Chemical weathering processes in the Chinese Loess Plateau

Covered by erodible loess and affected by significant seasonal climate variations,chemical weathering in the Chinese Loess Plateau(abbreviated as CLP)has important effects on the hydrochemistry of the Yellow River and the global carbon cycle.However,chemical weathering processes in the CLP are still unclear.Based on 296 river water samples in the CLP in the different seasons,hydrochemistry,weathering pro-cesses,and their controlling factors were revealed.River waters in the CLP exhibited slightly alkalinity(pH=8.4±0.5)with much high total dissolved solids(TDS)values(691±813 mg/L).The water types of river water in the CLP were primarily SO42--Cl--Na+,HCO3--Ca2+-Mg2+,and SO42--Cl--Ca2+-Mg2+.According to the forward model,evaporite dissolution has the largest contri-bution(55.1％±0.2％)to riverine solutes in the CLP,then followed by carbonate weathering(35.6％±0.2％)and silicate weathering(6.5％±0.1％).For spatio-temporal variations,the contribution of evaporite disso-lution in the CLP decreased from northwest to southeast with higher proportion in the dry season,car-bonate weathering increased from northwest to southeast with a higher proportion in the wet season,and silicate weathering showed minor spatio-temporal variations.Ca2+and Mg2+were affected by car-bonate precipitation and/or incongruent calcite dissolution,and about 50％of samples exhibited cation exchange reactions.The physical erosion rate in the CLP,which was 372±293 t km-2·yr-1,varied greatly and was greater than those of other worldwide rivers.Chemical weathering rates in the CLP showed an increasing trend southward.During the wet season,high runoff led to the release of evaporite and car-bonate from loess,while the interfacial reaction kinetic limited the increase of the silicate weathering rates.The CO2 consumption budget by carbonate weathering(6.1×1010 mol/yr)and silicate weathering(1.6 ×1010 mol/yr)in the CLP accounted for 0.29％and 0.08％of the global carbon cycle,respectively.Meanwhile,the weathering proportion by sulfuric acids was relatively high with a CO2 release flux of 6.5 × 109 mol/yr.By compiling the data,we propose that the interfacial reaction kinetic and runoff con-trol CO2 consumption rate by silicate and carbonate weathering,respectively.These results contribute to the understanding of modern weathering processes of loess in the CLP,thus helping to deduce the envi-ronmental and climatic evolution of the basin.