Spatial And Temporal Distribution Characteristics And Sources Of Organic Matter In Sediments Of The Yellow River Estuary

Lipid-like biomarker compounds in marine sediment organic matter can better preserve information about the carbon skeleton of the original biomass and are the ideal proxy for marine paleoenvironmental and palaeoclimatic studies.Based on information related to the molecular composition parameters of lipid-like biomarkers in sediments,it is important to indicate the composition structure and biogeochemical transformation processes of organic matter in marine sediments for applications in evaluating regional ecological and environmental changes and marine global history studies.The coastal zone at the edge of the continental shelf is a complex area of mixed input of organic matter from terrestrial,oceanic,and anthropogenic sources,and belongs to one of the most active global biogeochemical carbon cycles.The study of the composition and sources of lipid-like biomarkers in the sedimentary organic matter of the marginal seas is of great scientific significance in revealing the historical processes of coastal ecological changes and evaluating the impact of regional ecological changes and anthropogenic influences on the composition of organic matter in the sediments of the marginal seas of the shelf.In this paper,sediments in the shallow waters of the Yellow River Estuary in the northwestern part of Laizhou Bay were selected for study,based on the dating results of ²¹⁰Pb and ¹³⁷Cs radionuclides.The spatial distribution characteristics of total organic matter(TOC,TN),stable isotopes(?¹³C_org,?¹⁵N_TN),and biomarkers of fatty acids,fatty alcohols,and aliphatic hydrocarbons in 70cm sediment column cores were studied and their sources were analyzed,and the impact of economic development in the Yellow River basin and along the coast on the sedimentary environment of the Yellow River Estuary was preliminarily analyzed.Further exploring the influence of anthropogenic factors involved in the composition of organic matter in the sedimentary environment of the Yellow River estuary over a 100-year scale,the results of the study indicate that:1.The TOC and C/N values of the total organic matter in the sediment core of the Yellow River Estuary are 0.04%?0.10%and 7.41?17.68,respectively.The composition parameters of organic carbon isotopes and nitrogen isotopes(?¹³C_org,?¹⁵N_TN)reveal that the organic matter composition of the Yellow River Estuary sediments is the type of organic matter input from terrestrial and marine endogenous sources.The comprehensive dating results of the ²¹⁰Pb and¹³⁷Cs sediment cores show that the content of organic matter in the sedimentary environment of the Yellow River Estuary between 1929 to 2015 showed an overall upward trend as the sedimentary depth became shallower.2.Three types of alkanes,including n-alkanes,isoprenoids,and UCM,were detected in free aliphatic hydrocarbons in the sediments of the Yellow River Estuary.The relative content of n-alkane is 0.34?2.28?g/g,and the carbon number distribution range is n-C₁₄?n-C₃₃.The composition parameters of aliphatic hydrocarbon biomarkers such as L/H,TAR,n-C₁₆ index,and UCM show that the organic matter in the sediments of the Yellow River Estuary is the type of organic matter that is mixed with petroleum hydrocarbon pollutants from marine,terrestrial and human activities.And it is greatly affected by the input of terrigenous organic matter.In addition,the isoprenoid composition parameters indicated the presence of petroleum hydrocarbon pollutants with a low degree of degradation in the sedimentary environment of the Yellow River Estuary.And as the sediment core section becomes shallower,the contents of organic matter from terrestrial higher plants and petroleum hydrocarbons from human activities show an increasing trend.3.Neutral n-alkanes and acidic n-alkanes were also detected in the bound organic matter of the Yellow River Estuary sediments,The carbon number distributions are n-C₁₄?n-C₃₇ and n-C₂₀?n-C₃₈,respectively.Both types of n-alkanes show obvious advantages over high-carbon n-alkanes,indicating that the bound n-alkanes in sediments mainly come from the input of terrestrial organic matter.According to the analysis of n-alkane composition parameters such as L/H,NAR,CPI,etc.,the bound n-alkanes in the sediments of the Yellow River Estuary are a mixed source of petroleum hydrocarbon input from human activities,terrestrial higher plant input,and marine endogenous organic matter while the n-alkanes derived from petroleum sources are the dominant contribution.And with the shallower depositional depth,the n-alkanes derived from petroleum hydrocarbons and terrestrial sources show an upward trend,which is consistent with the analysis results of free aliphatic hydrocarbons.4.Bound fatty alcohols and fatty acid components were also detected in sediment samples from the Yellow River Estuary.The composition parameters such as L/H and TAR_FA show that the bound fatty alcohols and fatty acids in the sediments of the Yellow River Estuary are mainly derived from endogenous organic matter in marine/fresh water,and have obvious main peaks at C_16:0 and C_18:0,indicating that marine The sources of algae and bacteria dominated the endogenous organic matter input.Combined with the sediment core dating results,the relative contents of fatty alcohols and fatty acids showed a decreasing trend as the sediment depth became shallower,Contrary to the above-mentioned variation characteristics of total organic matter parameters and alkane parameters.From another perspective,the main reason for the increase in the relative content of organic matter in the sedimentary environment of the Yellow River Estuary from 1968 to 2015 was the input of exogenous organic matter.5.To sum up,the analysis results of the biomarkers in the sediment column core of the Yellow River Estuary show that the organic matter content of the Yellow River Estuary is low,and the degree of marine organic pollution is still relatively light.Regional economic development and human factors have a greater impact on the composition of organic matter in the shallow waters of the Yellow River Estuary.A combination of multiple biomarker parameters showed that the input of petroleum hydrocarbon pollutants was an important source of aggravating the level of organic pollutants in the Yellow River Estuary.