Characteristics Of Biomarkers In Wetland Sediments Of Yangtze Estuary And Their Significance In Indicating The Source Of Carbon Pool

Coastal wetlands sustain the highest rates of carbon sequestration per unit area of all natural systems primarily.The area of coastal wetlands accounts for only 0.5%of the total ocean area,but 50%?71%of the global marine sediment carbon burial derives from it.The main source of organic carbon can be characterized as either local or exogenous,but their contribution varies greatly in different costal ecosystems.Heterogeneous chemical structures and degradation of organic carbon molecules leads to different selective preservation of specific organic compounds.Therefore,exploring the source and degradation mechanism of organic carbon in wetlands is of great significance when it comes to evaluating the carbon pool capacity.The sediments of Yangtze Estuary were affected by both seawater and rivers,resulting in the complex sources of organic carbon.Hence,this research selected the intertidal wetland of Yangtze Estuary as the main research area,and performed spatial samplings seasonly to investigate the composition and characteristics of typical biomarkers in sediments and the stable isotope of lignin phenols,which assisted in quantifying the terrestrial,local and marine source of organic carbon in the wetland of Yangtze Estuary.What's more,this research also concluded the burial and degradation modes of organic carbon and refines the contribution sources of carbon pool.The main results are as follows:The average of n-alkanes(n-?Alk)in surface sediments was 7.348±5.927?g/g,which followed a decreasing trend from inland to estuarine along the Yangtze River Diluted Water.The short-chain showed an obvious even carbon preference,while the long-chain showed an obvious odd carbon preference.The indexes of n-alkanes such as ACL,TAR,1/Pmar-aq and AI indicated the carbon pool consists primarily of terrigenous sediments.The average lignin phenols normalized to OC(?₈)in surface sediments were 6.04±1.73mg/100mg OC.As an essential determinator of the content of organic carbon pool in Yangtze Estuary,lignin phenols were the lowest in summer(2.41±1.33 mg/100mg OC)and the highest in spring(10.63±6.93 mg/100mg OC).S/V and C/V implied that the majority of lignin originate from angiosperms and herbs,supplemented by angiosperms and woody plants.The indicators of degradation,such as(Ad/A1)v,(Ad/A1)s,P/(V+S),confirmed the extensive degradation in surface sediments.Similarly,the lignin degradation was lower in summer and autumn which mainly came from fresh plants.This research implemented a three-end-member mixing model based on Monte Carlo(MC)simulation to measure the variance of organic carbon inputs to the Yangtze Estuary.It could be concluded that organic carbon inputs consist of 55.0%±13.4%terrestrial particulate matter,38.8%±13.7%marine plankton and bacteria and 5.7%±2.2%local plant source.The terrestrial source was dominated by particulate matter carried by the runoff of the Yangtze River.The high proportion of marine source was located on the north bank of the Yangtze Estuary,which was greatly affected by seawater tide.The compound-specific isotope analysis(CSIA)was proposed to analyze lignin-derived phenols,which were made up of 81.7%±8.1%C₃ plant and 18.2%±8.1%C₄plany,respectively.The organic carbon transported by the runoff of the Yangtze River is mainly from herbaceous tissue of angiosperms,mixed with a small amount of woody tissue of angiosperms.In the area with large vegetation biomass,local vegetation is a critical input of organic carbon.Partial lignin from the runoff of the Yangtze River has been transported and deposited for a long time,while the lignin from angiosperms is relatively fresh.On the other hand,organic carbon from herbaceous tissue is either transported over a short distance or imported from the local vegetation.N-alkanes and lignin were stable in sediment organic carbon pool.n-?Alk was low in surface whereas high at the depth of 10?25cm.With the increase of depth,the odd-to-even carbon preference of alkanes in the short chain decreased,which was caused by the declining microbial activities and the increasing maturity of fatty acid reduction process.TAR,ACL,1/Pmar-aq,AI and other indicators showed that terrestrial organic carbon was relatively stable.Compared with woody plants,the organic carbon input from herbaceous plants is easier to decompose.The content of lignin phenols increased first and then decreased as depth increases.It showed the significance of local plants input.As burial time went by,lignin content decreased after microbial decomposition activity.The degradation indexes,such as(Ad/Al)v?(Ad/Al)s?P/(V+S)?3,5-Bd/V,showed that the degradation process of lignin phenols restricts the lignin content to a certain extent.With the increase of burial depth,lignin had a certain degree of degradation.Based on the three-end-member mixing model,the contribution of marine source decreased first and then increased slightly in the vertical direction.Due to the high stability and maturity of terrestrial organic carbon,the contribution of terrestrial particulate matter did not change slightly and decreased.At the depth of 10-15cm,the vegetation residued and plant roots from local sources were decomposed and transformed into organic carbon,and the proportion had increased significantly.In conclusion,the organic carbon pool of wetlands in Yangtze Estuary is mainly from terrestrial source.Firstly,rivers dominated by the Yangtze River carries a large amount of sediments and suspends particulate matters from the middle and upper reaches.Eventually,organic carbon in estuarine wetlands is composed of what accumulates at the estuary.Secondly,in the coastal and parts of northern coastal,organic carbon embraces the marine sources of surface sediments.Thirdly,wetland vegetation is also a supplementary source of sediment organic carbon.As for the burial characteristics of wetlands in Yangtze Estuary,terrigenous input contributed greatly,while there has been extensive degradation in the early stage of input.