『Abstract
Chemical and strontium isotopic compositions of Nanpanjian River
and Beipanjiang River in the upper reaches of Xijiang River draining
the Yunnan-Guizhou Plateau, Southwest China, were measured with
a main purpose to understand the rock weathering and associated
CO2 consumption rates in the karstic landform-dominated
area. The chemical ion composition of the river water is characterized
by dominance of Ca2+, Mg2+ and HCO3-, and SO42-,
which account for more than 90% of total ion concentration. 87Sr/86Sr
ratios of Nanpanjiang River water range between 0.7080 and 0.7140,
while those of most Beipanjiang River water vary within a limited
range from 0.7075 to 0.7085.
Weathering of carbonate rocks dominate chemistry of major ions
especially in the water of Beipanjiang River, whereas weathering
of silicate minerals in the upper reaches of Nanpanjiang River
basin is obvious in addition to carbonate weathering, mainly according
to the Sr isotopic composition of the river water. Analyses of
the stoichiometry of the river water show that the water chemistry
is controlled by carbonate dissolution not only by carbonic but
also by sulfuric acid, and indicate that sulfuric acid plays an
important role in carbonate weathering. The sulfate in river water
is mainly from atmospheric input by coal-combustion industries
and from oxidation of sulfide minerals during weathering of coal-containing
strata and coal mining in the catchment. The chemical weathering
rates of silicate and carbonate and associated CO2
consumption rates by both carbonic and sulfuric acid and by only
carbonic acid are respectively estimated. The results show that
chemical weathering rates of carbonate and silicate weathering
in Beipanjiang River basin are higher than those of Nanpanjiang
River basin, showing linkage of geology and geography in the catchment
to crustal weathering. The involvement of sulfuric acid in carbonate
weathering greatly enhances the carbonate weathering but lowers
the CO2 consumption rates, indicating that
sulfide acid is an important agent to rock weathering and the
role of sulfide acid should further be clarified in view of the
regional and global budget of CO2.
Keywords: Yunnan-Guizhou Plateau; River water geochemistry; Strontium
isotope; Chemical weathering; CO2 consumption
rate』
1. Introduction
2. General setting
3. Sampling and analytical procedures
4. Results
4.1. Chemical compositions
4.2. Strontium concentrations and isotopes
5. Discussion
5.1. Source of solutes
5.1.1. Atmospheric inputs
5.1.2. Anthropogenic inputs
5.1.3. Rock weathering inputs
5.1.4. The role of sulfuric acid in rock weathering
5.2. Chemical budget and chemical weathering rate estimation
5.2.1. Chemical budget
5.2.2. Chemical weathering and CO2 consumption
rate
6. Conclusions
Acknowledgments
References