Research On Weathering And Erosion In Wujiang Karst Watershed

Combined with water chemical indicators?anions,cations,carbonate saturation index,pCO₂?,multiple isotopes(?¹³C-DIC??¹⁵N-NO₃^-and ?¹⁸O-NO₃^-?834S-SO₄^2-and ?¹⁸O-SO₄^2-),?¹³C and ?¹⁵N of suspended and surface sediment,major and trace elements in the surface sediments in the Wujiang watershed,this paper analyzed the sources and influence factors of major ions,assessed the chemical weathering rates and CO₂ consumption rates,deciphered the source and flux characterization of POC and evaluated the physical weathering rates,constrained the sediment provenance and studied the chemical weathering and influence factors.The main conclusions are as follows:The main hydrochemical types were HCO₃^--Ca²⁺·Mg²⁺ and HCO₃^-·SO₄^2--Ca²⁺·Mg²⁺,with carbonate dissolution as a dominant influence factor.The ?¹³C-DIC values averaged-8.67‰;?34S-SO₄^2-and ?¹⁸O-SO₄^2-averaged-1.21‰and 4.70‰,respectively;?¹⁵N-NO₃^-and ?¹⁸0-NO₃^-averaged 7.21‰ and 3.46‰,respectively.The stable isotopic mass-balance model showed that 63%of DIC was derived from carbonate,73%of SO₄^2-was mainly derived from sulfides oxidation,and 65%of NO₃^-was mainly derived from soil organic matters.The results of hydrochemistry and carbon-sulpher-nitrogen isotopes indicated that the major chemical weathering process was carbonate dissolution by carbonic acid,and sulfuric and nitric acids were also involved in carbonate dissolution.The mean weathering rates of silicate and carbonate were 7.2 t/?km²·yr?and 76 t/?km²·yr?.The mean CO₂ consumption rates of silicate and carbonate was 1.08×10⁵ mol/?km²·yr?and 4.33×10⁵ mol/?km²·yr?,which were higher than carbonate weathering by carbonic acid alone.This indicated that sulfuric and nitric acids enhanced weathering but decreased CO₂ consumption.The carbon and nitrogen isotopes of particulate organic matter?POC?indicated different sources between suspended and surface sediment:67%of POC in the suspended were originated from phytoplankton,while 77%of POC in the surface sediment was originated from C3 and C4 plants detritus.The erosion modulus of POC in the Wujiang watershed was 0.13 t/?km²·yr?,which was lower than other world large rivers with exception of Yenisey River and Ob River.The cascade of reservoirs and climate exerted a significant influence on the POC flux in the Wujiang watershed.The POC burial in the reservoirs might be taken into account for the regional carbon cycle regardless of the operating time limit of reservoirs.Based on the suspended matter concentration and discharge of the Wujiang river mouth,the estimated physical erosion rate was 17.5 t/?km²·yr?.It was much lower than the chemical weathering rate,indicating that chemical weathering was the controlling process in the Wujiang watershed.The geochemical characteristics of major,trace and rare earth elements of the Wujiang sediments indicated the typical terrigenous sources.The sediment sources had a spatial variation with more mafic components in the upper reaches whereas felsic components in the lower reaches,which was consistent with the geological structure of the Wujiang River.The CIA values of the Wujiang sediments ranged from 63 to 86,indicating moderate to intense weathering.Climate and parent rock were important influence factors of chemical weathering.