Hydrogeochemistry And Mineral Thermochronology Constraint On Weathering,Erosion And Provenance In The Pearl River Basin Since Cenozoic

As mediums connecting the land and the ocean,rivers transport terrestrial materials into the ocean,which are important parts of the global geochemical cycle.The modern Pearl River originates in the southeast of the Tibet Plateau,flows through the South China Plate,and finally flows into the South China Sea.The Pearl River sediments record the uplift history of the Tibet Plateau,the evolution of the Asian monsoon,and information about weathering,denudation,transport,and deposition.The sediments also have been an important origin of the northern South China Sea since the Cenozoic.In view of the complex sediment composition and the source-sink system,this thesis adopts a variety of research methods,including hydrogeochemistry,heavy minerals,zircon U-Pb dating,and muscovite ⁴⁰Ar/³⁹Ar dating.In this study,hydrogeochemical analysis of dissolved matter in the Pearl River was carried out to reveal the chemical weathering process and carbon sink flux in the Pearl River Basin.Detrital zircon U-Pb dating and muscovite ⁴⁰Ar/³⁹Ar dating were performed on modern sediments in the Pearl River Basin to analyze the contribution of each tributary to the sediment flux of the main stream,and to reveal the modern erosion model of the Pearl River and its controlling factors.Then we combined the relevant data of the core samples in the Pearl River Mouth Basin and sediments in Pearl River to reconstruct the evolution history of the paleo-Pearl River,to reveal the evolution of the source-sink system of the Pearl River Basin and the Pearl River Mouth Basin,and to explore the relationship between the development of the paleo-Pearl River and tectonic activity,the Asian monsoon and the topography of South China.Hydrogeochemical studies have shown that river water in the Pearl River Basin is generally neutral to weakly alkaline.The hydrochemical types of the Xijiang and Beijiang rivers are Ca-HCO₃^-type,while the waters of the Dongjiang River are Na,Ca-HCO₃^-type.The spatial variation of the main ion concentration,Sr concentration,and ⁸⁷Sr/⁸⁶Sr ratio in river water is mainly related to the lithology distribution of basement rocks in the basin.The concentrations of most major ions were lower in the high-water period than in the low-water period.The seasonal variation of ions is jointly affected by factors such as climate,runoff,vegetation characteristics,and weathering erosion rate.The construction of the inversion model shows that the weathering rate of silicate rocks（SWR）in the Xijiang River Basin is 0.81 t/km²/year,and the weathering rate of carbonate rocks（CWR）is 18.62 t/km²/year.The SWR in the Beijiang River Basin is 2.65 t/km²/year,and the CWR is 16.47 t/km²/year.The SWR in the Dongjiang River Basin is 2.57 t/km²/year,and the CWR is 7.64 t/km²/year.In terms of seasonal variation,compared with the increase amplitude of runoff in the wet season,the increase amplitude of SWR in the Pearl River Basin was larger,while that of CWR was smaller.The temporal and spatial variation of rock chemical weathering rate is controlled by various factors such as bedrock lithology,climate,temperature,runoff,rainfall,and vegetation distribution.The CO₂ flux consumed by chemical weathering of rocks in the Pearl River Basin is 224.86×10⁹ mol/year.The CO₂ flux consumed by the weathering of silicate rocks and carbonate rocks in the Pearl River Basin is 47.28×10⁹ mol/year and 177.58×10⁹ mol/year,respectively,accounting for0.23%and 0.85%of the global CO₂ flux consumed by rock weathering.Sulfuric acid has an important impact on the chemical weathering of rocks and carbon sinks in the Pearl River Basin.On a time scale of 10⁷ years,the net carbon sink flux in the Pearl River Basin is 8.27×10⁹ mol/year.The net carbon fluxes of the Beijiang and Dongjiang basins are3.93×10⁹ mol/year and 8.27×10⁹ mol/year,respectively,while the net CO₂ release flux of the Xijiang basin is 4.31×10⁹mol/year.Based on the diversity of detrital muscovite ⁴⁰Ar/³⁹Ar ages and zircon U-Pb ages in the main stream and various tributaries of the Pearl River,the sources of the main stream sediments are revealed.The detrital muscovite ⁴⁰Ar/³⁹Ar dating data show that the Yujiang River is an important source of muscovite in the main reaches of the Xijiang River,while the Guijiang River and Hejiang River are not important sources.The zircon U-Pb dating data of detrital zircons show that Yujiang and Hejiang are important origin areas for zircons from the main stream of Xijiang River,while Hongshuihe River and Guijiang River are not the main provenance areas.The difference may be due to the difference in the transport rate of detrital muscovite and zircon in the river,the difference in the resistance of mineral grains to physical and chemical weathering,and the age composition and thermal history of the rock in the source area.The southeastern margin of the Tibet Plateau is characterized by active tectonic activities and steep terrain.Surprisingly,the Hongshui River,which flows through the southeastern margin of the Tibetan Plateau,is not the main source of sediments from the main stream of the Xijiang River.The"young"erosion patterns recorded by suspended sediments shows the Liujiang River is the most important source of suspended sediment in the main stream of the Xijiang River.The Liujiang River Basin is characterized by high rainfall,runoff,and widely distributed farmland.The"old"erosion model recorded by mineral thermochronology shows that the summer monsoon in the early Holocene was stronger than today,and the summer monsoon advanced westward to the eastern valley of the Tibet Plateau,resulting in enhanced summer rainfall in the Yujiang River Basin during the early Holocence period.The high contributions of muscovite and zircon minerals may be related to the increase in rainfall and runoff caused by the enhanced summer monsoon during the early Holocene period.In addition,the Neolithic human sites in Yujiang are densely distributed,which is consistent with its large sediment flux.Therefore,on the millennium scale,the erosion pattern of the Pearl River Basin is mainly affected by climate and human activities.Structure and bedrock lithologies may be not important controlling factors for the erosion of the Pearl River Basin.This study does not rule out the influence of tectonic evolution over a longer geological period on the erosion pattern of the drainage system.On the basis of previous studies,comparative analysis of the mineral thermochronology data of the Pearl River Basin and the Pearl River Mouth Basin revealed the evolution history of the paleo-Pearl River,analyzed the Cenozoic source-sink system in the Pearl River Mouth Basin,and discussed the relationship of paleo-Pearl River development,tectonic activities,Asian monsoon and South China topography.（1）During the Eocene Wenchang Formation period,the eastern part of the Cathaysia plate was relatively high,and the paleo-Pearl River developed the paleo-Dongjiang River and the paleo-Beijiang River.Due to strong rifting,a series of uplifts and depressions developed in the Pearl River Mouth Basin.The sediments in the Zhu I Depression and Zhu II Depression mainly come from the uplifted highlands in the basin.The sediments in the southern part of the Zhu III Depression mainly come from the intrabasin uplift（Shenhu uplift）and Hainan Island.（2）During the depositional period of the early Oligocene Enping Formation,the southeastern Tibet Plateau began to uplift rapidly.The paleo-Xijiang River began to develop.The paleo-Pearl River provided sediments to the Zhu I Depression,and the sediments could reach as far as the Panyu Uplift.The northern area of the Zhu III Depression received sediments from the Yunkai Block,and the southern area received sediments from Hainan Island and the Shenhu Uplift.（3）During the depositional period of the Zhuhai Formation in the late Oligocene,tectonic activities,such as the uplift of the Tibet Plateau and the rift stage of northern margin of the South China Sea,caused the landform of South China to tilt eastward,and the paleo-Xijiang River expanded westward to the Hongshui River Basin.The Pearl River provided sediments to most areas in the central part of the Pearl River Mouth Basin.The sediments could reach as far as the Liwan Depression in the south and the northern area of the Zhu III Depression in the west.The eastern part of the basin mainly received sediments from Dongsha uplift and Taiwan Island,while the western part of the basin received sediments from the paleo-Pearl River,Shenhu uplift and Yunkai block.（4）During the Miocene Zhujiang Formation,the uplift of the Tibet Plateau led to the westward expansion of the Pearl River system.The paleo-Pearl River gradually evolved into the current model.The uplift of the Tibet Plateau also led to the enhancement of the East Asian summer monsoon and the increase of the deposition rate of the South China plate.The paleo-Pearl River provided sediments to most areas of the Pearl River Mouth Basin.