the ancient climate research in recent years. Using travertine microlayer to restore the ancient climate change has become the current trend. At present, there has a lot of achievements about meteogene-type travertine in the international, but lack for research for the the relations of environmental climate change indicated by thermogene-type travertine.The thermogenic travertine is formed during the transport process of underground hot water, which contains deep-source CO2 and has a high temperature, among which the CO2 releases and facilitates the deposition of CaCO3, and than lead to laminated accumulation of travertine. The sedimentary characteristics are long standing time underground,strong circulation and complex interaction between rock and water.The research object in this paper is thermogenic travertine. We conduct systematic research on the paleoclimate changes during 25-20 ka B.P.,which is preserved in the travertine deposition at the Qilian Shule river, through combining analysis of the data from travertine carbon and oxygen stable isotopes, 14 C dating and uranium disequilibrium dating method.It is concluded the following main knowledge:(1)Travertine δ13C value perform good negatively linear relationship with the rainfall capacity. The rainfall capacity is large in the study area for two reasons: 1. the deepen thermal low depression of the Tibet plateau facilitated southwest monsoon to move northly, meanwhile, the southeast direction airflow in the north advanced towards the northwest inland, namely the southeast monsoon and southwest monsoon enhanced at the same time; 2. the influence of the westerly circulation made the northwest annual precipitation increase greatly.(2)Though the oxygen isotope analysis of travertine of the Qilian Shule river, the δ18O values from the medium- thick dense white-milky light color lamina of the travertine was negative, indicating a larger precipitation in that period and a warmer climate, maybe in summer season. The δ18O values of travertine change showed a negative correlation with temperature.(3)Through to the travertine deposition rate in seasonal change study found at the Qilian Shule river, temperature and precipitation become the main factors that influence the travertine deposition rate. Travertine deposition rate shows a fast increase in the rainy season and a slow one in dry season,which is opposite to the regular found in research on the travertine of Wangsijing in Tianjin, but similar to the regular of meteogene-type travertine in Japan.(4)Through the carbon and oxygen stable isotope analysis, it is obtained that synchronicity exists between travertine δ13C values and δ18O values, the two curves were positively correlated.(5)We built the standard for naked recognition of the travertine annual-layer with the color change, The thin loose brown lamina and the medium- thick dense white-milky light color lamina, representing the climate change during one year.(6)Travertine in the study area formed during the period of 25-20 kaB.P. according to 14 C method test and analysis, belongs to the Last Glacial Maximum(LGM). Combined with age data and δ13C、δ18O values, we deduce that during 24.9-24.5 kaB.P. climate appeared with warm wet- dry and cold- warm wet- humid change. The information preserved in travertine has instruction significance for the research on ancient climate environment of quaternary paleoclimate during the LGM.