Abstract
The dissolved molybdenum (Mo) contents and Mo isotope in water samples from the upper Xijiang River (XJR), draining the carbonate terrain, southwest China, are reported to investigate the seasonal and spatial variations, sources, ion budget, and isotopic fractionation of dissolved Mo. The results show that the Mo concentrations (5.3–18.9 nmol/L) exhibit an extensive variation along the mainstream without significant spatial pattern, but the Mo concentrations are slightly higher in the dry season than in the wet season caused by the dilution effect. There is a slight spatial tendency for δ98/95Mo to become higher along the mainstream (0.51–1.78%), while the seasonal variations in δ98/95Mo values of NPR (Nanpanjiang River) reach and BPR (Beipanjiang River) reach can be identified higher in the dry season but lower in the wet season. Based on the hydro-geochemical analysis, the sources of dissolved Mo are identified as the carbonates and sulfide/sulfate minerals weathering with a seasonal contribution. Moreover, our results suggest there is no significant Mo isotopic fractionation during weathering and riverine transportation. The calculation of Mo budget demonstrates that the dissolved δ98/95Mo of river draining the carbonate terrain is underestimated, which could significantly influence the redox history of oceans by Mo isotope model.
Highlights
In the last few decades, isotope hydrology has developed from the application of analytical techniques or methods in physics to problems of Earth and environmental science
The variations in DO were quite significant (4.9–12.4 mg/L) due to the large difference of water flow velocity in the wet season, while the variations in DO were relatively weak in the dry season (6.6–9.2 mg/L)
Our results show that the dissolved Mo concentrations vary from 5.3–18.9 nmol/L and the δ98/95 Mo values vary from 0.51% to 1.78%
Summary
In the last few decades, isotope hydrology has developed from the application of analytical techniques or methods in physics to problems of Earth and environmental science. With the rapid development of analytical techniques, many more methods were joined to the toolbox of isotope hydrologists, such as carbon isotopes in the evolutionary process of geothermal water and the carbon cycle of reservoir system [4,5], the applications of the strontium stable isotopes (87 Sr/86 Sr isotopic ratio) in surface and groundwater systems [6,7,8,9], and the nitrogen (δ15 N) and sulfur stable isotopes (δ34 S) in tracing water pollution and its sources [10,11,12,13]. Mo isotope systematics has become increasingly important as a new proxy
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