Chemical Characteristics Of Carbon And Nitrogen Elements In Surface Sediments Of Zhuang He Estuary And Their Implications For The Source Of Organic Matter

The estuarine area at the intersection of river and sea is a good place for breeding organic matter from different sources,and also an ideal area to study the mechanism of organic matter generation,migration and transformation.C/N and?13C are widely used as important indicators to study the source of organic carbon,and n-alkanes,as highly stable biomarkers,are also good markers to indicate the source of organic matter.In this thesis,the Zhuang river estuary in small River Basin was selected as the study area.Samples were collected at the confluence of the estuary and in different directions of the river east and west.The distribution of organic matter in surface sediments was studied according to the grain size,total organic carbon(TOC)and total nitrogen(TN)of surface sediments.At the same time,carbon and nitrogen elements such as C/N,?¹³C and n-alkanes are combined with biomarkers to more accurately explore the source of organic matter in the surface sediments of Zhuang Estuary.The conclusions are as follows:(1)The silt content in surface sediments of Zhuang river estuary is the highest,followed by sand and clay,with an average of 59.42%,24.32%and 15.8%,respectively.The distribution range of organic carbon and organic nitrogen in Zhuang river Estuary is0.46%-1.7%and 0.04%-0.16%,respectively.The distribution of TN is highly consistent with TOC.Through linear correlation fitting of sediment with TOC and TN,it is found that fine-grained sediments have stronger adsorption effect on organic matter and higher organic matter content.(2)The C/N values of surface sediments in the Zhuang river estuary ranged from 8.86 to15.36,with an average of 11.98,indicating that the source of organic matter in the study area was a mixed input of Marine and terrigenous sources,with Marine sources as the main source and terrigenous sources as the second.The?¹³C of sediments in the study area ranges from-21.2‰to-24.4‰,indicating that the organic matter in this area is derived from a mixture of Marine and terrigenous sources.Based on the?¹³C value of sediments,the average contribution rate of organic matter in surface sediments of The Zhuang River is 62.22%and37.78%,respectively,by means of two-end element analysis.Therefore,it can be concluded that the organic matter in the Zhuang River is mainly from Marine sources.(3)The contents of n-alkanes,short-chain alkanes(C₁₁-C₂₃)and long-chain alkanes(C₂₅-C₃₅)in the surface sediments of zhuang River estuary are 722-8706ng/g,433-5532ng/g and 243-2529ng/g,respectively.The carbon number of alkanes in the study area is distributed between C₁₀and C₃₅,and the chromatographic peak type diagram shows a bimodal pattern,indicating the mixed source of organic matter.The average of the total CPI was 1.46.The average Pmar-aq of sediment is 0.73,indicating that the source of organic matter in the study area is a mixture of freshwater plants and Marine plankton.The average AI value was 0.31,indicating that herbaceous plants were the main source of higher terrestrial organic matter in the study area.?T/?M values range from 0.47 to 1.09;TAR is between 0.22 and 1.11,reflecting the weak terrigenous influence.