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Geochemical Records of Environmental Evolution in the Last 2. 8 Ma from the Co Ngoin, Central Tibetan Plateau

A series of independent faulted inland lake basins are developed in the central Tibetan Plateau, among which the Co Ngoin Basin is located at the margin of the Indian monsoon with thick lacustrine sediment. The sediment is a perfect record for studies on the paleoenvironmental changes and tectonic-climatic cycles. In this paper, the authors present a high-resolution vertical distribution of Sr, Rb and Zr concentrations, Rb/Sr and Zr/Rb ratios and δ13C, TOC in the sediment from the Co Ngoin, Central Tibetan Plateau, and by a combination with the lithological properties, grain size distribution and clay mineralogy, discuss the chemical weathering process and reconstruct the history of the weathering intensity and paleoclimatic change in the last 2. 8 Ma. The sedimentary proxy records indicate that there are three strong environmental evolution processes. In the core depth of 197~170m (about 2. 8~2. 5 Ma) , the relatively low Sr content, and high Rb/Sr and Zr/ Rb ratios show a low intensity of chemical weathering in the Co Ngoin watershed, and the values of δ13C and TOC indicate a warm-dry to cool-wet climate. The sudden occurrence of subalpine coniferous forest shown from the core from the Co Ngoin and apparent changes in lithology with coarse sand and gravel sediments reflected a strong rising. The higher Sr content, lower Rb/Sr and Zr/Rb ratios reflect a relatively strong chemical weathering period between 2. 5 Ma and 0. 8 Ma (in the core depth of 170~38. 5 m), and the δ13C and TOC reveal a warm/cool and wet plateau climate. The lithology was characterized by mud and silt to fine sandy mud, probably indicating a process of denudation and relatively tectonic stable epoch of the Plateau. In the depth of 38. 5~0 m (about 0. 8~0 Ma?) , the relatively low Sr content, high Rb/Sr and Zr/Rb ratios record a relatively low intensity of chemical weathering, and the values of δ13C and TOC reflect a cold climatic period. The coarse sand and gravel sediments indicate a strong tectonic movement again. The plateau uplifted to above 4000 m a. s. 1. , shaping the basic topographic pattern of modern plateau.