『Abstract
Water samples from the Fraser, Skeena and Nass River basins of
the Canadian Cordillera were analyzed for dissolved major element
concentrations (HCO3-, SO42-, Cl-, Ca2+,
Mg2+, K+, Na+), δ13C
of dissolved inorganic cation (δ13CDIC),
and δ34S of dissolved sulfate (δ34SSO4) to quantify chemical
weathering rates and exchanges of CO2 between
the atmosphere, hydrosphere, and lithosphere. Weathering rates
of silicates and carbonates were determined from major element
mass balance. Combining the major element mass balance with δ34SSO4(-8.9 to 14.1‰CDT)
indicates sulfide oxidation (sulfuric acid production) and subsequent
weathering of carbonate and to a lesser degree silicate minerals
are important processes in the study area. We determine that on
average, 81% of the riverine sulfate can be attributed to sulfide
oxidation in the Cordilleran rivers, and that 25% of the total
weathering cation flux can be attributed to carbonate and silicate
dissolution by sulfuric acid. This result is validated by δ13CDIC values (-9.8 to -3.7‰VPDB)
which represents a mixture of DIC produced by the following weathering
pathways: (i) carbonate dissolution by carbonic acid (-8.25‰)>(ii)
silicate dissolution by carbonic acid (-17‰)≒(iii) carbonate dissolution
by sulfuric acid derived from the oxidation of sulfides (coupled
sulfide-carbonate weathering)(+0.5‰).
δ34SSO4
is negatively correlated with δ13CDIC
in the Cordilleran rivers, which further supports the hypothesis
that sulfuric acid produced by sulfide oxidation is primarily
neutralized by carbonates, and that sulfide-carbonate weathering
impacts the δ13CDIC of rivers.
The negative correlation between δ34SSO4 and δ13CDIC
is not observed in the Ottawa and St. Lawrence River basins. This
suggests other factors such as landscape age (governed by tectonic
uplift) and bedrock geology are important controls on regional
sulfide oxidation rates, and therefore also on the magnitude of
sulfide-carbonate weathering - i.e., it is more significant in
tectonically active areas.
Calculated DIC fluxes due to Ca and Mg silicate weathering by
carbonic acid (38.3×103 mol C・km-2・yr-1)
are similar in magnitude to DIC fluxes due to sulfide-carbonate
weathering (18.5×103 mol C・km-2・yr-1).
While Ca and Mg silicate weathering facilitates a transfer of
atmospheric CO2 to carbonate rocks, sulfide-carbonate
weathering can liberate CO2 from carbonate
rocks to the atmosphere when sulfide oxidation exceeds sulfide
deposition. This implies that in the Canadian Cordillera, sulfide-carbonate
weathering can offset up to 48% of the current CO2
drawdown by silicate weathering in the region.』
1. Introduction
2. Chemical weathering pathways
2.1. Carbonic acid-based weathering
2.2. Sulfuric acid-based weathering
3. Study area
3.1. Watershed characteristics
3.2. Basin geology
4. Methods
4.1. Field methods
4.2. Water samples
4.3. Laboratory analyses
4.3.1. Cations and anions
4.3.2. Carbon isotopes
4.3.3. Sulfur isotopes
5. Results
5.1. Major elements
5.2. Dissolved inorganic carbon and δ13CDIC
5.3. Dissolved sulfate and δ34SSO4
6. Chemical weathering rates
6.1. Rock types contributing to weathering products
6.1.1. Mass balance
6.1.1.1. Step 1: Atmospheric input
6.1.1.2. Step 2: Apportion SO4*
to sulfide and evaporite dissolution
6.1.1.3. Step 3: Apportion cations and DIC to weathering of
carbonates and silicates by sulfuric acid
6.1.1.4. Step 4: Apportion cations and DIC to weathering of
silicates by carbonic acid
6.1.1.5. Step 5: Apportion cations and DIC to weathering of
carbonates by carbonic acid
6.2. Pollution inputs
6.3. Mass balance results
6.4. Validation of results
6.4.1. Carbon isotopes
6.4.2. End-member definition
6.4.3. Measured δ13CDIC
6.4.4. Expected δ13CDIC
7. CO2 fluxes due to chemical weathering
8. Conclusions
Acknowledgments
References