Climatic Changes Documented By Sediments From Genggahai Lake Since The Late Clacial,Northeastern Tibetan Plateau

Studies on palaeoclimatic and environmental changes play a significant role in understanding of changes in atmospheric circulation patterns and processes and mechanisms of regional climate responses to global changes. The Gonghe Basin is located in the northeastern Qinghai-Tibetan Plateau, which is influenced climatically by both the Asian monsoon and the westerlies circulation. This area is very important for understanding of the climatic linkages between high latitudes and low latitudes of the Northern Hemisphere. Genggahai Lake, a shallow grass-type lake, is situated at the central Gonghe Basin. The simple hydrologic pattern and aboudant biodiversity make it be sensitive to the changing globle climate system, and hence an ideal site to study environment changes.Two parallel cores were recovered at central Genggahai Lake in January2008. Submerged plant (P. pectinatus) remains were used for AMS14C dating. Measurement of the organic debris from lake surface deposit yielded an age of1010±35years, as deemed a'reservoir effect'age. The core covered16700years before present after a subtraction of the'reservoir effect'. The samples from core GGH-A have been analyzed for carbonate content, grain size, magnetic susceptibility, macrofossils, total organic carbon (TOC) and total nitrogen (TN) contents and carbon isotope of bulk organic matter (δ13Corg), as well as stable isotopic compositions of mollusc shells, stem encrustations, and <38μm and<160μm fractions.Based on a survey of aquatic plant community in Genggahai Lake at present days, the relationship between stuctures and components of acquatic communities and water depth was determined. Lake-level fluctuations were reconstructed using biomacrofossils and geochemical proxies. Lake productivity was affected primarily by successions of the plant community, which in turn depended on the change in lake level. Variations in the δ18O values of different types of carbonate are affected primarily by temperature, rainfall ammount and δ18O composition of input water. However, they generally indicate changes in lake hydrologic statues for Genggahai Lake, alhtough influences of these factors are variable at different stages. In combination with the lake level changes, the δ18O record of Genggahai Lake was employed to indicate changes in moisture souces and atmospheric circulation patterns in the study area. Four major stages of climatic changes since the late Glacial were delineated from the multiple proxies:(1) Between16.7and15.3cal ka BP, a dry and windy climate existed in the study area, possibly indicating a weakness of the Asian summer monsoon.(2) From15.3to11.4cal ka BP, the low lake level, dominance of Chara spp. and the more-depleted δ18O values indicate that the climate was still cold and dry despite a slight enhance in the Asian summer monsoon. This period corresponded to the late Glacial.(3) Between11.4and6.4cal ka BP, the lake sustained an overall high lake level. During this period, a remarkable decline in the summer monsoon was inferred from a sharp lowering of lake level from9.2to7.4cal ka BP. The plant community experienced a succession pattern of scare and/or absent submerged macrophytes, Chara spp., to sumerged vascular plants, reflecting changes in lake level. Similarly, changes in the palaeoproductivity were consistent with the succession of plant community. An overall more negative δ18O values were recorded during this period. Generally, the climate was warm and humid, suggesting that the Asian monsoon was strengthened at that time.(4) An overall decline in lake level appeared since6.4cal ka BP, accompanying the frequent fluctuations of palaeoproductivity and the episodically depleted δ18O values. During this period, the Asian summer monsoon was generally weakened, whereas the westerlies or air masses from high latitudes seemed to episodically dominate the study area. The climate was cold and dry accompanying abrupt changes. Compared with other records from adjacent regions and high and low latitudes, the climate history documented by the Genggahai lake sediments is largely consistent with the long-term pattern of the Asian monsoon, punctuated by abrupt climatic changes on centennial to millennial timescales. Meanwhile, the abrupt climatic changes at Genggahai Lake, to a certain extent, appear to be associated with the ice-rafting events in the North Atlantic Ocean, reflecting sensitive responses of the regional climate to global changes.The climatic change in the study area responds sensitively to variations in atmospheric circulation patterns due to its special location. The intensified Asian summer monsoon exerted an great influence on the climate in the region during the early Holocene. In contrast, it was affected apparently by the Westerlies during the middle and late Holocene. Changes in atmospheric circulation patterns are controlled primarily by the intensity of orbitally-induced summer insolation, and may also be affected additionally by the interactions between ocean and atmosphere and the southward and northward shifts of the Intertropical Convergence Zone.The millennial-scale climate changes during the middle and late Holocene may be assosiated with Atlantic meridional overturning circulation at high latitudes. The connection could be established through the Westerlies circulation, which transmits cliamtic signals from the North Atlantic Ocean to central and eastern Asia, reflecting that a coulped system of ocean-atmosphere-land is of importance in the study on global changes.