Abstract
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 lowestTZsil+rates, (ii) the Northern sub-basins and the Mbei sub-basins, which drain the southern edge of the tectonically quiescent South Cameroon Plateau, show intermediateTZsil+rates and (iii) the Southern sub-basins characterized by steeper slopes record the highestTZsil+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.
Highlights
Over geological time scales, chemical weathering acts as a major player of the global biogeochemical cycles of elements in the Earth’s Critical Zone
The present study provides the opportunity to explore the variability in weathering fluxes and rates in the large Ogooué River Basin as well in the neighboring, smaller Mbei River Basin with respect to geomorphology, tectonics, and lithology
The remaining filtered water was splitted into three aliquots: one unacidified aliquot of 30 mL for major anion analyses, one acidified aliquot of 1L for cation, trace element, and 87Sr/86Sr analyses, and one acidified aliquot of 60 mL stored in an amber glass tube for dissolved organic carbon (DOC) analysis
Summary
Chemical weathering acts as a major player of the global biogeochemical cycles of elements in the Earth’s Critical Zone. Especially in the humid tropics, these low-relief settings favor the formation of deep regolith covers, chemically depleted in base cations and limiting water-bedrock interactions due to slow water percolation from the surface to the bedrock (e.g., Stallard and Edmond, 1987; Braun et al, 2005, 2012; West, 2012; Riebe et al, 2017) While these hot and humid cratonic areas commonly exhibit low silicate weathering rates by comparison with orogenic areas (e.g., Gaillardet et al, 1999b; Moon et al, 2014) they dominate the intertropical regions surface area and, represent a significant proportion of the global delivery of dissolved matter to the oceans (Milliman and Farnsworth, 2011; Von Blanckenburg et al, 2015). According to modeling results (Goddéris et al, 2008) the net weathering budget of the intertropical cratonic areas and their role on the long term CO2 budget may have been underestimated and need to be investigated in more detail to characterize their potential role in the Cenozoic global climate evolution
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