Geochemical Characteristics Of Muddy Sediments In The Late Quaternary Qiantang River Incised-valley And Its Geological Significance

The fills of incised valleys,developed in estuary and coastal areas,represent critical stratigraphic archives,in response to changes in paleoenvironment,paleoclimate,and sea level.The Late Quaternary Qiantang River incised valley,developed in the coastal plain of Zhejiang province,provides a representative area for the study of land-sea interaction in the eastern coastal areas of China.Since the Late Quaternary,the plains on both banks of the river act as one of the most important Neolithic cultural areas for rice cultivation in China.Nowadays,the incised valley has been buried and the Hangzhou city is flourishing at this area.The study of this incised valley not only has important scientific value,but also has certain social and economic benefits.Previous studies mostly focused on the features of the fill,sedimentary facies types and distribution,the evolution patterns of the incised valley,the main controlling factors,and shallow biogas.However,the depositional conditions of the fill,including palaeo-salinity,redox conditions,as well as composition and source of sedimentary organic matter,are not fully understood.In addition,the stratigraphy records of extreme climatic events and the past human-environment interaction require further study.In this study,a sediment core(SE3)has a depth of 64.0 m was retrieved from the incised valley to investigate the palaeo-salinity,redox conditions,the organic matter composition and sources,extreme event deposits,and the shallow biogas,based on the organic-inorganic geochemical parameters of the muddy sediments,obtained from the core,including total organic carbon content(TOC),total carbon content(TC),total sulfur content(TS),bulk organic stable carbon isotope,relevant saturated hydrocarbon biomarkers,isotopes of O,and C in carbonates,major and trace elements,pyrite content,sulfur isotope composition of pyrite.In addition,this study also aims to compare the applicability of palaeo-salinity reconstructive geochemical methods and related indicators from different depositional environments in the Late Quaternary Qiantang River incised valley and the Palaeocene Bohai Bay Basin,as the hypo-thermally evolved sediments.The sedimentary sequence is divided into five sedimentary units,i.e.units I?IV,in ascending order,based on their lithological features and variations in the vertical distribution of multiple palaeo-salinity indicators.Unit I consists of floodplain facies,deposited in fresh water;unit II is including the paleo-estuary facies,deposited in brackish water;unit III is deposited in the normal marine salinity,is probably storm deposit;units IV and V both consist of nearshore facies,however,unit IV is deposited in the normal marine salinity,while,unit V is deposited in the brackish to normal marine salinity.Based on our findings,the palaeo-salinity changes in the fill of the Late Quaternary Qiantang River incised valley are mainly due to the combined action of fresh water from the land-based river and seawater.Compared to the lacustrine mudstone samples from the Bohai Bay Basin,the palaeo-salinity reconstructions of the Late Quaternary Qiantang River incised valley is more challenging,due to lack of biomarkers in this sequence.In contrast,the sedimentary environment and waterbody energy of the lacustrine mudstone samples from the Bohai Bay Basin are relatively more stable.Moreover,most parts of the organic matters in the mudstones,derived from the input of autotrophic organisms in the lake,retain biomarker compounds that can be used as indicators to study the changes in palaeo-salinity.The biomarker methyl-trimethyl-tridecyl-chromans(MTTCs)is found to be effective in characterising the palaeo-salinity differences in the low thermal degree lacustrine mudstone samples from the Bohai Bay Basin.Hence,a new ternary diagram of mono-,di-,and trimethyl-MTTCs is established to indicate the changes in the palaeo-salinity of the depositional environments.Regarding the reconstruction of redox conditions in the fill of the Late Quaternary Qiantang River incised valley,the related parameters of trace and rare earth elements fail to respond significantly to the changes in redox conditions.This is closely related to the strong sea-land interactions,the strong influence of land source debris input,and the depositional background of multiple sources in the fill of the Late Quaternary Qiantang River incised valley.In contrast,the redox conditions of different units in the SE3 core can be easily distinguished by the biomarker compound parameter of Pristane/Phystane(Pr/Ph).Changes in the palaeo-salinity and redox conditions tend to respond synergistically to the changes in sedimentary organic matters.The rock pyrolysis parameter hydrogen index(HI)indicates that the sedimentary organic matters of units I?V in the SE3 core are dominated by terrestrial plant input,reflecting the feature of sedimentary organic matters in the estuarine and coastal environments are mainly originated from the land-based river input.The bulk rock organic carbon isotopes and saturated hydrocarbon biomarkers parameters effectively reveal the differences in the contributions of C₃ and C₄ plants,as well as the contributions of terrestrial plants and aquatic algae/microbes in the organic matters from different sedimentary units.Indeed,the bulk rock organic carbon isotopes and saturated hydrocarbon biomarkers parameters strongly confirm that the sediments in unit III are deposited during an extreme event,while,the sediments of IV and V units are deposited in a shallow marine environment,however,significant differences are existed in these two units.In addition,the development of the paleo-estuary environment is reconfirmed by the sedimentary organic matter composition to be significantly different from that of the underlying channel-flood plain sediments and the overlying shallow marine sediments,with a"transitional"feature.Variations in the sedimentary organic matter input from the Late Quaternary Qiantang River incised valley are mainly controlled by the changes in palaeo-climate,palaeo-vegetation,and sea level fluctuations.Based on multiple evidences of palaeo-salinity,redox conditions,sedimentary organic matter,sulphur isotopes,and palaeontology,we confirm that unit III is deposited during an extreme storm event.The reliable age data from this unit(e.g.unit III)is not yet known and is speculated to be in the range of 8000-9500 a BP.The further study of the sedimentary records from the storm events in this sequence may bring new breakthroughs in understanding of the past human-environment interaction in the study area.Based on the above results,the generation and preservation of shallow biogas is discussed in the study area.Based on the finding from this study,low contribution of soluble organic carbon results in production of the shallow biogas,in this area.Favourable stratigraphic temperature and timing of biogas generation are considered as key elements for the production and preservation of shallow biogas in the study area.The results from this study confirm that the fills of the Late Quaternary Qiantang River incised valley can be considered as well-preserved archives to study the palaeoenvironment,paleoclimate,and sea level changes in the eastern coastal plain of China.In future work,we will focus on exploring the integrated evolution of sedimentary organic matter,extreme climate events,sea level change,human-environment interaction,and their coupled relationship in response to global events.