| The Northern Jiangsu Plain is an important part of the coastal plain in the east of China,which is sensitive to the sea level change.There is no consensus on the evolution of the Holocene sedimentary environment in the North Jiangsu Plain due to the problems of dating and elevation errors.In this paper,the chronological framework of the Holocene strata in the Jianhu Gangxi section JH1601 core and JH1701 core are established by AMS14C dating of various materials.The geomorphologic evolution history of the early-middle Holocene sea-land transition in the area was discussed by means of RTK absolute elevation survey,lithology description,high-resolution geochemical element(XRF)scanning,foraminifera and grain size analysis.The results show that the northern Jiangsu Plain experienced an obvious transgression and regression during the Holocene.Based on the lithologic changes of the Borehole and multi-index analysis,the sedimentary environment can be divided into four stages:(1)Early Holocene(12500?11620 cal a BP)gray-black clay silty sediment indicates that the study area is a sedimentary environment with abundant organic matter,a low Ca/Ti ratio of marine elements,and foraminifera fossils The absence indicates that the study area has not been affected by seawater at this time.(2)During the early Holocene(8000?7100 cal a BP),sea level is in a rapid rising phase.The Ca/Ti ratio increases rapidly at about 8000 cal a BP,and the foraminifer's fossils begin to appear,indicating that the coastline has reached the study area,the sea beginning of the invasion.About 7600 cal a BP,the oyster layer enriched in the section indicates the sedimentary environment of the intertidal zone of the study area,reflecting the sea level of about-3.18 m.The 7600?7100 cal a BP core is composed of clay.The main sedimentary characteristics indicate that the hydrodynamic conditions are weak at this time.The foraminifera contents of common species in shallow sea increases,and the Ca/Ti ratio shows a slight downward trend,indicating that the study area is in a stable shallow marine sedimentary environment.(3)During the mid-Holocene mid-term(7100?6500 cal a BP),the sea level is in a stable or slowly rising stage,the sediment source in the coastal area increases,the coastline begins to withdraw eastward.And the sediment particle size becomes larger,hydrodynamic Gradually enhanced,the drilling lithology is the interactive parallel layering of silt and clay,the Ca/Ti ratio fluctuates upward,and the foraminifera is mainly composed of intertidal zone,indicating that the sedimentary environment of the study area has been turned from the shallow sea to the Intertidal deposition.Moreover,the northern Jiangsu plain enters the sea retreat mode.(4)In the late Middle Holocene(after 6500 cal a BP),the Xigang shell sand embankment(7400?6300 cal a BP,about 5 km east of the core)began to expose the sea surface,gradually separating the study area from the ocean.The coastline was forced to withdraw to eastward,the Jianhu area became land.The Ca/Ti ratio in the core decreased rapidly,and the foraminifer's abundance decreased or even disappeared.It shows that from about 6500 cal a BP,the study area has entered the evolution mode of the intertidal zone-tidal zone to the river-lake sedimentary environment.In order to construct a precise chronological framework of the Holocene strata in the North Jiangsu Plain,the high-precision chronological-depth model of the early-middle Holocene drilling is calculated by using AMS14C dating data and Bacon Program.In order to ensure the accuracy of the XRF scanning results,the trend analysis of the scanned elements and the traditional XRF elements was carried out to verify their reliability.The results showed that the trend changes of Ca,Ti,Fe,Al,Si and s were consistent In addition,comparing the results of element scanning with the clay minerals in the borehole,we found that Montmorillonite has a relatively consistent changing trend with Ca,illite and K elements. |
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