Paleoflora And Palaeogeography Since The Last Glacial Peak In Jianhu Area, Jiangsu

The study of paleoclimatic and environmental changes since the last glacial maximum has always been one of the hot topics for international scholars.After careful field investigation,we selected Gangbei Section of the Jianhu in Jiangsu Province(GB Section and GB2 Section)for study.The chronostratigraphic sedimentology and biostratigraphic methods were used to compare with the Qingfeng Section which is the standard section of the northern Jiangsu Province.AMS14C dating results establish a regional stratigraphic sequence.Simultaneous analysis of algae,sporo-pollen,zircon source tracer and other environmental proxy indicators.Based on the pollen records of the GX2 Section,nearby Gangbei named Gangxi,high-resolution reconstruction of paleophyte evolution processes since the last glacial maximum in the Jianhu region was undertaken to explore the changes in the coastline location and the surrounding geomorphic environment,which can be used to predict the future environmental change of the Jianhu region.Changing trends are of great practical significance.The conclusions mainly include the following aspects:? In Gangbei and the surrounding areas,the northern subtropical evergreen,deciduous broad-leaved mixed forests have been developed during the early and middle Holocene.In the late Holocene,the composition of evergreen broad-leaved trees decreased,while the deciduous broad-leaved components increased,and the vegetation evolved into a deciduous broad-leaved forest containing a small number of coniferous trees and evergreen broad-leaved trees.? There were abundant micro-algal fossils in the sporopollen analysis of the GB and GX2 section,the most important are the salty aquatic algae such as the Spiniferites and Multispinula.At the same time a small but continuous occurrence of freshwater algae such as Pediastrum and Zygnema.Algae fossils have a good indication of the landscape environment at that time.It can be speculated that the paleoenvironment of the GB and GX2 section during the early Holocene was a shallow sea.It can be considered that the location of the GB and GX2 section may be a land-sea transitional zone.? The U-Pb age distribution of detrital zircon in sediments of the Huai River can be well corresponded with the GB2 section samples,and similar to that of the waste Yellow River.while the sediments from the lower reaches of the Yangtze River and the Yellow River are obviously different from those of the Gangbei sediments,indicating that the source of the sediments in the Gangbei region is mainly from the Huai River.Although the changes of shoreline is directly related to the sea surface fluctuations,the land-forming effects of sediments brought about by the Huai River basin in this area are directly related to the changes in the shoreline.? The evolution process of the geomorphologic environment in the Jianhu region can be roughly considered as:8500-7500 cal.a BP,the sea surface is rising,the GB and GX2 sections are located in the shallow sea;7500-6800 cal.a BP,the rising rate of the sea surface is slower.The Huai River develops,and the terrestrial matters increases.The shoreline recedes eastward.the GX2 section is located in the upper part of the intertidal zone;6800-5800 cal.a BP,The shoreline continues to recede eastward with a period of swing.The GX2 section of the Jianhu was mainly located in the supra tidal zone and the lagoon develops.5800-4200 cal.a BP,the sea water has already withdrawn from the GX2 section of Jianhu region.And the lagoon gradually turns into a freshwater lake;4200-3800 cal.a BP,influenced by sediment deposition,the lake surface is reduced,and the water area is reduced to become a shallow lake-Marsh environment;3800-400 cal.a BP,Jianhu region is developed into a freshwater marsh environment.