Peat Development Characteristics And Regional Environment Evolution During The Holocene In Altai Mountains, Xinjiang

Climate changes become a major environment problem recently. With the development of a series of international programs on climate changes, reconstruction of past climate changes becomes an important component of the globle climate changes. Because they contain the remains of different type plants that are usually sensitive to changes in climate, well preserved peat sequences can serve as good archives recording past climate changes. Xinjiang Province in northwestern China, an important part of arid areas in central Asia, is especially sensitive to climate changes. The Altai Mountains, in the northern of Xinjiang, provide good geological conditions for the development of peatlands, owing to their special geographic position, geologic structure and climate characteristics. Consequently, the peat in the region can provide ideal geological archives for obtaining evidence for climate changes in mountain regions of northwestern China. The investigation of peat resources in Altai Mountains has been carried out by the research team from Northeast Institute of Geography and Agroecology, Chinese Academy of Sciences in August, 2014. Seven major peat bogs were investigated, and 36 peat cores were obtained from the Altai Mountains region. In this study, Tielishahan peat bog was selected as the sample point, due to the fact that the peat bog is managed by Khanas National Nature Reserve of Xinjiang,and less affected by human activities, and well-preserved modren vegetation exist in the region. A 395 cm continuous peat sequence collected in the peat bog, is served as the research object.According to the lithology and the variations of organic matter(OM) and humification degree(HD) of the peat sequence, five peat samples(three bulk peat samples and two selected plant macrofossil samples) were selected for AMS 14 C age dating to establish the chronology framework of the sequence. The peat development characteristics during the Holocene are discussed by ways of the analysis of peat density, ash content, OM, total organic carbon(TOC), total nitrogen(TN), total phosphorus(TP) and peat HD with 1 cm or 2 cm resolution. Moreover, high resolution records of the combination of TOC, HD, n-alkane distributions and pollen are employed for a reconstruction of the Holocene vegetation and climate changes in the Altai Mountains region.The results of peat development characteristics and regional climate changes during the Holocene recorded by peat in the Altai Mountains indicate that extreme drought conditions dominated by desert vegetation occured during the early Holocene(before ~8.0 cal kyr BP). Serious peat mineralization and decomposition degree indicate eutrophic peat developed in this period. Then the climate conditions were better than that in the early Holocene and large amounts of hygrocolous herbaceous plants developed in the mid-early Holocene(8.0~5.5 cal kyr BP). The climate then became generally warm and dry in the late mid-Holocene(5.5~4.0 ka), when woody and desert herbaceous vegetation were the major contributors to the peat. The eutrophic peat development stage in these two periods is evident from increased peat OM and nutrient contents in the peat sequence. In the late Holocene(4.0~1.0 cal kyr BP), the regional climate entered into cold-wet stage with abundant hygrocolous herbaceous vegetation input. From 4.0 to 3.5 cal kyr BP, the the vegetation in the region was dominated by mosses with the characteristics of higher OM concentrations but lower nutrient contents and recalcitrance to decomposition. Therefore, the peat development in this period belonged to the typical oligotrophic peat development. Then, mesotrophic peat developed as indicated by the input of both mosses and sedges from 3.0 to 1.0 cal kyr BP. A gradual shift to warm and dry conditions occurred during the last 1.0 kry, when large amounts of woody and desert vegetation developed again. The eutrophic peat developed resulting from higher peat nutrient contents in recent years.The comparision of climate changes in Altai Mountains region with that in nearby Mountains regions and other regions in Xinjiang and Asian summer monsoon regions incdicate that the reconstructed Holocene climate characteristic in the Altai Mountains region recorded by peat is coincident with the climate and temperature-moisture patterns in surrounding arid central Asian regions, which in out-of-phase with Asian summer monsoon regions. We suggest that the climate changes in Altai Mountains, Xinjiang, are hardly influenced by Asian summer monsoon. The moisture changes in arid western China have a good linkage with the North Atlantic Ocean sea surface temperatures(SSTs) via the westerlies. However, owing to the mountainous character of the study areas, high altitude, mountain microclimate characteristics and glacial snow and meltwater, and other local factors, the climate changes in the Altai Mountains region recorded by peat have not always been concordant with variations of North Atlantic Ocean SSTs. We propose that water-heat patterns in the central Asia mountain region were mainly influenced by summer insolation in the Northern Hemisphere and that temperature-induced evaporation might have played a dominant role in regional moisture changes during the Holocene.