Geochemical Characteristics and Its Paleoenvironment of Wetland Sediment in Caotanhu Wetland,Shihezi City in Xinjiang Province of China

Soil organic matter,total organic carbon(TOC) and total nitrogen(TN) are the important indexes to research the change of dry-moist palaeoclimate change and analyze the reason of the wetland degeneration.The Caotanhu wetland in Shihezi,which is located at the transitional zone between the middle section of Tianshan Mountains and the Junggar Basin in Xinjiang,is selected as the research area.Using the chemical analysis methods,the high resolution record of soil organic matter,TOC and TN data since 4 000 a B.P.was obtained.Combined with pollen data,the information of paleoclimate change was extracted.The results revealed during 500-1080 a B.P.and 2190-3800 a B.P.,soil organic matter and TN were at their higher values,suggesting that the climate became humid and the wetland developed with abundant freshwater aquatic plants,which contributed to the peat accumulation.But during 1080-2190 a B.P.,the lowest values of soil organic matter and TN indicated that a dry climatic condition was not favorable for the accumulation of peat.Once the wetland has been converted into farmland,the great loss in soil organic matter and TN happened,which indicates that human reclamation is one of reasons of the degradation of wetlands.