『Abstract
Lithology is an important characteristic of the terrestrial surface,
whose properties influence chemical weathering rates. Specifically
non-silicate minerals may contribute significantly to the weathering
derived fluxes from silicate-dominated lithological classes. The
Japanese Archipelago consists of predominantly silicate-dominated
lithologies with a high proportion of volcanics. However, the
spatially explicit representation of chemical weathering rates
remains difficult for such a large region, because many controlling
factors on chemical weathering rates are correlated with each
other. Due to the spatial heterogeneity of lithology, a multi-lithological
model approach to estimate spatially explicit chemical weathering
rates for unmonitored areas is applied here. To achieve this,
hydrochemical data of 381 catchments are used to train a set of
models, recognizing the contribution of a variety of proposed
factors influencing chemical silicate rock weathering rates (CSRWR:
cations plus dissolved silica flux). The monitored catchments
cover 〜44% of the Japanese Archipelago. Cation chemical weathering
rates (excluding Si) are linearly correlated with CSRWR and show
outliers if basic volcanics or pyroclastic flows are present due
to increased silica release rates.
Lithology and runoff are identified as the strongest predictors
for chemical weathering rates. Temperature and gradient of slope
are of less relevance for the regional scale prediction while
further proposed factors like soil properties or land cover are
not identified as major predictors. Latter findings are partly
attributed to geodata quality, low availability of parameter values
as well as spatial correlations of proposed controlling factors
with lithology or runoff.
The calculated average CSRWR of the Archipelago is 〜25 t km-2
a-1 and ranges from 5.9 to 107 t km-2 a-1
in monitored catchments. Weathering rates per lithological class
as a function of runoff can be grouped into three classes: a)
pyroclastic flows showing the highest chemical weathering rates;
b) alluvial deposits, mixed sediments and basic to intermediate
volcanics with medium rates; and c) metamorphics, siliciclastic
sediments, acid volcanics, and plutonics and unconsolidated sediments
(other than alluvial deposits), showing the lowest rates. The
recognition of lithogenic sulfur would add 9.7% to CSRWR of considered
catchments. Results suggest that the lithological classes acid
volcanics and unconsolidated sediments contribute above average
to the sulfur fluxes. Possible biases of this observation are
discussed.
The contribution of Ca-fluxes from non-silicate calcic minerals
(named Ca-excess, Ca-fluxes in addition to silicate Ca-fluxes)
is about 10% of the CSRWR on average and is attributed by a wide
value range. The calculated ratio “Ca-excess to total Ca-fluxes”
from chemical weathering averages around 62%, 75%, 56%, 83% and
84% for the lithological classes acid plutonics, metamorphics,
siliciclastic sediments, mixes sediments and acid volcanics, respectively.
This suggests a major Ca-contribution from non-silicate calcic
minerals for these lithological classes. Phosphorus release from
rocks due to chemical weathering is estimated to between 1 kg
P km-2 a-1 and 390 kg P km-2
a-1. The P-release patterns in dependence of runoff
per lithological class are different from CSRWRs due to differences
of applied P-content in rocks. The identified spatial P-release
pattern suggest that the consideration of dynamic and spatially
resolved P-release rates by chemical weathering might improve
ecosystem studies. Later findings may be of importance for analysing
the influence of P-release from rocks on the climate system via
ecosystem functioning on geological time scales. A first application
of the P-release model to the global scale suggests an annual
release of 1.6 Mt P (13.8 kg P km-2 a-1)
by chemical weathering of silicate dominated lithological classes
(excluding carbonate sedimentary rocks).
Keywords: Chemical weathering; Japan; Lithology; Phosphorus; Sulfur;
Calcium』
1. Introduction
2. Data and methods
2.1. Overview
2.2. Handling of geodata
2.3. Mass balance
2.4. Modeling technique
2.5. Calculation of Ca-excess fluxes
2.6. Application of the derived models to the Japanese Archipelago
3. Results and discussion
3.1. Overview
3.2. Calculated CSRWR based on the monitoring data
3.3. Correlations of CSRWR
3.4. Predicted CSRWR by the applied models
3.4.1. Overall interpretation for the monitored catchments
3.4.2. Average CSRWR predictions per lithological class
3.4.3. Physical interpretation of the b-estimates of the models
3.4.4. Residual analysis
3.5. Application to the Japanese Archipelago
3.6. The influence of excess-Ca from non-silicate calcic minerals
on CSRWR of silicate-dominated lithological classes
3.7. Contribution of sulfur to CSRWR&S
3.8. Liberation of lithogenic phosphorus by chemical weathering
3.9. Application to the global scale
4. Conclusion
Acknowledgements
Appendix A
Appendix B
Appendix C
Appendix D
Appendix E
References