Seasonal contributions of catchment weathering and eolian dust to river water chemistry, northeastern Tibetan Plateau: Chemical and Sr isotopic constraints

River waters collected weekly over the whole year of 2007 from the Buha River draining to Lake Qinghai on the northeastern Tibetan Plateau were analyzed for major ions and Sr isotopes. Dissolved loads in the river exhibit distinct seasonal variability in major cation ratios and Sr isotopes over the 1 year period, reflecting seasonal differences in relative inputs from various sources and weathering reactions in the catchment. Distinct geochemical signatures suggest that eolian dust may affect river water chemistry significantly, resulting in a twofold increase influx of dissolved loads during spring relative to winter. It is noticeable that both the lowest and the highest Sr-87/Sr-86 values of the Buha River waters occurred in the monsoon season, indicating a sensitive response of carbonate versus silicate weathering sources to hydrological forcing on a seasonal basis. A significant decrease in Na/cation, together with lower Sr isotope ratios, is consistent with a greater proportion of carbonate weathering relative to silicate weathering in the early monsoon season. High temperature and increased rainfall during the peak of the monsoon facilitate an increased proportion of ions derived from silicates, partly from groundwaters, to river water. In other seasons, elemental and 87Sr/86Sr ratios vary much less, indicating a constant ratio of silicate to carbonate weathering, consistent with limited variation in discharge. Our results highlight that in a semiarid region where climatic conditions vary seasonally, in addition to silicate and carbonate contributions, supply from eolian dust may also play a significant role in controlling seasonal variations in chemistry of river waters.