Spatio-temporal Variations In Riverine Mg Isotopes Within The Yangtze River Basin And Its Tracing Continental Carbonate Weathering

Chemical weathering regulates global carbon budget and climate change by consuming atmospheric CO₂ as a negative feedback mechanism,maintaining the long-term climate stability and habitability of the Earth system.In particular,silicate weathering induced by tectonic uplift is thought to have driven climate change on the million-year scale.On short time scales（ten thousand years-hundred years）,carbonate weathering is a very important process of carbon sink.Due to the rapid dissolution kinetics of carbonate minerals,carbonate weathering can respond rapidly to short-term events such as hydrological process and land use,and is therefore considered a core mechanism for explaining climate change at short time scales.Thus,the importance of carbonate weathering for the carbon cycles cannot be ignored,especially in the current century,when uncertainties in climate and environmental change have increased significantly and when intensified human activities have greatly perturbed the composition of atmospheric chemistry and surface processes.Therefore,it is crucial to constrain and quantify continental carbonate weathering,in order to better understand the mechanisms of the carbon cycling and climate change on short time scales.River catchments are areas of convergence of physically eroded and chemically weathered materials,carrying the exchange and circulation of matter and energy.The geochemical characteristics of rivers can reflect the processes of surface denudation and weathering.Constraining carbonate weathering processes by studying the elements and isotopic composition within the basin systems can contribute to an in-depth understanding of the relationship and mechanisms of global carbon cycling and climate change on short time scales.However,the constraining of carbonate weathering processes in watersheds in previous studies is far from adequate,and more field studies are needed to explore robust alternative proxies to better trace carbonate weathering.With the improvement of analytical techniques,magnesium（Mg）isotopes have been developed rapidly,and widely used in tracing surface processes and chemical weathering.Riverine Mg isotopes are sensitive to the dissolution of primary minerals/rocks and have the potential to trace carbonates weathering.However,riverine Mg isotopes are significantly fractionated during supergene geochemical processes,including secondary mineral formation,ion exchange,adsorption,and biological effect.Further evaluation of these fractionation processes is needed to test the effectiveness and robustness of riverine Mg isotopes in tracing continental carbonate weathering.Previous studies on riverine Mg isotopes have mostly focused on small basins with single climate and lithology,but systematic Mg isotopic studies on large river basins are rare,which limits the knowledge of how Mg isotopes respond to carbonate weathering at a continental scale.In addition to spatial studies,previous studies on the time-series variation of Mg isotopes in river water also have shortcomings,such as low temporal resolution,concentrated in glacial and frozen regions,and affected by upstream human hydropower activities.Large river catchments represent the continental crust basin,and brings together the main information on terrestrial crust weathering.Field studies of Mg isotopes in large river basins can help quantify the Mg isotopic signatures of continental weathering fluxes into the oceans and its impact on the global Mg cycle,which is of great significance to further explore the relationship between continental carbonate weathering and global carbon cycle and climate change.As the world’s third largest and the Asian longest river,the Yangtze River basin originates from the Tibetan Plateau and crosses three major geomorphic landforms in China from west to east,eventually merging into the Pacific Ocean,delivering large amounts of dissolved and suspended loads to the ocean and having a significant impact on the global material cycle and carbon budget.The Yangtze River basin is characterized by significant differences and change in lithology,altitude,and climate,so is an ideal natural setting to test the response of river Mg isotopes to continental carbonate weathering.This dissertation investigated the spatial and temporal variations of river Mg isotopes within the Yangtze River basin,combining dissolved elements,Sr isotopes,hydrological data,and global riverine Mg-Sr isotopic data to explore the factors controlling riverine Mg isotopes.This study is helpful to further understand the relationship between modern continental carbonate weathering and catchment erosion,providing new insights into how carbonate weathering regulates global carbon budget and climate change on short time scales,especially in the context of present global warming and uplift of the Tibetan Plateau since the Cenozoic.The main achievements and conclusions obtained in this dissertation are as follows:（1）The spatial and temporal variations of Mg isotopic compositions in river waters within the Yangtze River basin were systematically reported for the first time.Spatially,the dissolved Mg²⁺in the headwater were dominated by evaporites dissolution,but Mg isotopes may be controlled by carbonate precipitation.The dissolved Mg²⁺in the mainstream and major tributaries mainly originates from carbonate rock weathering,followed by silicate rock weathering,with relatively little influence from both atmospheric input and human activities.Conservative mixing models based on Sr-Mg isotopes suggested that the riverineδ²⁶Mg values of the mainstream and major tributaries were mostly close to or covering the carbonate-dominated catchments end-member and responded well to carbonate weathering rate（CWR）and intensity（CWI）,indicating that carbonate weathering dominated the Mg isotopes in the Yangtze River,with limited fractionation processes.（2）In terms of time series,riverineδ²⁶Mg values at the Zhenjiangguan station in the upper reaches of the Min Jiang showed a systematic seasonal variation,which shows that the river water is enriched with heavy Mg isotopes during monsoon period and the river water is enriched with light Mg isotopes during non-monsoon period.In addition to increasing runoff and enhancing carbonate weathering,the heavy rainfall events during the monsoon also intensified the production of physical erosion materials.After excluding the formation of clay and carbonate minerals,the increase in riverineδ²⁶Mg values during the monsoon was preliminarily attributed to the response to sensitive response to hydrological events,i.e.,the preferential adsorption of ²⁴Mg by Mg-Na ion exchange created by fluvial sediment.（3）Finally,we extended the concept of the Yangtze River to global large river basins.Due to the reservoir effect,global rivers with high Mg flux and carbonate weathering contribution were closer to the carbonate end-members,corresponding to the significant dissolution of carbonates,thus buffering the fractionation processes.Large river basins with high CWI values all had low physical erosion rates（PER）and direct contributions from carbonate end-member,while rivers with low CWI values were associated with higher PER environments.Furthermore,there is a strong negative correlation betweenδ²⁶Mg and CWI values（r²=0.60）in global large river basins,indicating that Mg isotopes in large rivers can effectively trace and quantify carbonate weathering intensity with robustness.The negative correlation between river waterδ²⁶Mg values and CWI observed in spatial and temporal studies was further explained by the coupling between carbonate weathering and erosion.These results demonstrated that riverineδ²⁶Mg values within the Yangtze River basin can be utilized to trace and quantify carbonate weathering on a continental scale.In the future of global warming and increased runoff,human activities and land use will further intensify carbonate weathering,thus the carbon sink associated with will be more significant,which will have a profound impact on the global carbon cycle and climate change.According to the relationship betweenδ²⁶Mg and CWI values,the Mg isotopic signatures of continental weathering fluxes into the ocean will become lighter,thus regulating the global Mg cycle in a short time scale.