Characteristics Of Pollen Assemblage And Climate Change In Holocene At The BHQ Core, Pingyao, Zhejiang

The Holocene is the nearest interglacial stage to us in Quaternary, and is also the most important period to modern humans, at which the process of development of human civilization was recorded clearly, we must first understand the laws of the Holocene climate change. It has become a practical and effective way.In eastern China, the natural environmental elements evolution is intense and the most sensitive area are located between30°～40°N. zhejiang is just located in this area, where the Subtropical highs and Siberian highs alternating control area. There are rich research achievements in this area about Holocene climate change. However, in the northern coast of Qiantang River in Zhejiang Province, the Holocene environmental research is relatively weak. BHQ core, is from a relatively closed watershed which is small interface by extra-pollen,in the Pingyao Town, Zhejiang Province, near the Liangzhu Ancient City. Through the analysis of pollen combination in detail, we try to reveal the characteristics of vegetation and paleoclimate evolution from early-middle Holocene in the south of Yangtze River Delta, and reveal the impact of human activities on the local vegetation. The aim is to provide evidence for the research of Holocene environment evolution and paleontology in Zhejiang Province.This research was based on the14m long core from the BHQ bore from Beihu Lake, Pingyao Town of Zhejiang Province. With4dating data by AMS14C method, palynological analysis was performed on49samples. It was found that there was much difference for palynological concentrations in different samples, and there were a wide variety of plants, totally93pollen types are recorded. Through analysis on characteristics of palynological assemblages and combining with lithology,3pollen zones and9pollen subzones were divided, and the environmental sequence of vegetation succession and fluctuation of climatic temperature change in early and middle Holocene for the study area was reconstruction.(1) In19-8.9m, about1500-7994cal. a B.P, vegetation in the study area was mainly conifer-deciduous broad-leaved forest, including Quercus(deciduous), Pinus, Altingiaceae and Pterocarya as well as Quercus(evergreen) and Ulmus, etc. There were not many evergreen trees, indicating that after the Younger Dryas Event at the later period of late Pleistocene, the research region entered Holocene, and climate quickly turned warm.(2) In8.9-6.7m, about7994-5780cal. a B.P., this period was the best period of hydrothermal condition in all the section, At this period, evergreen and deciduous broad-leaved mixed forest were dominant, including guercus(deciduous), Quercus(evergreen), Pinus, Altingiaceae, Pterocarya and Ulmus. And the species and quantity of evergreen trees were far more than the above period, indicating the warm time and humid climate.(3) During the6.7-4.775m sedimentary time period, About5780-4029cal. a B.P., there was apparent fluctuation with palynological assemblages, and the vegetation in the study area mainly contained deciduous broad-leaved forest such as Quercus(deciduous), Altingiaceae, Pinus and Pterocarya. The number of Quercus(evergreen) dwindled rapidly. It shows that the vegetation type in the research area had evolved into deciduous and broad-leaved forests. It reflects that the area experienced relatively climatic deterioration event. And the high concentration of aOryza(>35μm) pollen during this period indicates a certain scale of ancient human activity in the research area.(4) The Concentricystes are almost recorded at all the samples in BHQ core. According to the algae concentration of Concentricystes, the BHQ core is divided into four zones:Concentricystes zone1is corresponded to the pollen subzone1a and1b; Concentricystes zone3is take corresponding to the pollen zone2; Concentricystes zone2is correspond to the pollen subzone lc and1d; Concentricystes zone4is correspond to the pollen zone3. Concentricystes zone1(19～16.925m) and Concentricystes zone3(8.925～6.7m)are high concentration, and average of19.22grain/g and16.66grain/g; Concentricystes zone2(16.925～8.925m) and Concentricystes zone4(6.7～4.775m) concentration is reduced, with an average of13.17grain/g and6.38grain/g. In study area, the changes of Concentricystes concentration show that the climate is not stable. Affected by the climate fluctuation, the underground water level rise frequently in the early-middle Holocene.(5) At pollen zone3b of BHQ core, Oryza pollen grains show "big bang", corresponding archaeological culture from the late Songze Culture to the early Liangzhu Culture. Presumably due to the humid to dry cool climate, water area reduced and land area increased. Dry and cold climate change of river and lake swings with sight, the humanity can live in the low-lying limnological region and plant rice. This also suggests that, in hot and humid subtropical areas, cold and dry climate is conducive to the development of civilization.(6) At the flood season of Tiaoxi River drainage basin, the pollen assemblages reflects the vegetation landscape at the sample point and its upstream mountain; At the normal or low flow period, the pollen assemblages mainly reflect the vegetation landscape surrounding area. So the pollen assemblages of BHQ core can both reflect the surrounding vegetation, and the western Tianmu Mountain vegetation.(7) According to the BHQ core, while the Chenopodiaceae pollen content increase speculation, may be associated with a rise in sea level. At the same time, Chenopodiaceae pollen content changes is a higher correlation with archaeological culture. However, due to the lack of related Marine microfossil, whether seawater directly affects the district, remains to be further research work.