TY - JOUR A1 - Sebag, David A1 - Garcin, Yannick A1 - Adatte, Thierry A1 - Deschamps, Pierre A1 - Menot, Guillemette A1 - Verrecchia, Eric P. T1 - Correction for the siderite effect on Rock-Eval parameters BT - Application to the sediments of Lake Barombi (southwest Cameroon) JF - Organic geochemistry : the international journal for rapid publication of current research in organic geochemistry and biochemistry N2 - Originally developed for use in the petroleum industry, Rock-Eval pyrolysis is a technique commonly applied to lake sediments to infer paleoenvironmental reconstructions. The standard Rock-Eval parameters provide information on the amount of total organic and inorganic carbon (TOC and MinC, respectively), and are usually interpreted as proxies for the source (aquatic or terrestrial) of the primary production of organic matter (Hydrogen Index vs Oxygen Index). Although this method usually provides valuable evidence, the common presence of siderite in tropical lake sediments can alter the primary signal of the sedimentary organic matter (SOM). Indeed, the CO2 and CO released by the pyrolysis of siderite are integral to the calculation of the SOM-related standard Rock-Eval parameters. In this study, we analyze sediments from a core collected in the Lake Barombi (southwest Cameroon) and describe the impact of siderite on standard Rock-Eval parameters. We propose a workflow that allows standard Rock-Eval parameters to be corrected, based on the analysis of thermograms. The proposed corrections provide siderite-effect-free parameters, accurately reflecting the changes in sedimentary organic matter composition. (C) 2018 Elsevier Ltd. All rights reserved. KW - Organic geochemistry KW - Rock-Eval pyrolysis KW - Van Krevelen diagram KW - Siderite KW - Tropical lake KW - Western Central Africa Y1 - 2018 U6 - https://doi.org/10.1016/j.orggeochem.2018.05.010 SN - 0146-6380 VL - 123 SP - 126 EP - 135 PB - Elsevier CY - Oxford ER - TY - JOUR A1 - Diaz, Nathalie A1 - Dietrich, Fabienne A1 - Sebag, David A1 - King, Georgina E. A1 - Valla, Pierre G. A1 - Durand, Alain A1 - Garcin, Yannick A1 - de Saulie, Geoffroy A1 - Deschamps, Pierre A1 - Herman, Frederic A1 - Verrecchia, Eric P. T1 - Pedo-sedimentary constituents as paleoenvironmental proxies in the Sudano-Sahelian belt during the Late Quaternary (southwestern Chad Basin) JF - Quaternary science reviews : the international multidisciplinary research and review journal N2 - Climate and environmental changes since the Last Glacial Maximum in the tropical zone of West Africa are usually inferred from marine and continental records. In this study, the potential of carbonate pedo-sedimentary geosystems, i.e. Vertisol relics, to record paleoenvironmental changes in the southwestern part of Chad Basin are investigated. A multi-dating approach was applied on different pedogenic organo-mineral constituents. Optically stimulated luminescence (OSL) dating was performed on the soil K-rich feldspars and was combined with radiocarbon dating on both the inorganic (C-14(inorg)) and organic carbon (C-14(org)) soil fractions. Three main pedo-sedimentary processes were assessed over the last 20 ka BP: 1) the soil parent material deposition, from 18 ka to 12 ka BP (OSL), 2) the soil organic matter integration, from 11 cal ka to 8 cal ka BP (C-14(org)), and 3) the pedogenic carbonate nodule precipitation, from 7 cal ka to 5 cal ka BP (C-14(inorg)). These processes correlate well with the Chad Basin stratigraphy and West African records and are shown to be related to significant changes in the soil water balance responding to the evolution of continental hydrology during the Late Quaternary. The last phase affecting the Vertisol relics is the increase of erosion, which is hypothesized to be due to a decrease of the vegetation cover triggered by (i) the onset of drier conditions, possibly strengthened by (ii) anthropogenic pressure. Archaeological data from Far North Cameroon and northern Nigeria, as well as sedimentation times in Lake Tilla (northeastern Nigeria), were used to test these relationships. The increase of erosion is suggested to possibly occur between c. 3 cal ka and 1 cal ka BP. Finally, satellite images revealed similar geosystems all along the Sudano-Sahelian belt, and initial C-14(inorg) ages of the samples collected in four sites gave similar ages to those reported in this study. Consequently, the carbonate pedo-sedimentary geosystems are valuable continental paleoenvironmental archives and soil water balance proxies of the semiarid tropics of West Africa. (C) 2018 Elsevier Ltd. All rights reserved. KW - Pedogenic carbonate nodules KW - Vertisol genesis KW - Soil water balance KW - Optical methods KW - Radiogenic isotopes KW - Quaternary KW - Monsoon KW - Western Africa Y1 - 2018 U6 - https://doi.org/10.1016/j.quascirev.2018.05.022 SN - 0277-3791 VL - 191 SP - 348 EP - 362 PB - Elsevier CY - Oxford ER -