Significant amounts of pedogenic Ca-carbonate nodules have been observed in the Far North Region of Cameroon in the carbonate-free watershed of the Mayo Tsanaga, thus a priori not favourable for carbonate nodules accumulations. These nodules are associated with a Clay-Rich Parent Material (CRPM), covering either a granitic (upstream) or a greenstone bedrock (downstream). In this peculiar context, the amount of pedogenic carbonate nodules represents large quantities of Ca and C. Therefore, determining the Ca sources for pedogenic carbonate nodules is a key point regarding the mechanisms leading to carbonate nodule precipitation and their role in the biogeochemical cycle of Ca. Three sites, two on granite and one on greenstone, were studied by combining Sr et Nd isotope systematics in order to assess the Ca sources of carbonate nodules and the origin of the CRPM associated with nodules.
Carbonate nodules have a distinct Sr isotopic composition in each profile, pointing to the contribution of a local source for Ca. Sr isotopic compositions of plagioclases and carbonate nodules display a concomitant variation in each profile, indicating that plagioclases acted as a Sr, and thus Ca, source for the carbonate nodules. Nevertheless, carbonate nodules have a higher Sr isotopic signature than plagioclases, implying the contribution of another more radiogenic Sr source. Sr and Nd isotope data from the CRPM show that it is a mixture of weathered bedrock compounds and Saharan dust. This result highlights two other potential Sr and Ca sources: biotite and Saharan dust. Calculations of their respective contributions demonstrate that in situ weathering significantly adds to the Ca source of nodules. This result contrasts with many other studies from similar settings that highlight the importance of allochthonous - marine, aeolian - inputs. This conclusion points out that a substantial part of Ca is transferred from the local bedrock to the carbonate nodules. This mechanism seems to have been possible as peculiar edaphic conditions preserved the primary Ca sources, making in situ weathering a significant process for providing Ca.