scholarly journals Contrasted Chemical Weathering Rates in Cratonic Basins: The Ogooué and Mbei Rivers, Western Central Africa

2021 ◽  
Vol 2 ◽  
Author(s):  
Jean-Sébastien Moquet ◽  
Julien Bouchez ◽  
Jean-Jacques Braun ◽  
Sakaros Bogning ◽  
Auguste Paulin Mbonda ◽  
...  
Keyword(s):  
Chemical Weathering ◽  
Natural Waters ◽  
Isotope Composition ◽  
Central Africa ◽  
River Basins ◽  
Tectonic Activity ◽  
Weathering Rates ◽  
Base Level ◽  
Solute Fluxes ◽  
Dissolved Load

Despite the absence of tectonic activity, cratonic environments are characterized by strongly variable, and in places significant, rock weathering rates. This is shown here through an exploration of the weathering rates in two inter-tropical river basins from the Atlantic Central Africa: the Ogooué and Mbei River basins, Gabon. We analyzed the elemental and strontium isotope composition of 24 water samples collected throughout these basins. Based on the determination of the major element sources we estimate that the Ogooué and Mbei rivers total dissolved solids (TDS) mainly derive from silicate chemical weathering. The chemical composition of the dissolved load and the area-normalized solute fluxes at the outlet of the Ogooué are similar to those of other West African rivers (e.g., Niger, Nyong, or Congo). However, chemical weathering rates (TZsil+ rate expressed as the release rate of the sum of cations by silicate chemical weathering) span the entire range of chemical weathering intensities hitherto recorded in worldwide cratonic environments. In the Ogooué-Mbei systems, three regions can be distinguished: (i) the Eastern sub-basins draining the Plateaux Batéké underlain by quartz-rich sandstones exhibit the lowest TZsil+ rates, (ii) the Northern sub-basins and the Mbei sub-basins, which drain the southern edge of the tectonically quiescent South Cameroon Plateau, show intermediate TZsil+ rates and (iii) the Southern sub-basins characterized by steeper slopes record the highest TZsil+ rates. In region (ii), higher DOC concentrations are associated with enrichment of elements expected to form insoluble hydrolysates in natural waters (e.g., Fe, Al, Th, REEs) suggesting enhanced transport of these elements in the colloidal phase. In region (iii), we suggest that a combination of mantle-induced dynamic uplift and lithospheric destabilization affecting the rim of the Congo Cuvette induces slow base level lowering thereby enhancing soil erosion, exhumation of fresh primary minerals, and thus weathering rates. The study points out that erosion of lateritic covers in cratonic areas can significantly enhance chemical weathering rates by bringing fresh minerals in contact with meteoric water. The heterogeneity of weathering rates amongst cratonic regions thus need to be considered for reconstructing the global, long-term carbon cycle and its control on Earth climate.

Chemical weathering rates of a soil chronosequence on granitic alluvium: III. Hydrochemical evolution and contemporary solute fluxes and rates

2005 ◽  
Vol 69 (8) ◽  
pp. 1975-1996 ◽  
Author(s):  
Art F. White ◽  
Marjorie S. Schulz ◽  
Davison V. Vivit ◽  
Alex E. Blum ◽  
David A. Stonestrom ◽  
...  
Keyword(s):  
Chemical Weathering ◽  
Hydrochemical Evolution ◽  
Soil Chronosequence ◽  
Weathering Rates ◽  
Solute Fluxes

scholarly journals K isotopes as a tracer for continental weathering and geological K cycling

2019 ◽  
Vol 116 (18) ◽  
pp. 8740-8745 ◽  
Author(s):  
Shilei Li ◽  
Weiqiang Li ◽  
Brian L. Beard ◽  
Maureen E. Raymo ◽  
Xiaomin Wang ◽  
...  
Keyword(s):  
Mass Balance ◽  
Chemical Weathering ◽  
Drainage Basin ◽  
Isotope Composition ◽  
Systematic Investigation ◽  
K Value ◽  
Weathering Intensity ◽  
Intensity Changes ◽  
Continental Weathering ◽  
Dissolved Load

The causal effects among uplift, climate, and continental weathering cannot be fully addressed using presently available geochemical proxies. However, stable potassium (K) isotopes can potentially overcome the limitations of existing isotopic proxies. Here we report on a systematic investigation of K isotopes in dissolved load and sediments from major rivers and their tributaries in China, which have drainage basins with varied climate, lithology, and topography. Our results show that during silicate weathering, heavy K isotopes are preferentially partitioned into aqueous solutions. Moreover, δ41K values of riverine dissolved load vary remarkably and correlate negatively with the chemical weathering intensity of the drainage basin. This correlation allows an estimate of the average K isotope composition of global riverine runoff (δ41K = −0.22‰), as well as modeling of the global K cycle based on mass balance calculations. Modeling incorporating K isotope mass balance better constrains estimated K fluxes for modern global K cycling, and the results show that the δ41K value of seawater is sensitive to continental weathering intensity changes. Thus, it is possible to use the δ41K record of paleo-seawater to infer continental weathering intensity through Earth’s history.

Age of river basins in Guadeloupe impacting chemical weathering rates and land use

2011 ◽  
Vol 26 ◽  
pp. S123-S126 ◽  
Author(s):  
Sétareh Rad ◽  
Olivier Cerdan ◽  
Karine Rivé ◽  
Gilles Grandjean
Keyword(s):  
Land Use ◽  
Chemical Weathering ◽  
River Basins ◽  
Weathering Rates

scholarly journals Supply-limited weathering regime in a tropical shields basin (Ogooué River basin, Gabon)

2020 ◽  
Author(s):  
Jean-Sébastien Moquet ◽  
Julien Bouchez ◽  
Jean-Jacques Braun ◽  
Sakaros Bogning ◽  
Auguste Mbonda ◽  
...  
Keyword(s):  
Chemical Weathering ◽  
Biogeochemical Cycles ◽  
Tectonic Activity ◽  
Silicate Weathering ◽  
Weathering Rate ◽  
Weathering Rates ◽  
Erosion Rates ◽  
The World ◽  
Weathering Intensity

<p>At the global scale and on geological time scales, mechanical erosion and chemical weathering budgets are linked. Together, these processes contribute to the formation and the degradation of the Earth’s critical zone and to the biogeochemical cycles of elements. While the weathering of hot and humid shields areas exhibit low weathering rates because of the depth of the mature depleted soil mantle there, shields areas dominate the continents areas over intertropical regions and, therefore, represent a significant proportion of the global delivery of dissolved matter to the oceans. In addition, these environments are under supply-limited conditions (the weathering rate is limited by the low rates of the erosion) and thus particularly sensitive to long-term variability erosion rates. Despite this importance, weathering-erosion budgets and rates estimation in these environments is sparse, and generally performed at a local scale (soil profiles) or, when performed at a larger catchment scale, the intra cratonic characteristics variabilities (e. g. the diversity of mechanical erosional regimes) are usually not singled out.</p><p>In the present study, we explored the variability of the weathering intensity of the Ogooué sub-basins (Western central Africa, Gabon) as a function of their geomorphologic, tectonic and lithological setting variability. We analyzed major and trace elements concentration and the strontium and neodymium isotopes of water, suspended matter sediments and bedload sampled in 24 Ogooué tributaries (September 2017 campaign). Our results show that shield areas exhibit a high variability of chemical weathering intensity, which follows the erosional regime characteristics of the studied sub-basins, likely related to their tectonic activity. Three regions can be distinguished: The Bateke plateau (East sub-basins - PB), is composed of pure sandstones (quartz) and is inert in term of tectonic activity and therefore in term of erosion and weathering budget; the northern sub-basins (NB) are subjected to low tectonic activity and exhibit slightly higher erosion and weathering intensity than PB region and, by comparison, southern sub-basins (SB) exhibits uplift activity which is traduced by more intensive erosion and weathering processes.</p><p>The annual dissolved solid budget of the Ogooué basin is ~2.52 t.yr<sup>-1</sup> for a rate of 11.7 t.km<sup>-2</sup>.yr<sup>-1</sup>. According to the source discrimination method performed based on the geochemical analysis, the atmospheric inputs contributes to around 20% to the TDS, the silicate weathering contribution dominates the dissolved exports throughout 70% of its production while the carbonates weathering lowly contributes to the TDS production.</p><p>By comparison to the other large shields rivers, this basin exhibit a lower range of chemical silicate weathering rate than most of the world’s large rivers, with values similar to those of the Congo River. This new dataset provides a key information to complete the World River chemistry database, which is limited for inter-tropical regions, especially in tectonically quiescent environments. Moreover, this study provides new data for tropical shields contexts allowing for the exploration of the interactions between erosion rates and climate in the control of continental weathering rates, and their relationships with long-term carbon cycle and short-term biogeochemical cycles.</p>

scholarly journals Chemical weathering and long-term CO2consumption in the Ayeyarwady and Mekong river basins in the Himalayas

2015 ◽  
Vol 120 (6) ◽  
pp. 1165-1175 ◽  
Author(s):  
Takuya Manaka ◽  
Souya Otani ◽  
Akihiko Inamura ◽  
Atsushi Suzuki ◽  
Thura Aung ◽  
...  
Keyword(s):  
Chemical Weathering ◽  
River Basins ◽  
Mekong River

Imprint of chemical weathering and hydrothermalism on the Ge/Si ratio and Si isotope composition of rivers in a volcanic tropical island, Basse-Terre, Guadeloupe (French West Indies)

2021 ◽  
Vol 577 ◽  
pp. 120283
Author(s):  
François Gaspard ◽  
Sophie Opfergelt ◽  
Celine Dessert ◽  
Vincent Robert ◽  
Yolanda Ameijeiras-Mariño ◽  
...  
Keyword(s):  
West Indies ◽  
Chemical Weathering ◽  
Isotope Composition ◽  
French West Indies ◽  
Tropical Island ◽  
Si Isotope

scholarly journals Under the surface: what we know about the threats to subterranean fishes in Brazil

2021 ◽  
Vol 19 (3) ◽  
Author(s):  
Maria Elina Bichuette ◽  
Jonas Eduardo Gallão
Keyword(s):  
Water Table ◽  
Food Restriction ◽  
Hyporheic Zone ◽  
Public Awareness ◽  
River Basins ◽  
Physical Change ◽  
Alluvial Sediments ◽  
Base Level ◽  
Threatened Fauna ◽  
Brazilian Biodiversity

Abstract The present work brings information on threats to the subterranean fishes in Brazil. Currently, at least 36 species are known, 22 of which are already formally described. Endemism is the rule for most of them. Regarding their conservation, these fishes are in general considered threatened: and most of the already formally described species are included in national lists of threatened fauna, and only four of them are included in the global list of the IUCN. Regarding habitats, Brazilian subterranean fishes occur in alluvial sediments (part of the hyporheic zone), shallow base-level streams, flooded caves, lakes in the water table, upper vadose tributaries, and epikarst aquifers. We detected 11 main threats, mainly related to agriculture, pasture, and hydroelectric plans, but unmanaged tourism and pollution are also significant threats. Two threats affect a high number of species (physical change of the habitat and food restriction). The river basins with the higher number of identified threats are the upper Tocantins (eight) followed by the upper Paraguaçu (six). Effective proposals to protect this neglected component of the Brazilian biodiversity are still scarce, such as monitoring projects and their function in the subterranean communities, besides education projects aiming to develop public awareness.

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