Palaeovegetation And Palaeoenvironment Reconstruction Of Sanjiang Plain Wetland During The Holocene

Since the mid-1980 s of the last century, reconstruction of palaeovegetation and palaeoenvironment, which is the important component section of global change research, has become a hot research field with the launch of a series of "past global environmental change" research program. Peat is a natural archive as past global change record, playing an increasing important role in palaeovegetation and palaeoenvironment research. Sanjiang Plain, located in Northeast China, is one of the largest freshwater marshes. Sanjiang Plain is in the East Asian summer monsoon region. Its unique geographical location and its sensitivity responding to climate change make Sanjiang Plain as an ideal testing ground for the study of palaeovegetation, palaeoenvironment and the regional response to global climate change. In this study, peat profile or cores have been drilled at different types of Sanjiang Plain wetlands(valley marsh, dish-shaped depression and lake marsh), namely Shenjiadian profile, Honghe peat core and Xingkaihu peat core. AMS14 C technique is used to construct age-depth models using peat bulk samples, a total of 23 AMS14 C data were get. Plant macrofossil analysis was performed with 1 cm or 2 cm resolution for each core. The δ13C analysis was conducted for the Honghe 1 peat core with 2 cm resolution. Through the reconstruction of palaeovegetation and palaeohydrology, we explored the main driving factors of the evolution of palaeovegetation, and investigate the major environmental factors controlling the wetlands surface wetness changes. In addition, we explored the evolution process of wetland ecosystem and forecast the future development trend under the global climate changes. The research can provide scientific support for the strategy formulation of wetland protection and recovery. The main conclusions are as follows:(1) Vegetation succession rule of Sanjiang Plain wetlandsThough the plant macrofossil and lithology analysis of the different sediment profile or cores of Sanjiang Plain wetland, we found that lower part of the core consists of black silt, and the component of upper part of the core is peat, which indicate that most Sanjiang Plain wetlands are evolved from water body. The emergent plant(Equisetum heleocharis) first grown in the water, companioning with small amount of Menyanthes trifoliate and Carex. With the continuous growth of these plants, the death residues sunk in the bottom of the pond and water lavel decreased consequently. Then a lot of marsh Carex plants invade and dominated, thus Carex community gradually replaced the aquatic Equisetum heleocharis community. Therefore, there are differences among the different type marshes with regard to vegetation succession processes, such as Carex- Drepanocladus aduncus community existed in the vegetation evolution of H-1, while it does not appear in other cores.(2) Environment change process of Sanjiang Plain during the HoloceneThrough carbon isotope analysis of plant macrofossil from the Sanjiang Plain sediment core, combined with the plant macrofossil DCA 1 axis score, these two indicators indicate environmental change of Sanjiang Plain experienced the following stages since the Holocene: 7000 cal yr BP—3800 cal yr BP period, the δ13C values and DCA 1 axis scores are large which indicate the environment is relatively dry; 3800 cal yr BP—1100 cal yr BP period, δ13C values and DCA 1 axis scores become smaller than the last period indicate environment become wet; 1100 cal yr BP—500 cal yr BP period, δ13C values and DCA 1 axis scores fluctuate frequently indicate the environment is in a alternating wet and dry state, but it is wet overall; 500 cal yr BP—0 cal yr BP period, δ13C values and DCA 1 axis scores sharply increase indicate environment become drier than the last period.(3) The main driving force for the vegetation and environmental changes of Sanjiang Plain wetlandsThrough GAM model analysis was used to to explore the influence of its auto-evolution and the external environment on vegetation succession during the development of wetland, the results show that the main driving forces affecting vegetation succession of Sanjiang Plain wetlands is the surface moisture.By comparision with the δ13C of Hani and Jinchuan peatland in Northeast, δ13C of Hongyuan in Tibetan Plateau, δ18O of Dongge Cave in Guizhou, hematite-stained grains percentage in Northern Atlantic Pacific sediments, and solar activity, we found that environmental changes recorded by plant macrofossil and carbon isotope of Sanjiang Plain wetland is basically consistent with others, especially with the record of Dongge Cave and solar activity, but there are some differences among them. The surface wetness changes of Sanjiang Plain wetland is mainly controlled by the East Asian summer monsoon and the solar activity.Plant macrofossil and carbon isotope of Sanjiang Plain have significant record the abrupt climate events during the mid and late Holocene, such as 2.8 ka BP event and the "Little Ice Age" event, which indicate the vegetation of Sanjiang Plain wetlands is sensitive to global climate change.