Consumption Of Atmospheric CO₂ By Chemical Weathering In The Bishuiyan River Basin:A Mixed Silicate And Carbonate Catchment

Global imbalance of carbon source and sink is an important issue in global change science. It is a necessary way to indentify the mechanisms of the global carbon cycle with the aim of responding to the global climate change. To date, the majority of carbon cycle studies have been conducted in the “missing carbon sink”,accounted for 10%²0% of carbon sink. Rock weathering process, as part of the terrestrial ecosystem, are also actively involved in the global carbon cycle, and is one of the composition of the missing carbon sink. This paper takes the Bishuiyan river basin, a mixed silicate and carbonate catchment, as the research object, to accurately estimate rock weathering carbon sink. In this paper, a detailed hydrochemical analysis was conducted in the Bishuiyan river basin to accurately estimate rock weathering carbon sink and investigate its influence factors.The physicochemical properties and hydrochemical characteristics of the rivers in Bishuiyan cathchment displayed the obvious spatial variability, due to the special geologic terrains（granitical to carbonated）. The upstream tributary Taiping is mainly recharged by allogenic water from granite region, resulted in lower values of pH, EC,TDS than the other sampling cross-sections in carbonate region. The main cationic compositions of the basin are Ca2+ and HCO3-, and the hydrochemical type is HCO3-Ca water. The concentration order of Taiping river is Na++K+>Mg2+, while Mg2+>Na++K+ in the other sampling cross-sections, except for Ca2+ and HCO3-. As for anions, SO42- and NO3- concentrations are higher, which source from human activities.However, the lowest is Cl- concentration. In addition, most of the ion components showed seasonal variation, derived from the influence of seasonal rainfall amount.The relative contribution of solute sources end-member to the total dissolved cations for the watershed have been quantitatively estimated with dissolved load balance models, showing the results as carbonate weathering > silicate weathering >atmospheric input to the whole catchment except the granite water which is the source of the Bishuiyan river, that the silicate weathering is prior to carbonate weathering. Itcan be concluded that stratigraphic lithology have the most controls on water chemistry in the basin.The DIC concentration of Bishuiyan basin ranged from 18.30 mg/L to187.11mg/L, and δ13CDIC ranged from-13.44‰ to-8.19‰. The sources of inorganic carbon（DIC） were mainly from the soil CO₂ released by biological activity and the carbonate dissolution, depended on the indication of carbon isotope. however,stratigraphic lithology and hydrothermal conditions dominate the contribution ratio of DIC in the river from the two sources. Water-rock equilibrium calculation also shows that the rapid kinetics of carbonate dissolution controls the dissolved inorganic C（DIC）and carbon sink of the basin. Moreover, human activities such as the release of acid gases, mining activities, can accelerate chemical erosion and modify biogeochemical cycling of substance.For the Bishuiyan River Basin, average silicate and carbonate weathering rates were estimated as 13.46)yr（kmt2× and 81.51）yr（kmt2×, and CO₂ consumption rates of silicate and carbonate were 192.51×103）yr（kmmol2× and 650.51×103）yr(kmmol2×, respectively. The proportion of the carbonate weathering sink accounted for the total carbon sink is of 77.14%, while the silicate weathering sink only 22.83%. Therefore, Carbonate rock weathering dominants CO₂ consumption in the river watershed. Rock chemical weathering rates and CO₂ consumption rates are determined mainly by rock types, precipitation, atmospheric temperature, amount of river runoff and characteristics of human activities within the drainage basin.