Chemical weathering processes in the Chinese Loess Plateau

Abstract

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 processes, 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 contribution (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 dissolution in the CLP decreased from northwest to southeast with higher proportion in the dry season, carbonate 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 carbonate 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 carbonate 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 control 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 environmental and climatic evolution of the basin.