『Abstract
Prediction of CO2-consumption by chemical
weathering is important to understand the global carbon cycle,
and island arcs are assumed to contribute significantly to the
transfer of atmospheric/soil CO2 to the oceans.
Previous work established empirical functions for bicarbonate-flux
and CO2-consumption in dependence of runoff
for five to six lithological classes. These functions were applied
in global studies. However, it has remained uncertain, if improvements
can be achieved by considering further factors or by an enhanced
lithological classification scheme. This study applies a new lithological
map of Japan with an enhanced lithological classification, a hydrochemical
data set representing 382 catchments (covering 〜44.4% of the Japanese
Archipelago) and a multi-lithological model approach to predict
bicarbonate-fluxes and CO2-consumption by
chemical weathering. Because of significant carbonate contents
in sediments (different from carbonate sedimentary rocks), acid
plutonics and metamorphics, firstly a bicarbonate-flux model approach
is established and, secondly, silicate and carbonate CO2-consumption
is estimated, based on the geochemical composition of rocks.
In accordance with previous studies on the catchment scale, the
most important factors controlling bicarbonate-fluxes are lithology
and runoff. The anion ratio HCO3-/(SO42- + Cl-) is identified
as the third most important predictor, because anion sources and
anion composition of river water have a great effect on bicarbonate
concentrations. And last but not least, potential predictors such
as gradient of slope, temperature and physical erosion explain
some part of the observed bicarbonate-flux variation. These potential
predictors have been discussed in literature, but quantification
of the effects of these factors remains difficult due to their
correlation with runoff and lithology. However, all tested models
reproduce a total of observed bicarbonate-fluxes within 10% on
the regional scale including the simplest model which recognizes
only lithology and runoff as predictors.
Model results suggest that bicarbonate-flux from the Japanese
Archipelago is about 6.61 t C km-2 a-1,
and CO2-consumption by chemical weathering
is about 6.05 t C km-2 a-1 (91.6% of bicarbonate-flux).
This CO2-consumption rate is 3.2 times above
the global average rate. The silicate to carbonate CO2-consumption
ratio is comparatively high. It amounts to 9.9 which is above
the global value. The latter one being 1 to 1.5. Carbonate sedimentary
rocks contribute only 1.3% to bicarbonate-fluxes due to their
low areal proportion (0.2%). Acid volcanic rocks (VA) show the
lowest bicarbonate-fluxes, on average, while basic and intermediate
volcanics (VB) as well as pyroclastic (PY) are in the range of
the Japanese average value. The carbonate weathering contribution
to bicarbonate-fluxes from mixed sediments, siliciclastic sediments,
metamorphics and acid plutonics is estimated to be 69.2%, 23.3%,
63.4% and 24.8%, respectively. Only an average carbonate CO2-consumption per lithological class is calculated
because no function based on typical predictors could be established
to calculate carbonate weathering contribution to silicate-dominated
lithological classes. This can be attributed to large spatial
differences in carbonate content in rocks and to corresponding
contributions to bicarbonate-fluxes. In the following, results
are compared to bicarbonate-flux models of previous studies and
identified differences are discussed. In conclusion, recognition
of carbonate abundance in silicate-dominated lithological classes
(including trace calcite) is important to calculate silicate/carbonate
CO2-consumption ratios. Presented results
are relevant for studies modelling CO2-consumption
by chemical weathering in context of the global C-cycle.
Keywords: Chemical weathering; CO2; Bicarbonate;
Lithology; Silicate; Carbonate』
1. Introduction
1.1. Weathering and the global carbon cycle
1.2. Controls on chemical weathering
1.3. Prediction of CO2-consumption by chemical
weathering
1.4. Objectives
2. Data and methodology
2.1. Overview
2.2. Hydrochemical data
2.3. Handling of geodata
2.4. Mass balance
2.5. Preanalysis
2.5.1. Selection of parameters for CO2-consumption
modelling
2.5.2. Selection of predictors for bicarbonate-flux models
2.6. Modelling technique
2.7. Calculation of the contribution of carbonates to bicarbonate-fluxes
2.7.1. Ca-proportion counterbalancing bicarbonate
2.7.2. The hypothetical maximal carbonate contribution to bicarbonate-fluxes
2.7.3. Calculation of silicate and carbonate CO2-consumption
2.8. Application of the bicarbonate runoff model on the Japanese
Archipelago
3. Results
3.1. Overview
3.2. Observed fluxes
3.3. Relationships between proposed predictors and bicarbonate-fluxes/concentrations
3.4. Predicted bicarbonate-fluxes
3.4.1. Monitored catchments
3.4.1.1. Average bicarbonate-flux predictions per lithological
class
3.4.1.2. Interpretation of parameter estimates
3.4.2. Application to the Japanese Archipelago
3.5. The influence of silicate and carbonate proportions on CO2-consumption
3.6. Evaluation: analysis of model residuals
3.7. Evaluation: comparison with previous bicarbonate-flux models
4. Discussion
4.1. General discussion
4.2. Observed bicarbonate-fluxes: local findings
4.3. Discussion of model results
4.3.1. Discussion of predictors and potential errors
4.3.1.1. Runoff
4.3.1.2. Lithology
4.3.1.3. Anion ratio AER
4.3.1.4. Temperature
4.3.1.5. Slope
4.3.1.6. Physical erosion
4.3.2. CO2-consumption and contribution
of carbonates
4.4. Comparison with previous models
4.4.1. Bicarbonate-flux models
4.4.2. Global CO2-consumption models
4.5. Error discussion considering factors not included in the
model discussion
4.5.1. Metamorphic CO2
4.5.2. Soils
4.5.3. Ecosystems and land cover
4.5.4. Mass balance
5. Conclusion
Acknowledgments
Appendix A. Distribution of precipitation and atmospheric hydrochemical
deposition patterns were calculated from the following references.
Atmospheric deposition was extrapolated using the product of concentration
in atmospheric deposition and annually averaged precipitation
Appendix B. Calculated Ca-excess for lithological classes SM,
SS, MT and PA. Ca-excess is assumed to be equivalent to bicarbonate-fluxes
from carbonates
Appendix C. Application of the Japanese CO2-consumption model
on the global scale
Appendix D. Supplementary data
References