@article{PAR00009665,
  title = {{E}lemental weathering fluxes and saprolite production rate in a {C}entral {A}frican lateritic terrain ({N}simi, {S}outh {C}ameroon)},
  author = {{B}raun, {J}ean-{J}acques and {M}ar{\'e}chal, {J}.{C}. and {R}iotte, {J}ean and {B}oeglin, {J}ean-{L}oup and {B}edimo, {J}.{P}.{B}. and {N}goupayou, {J}.{R}.{N}. and {N}yeck, {B}. and {R}obain, {H}enri and {S}ekhar, {M}. and {A}udry, {S}. and {V}iers, {J}.},
  editor = {},
  language = {{ENG}},
  abstract = {{T}he comparison between contemporary and long-term weathering has been carried out in the {S}mall {E}xperimental {W}atershed ({SEW}) of {N}simi, {S}outh {C}ameroon in order to quantify the export fluxes of major and trace elements and the residence time of the lateritic weathering cover. {W}e focus on the hillside system composed of a thick lateritic weathering cover topped by a soil layer. {T}his study is built on the recent improvements of the hillside hydrological functioning and on the analyses of major and trace elements. {T}he mass balance calculation at the weathering horizon scale performed with the parent rock as reference indicates (i) strong depletion profiles for alkalis ({N}a, {K}, {R}b) and alkaline earths ({M}g, {C}a, {S}r, {B}a), (ii) moderate depletion profiles for {S}i, {P}, {C}d, {C}u, {Z}n, {N}i and {C}o, (iii) depletion/enrichment profiles for {A}l, {G}a, {G}e, {S}n, {P}b, {LREE}, {HREE}, {Y}, {U}, {F}e, {V}, {C}r, {M}n. {I}t is noteworthy that (i) {M}n and {C}e are not significantly redistributed according to oxidative processes as it is the case for {F}e, {V} and {C}r, and (ii) {G}e is fractionated compared to silica with enrichment in {F}e-rich horizons. {T}he 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 ({F}e, {V}, {C}r) and iron oxide related elements like {T}h. {T}he overall current elemental fluxes from the hillside system at the springs and the seepage zones are extremely low due to the inert lateritic mineralogy. {N}inety-four percent of the whole {N}a 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. {T}he silica hillside flux is 300 mol/ha/yr and can be mostly attributed to the plagioclase and kaolinite dissolution. {A}l and {G}a are exported from the lateritic regolith and maybe due to the dissolution of kaolinite crystals. {C}ompared to the other immobile elements ({Z}r, {H}f, {N}b and {T}h), {T}i is significantly exported. {A}mong redox-sensitive elements ({F}e, {V}, {C}r, {M}n, {C}e), only {C}e and {M}n are exported out of the hillside system. {T}he other elements ({F}e, {V}, {C}r) are likely able to be mobilized but over a short distance only. {R}b, {S}r, {B}a, {N}i, {C}u, {Z}n are affected by export processes. {LREE} and {Y} are exported but in very low amounts (in the range from mu mol/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 {N}simi {SEW} with the immature ridge top weathering profile from the {M}ule {H}ole {SEW} ({S}outh {I}ndia), 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 {M}ean {A}nnual {R}ainfall, {MAR}) and evapotranspiration on the aggressiveness of the draining solutions. {C}onsidering (i) the contemporary {N}a 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 (sigma(r)) are of 22 mm/kyr for {M}ule {H}ole {SEW} and 2 mm/kyr for {N}simi {SEW}, respectively. {E}ven with a very low {R}/{MAR} ratio (0.04) compared to {N}simi, the chemical weathering at {M}ule {H}ole is active and related to the groundwater exports. {A}t {M}ule {H}ole, plagioclase crystals are still present in the saprolite and the soil cover leading to a diffuse weathering front. {T}he high {N}simi {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 centimetres (sharp weathering front) leading to a mature saprolite. {F}or the {N}simi {SEW}, if we consider (i) the low contemporary saprolite production rate (2 mm/kyr), (ii) the {M}iocene age (average 15 {M}yr) of the {S}outh {C}ameron {P}lateau landscape and (ii) the limited movement of {A}frica continent since {E}ocene, the long term saprolite production rate should have remained in its lower range, from 2 to 10 m/{M}yr. {T}his 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. {C}limate 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.},
  keywords = {{CAMEROUN}},
  booktitle = {},
  journal = {{G}eochimica et {C}osmochimica {A}cta},
  volume = {99},
  numero = {},
  pages = {243--270},
  ISSN = {0016-7037},
  year = {2012},
  DOI = {10.1016/j.gca.2012.09.024},
  URL = {https://www.documentation.ird.fr/hor/{PAR}00009665},
}