『Abstract
Large river sediments are mostly derived from tectonically active
mountain belts, but then undergo a series of sedimentation, temporary
storage and reworking episodes on their journey to the ocean.
The long transfer time of these sediments through active floodplains
might result in a significant chemical maturation via weathering
reactions, which is of critical importance for biogeochemical
cycles at the Earth surface. This study reports the chemical composition
of river sediments from different locations throughout the courses
of the main tributaries of the Amazon Basin. Sampling along river-profiles
yields access to the whole grain size and chemical composition
range of ricer sediment. Here, weathering intensities (i.e. losses
of Na, K, Mg and Ca) associated with chemical weathering in floodplains
are (1) examined as a function of grain size, and (2) integrated
over the whole grain size range, for three selected long river
reaches flowing through the foreland and the lowland of the Amazon
Basin: the upper Maranon(最初のnの頭に〜), the Beni
and the lower Madeira ricers. A relatively small Na loss through
plagioclase dissolution is observed in the Madeira reach, while
an important Ca loss due to carbonate dissolution occurs in the
two foreland reaches (Maranon(最初のnの頭に〜) and
Beni). No significant loss of K and Mg is observed in any of the
reaches, showing the low alterablity of primary K and Mg-bearing
minerals and suggesting retention of K and Mg in the particulate
phase by secondary minerals. The combination of these findings
with previously reported data on the downstream change of dissolved
Na, K, Mg and Ca fluxes suggests that chemical weathering of “stable”
alluvial deposits could also significantly contribute to the weathering
flux generated in foreland and lowland areas. The comparison between
the three Amazon reaches and the Gangetic plain tends to show
that the features observed in the Amazon are valid on a global
scale. Finally, we show that, although resulting in a relatively
small change in the chemical composition of the river sediment,
silicate weathering in the lower Madeira floodplain can lead to
a CO2 drawdown equivalent to ca. 10% of the
total CO2 consumption flux of the whole Madeira basin.
Keywords: Amazon River; Floodplains; Chemical weathering; Geochemical
fluxes; Grain size』
1. Introduction
2. The Amazon system: setting, sampling, and methods
2.1. The Amazon Basin
2.2. Sampling sites and river reaches
2.3. Sampling and analytical methods, and origin of hydrological
data
2.4. The hypothesis of a sedimentary steady-state for the Amazonian
floodplains
2.5. Chemical proxies for grain size and weathering
3. Results
3.1. Relationships for each sampling site
3.2. Comparison between sampling sites
4. Discussion
4.1. Grain size-specific floodplain weathering intensities
4.2. Grain size-integrated floodplain weathering intensities
4.3. Weathering processes during transfer of sediments through
the Amazon floodplains
4.4. Comparison with floodplain weathering fluxes determined
from river dissolved loads
4.5. Comparison between different river systems and potential
controls on chemical weathering in the plains
4.6. CO2 flux associated with silicate weathering
of river sediments in floodplains
5. Conclusions
Acknowledgments
Appendix A. Regression of X/Al-Al/Si relationships and uncertainties
Appendix B. Grain size-integrated parameters indexing the changes
of chemical composition of river sediment
References