『Abstract
Rock weathering by carbonic acid is thought to p@lay an important
role in the global carbon cycle because it can geologically sequestrate
atmospheric CO2. Current model of carbon
cycle evolution usually assumes that carbonic acid is the major
weathering agent and that other acids are not important. Here
we use carbon isotopic evidence and water chemistry of springs
and rivers from the Beipanjiang River basin (Guizhou Province,
Southwest China) to demonstrate that sulfuric acid is also an
important agent of rock weathering. The δ13C of dissolved
inorganic carbon (DIC) in the water samples ranges from -13.1‰
to -2.4‰, and correlates negatively to [HCO3-]/([Ca2+]
+ [Mg2+]) ratios and positively to [SO42-]/([Ca2+]
+ [Mg2+]) ratios. These relationships are interpreted
as mixing diagrams between two reactions of carbonate weathering,
using carbonic acid and sulfuric acid as a proton donor, respectively.
Mixing proportions show that around 42% of the divalent cations
in the spring water from Guizhou are originated from the interaction
between carbonate minerals and sulfuric acid. It is shown that
40% of this sulfuric acid is derived from the atmosphere and has
an anthropogenic origin. The remaining 60% are derived from the
oxidative weathering of sulfide minerals in sedimentary rocks.
Our results show the positive action of sulfuric acid on the chemical
weathering of carbonate. Particularly, we show that sulfuric acid
generated by coal combustion has increased by almost 20% the weathering
rates of carbonate in Southwest China. This is a clear evidence
that human activities are changing the weathering rates of rocks
and demonstrates a negative feedback on the acidification of the
ocean by greenhouse gases. Because of the involvement of sulfuric
acid in weathering reactions, 63% of the alkalinity exported by
rivers is derived from carbonate, instead of 50% when atmospheric
CO2 is the only acid involved in chemical
weathering of carbonate. In the Guizhou Province, the weathering
of carbonate is thus, at least transiently, a net source of CO2 to the atmosphere. When extrapolated at global
scale, sulfuric acid-induced carbonate weathering could counterbalance
a significant part of the CO2 consumed by
silicate weathering. This paper highlights the competition between
silicate weathering by carbonic acid and carbonate weathering
by sulfuric acid for the regulation of the atmospheric CO2 level.
Keywords: δ13C; carbonate weathering; sulfuric acid;
Beipanjiang River; carbon cycle; anthropogenic activities』
1. Introduction
2. Geographical setting of drainage catchments
3. Samples and analyses
4. Results
4.1. Chemical composition of water
4.2. δ13C of DIC and POC in river and spring water
samples
5. Discussion
5.1. Theoretical sources of DIC in river and spring water
5.2. Sources of carbon to the rivers of Guizhou Province
5.3. Mixing diagrams and mixing proportions
5.4. Anthropogenic impact on carbonate weathering
6. Conclusions and consequences for atmospheric CO2
budget
Acknowledgments
Appendix A. Estimates of the contributions of carbonate and silicate
weathering and corrections of the atmospheric and anthropogenic
inputs
References