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Elemental weathering fluxes and saprolite production rate in a Central African lateritic terrain (Nsimi, South Cameroon)

Abstract : The comparison between contemporary and long-term weathering has been carried out in the Small Experimental Watershed (SEW) of Nsimi, South Cameroon in order to quantify the export fluxes of major and trace elements and the residence time of the lateritic weathering cover. We focus on the hillside system composed of a thick lateritic weathering cover topped by a soil layer. This study is built on the recent improvements of the hillside hydrological functioning and on the analyses of major and trace elements. The mass balance calculation at the weathering horizon scale performed with the parent rock as reference indicates (i) strong depletion profiles for alkalis (Na, K, Rb) and alkaline earths (Mg, Ca, Sr, Ba), (ii) moderate depletion profiles for Si, P, Cd, Cu, Zn, Ni and Co, (iii) depletion/enrichment profiles for Al, Ga, Ge, Sn, Pb, LREE, HREE, Y, U, Fe, V, Cr, Mn. It is noteworthy that (i) Mn and Ce are not significantly redistributed according to oxidative processes as it is the case for Fe, V and Cr, and (ii) Ge is fractionated compared to silica with enrichment in Fe-rich horizons. The calculations performed for the topsoil with iron crust as parent material reference reveal that the degradation of the iron crust is accompanied by the loss of most of the constituting elements, among which are those specifically accumulated as the redox sensitive elements (Fe, V, Cr) and iron oxide related elements like Th. The overall current elemental fluxes from the hillside system at the springs and the seepage zones are extremely low due to the inert lateritic mineralogy. 94% of the whole Na flux generated from the hillside corrected from atmospheric deposits (77 mol/ha/yr) represents the current weathering rates of plagioclase (oligoclase) in the system, the other remaining 6% may be attributed to the dissolution of hornblende. The silica hillside flux is 300 mol/ha/yr and can be mostly attributed to the plagioclase and kaolinite dissolution. Al and Ga are exported from the lateritic regolith and maybe due to the dissolution of kaolinite crystals. Compared to the other immobile elements (Zr, Hf, Nb and Th), Ti is significantly exported. Among redox-sensitive elements (Fe, V, Cr, Mn, Ce), only Ce and Mn are exported out of the hillside system. The other elements (Fe, V, Cr) are likely able to be mobilized but over a short distance only. Rb , Sr, Ba, Ni, Cu, Zn are affected by export processes. LREE and Y are exported but in very low amounts (in the range from µmol/ha/yr to mmol/ha/yr) while HREE and U are exported in negligible quantities. A first attempt is carried out to compare the mature ridge top profile from Nsimi SEW with the immature ridge top weathering profile from the Mule Hole SEW (South India), developed on similar granodioritic basement, in order to get deeper insight into (i) the contemporary saprolite production rates and (ii) the combined effect of precipitation (in terms of Mean Annual Rainfall, MAR) and evapotranspiration on the aggressiveness of the draining solutions. Considering (i) the contemporary Na flux as representative of the dissolution of plagioclase crystals and conservative during saprolitization processes and (ii) steady state of the inter-annual recharge (R) over a 10 years period, the current saprolite production rates ( ) are of 22 mm/kyr for Mule Hole SEW and 2 mm/kyr for Nsimi SEW, respectively. Even with a very low R/MAR ratio (0.04) compared to Nsimi, the chemical weathering at Mule Hole is active and related to the groundwater exports. At Mule Hole, plagioclase crystals are still present in the saprolite and the soil cover leading to a diffuse weathering front. The high Nsimi R/MAR ratio (0.2) allows the solution to be still aggressive with respect to the plagioclase and other weatherable minerals at the bedrock interface resulting in their complete breakdown in a few centimeters (sharp weathering front) leading to a mature saprolite. For the Nsimi SEW, if we consider (i) the low contemporary saprolite production rate (2 mm/kyr), (ii) the Miocene age (average 15 Myr) of the South Cameron Plateau landscape and (ii) the limited movement of Africa continent since Eocene, the long term saprolite production rate should have remained in its lower range, from 2 to 10 m/Myr. This suggests that, for thick weathering profiles the migration of the weathering front into the bedrock occurs at a relatively uniform rate regardless of present-day climatic conditions. Climate variation leading to the alternation of setup of savanna or humid forest will have an effect on physical erosion rather than chemical erosion for such deep weathering profiles.
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Jean-Jacques Braun, Jean-Christophe Maréchal, Jean Riotte, Jean-Loup Boeglin, Jean-Pierre Bedimo Bedimo, et al.. Elemental weathering fluxes and saprolite production rate in a Central African lateritic terrain (Nsimi, South Cameroon). Geochimica et Cosmochimica Acta, Elsevier, 2012, 99, pp.243-270. ⟨10.1016/j.gca.2012.09.024⟩. ⟨hal-00806642⟩

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