A Study On Holocene Vegetation And Climate Change Of Ordos Plateau, As Inferred By Sedimentary Record From Bojianghaizi Lake Of Inner Mongolia, China

High resolution Holocene climate sequences were needed in marginal monsoon area of China for that not only the area were interacted by those climate subsystems including the winter monsoon, the summer monsoon and the westerlies, but also for that controversies were still existed among those published Holocene sequences. This paper presents a high-resolution Holocene vegetation and climate reconstruction from the sediment of Bojianghaizi Lake (39°46’～48’N,109°17’～22’E), a closed lake located in the undulating plateau in the central north area of the Ordos Plateau, Inner Mongolia, China. A 438 cm-long core was drilled from the north central part of the lake and proxies including grain-size, loss on ignition (LOI) and pollen were investigated on it. Conclusions and understandings obtained in this study are:1,On the basis of 10 valid AMS14C dates and the proxy’s investigations, Holocene climate sequences and lake evolution were recontructed. That is, aeolian environment dominated and desert prevailed before 11,820 cal. a B.P., as inferred by the aeolian sand deposit and the lowest organic matter content and the lowest total pollen concentration. The lake came into being and the vegetation ameliorated a little since 11,820 cal. a B.P. as indicated by the fluvial sediment and the small rise of organic matter content and total pollen concentration. High lake level persisted and broadleaf woodland steppe prevailed between 11,390 cal. a B.P. and 7510 cal. a B.P., as inferred by the lacustrine silt deposit, the ～10% broadleaf pollen percentages, the peak value of pollen concentration, the increased organic matter content and the high carbonate contents. During 7510～6800 cal. a B.P., the lake regressed and the vegetation deteriorated into steppe or desert steppe under a gradually drying climate, those were reconstructed on the basis of the coarsening of sediment, the decrease of broadleaf pollen percentages, the decrease of organic matter and carbonate contents. The period between 6800 cal. a B.P. and 1590 cal. a B.P. might be fluctuantly drying with a remarkable humid phase of lake transgression during 5000-4000 cal. a B.P. and a extreme dry phase of 4000-1590 cal. a B.P., the former humid phase was reconstructed on the base of the high lake terrace and the latter dry phase was mainly recovered on the sedimentary hiatus. The past 1590 cal. a B.P. might evidenced a vegetation and climate amelioration and lake transgression, as inferred by the increase of broadleaf pollen percentages and total pollen concentration, and the fining of sediments. It should be noted that the humidity of past 1590 yrs might be still lower than that of Early Holocene and the period between 5000 cal. a B.P. and 4000 cal. a B.P.2,The BJH01 sequence exhibits humid Early Holocene and a drying middle to late Holocene. This was in broad consistent with the regional climate sequences in marginal monsoon area. These might be in close relationship to the increase sea surface temperature of west tropic Pacific, both as close responses to the increased north hemisphere summer insolation.