The late Pleistocene – Holocene paleoclimate reconstruction in the Adamawa plateau (Central Cameroon) inferred from the geochemistry and mineralogy of the Lake Fonjak sediments

Abstract

In the aim of improving the understanding of paleoclimate variations in the transitional zone between the sahelian and sub-equatorial rainforest belts, paleoclimate conditions of the Adamawa plateau were reconstructed from geochemical and mineralogical records coupled with radiocarbon dating of lacustrine sediments since 13,339 cal yr BP. The Lake Fonjak depositional sequence displays eight lithofacies made up of: light greenish grey clayey mud; black clayey mud mostly composed of volcanic ash; light grey silty clayey mud; dark grey sandy layer with coarse rounded fragments of volcanic rocks; light grey silty mud; dark grey silty mud; light grey silty mud; dark grey silty mud with few rootlets, and an herbaceous peat. In this depositional sequence, organic matter contents range from 9% to 59%. A correlation between the age-depth model and lithofacies of this depositional sequence suggests Late Pleistocene sediments at the base and Late Holocene sediments at the top. These Holocene sediments exhibit in XRD data various minerals assemblages with relative contents varying from 15% to 42% for quartz; 19%–37% for kaolinite; 12%–37% for micas; 5%–18% for serpentine; 1%–9% for feldspars; 5%–7% for gibbsite and 11% for olivine. As far as detrital minerals are concerned, they are: quartz, orthoclase, siderite, vivianite, hornblende, plagioclase and opaque minerals. Major element abundances in decreasing order in these sediments are as follow: silicium, aluminium, iron and titanium. The highest content of SiO2, Fe2O3, MnO, CaO, Na2O and K2O are recorded within either the volcanic ash layer or its overlapped lithofacies; whereas, P2O5, MgO, TiO2 and Al2O3 exhibit their highest values at the top of the sequence. Trace elements vary little in these sediments, except for Co which rises abruptly within the volcanic ash layer. However, Ni, Co, Cr and Cu are twice to three times enriched with respect to average shale composition; whereas V and U are depleted. REE show complex behavior, alternatively increasing and decreasing, except for Ce; and Eu with positive anomaly. The geochemical classification of these sediments ranges them within Fe-shales with few litharenite. They were deposited under dysoxic to suboxic conditions, except around 4200 cal yr BP and around 2500 cal yr BP where these conditions were anoxic. The Lake Fonjak sediments derive from highly weathered mafic rocks probably andesite as suggested by PIA: 95.32–99.10%, CIA: 86.46–97.45%, CIW: 95.70–99.12% and La/Sc vs. Co/Th diagram. This sedimentation took place within three paleoclimate phases: 13,339–4400 cal yr BP; 4400–2400 cal yr BP; and 2400 cal yr BP to present. This paleoclimate was predominantly humid and marked either by the slowdown of rainfall and an intensification of the dry season, or by the reinforcement of rainfalls.