Silicon isotopic composition of dissolved silicon and suspended particulate matter in the Yellow River, China, with implications for the global silicon cycle

The silicon isotopic composition of dissolved silicon and suspended particulate matter (SPM) were systematically investigated in water samples from the mainstem of the Yellow River and 4 major tributaries. The SPM content of the Yellow River varied from 1.4 to 38,560 mg/L, averaging 3568 mg/L, and the delta Si-30 of suspended particulate matter (delta Si-30(SPM)) varied from 0.3 parts per thousand to -0.4 parts per thousand, averaging -0.02 parts per thousand. The major factors affecting the SPM content and the delta Si-30(SPM) values in the Yellow River were inferred to be the mineralogical, chemical and isotopic characteristics of the sediments from the Loess Plateau and a combination of the climate and the flow discharge of the river. The major ions in the Yellow River water were Na+, Ca2+, Mg2+, HCO3-, SO42- and Cl-. High salt concentration was observed in samples from the middle and lower reaches, likely reflecting the effects of evaporation and irrigation because the Na+, Mg2+, SO42- and K+ concentrations were correlated with the Cl- concentration. The dissolved Si concentration (D-Si) increased downstream, varying from 0.016 to 0.323 mM. The delta Si-30 of dissolved Si (delta Si-30(Diss)) varied from 0.4 parts per thousand to 2.5 parts per thousand, averaging 1.28 parts per thousand. The major processes controlling the D-Si and delta Si-30(Diss) of the Yellow River are (a) the weathering of silicate rocks, (b) the formation of phytoliths in plants, (c) the evaporation of water from and the addition of meteoric water to the river system, which only affects concentrations, (d) the adsorption and desorption of aqueous monosilicic acid on iron oxide, and (e) the dissolution of phytoliths in soils. The D-Si and delta Si-30(Diss) values of global rivers vary spatially and temporally in response to changes in climate, chemical weathering intensity and biological activity. The moderately positive delta Si-30(Diss) values observed in the Yellow River may be attributed to the higher rates of chemical weathering and biological activities that have been observed in this catchment in comparison with those of other previously studied catchments, excluding the Yangtze River. Human activities may also potentially influence chemical weathering and biological activities and affect the D-Si and delta Si-30(Diss) values of the major rivers of the world. Further river studies should be performed to gain a better understanding of the global Si isotope budget. (C) 2011 Elsevier Ltd. All rights reserved.