The African Holocene Humid Period in the Tibesti mountains (central Sahara, Chad): Climate reconstruction inferred from fossil diatoms and their oxygen isotope composition

The climate of the African Holocene Humid Period (AHHP) is reconstructed in the Tibesti Volcanic Massif (TVM) in the central Sahara from well-preserved diatomaceous deposits in the two crater palaeolakes of Trou au Natron at Pic Toussidé and Era Kohor at Emi Koussi. The two records cover the period from ∼9500 to 4500 cal yr BP. Climate and palaeoenvironmental changes during this period were inferred from diatom assemblages, interpretation of variations in their oxygen isotope composition (δ18Odiatom), reconstruction of lake water conductivity from diatom-based transfer functions, and estimation of the lake water balance (Evaporation/Inflow ratio, E/I). Our findings provide evidence for two distinct lacustrine episodes. During the early to mid-Holocene transition, low δ18Odiatom values, high percentages of planktonic diatoms, low lake water conductivity and a positive water balance (E/I < 1) suggest wet conditions, which were likely related to the optimum of the AHHP. From the mid-to late Holocene transition, an aridification trend is revealed by increasing δ18Odiatom values, high percentages of benthic diatoms and a negative water budget (E/I > 1), occurring as early as 6500 cal yr BP and intensifying after 5300 cal yr BP. Moreover, our data show on average a decrease in precipitation amounts of ∼35% between the peak and the end of the AHHP in the Tibesti region. This timing of the AHHP in the mountainous Tibesti is consistent with the aridification of the central Sahara recorded at lowland sites, which has mainly been related to the southward retreat of the Intertropical Convergence Zone (ITCZ) and the associated African monsoonal rainfall belt, following the gradually declining summer insolation that led to the termination of the AHHP. Our results prove the existence of Holocene lakes in the TVM craters that developed contemporaneously with the lakes of the Chadian basin and the Libyan Sahara. On a broader scale, our data share similar hydroclimatic patterns with studies from the eastern and northern Sahara.