『Abstract
This study is a geochemical investigation of the Andean and Foreland
basins of the Amazon River at high spatial and time resolution,
carried out within the framework of the HYBAM research program
(Hydro-geodynamics of the Amazon Basin). Monthly sampling was
carried out at 27 gauging stations located in the upper tributaries
of the Amazon Basin (from north to south: the Napo, Maranon（最初のnの頭に〜）, Ucayali, Madre de Dios-Beni and Mamore
Rivers). The aim of this paper is to estimate the present-day
chemical weathering rate (CWR), as well as the flux of CO2 consumption from total and silicate weathering
in the Andes and Foreland Amazon basins, and to discuss their
distribution as a function of geomorphic and structural parameters.
Based on the forward method, the Napo and other Ecuadorian basins
present high silicate weathering rates in comparison with the
other basins. We confirm that the Maranon（最初のnの頭に〜）
and Ucayali Rivers control the Amazon hydrochemistry due to the
presence of salt rocks and carbonates in these basins. The Madre
de Dios, Beni and Mamore basins do not contribute much to the
Amazon dissolved load. This north to south CWR gradient can be
explained by the combination of decreasing weatherable lithology
surface and decreasing runoff rates from the north to the south.
The foreland part of the basins (or Mountain-Lowland transition)
accounts for nearly the same proportion of the Amazon silicate
chemical weathering and carbonate chemical weathering fluxes as
the Andean part. This result demonstrates the importance of the
sediment accumulation areas in the Amazon Basin weathering budget
and can be explained by the occurrence of a higher temperature,
the deposition of fresh sediments from Andean erosion and a higher
sediment residence time than in the upper part of the basin. With
a total CO2 consumption rate of 744.103
moles km-2 year-1 and a silicate CO2 consumption rate of 300.103』 moles
km-2 year-1, the Upper Amazon River (Andes
+ Foreland part) is the most intense part of the Amazon Basin
in terms of atmospheric CO2 consumption by
weathering processes. It is an important CO2
sink by weathering processes but accounts for only somewhat more
than half of the CO2 consumption by silicate
weathering of the Amazon Basin. This result points out the importance
of the Lowland part of the basin in the inorganic C silicate budget.
The Upper Amazon accounts for 2-4％ of the world's silicate CO2 consumption, which is the same proportion as
for the southern and southern-east Himalaya and Tibetan plateau.
Keywords: Andes; Foreland; Amazon Basin; Hydrochemistry; Chemical
weathering; CO2 consumption』
1. Introduction
2. Studied area
2.1. Main geographical features
2.2. Geology and lithology
2.3. Hydroclimatology
3. Material and methods
3.1. Origin of data
3.2. Acquisition of hydrometric data and sample analysis
4. Results and discussion
4.1. General considerations for the physicochemical characteristics
of the sampled rivers
4.1.1. Ion charge balance and dissolved chemical concentrations
4.1.2. Ternary distribution of major elements
4.1.3. Comparison to global rivers
4.2. Sources of major ions in river waters
4.2.1. Anthropogenic input
4.2.2. Biogenic input
4.2.3. Atmospheric input
4.2.4. Weathering input
4.2.4.1. Evaporite dissolution, pyrite oxidation and hot spring
acid inputs
4.2.4.2. Silicate weathering
4.2.4.3. Carbonate weathering
4.2.4.4. Validation of the forward method
4.3. Calculation of the CO2 consumption
rates by weathering
5. Summary and conclusions
5.1. The weathering budget
5.2. Andean and Foreland fluxes
5.3. CO2 consumption rates by weathering
5.4. Inorganic carbon consumption of the Upper Amazon (Andes
and Foreland basins) in the world budget
Acknowledgments
Appendix 1. Step by step description of the forward method and
of other calculations
References