The Geochemical Characteristics Of DissolvedInorganic Carbon And ?¹³C_DIC In Karst River

River system is important to global biogeochemical cycles,and land erosion is the main input channel to the ocean.Recent studies suggest that vertical carbon?C?fluxes?1.8 PgCyr-1?released into the atmosphere are twice the lateral fluxes?0.9 PgCyr-1?to the ocean,accounts for 43% of the C degassing flux from inland waters including rivers,lakes and wetlands,and acts as an important role in the atmospheric C budgut.At the local scale,riverine C is a significant source of energy to riverine ecosystems?i.e.for aquatic biota?and closely influences the geochemical nature of the aquatic environment.The karst drainage basin in southwest China is characterized by outstanding geochemical sensitivity and ecological fragility,where natural weathering is the main force of the provenance of substance in the water,while,the human disturbance make the geochemistry and C sources in the river more complex.Relying solely on major ion composition is not enough for distinguishing different C sources in the river,while stable C isotopes can help to trace the source of the matters in the rivers.This article selects Sancha River Basin as the research object which located in the karst-dominated terrain of Guizhou province,southwest China.The conductivity?EC??pH?the total dissolved solids?TDS??dissolved inorganic carbon?DIC?and ?¹³C were determined from the river water to analyze the source of dissolved inorganic carbon?the proportion of sulfuric acid dissolution of carbonate rocks and its contribution to DIC concentration in February 2014?dry season?and August 2014?wet season?.Moreover to understand the distributions and influences of pCO₂ in medium and small karstic rivers and estimate the CO₂ emission flux from Sancha River to atmosphere.The results are showed as follows:?1?The average temperature value of water samples in dry season and wet season of Sancha River Basin is 15.7?.The average value of pH is 7.8,slightly alkaline.The average value of conductivity is 487 us/cm.The average concentration of total dissolved solid is 340.46 mg/L,excced than the world average value of 100 mg/L.The average value of dissolve inorganic carbon is 2.17mmol/L.The results show that EC?pH?TDS and DIC in dry season were higher than that in wet season.?2?DIC concentration is mainly affected by dissolution of carbonate minerals and soil CO₂.According to carbon isotopic composition of soil CO₂ and carbonate rock,61% of the DIC is calculated to originate from soil CO₂,respectively,and 39 %of the DIC is calculated to originate from carbon rock in dry season,while,52% of the DIC is calculated to originate from soil CO₂,respectively,and 48% of the DIC is calculated to originate from carbon rock in wet season.?3?The contributions of carbonate dissolution by sulfuric acid to total?Ca2++Mg2+?and DIC in groundwater is 73% and 46% in dry season.While the contributions of carbonate dissolution by sulphuric acid to total?Ca2++Mg2+?and DIC in surface water ranged is 56% and 29%,respectively,demonstrated that sulphuric acid is an important agent of carbon rock weathering.The results of this study have a great significance for the protection and development and utilization of water resources in the Sancha River and for karst carbon cycle research.?4?The pCO₂ in the surface water ranges between 300 and 10000?atm,with an average value of 3100?atm.The pCO₂ values in the wet season were higher than that in dry season.The analyses of supersaturated CO₂ and apparent oxygen utilization demonstrate that pCO₂ was controlled by the carbonate system in the dry season.The in-situ aerobic respiration was one of the significantin fluencing factors in the wet season,and the supersaturated CO₂ may attribute to the flushing of carbon dioxide from soils.?5?The CO₂ emission flux from Sancha River to atmosphere was estimated about 0.9-1.7×109mol and 10.8-20.3 MgC·ha-1·a-1.The water-to-air CO₂ outgassing flux from Sanchahe River was higher than that from large rivers?i.e.Amazon River?Yangtze River?and lower than that from streams?i.e.Houzhai River?G?ddtj?rn River?.The results indicate that the CO₂ emission fluxes from surface water systems are influenced by the scale of rivers,and the contribution to the regional carbon cycling brought by the medium and small river may be underestimated for a long time.