Diatom, phytolith, and pollen records from a 10Be/9Be dated lacustrine succession in the Chad basin : insight on the Miocene–Pliocene paleoenvironmental changes in Central Africa Alice Novello, Anne-Elisabeth Lebatard, Abderamane Moussa, Doris Barboni, Florence Sylvestre, Didier Bourles, Christine Paillès, Guillaume Buchet, Alain Decarreau, Philippe Duringer, et al. To cite this version: Alice Novello, Anne-Elisabeth Lebatard, Abderamane Moussa, Doris Barboni, Florence Sylvestre, et al.. Diatom, phytolith, and pollen records from a 10Be/9Be dated lacustrine succession in the Chad basin : insight on the Miocene–Pliocene paleoenvironmental changes in Central Africa. Palaeogeogra- phy, Palaeoclimatology, Palaeoecology, Elsevier, 2015, 430, pp.85-103. 10.1016/j.palaeo.2015.04.013. hal-01691621 HAL Id: hal-01691621 https://hal.archives-ouvertes.fr/hal-01691621 Submitted on 29 Mar 2019 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Accepted manuscript now published in Palaeogeography, Palaeoclimatology, Palaeoecology (2015) 430: 85 - 10310.1016/j.palaeo.2015.04.013 Diatom, phytolith, and pollen records from a 10Be/9Be dated lacustrine succession in the Chad basin: insight on the Miocene-Pliocene paleoenvironmental changes in Central Africa Alice Novello1, 2, Anne-Elisabeth Lebatard2, Abderamane Moussa3, Doris Barboni2, Florence Sylvestre2, Didier L. Bourlès2, Christine Paillès2, Guillaume Buchet2, Alain Decarreau4, Philippe Duringer5, Jean- François Ghienne5, Jean Maley6, Jean-Charles Mazur2, Claude Roquin5, Mathieu Schuster5, Patrick Vignaud1 1 iPHEP, Institut de Paléoprimatologie, Paléontologie Humaine : Evolution et Paléoenvironnements, UMR 7262 CNRS-INEE - Université de Poitiers, 6 rue Michel Brunet, 86073 Poitiers Cedex 9, France 2 Aix-Marseille Université, CNRS-IRD UM 34 CEREGE, Technopôle de l’Environnement Arbois- Méditerranée, BP80, 13545 Aix-en-Provence, France 3 Département de Paléontologie, Université de N'Djamena, BP 1117, N'Djamena, Chad 4 IC2MP, Institut de Chimie des Milieux et Matériaux de Poitiers, UMR 7285 CNRS - Université de Poitiers, 4 rue Michel Brunet - TSA 51106, 86073 Poitiers Cedex 9, France 5 Institut de Physique du Globe de Strasbourg, UMR7516, CNRS - Université de Strasbourg/EOST, 1 rue Blessig, 67084 Strasbourg Cedex, France 6 IRD & Département Paléoenvironnements, Institut des Sciences de l'Evolution de Montpellier, UMR 5554 CNRS, Université de Montpellier II, 34095, Montpellier, Cedex 5, France *Corresponding author: A. Novello, Email: [email protected] Abstract A discontinuous 200m-long borehole drilled in the Bol Archipelago (13°N, Lake Chad) provided 25 samples which were dated with the 10Be/9Be method and analyzed for their micro-biological content. The dating provided ages ranging from 6.3±0.1 to 2.6±0.1 Ma, a period contemporaneous to the Pliocene fossil localities located in the current Chadian Djurab desert (16-17°N). Well-preserved diatom assemblages first occurred at 4.7±0.1 Ma and remain dominated by the freshwater planktonic genera Aulacoseira and Stephanodiscus until the end of the Pliocene. This supports the recurrences of lacustrine conditions at Bol during all the Pliocene. The presence of pelite and argillaceous deposits in the core before 4.7±0.1 Ma, however, suggests that the lake settled earlier, at least since 6.3±0.1 Ma. The abundance of Afromontane pollen taxa at 4.2 ±0.1 Ma and the occurrence of trapeziform polylobate phytoliths throughout the sequence suggest significant vegetation inputs from the southern highlands. The importance of kaolinite in the clay sediments also indicates that a water supply predominantly from the south already existed at that time. Phytolith assemblages are all dominated by lobate grass silica short cells and by blocky and elongate types, which attest to the presence of herbaceous-dominated vegetation around Bol and/or in the southern drainage basin during the Pliocene. This result is also supported with the pollen assemblage described at 4.2±0.1 Ma, which shows highest affinity for the savanna biome. Moreover, low values for the Xerophytic grass phytolith index indicate the presence of mesophytic grass communities in this vegetation. Significant variations in the abundance of blocky and elongate phytoliths suggest local alternations of fully lacustrine and marshy conditions at Bol. Particularly between 3.6 and 2.7 Ma the abundance of silicified bulliform cells associated with the absence of diatoms support the hypothesis of a significant lacustrine reduction at Bol favoring the increasing of local marshy vegetation. Keywords: Lake Chad, Sahara-Sahel, Miocene-Pliocene, vegetation, climate, hominin 1 Accepted manuscript now published in Palaeogeography, Palaeoclimatology, Palaeoecology (2015) 430: 85 - 10310.1016/j.palaeo.2015.04.013 1 Introduction layers useful for intersite paleoenvironmental First hominins appeared during the late comparisons. Biomarker data from the marine Neogene, when significant vegetation changes record DSDP231 off the East African coast occurred in Africa (e.g. Bonnefille, 2010). indicate two major and distinct steps in the These changes notably include the appearance expansion of C4 biomass in East Africa: one and expansion of C4 grasses (Cerling et al., during the Tortonian (11-9 Ma) and one during 1997; Feakins et al., 2005; Edwards et al., 2010; the Pliocene (4.3-1.4 Ma) (Feakins et al., 2013). Cerling et al., 2011; Strömberg, 2011; Dupont Yet the increase in C4 biomass was not et al., 2013; Feakins et al., 2013), which are necessarily linked with an expansion of C4 currently part of tropical grasslands and grasses but rather with a greater abundance of savannas at low altitude (Sage, 2004). Amaranthaceae and Chenopodiaceae Moreover, marine records off the African coasts (xerophytic C4 forbs) in the vegetation as indicate high climate variability in tropical precised by pollen data from the same record Africa during the late Neogene (Feakins and (Bonnefille, 2010). deMenocal, 2010). This environmental variability is revealed by a succession of humid In Chad (Central Africa), the hominin record is and arid phases tuned with precessional less documented than in the East African rift cyclicity of the Earth’s orbit (deMenocal, 2004). system but nevertheless significant. It includes Global vegetation and climate changes may Sahelanthropus tchadensis and have triggered hominin speciation and adaptive Australopithecus bahrelghazali, which are the features (see review by Maslin et al., 2015). only two hominin species described in tropical Particularly, the spread of C4 grass savannas Africa outside the East African rift valley may have triggered the appearance of (Brunet et al., 2002; Vignaud et al., 2002; bipedalism in early hominins (e.g. Bonnefille, Brunet et al., 2005). S. tchadensis is what’s 2010; Cerling et al., 2011; Dominguez-Rodrigo, more the oldest hominin species described so 2014). Among the factors that may have far (Brunet et al., 2002). Fossil faunal contributed to the increase of C4 grasses in assemblages associated with the Chadian African landscapes, there is the decrease in the hominins bear ages around 7 Ma for S. length but the intensification of the rainy tchadensis and of 3.5 Ma for A. bahrelghazali, season, or so called seasonality (Beerling and respectively (Brunet et al., 1995; Vignaud et al., Osborne, 2006), and the subsequent 2002). The authigenic 10Be/9Be dating method enhancement of the fire activity (Archibald et applied to the fossiliferous areas of northern al., 2009). Hydrological changes and the Chad produced after absolute ages that were decrease of pCO2 recorded at the end of the consistent with the ages provided by faunas Miocene (Pagani et al., 1999) may also have (Lebatard et al., 2008; Lebatard et al., 2010). been significant in the spread of C4 plants in the African vegetation (Tipple and Pagani, 2010). Paleoenvironmental studies at Chadian hominin As the hominin record is remarkably well sites (faunal composition and structure, dental documented in the East African rift system, mesowear and isotopes) concluded to the much attention was given to this region for existence of a diversified vegetation cover, with reconstructing late Neogene a mosaic of forests, woodlands, grasslands, up paleoenvironmental changes and especially C4- to desert conditions, in close relationship with grass expansion in Africa (Bonnefille, 2010). aquatic/lacustrine areas in northern Chad during Indeed, about ten early hominin species from the Messinian (Vignaud et al., 2002; Schuster et this region were described between the al., 2006; Jacques, 2007; LeFur et al., 2009; Messinian and the early Pleistocene, including Schuster et al., 2009; Blondel et al., 2010; Otero three of the first hominins: Orrorin tugenensis et al., 2010), and open C4-vegetations later on (Kenya, ca 6 Ma, Senut et al., 2001), during the Pliocene (Brunet et al., 1997; Zazzo Ardipithecus kaddaba, and Aridipithecus et al., 2000; Geraads et al., 2001; Lee-Thorp et ramidus (Ethiopia, ca 5.5-4.4 Ma, White et al., al., 2012), in association with