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The Organinc Carbon And Organic Carbon Isotope Records Of XYC Core In The Downstream Of Cao'e River

Posted on:2018-01-11Degree:MasterType:Thesis
Country:ChinaCandidate:J WangFull Text:PDF
GTID:2310330518973690Subject:Physical geography
Abstract/Summary:PDF Full Text Request
XYC drilling core(120°53'14.1"E,30°02'49.4"N)is located in Xing Yi Cun village,which is in the downstream of Cao'e River,the middle of Ningshao Plain,Northern Zhejiang Province.The length of the XYC core is 34.34m,1717 samples was collected from the core with a sample spacing of 2cm.According to the previous researches,the core could be divided into 4 lithologies,they are clay silt,silt,sandy silt and silty sand.The 4 lithologies are alternately distributed in the whole borehole,which can be divided into 10 lithologic layers.The 10 lithologic layers are marked as?-? from the bottom to top,and the layer ? was not been analyzed for been destabilized.This paper selected total organic carbon(TOC)and organic carbon isotope(813C)and color experimental analysis,combined with depositional facies and AMS14C dating,to discusses the evolution of the climate and environment during early to middle Holocene in the XYC area,and its purpose is to reveal the environmental evolution of the study area.Through the analysis of various indicators,combined with the conclusions of previous studies of the surrounding area,we obtained the following conclusions:(1)The TOC test of 1717 samples show a significant difference in TOC content between lithologic layers.The TOC contents of the layer ? and layer ? are the lowest,which are 1.02 g.kg-1 and 1.57 g.kg-1 respectively.The highest TOC content are layer ? and layer ?,which are 12.57 g.kg-1 and 14.62 g.kg-1 respectively.The TOC contents of layer ?,layer ? and layer ? are more than 5 g.kg-1.The TOC contents of layer ? and layer ? are between 2 g.kg-1?4 g.kg-1.The mean value of the whole drilling core is 7.29g.kg-1.According to the TOC curve of XYC core,the TOC contents show a rising trend from the bottom to top of the drilling core.Based on the longitudinal variation of the TOC content change process in the XYC core,the sedimentary environment can be divided into 5 stages,they are a,b,c,d,f:Stage a is 34.34-32.08m(layer ?),stage b is 32.08-27.72m(layer ?),stage c is 27.72-22.60m(layer ?,layer ? and layer ?),stage d is 22.60-9.22 m(layer ?,layer ? and lower part of layer?),stage f is 9.22-2.20m(upper part of layer?and layer?).Among the 5 stages,the stage f is of highest content of TOC and shows quick and sharp fluctuation;The stage d is low and stable.Stage c,b,a composed a "valley-peak-valley" trend,but with the slow wave.(2)The organic carbon isotope(813C)values in the XYC core distribute between-28.1‰?-13.7‰,and most values are between-26.00‰?-23.00‰.The minimum value of-28.1‰ appears in layer?,the maximum value of-13.7‰ appears in layer?,and the mean value is-24.4‰.The ?13C mean values in different lithologic layers are also different.The layer ? is the highest,which is-23.37‰.The lowest is layer?,which is-26.60‰.From the vertical change,?13C and TOC are consistent,but at the most stable stage of TOC,the stage d,the ?13C values are more higher,so it can be recognized a stage named stage e.Therefore,the vertical changes of ?13C value can be divided into a,b,c,d,e,f six stage.The stage a is 34.34-32.08m(layer ?),and the mean value of 513C is negative,which is significantly lower than the average value of the whole core.The stage b is 32.08-27.72m(layer?),and the 813C mean value is slightly lower than that of the whole core;At this stage,the ?13C value is still negative,but is higher than the stage a and more stable.The stage c is 27.72-22.60m(layer ?,layer ? and layer ?),and the ?13C value of the whole stage is positive,which is close to the average value of the whole core.As a whole the fluctuation increases.The stage d is 22.60-13.32m(layer ? and layer ?),which is more positive than the previous stage,and values of this stage fluctuate near the mean value of the whole core.The stage e is 13.32-9.22m(lower part of layer ?),the organic carbon isotope values are obvious increasing,and it is the most positive stage of the whole core.The stage f is 9.22-2.20m(upper part of layer ? and layer ?),the 813C values of this stage turn to negative obviously compared with the stage e;The overall 813C values are low and fluctuating trend rises;Then the ?13C values increase obviously.The ?13C values of XYC core have experienced a sharp change at the bottom and top,which means that the environment of the two stages has been changed obviously.(3)There is a certain correlation between the data of XYC core.The high values of TOC correspond to the higher contents of clay.The high values of L*usually correspond to the lower TOC values,so that the TOC and ?13C change characteristics of XYC core are the same to those previous study.The characteristics of sedimentary facies have a good correspondence.Stage a,b according to TOC and ?13C correspond to river facies deposition;Stage c,d correspond to tidal flat deposition;Stage e corresponds to shallow marine sediments;Stage f corresponds to lacustrine sediments.Based on the AMS14C chronology test and deposition rate,and using the method of interpolation and extrapolation,the bottom age of XYC core is 9690 cal.a BP,the top age in layer ? is 5131cal.a BP,so the drilling core are the sediments during early and middle Holocene.(4)Based on the analysis of TOC and organic carbon isotopic data,combined with chromaticity data,grain-size data,sedimentary facies division,a small amount of foraminifera data and predecessors' conclusions,the environmental evolution of XYC core can be divided into four stage:The stage I is 9690?9107 cal.a BP(34.34-27.72m).During this time,it was a river facies environment,which could be further divided into two sub-sections:The data of the earlier stage fluctuated greatly and the environment was poor.The earlier stage(9690-9612 cal.A BP)is in an sedimentary environment of the river bed facies.In this stage,the sedimentary materials were coarse,the hydrodynamic force was strong,the TOC content was low,the ?13C values were negative and the environment was cooler;At the later stage(9612?9107 cal.A BP),the grain size was thinner,the hydrodynamic force became weakened and unstable,the TOC content increased and the 813C values increased but still negative,which means that the environment became warmer.The stage ? is 9107?8613 cal.a BP(27.72m-13.32m).which is a tidal flat environment,because the salt-brackish species Ammonia beccarii vars.being found at this stage.And this stage could be further divided into two sub-sections:The earlier stage is 9107?8932 cal.a BP.During this time,the TOC content values reduced but the ?13C values increased.The later stage is 8932?8613 cal.a BP,during this time,the TOC content values increased slightly and stayed stable,and the 813C values were stable and the mean value was around the mean value of the whole core.In a word,the environment of this stage was low tide gradually transiting to climax tide,which indicated that the climate continued changing to warm and the sea level was rising gradually.The stage ? is 8613?7330 cal.a BP(13.32-9.22m).During this time,the TOC content values were stable,but the ?13C values were the most positive in the XYC core.And the ?13C values were close to those of marine algae.The marine species Elphidium advenum could be found.The contents of fine grain deposition increased,and the hydrodynamic was stable,which stood for shallow sea environment.Based on all analysis above and compared to the researches in surrounding areas,we speculate that this time should correspond to the Holocene high sea level,and the climate was warm and humid.The stage IV is 7330-5131 cal.a BP(9.22-2.20m).During this time,the TOC content values increased rapidly but the ?13C values turned to negative significantly.The grain size changed from thin to coarse,and the a*and b*values firstly decreased then increased,and two peat layers mean that the environment changed from lagoon to freshwater lake.In addition,the process of early and middle Holocene environmental changes in XYC drilling core also recorded the Holocene Megathermal(8.6-5.6 ka)and the 5.5 ka BP event,which was in good agreement with the environmental records of BHQ drilling core on the north bank of Hangzhou Bay.
Keywords/Search Tags:The middle and lower reaches of Cao'e river, Total organic carbon, Organic carbon isotope, Environmental change
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