『Abstract
The Min Jiang is a major headwater tributary of the Yangtze (Chang
Jiang). Its source is in the undeveloped eastern Tibetan Plateau,
but it flows through the heavily populated Sichuan (Four Rivers)
Basin in its way to join the Yangtze main channel. The dissolved
major element composition was determined in the Tibetan headwaters
at the rising stage and at four hydrologic stations in a monthly
time series. The Min Jiang is alkaline and has high dissolved
load compared to other world rivers. Carbonate weathering dominates,
especially in the source region of the MinJiang main channel,
and silicate weathering and evaporite dissolution gain some importance
in the Dadu He tributary. The Si/(Na*+K) ratios of
the dissolved load and the clay mineralogy of the bed material
suggest that silicate weathering in the watershed is superficial,
i.e. soluble cations are being preferentially leached. Many major
elements show lower concentrations at high summer discharge nut
there is also additional input, such that the concentration-discharge
relationship is not purely a result of dilution. The discharge-weighted
total dissolved solid (TDS) flux, based on data from the three
upstream monitoring stations, is 10×106』 tons/year,
〜6% of the Yangtze at mouth, and the chemical denudation rate
as measured by TDS yield is 115 tons/km2/year for the
Min Jiang, higher than the average Yangtze basin (85 tons/km2/year).
The common extrapolation using annual average discharge and spot
sampling of dissolved and suspended material during rising or
falling stage can reasonably estimate chemical fluxes to ±20%
but seriously underestimate the physical flux. Sampling the peak
flash flow is critical for the suspended particulate material.
Concentrations of SO4, Cl and Ca are significantly
elevated as the river debouches onto the populated Sichuan Basin.
Keywords: Major element geochemistry; Hydrologic station; Tibetan
Plateau; Time series; Changjiang; Monitoring
1. Introduction
2. Natural setting
2.1. Topography and geology
2.2. Climate and soil
3. Method
4. Results and discussion
4.1. Dissolved major elements
4.2. Sources of solutes in the Min Jiang system
4.3. Temporal variation in composition
4.4. Flux calculations and uncertainty analysis for spot sampling
4.5. Upstream monitoring stations versus Gaochang station
5. Conclusion
Acknowledgments
References