Sr fluxes and ~(87)Sr/~(86)Sr characterization of river waters from a karstic versus granitic watershed in the Yangtze River

摘要

The watershed in the southern Jiangxi Province (Jiangxi Province is called simply Gan) (SGW) and the watershed in the central Guizhou Province (Guizhou Province is called simply Qian) (CQW) are two subtropical watersheds of the Yangtze River in China. Both watersheds have similar latitudes and climate, but distinct differences in basin lithology. These similarities and differences provide a good natural laboratory in which to investigate weathering processes and Sr end-members in river waters. This work aims to identify and contrast the sources, fluxes and controls on Sr isotopic composition in the river waters of these two areas. Results showed that the ~(87)Sr/~(86)Sr in the SGW waters ranged from 0.716501 to 0.724931, with dissolved Sr averaging 27μg l~(-1). Rhyolites and granites are two major sources for the dissolved Sr. The SGW waters receive 42% of their Sr from silicates weathering, 32% from carbonates and 3.2% from evaporites. ~(87)Sr/~(86)Sr in the CQW waters has a lesser variation from 0.707694 to 0.710039, but higher Sr contents (average of 208 μg l~(-1)). Dolomite, limestone and dolomitic limestone are major sources of Sr in the waters. The CQW waters receive 69% of their Sr from carbonates, 1.7% from silicates and 0.9% from evaporites. The chemical erosion rate and Sr flux in the CQW are 1221 km~(-1) a~(-1) and 0.0791 km~(-2) a~(-1), respectively, which are higher than those of the SGW (56 tkm~(-2)a~(-1) and 0.021 t km~(-2)a~(-1), respectively). These data suggest that the intensive carbonates weathering occurred in the karstic area in the upper-reach of the Yangtze River exert great influence on the high Sr concentration and low Sr isotopic ratios in the River.