In the Bhs horizon of the 335-year-old soil, the high content of free Fe (Fedcb = 44 g•kg−1) is in the same order of magnitude than in a weathering sequence in Cameroon (20–85 g•kg−1) (Opfergelt et al., 2009), for which the variations of δ30Si values in the clay fraction due to adsorption onto Fe oxides are known (Opfergelt et al., 2010).