| The radial sand ridges of South Yellow Sea located in the offshore of Jiangsu Province,were formed by the river-sea interaction.The paleo Yangtze river enterned into the South Yellow Sea through Jiangsu region during Late Pleistocene period,the river carried a large amount of fine sand and silt composition,as a result,formed a large-scale estuary and delta system.Following the Holocene rising of sea level and tidal current field of unique pattern of radial spread and radial aggregation tidal wave systems,the radial sand ridges were developed gradually,which were described as radial spread distribution condition of sand ridges and tidal channels.The ridial sand ridges contain a wealth of environment changes information because it was influenced by the long-term land-sea interaction;it has great scientific significance because of regional environment changes can be revealed by studying on it,and also concerned of that,the Jiangsu area as the developed province of economic level,nowadays the economic structure has transferred from Yangtze river dominated to South Yellow Sea dominated,as the main and important part of marine economy development,the development and operation of sea harbour is related with the marine environment conditions,and the background of global and country economic development,the studies on the radial sand ridges of South Yellow Sea also have a large production and application significance.The sediment core of 07SR09 is located in the sea bottom of Lanshayang tidal channel,the main channel of radial sand ridges of the South Yellow Sea,and the main navigation channel of the Yangkou harbour.Thus,the study on the sediment environmental evolution of Lanshayang tidal channel is very necessary and important.By analyzing the sedimentary facies and testing experimental indicators of core 07SR09,with analyses former published literatures,the evolution characteristics of the depositional environment of the area since the late Pleistocene are researched.Based on the comparison of the sea chart of different historical period of this area,sea bottom sediment samples,different related cores,the characteristics of erosion-deposition changes and stability of the Lanshayang tidal channel in different time scales are studied.The great significance of Yangkou harbour has been built up and the operation for the different regions is demonstrated at last.The main conclusions of this thesis have been summarized as following:(1)Core 07SR09 can be divided into three sections based on core catalog,foraminifera and shell identification.The top section is from 0 to 1838cm,the middle section from 1838 to 2447cm,and the bottom section is from 2447 to 4591cm.According to the sedimentary structure,foraminifera assemblages and the different shell species,the sedimentary environment from top section to bottom section of core 07SR09 are as follows:marine sedimentary environment,terrestrial sedimentary environment and fluvial sedimentary environment.According to the analysis of 14C dating,the division age of the top and the middle section is 7713±45 a cal B.P.-18135±305 a cal B.P.,approximately about 12000 years,belonging to the late Pleistocene,near the early period of Holocene;the division age of the middle and the bottom section is 18135±305 a cal B.P.-31094±85 a cal B.P.,approximately about 22000 years,belonging to the late Pleistocene.(2)Based on the trend changes of fine sand content from grain size analysis,the content of fine sand is the most one of core 07SR09,up to 54.9%,the content of fine sand from the top section to bottom section are as follows:64.3%,57.2%and 48.8%,showing a gradually reduced of change trend.Combined the former papers,the sediment should be the paleo Yangtze River,and this change showed that the source of paleo Yangtze River gradually reduced from the bottom section to the top section since the late Pleistocene,We argue that this area was influenced by deposition after the southward movement of the paleo Yangtze River,most likely from the paleo Huanghe River,or the finer particles brought by tidal action.And these later deposited fine particles diluted the original sediment grain size composition of this area.(3)Susceptibility experimental results show that the average magnetic susceptibility of core 07SR09 is 8.18 × 10-7m3/kg,the top section is 4.76 × 10-7m3/kg,the middle section is 4.77 × 10-7 m3/kg,and the bottom section is 11.89 × 10-7 m3/kg.Combined the change trend of fine sand content,the value of the bottom section is the highest one showed that the most strong hydrodynamic power between the three sections,and from the bottom section to the top section,the dynamic conditions of sedimentary get more and more stable.(4)C/N ratio analysis showed that the average value of core 07SR09 is 8.62,so we inferred that the sedimentary environments are mainly marine conditions.From the top section to bottom section the C/N ratio is as follows:8.94,10.74 and 6.75,the middle section is the highest value showed that the section is the typical terrestrial sedimentary environment,but the lowest value the bottom section is caused mainly due to the sampling location focused on the lower part of the bottom section,so we can't get the conclusion that the bottom section is a typical marine sedimentary environment.(5)Studies on the migration of the paleo Yangtze River,several related cores in nearby regions are collected and analyzed in stratigraphy division and AMS 14C,the beginning period of migration of the paleo Yangtze River is roughly 30 000 a BP by AMS 14C dating,late Pleistocene stage,MIS3.(6)To analyze the different time scales of erosion and deposition of Lanshayang tidal channel evolution has showed that,in the short time scale of historical period,the results of the comparison of sea chart among different historical period of this area showed that the net erosion-deposition rate is +0.085 m/a from 1963-68 to 1979 years,reflected a minor-deposition status in Lanshayang tidal channel;the net erosion-deposition rate from 1979 to 2005 is +0.019 m/a,significantly decreasing than the previous period,reflected that the hydrodynamic condition of Lanshayang tidal channel get more stable than before.Thus,the Lanshayang tidal channel can be used as an excellent natural deep water navigation channel,because of the stability and the minor changes in water depths of it.And in the long time scale,by analyzing the sea bottom sediment samples and related cores,the conclusion that the sea area conditions of the whole Lanshayang tidal channel and Yangkou harbour nearby is stable relatively.In order to protect and ensure the normal and convenient operation of Yangkou harbour,it should be attracted attention on the rapid deposition status caused by sudden natural disasters or artificial activities influence. |
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