The origin of Vertisols and their relationship to Acid Sulfate Soils in the Senegal valley

International audienceIn the Senegal valley, it is commonly considered that the Acid Sulfate Soils of the delta are fossil soils overlain by more recent sediments, and that the Vertisols, which abruptly overlie a thick sandy horizon, result from a change in the sedimentation mode of the river. However, we show that both soils belong to the same pedological system extending from the delta to the limit of the last marine transgression. This conclusion is based on (1) the study of soil profiles intermediate between Acid Sulfate Soils and Vertisols along a 100-m sequence in the delta, (2) the mineralogy of the clay fraction ( < 2 Am) in a 200-km transect along the river, and (3) the similarity of the sand size distribution across the textural discontinuity between the horizons. The following processes are involved in the pedological transformations: (1) development of acidity by oxidation of pyrite, (2) neutralization of acidity initially by the carbonate in shell beds, and later by the hydrolysis of easily weatherable silicate clays. The slightly alkaline river water precipitates kaolinite and later smectite at the contact between the strongly acidic and slightly alkaline environments. This results in the formation of a superficial vertic clay horizon surmounting a sandy horizon. Therefore, we emphasize that the soil morphology results from development and control of acidity and not from changes in past climates

Geochemistry of clay dunes and associated pan in the Senegal Delta (Mauritania)

Aeolian deflation is very active in the Senegal delta and results in the formation of many clay dunes with high salt concentration. The chemistry of soil and groundwater was studied along a sequence from a pan to the associated clay dune in the Diawling park. Equilibria computation and field and SEM observations indicates that the changes observed in the chemical facies of the solution are due to the following precipitation sequence : calcite&gt;gypsum&gt;halite. A progressive but significant fixing of sodium onto the clay particles is observed. However, the calcite deposite in the soil is limited by exhibition of the potentiel acidity arising from the former mangrove. It locally results in high equilibrating CO2 pressure. This study provided an opportunity to test succesfully a new extension to the Debye-Hückel law for concentrated chlorurated sodic waters. (Résumé d'auteur

The origin of Vertisols and their relationship to Acid Sulfate Soils in the Senegal Valley

In the Senegal valley, it is commonly considered that the Acid Sulfate Soils of the delta are fossil soils overlain by more recent sediments, and that the Vertisols, which abruptly overlie a thick sandy horizon, result from a change in the sedimentation mode of the river. However, we show that both soils belong to the same pedological system extending from the delta to the limit of the last marine transgression. This conclusion is based on (1) the study of soil profiles intermediate between Acid Sulfate Soils and Vertisols along a 100-m sequence in the delta, (2) the mineralogy of the clay fraction $( < 2 \mu m)$ in a 200-km transect along the river, and (3) the similarity of the sand size distribution across the textural discontinuity between the horizons. The following processes are involved in the pedological transformations: (1) development of acidity by oxidation of pyrite, (2) neutralization of acidity initially by the carbonate in shell beds, and later by the hydrolysis of easily weatherable silicate clays. The slightly alkaline river water precipitates kaolinite and later smectite at the contact between the strongly acidic and slightly alkaline environments. This results in the formation of a superficial vertic clay horizon surmounting a sandy horizon. Therefore, we emphasize that the soil morphology results from development and control of acidity and not from changes in past climates