A Dissertation Submitted To Chinese Academy Of Geological Sciences

The Hongyuan profile (N:32°46.834’, E:102°30.954’) is7.88m thick and located near Hongyuan Country of Sichuan province. The present-day climate at Hongyuan is cold with a mean rainfall of600-800mm and an annual mean temperature of10.3℃. The profile is lithologically composed of three parts: the peatland at the the upper part (0-4m), the silk with orangnic at the middle part(4-5m), and underlaying lake sediment(5-7.88m).AMS14C results show that lower age of profile is22.4ka BP and the peatland began to develope in11.6ka BP. The values of LOI and TOC、LOI and lightless have a good correlation, indicate that LOI and lightless are ideal proxies for reflecting the changes of climate. The pollen results suggest that local vegetation has experienced three distinct stages, from subalpine meadow(16.5-11.7ka BP) to alpine grasslands (11.7-8.3ka BP), and to alpine meadow(8.3-0ka BP). The ratio of A/Cy can be used as an indicator to distinguish from alpine meadow to alpine grasslands.The Hongyuan profile can be divided into three parts on the basis of loss on ignition, TOC, lightless, organic matter δ□13C. The lower part (22.4ka BP to15.0ka BP) have higher lightless and lower LOI and TOC, which are about51.0,4.23and1.17on average, respectively. For the middle part(15.0ka BP to11.7ka BP), the value of lightless LOI and TOC vary from18.5-47.6,9.8-64.3,4.35-42.3, respectively. The top part (11.7ka BP to0ka BP) have lower lightless and higher LOI and TOC, which is about23.1,73.3,39.7, respectively. The A/Cy ratio indicated a general cooling trend throughout the holocene, this process is mainly affected by summer insolation in lower latitudes.Hongyuan profile recorded a number of cold events which are10.3ka BP,8.2ka BP,7.4ka BP,5.1ka BP,4.2ka BP,2.5ka BP. Those cold events can compared with the ice-rafted debris events in North Atlantic. The holocene optimum of the Tibetan Plateau occurred in the early Holocene, while occurred in the mid-late Holocene in northwest arid area, which is dominated by different atmospheric circulation.