Late Holocene Vegetation And Climatic Evolution In The Source Region Of Yellow River

Qinghai-Tibet Plateau is one of the most sensitive areas that responses to the global climatic change and its own Holocene climatic change absorbed broad attention. Although its climatic change through the Holocene is consistent with the global change, as its special location it has distinguished characteristics, for instance, frequent climatic fluctuation, regional variations, significant difference from southeast to northwest. Our work chooses the source region of the Yellow River as study area as well as takes Erling Lake, Xingxinghai Lake and Xianchengbei area as study sites, uses pollen analysis, TOC, carbonate and magnetic susceptibility to obtain the environmental evolution sequence.Surface Pollen analysis showed that the surface pollen samples can reflect the characteristics of the surrounding vegetation community samples, which can also serve as good proxy indicators. The main pollen types are: Ephedra(Ephedraceae), Ranunculaceae(Ranunculaceae),Thalictrum(Thalictrum),Chenopodiaceae(Chenopodi aceae),Artemisia(Artemisia),Gramineae(Poaceae),Asteraceae(Asteraceae),smartweed(Polygonum),sedge(Cyperaceae),mustard family(Brassicaceae), Asteraceae(Compositae), Aster(Aster), Taraxacum(Taraxacum), Urtica(Urtica), lily family(Liliaceae), umbrella Section(Umbelliferae).Sedge has moderate representative, Artemisia and Chenopodiaceae have super representative, Gramineae has low representation.Paleoclimate proxies analysis showed that the Yellow River source region has experienced four climatic phases through the past 1900 years:dry and cold-old and wet-arm and wet-ry and cold, the main vegetation is alpine steppe and alpine meadow two categories, vegetation turned from alpine steppe to alpine meadow in Erling Lake area, Xingxinghai Lake area was mainly covered by alpine meadow and the vegetation in Xianchengbei area turned from alpine swamp meadow to alpine meadow. Vegetation distribution significantly showed its horizontal and verticalvegetation zones characteristics. Overall climate in the past 1900 years was dry and cold, the monsoon became strong after 1000 AD which eased the driught condition,. Monsoon became weakand climate turned dry and cold fater 1440. Four humid perion had appeared which are: around 383 AD, around1070, 1241-1380 AD and 1600-1700 AD.