Nd-isotope evidence for the distal provenance of the historical (c. <3000BP) lateritic surface cover underlying the Equatorial forest in Gabon (Western Africa)
Résumé
Surficial formations in Gabon, as well as in other places of western Central Africa include a ubiquitous, homogeneous and 1-3 m-thick clayey to sandy lateritic surface cover known as the 'Cover Horizon'. From C-14 radiometric dating it has been concluded that the emplacement of this unit was correlative with a major environmental crisis which affected Central Africa c. 3000-2000 years ago. Be-10 and Nd-isotopic analyses have been performed to provide new constraints on the age and origin of this layer. Six samples from two depth profiles investigated for 10Be exhibit an almost constant concentration consistent with a very recent deposition age. Nd-isotopic analyses performed on the silt to clay fraction of eleven samples from widely spaced locations over Gabon attest for mildly radiogenic signatures (epsilon Nd = -23 to -17) in ten of them, and a slightly radiogenic signature (epsilon Nd = -9) in one sample. T-Dm model ages range from 1.6 to 2.6 Ga, and a perfect discrimination is observed between the Nd-isotopic signature of the Cover Horizon and that of the underlying Congo Craton. This makes an aeolian origin as the most probable for the Cover Horizon. The average epsilon Nd (c. -20) is however rather unusual for aeolian sediments or aerosols. A possible source of particles is therefore tested by considering the present-day atmospheric flux over Gabon and adjacent regions. Combined atmospheric modeling and Nd-isotopes leads to the conclusion that the fine fraction of the Cover Horizon could have originated from the northern part of the Namib desert.