Preliminary Study On Lignin Phenols In Sediments For Tracing Terrigenous Organic Matter And Reconstructing Environmental Change

Burial of terrigenous organic carbon on continental shelves plays an important role in the global carbon cycle.To better constrain the characteristics and distribution of terrigenous organic carbon in Bohai Sea,Yellow Sea and East China Sea shelves, lignin phenols in sediments were analyzed,combined with grain size,OC%,TN% andδ¹³C.Additionally,organic geochemical research on sediment cores from river catchments and continental shelves would help to understand the factors controlling integration,transport and burial of terrigenous organic carbon.Hence,those parameters in cores from middle Changjiang River and Yellow Sea were analyzed for tentative studies.The contributions of terrigenous organic matter to sedimentary organic matter were 3%～69%in Bohai Sea,Yellow Sea and East China Sea.Higher values were found in the main accumulating areas of fluvial sediments,including the Changjiang estuary,modern and old estuaries of Yellow river and Zhejiang-Fujian coastal zone. On the contrary,lignin concentrations were lower in central Bohai Sea,northeastern and central Yellow Sea and offshore stations in East China Sea.Generally,lignin in research areas originated from mixed nonwoody and woody angiosperms and was highly degraded.In shelf zone adjacent to Changjiang estuary,soil-derived organic matter dominated the terrigenous organic matter.Distinct Pd/Pon values were found for Bohai Sea and East China Sea,mainly due to differences in suspended sediments from Yellow river and Changjiang river.Illustrated by this proxy,terrigenous organic carbon in south Yellow Sea were derived from Changjiang river in areas south to 33°N,from old Huanghe estuary in coastal zone and from modern Huanghe river at offshore stations north to 33°N.As a widely used tracer of terrigenous organic matter, the burial of lignin in East China Sea was rather efficient,probably associated with the recalcitrant property of terrigenous organic matter and relatively high sedimentation rates in Changjiang Estuary and Zhejing-Fujian muddy zone.Lignin characteristics of a peat core collected from Lake Dajiu in the middle stream of Changjiang river were studied to investigate the generation and preservation of terrigenous organic matter in past 12.8ka.C3 vegetation was the main source of the organic matter and nonwoody angiosperms were the dominant vascular pants.The variation of OC%corresponded with vegetation parameters in different climatic conditions,which suggested the important influences of vegetation changes on OC%. (Ad/Al)v and(Ad/Al)s indicated moderate oxidation degradation states of lignin, while good correlations forΛ8 vs.P/(V+S) and DHBA/V suggested demethylation/demethoxylation should be the main decomposition pathway.Higher microbial alterations during warm and wet Holocene Climate Optimum were also indicated compared with those in cool and dry periods.In another core from Swan Oxbow in middle reach of Changjiang river,organic geochemcial characteristics were influenced by natural environment evolution and human activities.Bottom sediments were characterized by coarse grain,low organic matter content and high degradation grade,indicated that they were deposited before channel migration of Changjiang river in 1972.The sediment grain size decreased and the organic matter showed increased concentration and became fresher upwards, maybe due to more terrigenous matter inputs and higher eutrophic levels.Agriculture development in 1980s might increased terrigenous organic matter inputs while the establishment of natural reservation zones in early 1990s reduced the terrestrial inputs. The 1998 extraordinary flood yielded a 2cm layer of much higher OC%,TN%,Σ8 and A8 while lower(Ad/Al)v.Thereafter a dam was built and resulted in reducing hydrodynamic force and grain size.Furthermore,another 2 flood deposit layers were inferred according to the characteristics of 1998 flood layer,maybe corresponding with upstream floods in 1974 and 1981.In a sediment core from Jiaozhou Bay,the source and burial of organic matter from 1840s～early 1910s were stable with low terrigenous inputs,which provided the background for assessing the influence of activated human activities during twenty century.After 1910s,terrigenous organic matter increased generally while autochthonous organic matter increased firstly but decreased after 1990 due to urbanization and industrialization of Qingdao.In addition,changes of lignin contents corresponded with that of decennary precipitation of flood season,suggested that the latter might be an important factor controlling the terrigenous input. Finally,E2 core from central Yellow Sea was analyzed.Lignin concentrations and C/N were higher from 118cm to 154cm,though both decreased at 117cm and 50cm upwards,accompanied with elevated degradation states.This indicated the contributions of terrigenous organic matter were higher at the bottom and reduced above 117cm.Comparison of C/N,OC%and lignin content indicated a reverse trend of autochthonous organic matter.It was suggested that such changes might be associated with channel migration of the Yellow river,temperature changes, construction of riverbanks,flood frequencies and human activities in history.In addition,the high OC%in upper layers could be explained by algal spring bloom.In conclusion,these core studies suggested that climate change was an important factor influencing the generation and preservation of terrigenous organic carbon, while the transport processes might be significantly regulated by factors such as precipitation in flood season,flood frequency,construction of riverbanks,agricultural and industrial activities and migration of river course.