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The major climatic forcing parameters on Earth climate are temperature and the atmospheric concentrations of CO2. Even if their evolutions covaried to the first-order, the geological record show periods with non-linear evolution between those two parameters. Such delinking requires accurate paleoclimate reconstructions with implications for the modelling studies of our future climate.pCO2 and Sea Surface Temperature (SST) reconstructions are usually quantified using proxies relying on both the organic matter produced by coccolithophores (UK37’ index and δ13Calkenones) and calcite of foraminiferal tests (δ11B, δ18O, Mg/Ca). These proxies have been very useful for a variety of paleoclimatic advances, yet present unresolved and potentially important biases. As an example, alkenone carbon isotopes are not able to register low to moderate pCO2 levels (Badger et al., 2019). This is notoriously a major issue for paleoclimate reconstructions of the last 6 My (Plio-Pleistocene period).Our approach is to use a unique archive – the coccoliths – for determination of coeval SST and pCO2. Coccoliths are small calcite plates produced by unicellular photosynthetic algae called coccolithophores. They are a very promising substrate to analyse for paleoclimate studies because they calcify in the uppermost water column and because their isotopic ratios are sensitive to both photosynthesis and calcification (Hermoso et al. 2020). Therefore, these isotopic ratios provide physiological and metabolic information about coccolithophores of the past. In order to infer paleoclimates from the sedimentary archives, we have to deconvolve the isotopic biological imprint (vital effect) from the environment signal. For the evaluation of the vital effects, we have undertaken a large-scaled culture experiments with various strains of coccolithophore grown under various CO2 concentrations and pH (Le Guevel et al. in prep). Even if we have managed culture until 1400ppm and 7.55 unit of pH, we were particularly interested in low pCO2 and high pH conditions because the bibliography is lacking of vital effect for Plio-Pleistocène applications. All the selected strains produce coccoliths within the size range of the one we find predominantly in the marine sediments throughout geological times.We document a large decrease of the carbon differential vital effect with the CO2 concentration increase between Gephyrocapsa oceanica and Coccolithus braarudii. This is consistent with previous studies but the absolute values are slightly different and we provide a more precise dataset at low to moderate pCO2 than the previous ones (Rickaby et al., 2010; Hermoso et al., 2016). We propose the first study of the oxygen and carbon vital effect of the Helicosphaera carteri group with combined CO2/pH changes. Taken together, the culture data and measurements of the isotopic composition of the calcite biominerals allows better paleoreconstructions of SST and aqueous CO2.

Methodological approaches that allow the characterization of environmental and climatic changes on complementary tem-poral and geographical scales are essential in the study of the response of ancient societies to changes in their environment.Here, through the combining taxonomic and isotopic (δ13 C) analysis of archaeological charcoal from the Mesolithic site of La Baume de Monthiver (Var, France) with sub-stratigraphic sampling, we highlight the potential of this method to provide a highly sensitive proxy for long-term environmental trends as well as more rapid climatic episodes. Thus, we observe at the site: (i) the general taxonomic composition of the tree cover for the study phase, (ii) a high-resolution isotopic signal highlighting short-term climatic evolution, interpreted as early Holocene warming and/or the “9.3 ka” dry climate event.; Les approches permettant de caractériser les évolutions environ-nementales et climatiques à différentes échelles temporelles/géographiques sont essentielles dans l’étude de la réponse des sociétés anciennes faces aux changements de leur milieu. Ici, à travers l’application combinée de l’analyse taxinomique et isotopique (δ13 C) des charbons archéologiques des charbons du site mésolithique de La Baume de Monthiver (Var, France) avec un échantillonnage sous-stratigraphique, nous soulignons le potentiel de cette méthode pour fournir un proxy de haute sensibilité renseignant les tendances environnementales à long terme comme les épisodes climatiques plus rapides.On observe ainsi sur le site : (i) la composition taxinomique du cou-vert arborée de la phase d’étude, (ii) une évolution climatique à court terme, interprétée comme le réchauffement du début de l’Holocène et/ou l’événement climatique sec de « 9,3 ka »

The Zambezi turbidite system (Mozambique Channel, Western Indian Ocean) is one of the largest turbidite systems in the world and yet still remains poorly understood. Newly acquired high-resolution multibeam bathymetry, seismic reflection and sedimentological data allowed to investigate the architecture evolution and depositional patterns since the Oligocene in order to understand the main forcing factors that control the deep sea sedimentation in the Mozambique Channel. It was found that the Zambezi turbidite system is composed of two adjacent depositional systems: the channelized Zambezi Fan and a semiconfined fan in the lntermediate Basin. Moreover, results and interpretations indicate: (1) important tectonic control since the Miocene that caused deep incision of the Zambezi Valley and limited overflow of turbidite currents; (2) an important influence of bottom-currents that induces scarcity of fine-grained turbidites, valley flanks erosion and widespread occurrence of sediment waves; (3) low turbidite activity for the last 700 kyr that shows no relationship with sea-level changes as turbidite activity occurred irrespective of glacial or interglacial periods; (4) peaks in terrigenous flux with maxima in local summer insolation, reflecting that monsoon controls the sediment inputs towards the deep marine depositional system; (5) an on-off evolution of the Zambezi Fan that demonstrates a depocenter shift from the distal Zambezi Fan to the proximal Intermediate Basin. All our findings underline the high complexity in depositional environments of the Zambezi turbidite system., Le système turbiditique du Zambèze (Canal du Mozambique, Océan Indien occidental) est l'un des plus grands systèmes turbiditiques au monde et reste encore mal compris. L'acquisition récente de données bathymétriques multifaisceaux à haute résolution, de données de sismique réflexion haute et très haute résolution et de données sédimentologiques a permis d'étudier l'évolution de l'architecture et l'organisation des dépôts depuis l'Oligocène afin de comprendre les principaux facteurs de forçage qui contrôlent la sédimentation en eau profonde dans le Canal du Mozambique. Le système turbiditique du Zambèze est composé de deux systèmes de dépôt adjacents : l'éventail du Zambèze ("Zambezi Fan") et un éventail semi-confiné ("ponded fan") dans un bassin intermédiaire face à l'embouchure du Zambèze. Les résultats et les interprétations indiquent : (1) un important contrôle tectonique depuis le Miocène responsable d'une sur-incision profonde de la vallée du Zambèze et de débordements limités des courants turbiditiques ; (2) une influence importante des courants de fond qui induisent la rareté des turbidites fines, l'érosion des flancs des vallées et l'apparition généralisée de "sediment waves" ; (3) une faible activité turbiditique au cours des 700 derniers kyr qui ne montre, en outre, aucune relation avec les changements du niveau de la mer, l'activité turbiditique s'observant indépendamment des périodes glaciaires et interglaciaires ; (4) des pics de flux terrigènes corrélés aux maxima d'ensoleillement estival local, indiquant que la mousson est le contrôle majeur des apports de sédiments vers le système de dépôt marin profond ; (5) une évolution "on-off" du l'éventail du Zambèze qui démontre un déplacement du dépocentre de la partie distale de l'éventail vers le bassin intermédiaire proximal. Ces résultats soulignent la grande complexité du système turbiditique du Zambèze en raison de l'impact de facteurs de contrôles multiples.

The main objective of the PhD thesis is the reconstruction of palaeoenvironment experienced during the Late Pleistocene, mainly during Marine Isotope Stage 3 (MIS 3), at northeastern Iberia, through the combination of different ecological approaches based on small-mammal assemblages (insectivores, bats and rodents). Three archaeological sites are included: Abric Romaní rock-shelter (Capellades, Barcelona); Teixoneres cave (Moià, Barcelona) and Arbreda cave (Serinyà, Girona). It is also included a paleontological site from a close chronology, Xaragalls cave (Vimbodí-Poblet, Tarragona). The aim is the integration of the available data of the studied archaeological sequences through palaeoecological and oxygen isotope compositions analysis for increasing our knowledge about the MIS 3 environment from a regional point of view and reach a better understanding of the climatic singularities of Iberia during last Neanderthals settlements. One of the focus is fixed in oxygen isotope compositions (δ18O) analyzed from enamel tooth phosphates of both modern and fossil rodent remains of these localities. Taphonomic and taxonomical analyses based on small-mammals are also performed to reach a complete ecological interpretation of these sites. Other methods currently use in palaeoenvironmetal reconstructions based on small-mammal assemblages were also employed, such as mutual ecogeographic range method, habitat weighting method, chorotypes classification and bioclimatic model method. Considering the isotopic results obtained, this work shows that two main factors should be considered in Iberian palaeoenvironmental reconstructions: the possible seasonality of the rodent assemblages, derived from the predator, and the regional particularities on δ18O values of meteoric waters in the Iberian Peninsula. Accordingly, a new methodological proposal for the reconstruction of palaeotemperatures from rodent tooth δ18O accumulated in the Iberian Peninsula was developed and after applied to the archeological and paleontological sites studied. The recorded palaeoenvironment show globally cooler and wetter conditions than nowadays but it is detected notable climatic stability in all sites, with only slight changes related to stadial-interstadial pulsations, reflected mainly in oscitations between Mediterranean and mid-European species abundances. Woodland formations are always abundant throughout the sequences. The continuous maintenance of woodland covertures besides slight climatic variations and globally stable rodent communities, demonstrate the singularities of Iberian Peninsula, in contrast to the high climatic instability experience in Eurasia. This work highlights that a combination of different local-scale palaeoecological proxies compared to larger-scale proxies is essential to obtain faithful environmental reconstructions., L'objectif principal de cette thèse de doctorat est la reconstruction paléoenvironnementale enregistrée pendant le Pléistocène supérieur dans la péninsule ibérique du nord-ouest. Le stade 3 de l’isotope marin (MIS 3) a principalement été étudié, en combinant différentes approches scientifiques basées sur l’étude des ensembles fauniques de petits mammifères (insectivores, chauves-souris et rongeurs). Les trois sites analysés sont: l'Abric Romaní (Capellades, Barcelone), la grotte des Teixoneres (Moià, Barcelone) et la grotte d'Arbreda (Serinya, Gérone). Un quatrième site paléontologique contemporain, la grotte Xaragalls (Vimbodí-Poblet, Tarragone), était également inclus. L'objectif est d'intégrer les données disponibles des séquences archéologiques aux données dérivées de l'approche paléontologique et de l'analyse isotopique de l'oxygène, afin de permettre une augmentation des connaissances environnementales du MIS 3 d'un point de vue plus strictement régional et pour mieux comprendre la singularité climatique de la dernière phase de Néandertal dans la péninsule ibérique. L'étude s'est concentrée sur l'analyse isotopique de l'oxygène (δ18O) de l'émail dentaire chez les rongeurs modernes et fossiles des sites archéologiques susmentionnés. L'analyse tafonomique et taxonomique basée sur les petits mammifères a également contribué à l'enrichissement des connaissances écologiques, permettant une interprétation plus large des sites analysés. D'autres méthodes, également utilisées, sur les restes fossiles de petits mammifères pour la reconstruction paléoenvironnementale sont la méthode commune des intervalles éco-géographiques, la méthode des habitats pondérés, la classification corotypique des organismes et la méthode basée sur le modèle bioclimatique. Au vu des résultats obtenus, ces travaux montrent qu'il faut prendre en compte deux facteurs importants dans la reconstruction du paléoenvironnement ibérique à partir d'analyses isotopiques de l'oxygène: la saisonnalité possible de l'accumulation de rongeurs, dérivée de leur prédateur, et les particularités régionales en termes de valeurs de δ18O des eaux météoriques de la péninsule ibérique. Une nouvelle proposition méthodologique utilisée pour la reconstruction de la paléo-température et obtenue de l'étude dentaire de rongeurs δ18Op dans la péninsule ibérique, après avoir été élaborée conformément aux dispositions, a été appliquée aux sites archéologiques et paléontologiques étudiés. Le paléoenvironnement enregistré montre des conditions généralement plus froides et plus humides qu'aujourd'hui et une homogénéité climatique importante a été identifiée dans tous les sites, avec de légères variations liées aux pulsations entre les stades et les interstades, qui se traduisent par des oscillations entre les abondances des espèces méditerranéennes et de Europe centrale. Les formations forestières sont toujours abondantes dans toutes les séquences. La persistance des couvertures forestières, ainsi que les légères variations climatiques et communautaires des rongeurs globalement stables, démontrent les particularités de la péninsule ibérique, contrastant avec la grande instabilité climatique enregistrée en Eurasie. Ce travail met en évidence le fait que la combinaison d’une analyse paléoécologique à l’échelle locale et d’une analyse à grande échelle permet essentiellement une reconstruction plus fidèle du paléoenvironnement.

This study presents a reconstruction of the middle to late Holocene sedimentation dynamics and paleoclimatic implications in Lake Ulaan, southern Mongolia for the first time. The dynamics is inferred from weathering intensity, Accelerator Mass Spectrophotometry (AMS) radiocarbon dating and estimated sedimentation rates. Result shows that a higher sedimentation rate of 4.6 cm.kyr‑¹ between 2.7 and 6.0 cal kyr BP and a lower sedimentation rate of 1.6-1.8 cm.kyr‑1 after 2.7-3.2 cal kyr BP are associated with a climatic shift from humid in the middle Holocene to arid in the late Holocene within the Lake Ulaan basin. This result from the Lake Ulaan basin in southern Mongolia correlates with the middle and late Holocene climatic records reconstructed from other lakes in Mongolia and Central Asia. Further investigation of lacustrine sedimentary sequences and more age data from Lake Ulaan are still needed to constrain better the Holocene sedimentation history in southern Mongolia and Central Asia. Cette étude propose une reconstruction de la dynamique de la sédimentation à l'Holocène moyen et final et de ses implications paléoclimatiques dans le lac Ulaan, au sud de la Mongolie. Cette dynamique est reliée à l'intensité des intempéries, à l'estimation de taux de sédimentation et est calée temporellement par datation au radiocarbone (AMS). Les résultats montrent qu'un fort taux de sédimentation équivalant à 4,6 cm/1000 ans, entre 2,7 et 6,0 années cal BP, et un taux de sédimentation très bas d’environ 1,6-1,8 cm/1000 ans après 2,7‑3,2 années cal BP, sont associés à un changement climatique évoluant d'une ambiance humide au milieu de l'Holocène à une ambiance aride à la fin de l'Holocène dans le bassin du lac Ulaan. Ce résultat est corrélé avec les relevés climatiques de l'Holocène moyen et final reconstitués à partir d'autres lacs en Mongolie et en Asie centrale. Une étude plus approfondie des séquences sédimentaires lacustres et des données temporelles sont encore nécessaires pour mieux contraindre l'histoire de la sédimentation holocène dans le sud de la Mongolie et en Asie centrale.

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.

International audience; Highlands, hydrographic systems and coastal areas have been hypothesised to form corridors across the hyperarid Sahara desert in North Africa, allowing dispersal and gene flow for non-xeric species. Here we aim to provide a genetic test for the trans-Saharan corridor model, and predict the location and stability of ecological-corridors, by combining phylogeography and palaeoclimatic modelling. The model was the Psammophis schokari (Schokari sand racer) group, fast-moving and widely distributed generalist colubrids occurring mostly in arid and semiarid scrublands. We combined dated phylogenies of mitochondrial and nuclear markers with palaeoclimatic modelling. For the phylogeographic analysis, we used 75 samples of P. schokari and P. aegyptius, and Bayesian and Maximum-Likelihood methods. For the ecological models, we used Maxent over the distribution of P. schokari and West African lineages. Models were projected to past conditions (mid Holocene, Last Glacial Maximum and Last Inter-Glacial) to infer climatic stable areas. Climatic stability was predicted to be mostly restricted to coastal areas and not spatially continuous. A putative temporary trans-Saharan corridor was identified in Eastern Sahara, with a more stable one along the Atlantic coast. Six parapatric lineages were identified within P. schokari, four occurring in North Africa. These likely diverged during the Pliocene. The Tamanraset River might have been a vicariant agent. African lineages may have experienced further subsequent diversification during the late Pleistocene. The main P. schokari refugia were probably located along the northern margins of the Sahara, allowing its North-to-South colonization. Trans-Saharan corridors seem to have played a role in P. schokari biogeography, allowing colonization of central Saharan mountains and Sahel. Some might have worked as refugia, and even the most stable corridors may have sections working as filters, depending on each climatic phase. We expect the use of trans-Saharan corridors to decrease for more mesic species or with less dispersal capabilities.

A potential human footprint on Western Central African rainforests before the Common Era has become the focus of an ongoing controversy. Between 3,000 y ago and 2,000 y ago, regional pollen sequences indicate a replacement of mature rainforests by a forest–savannah mosaic including pioneer trees. Although some studies suggested an anthropogenic influence on this forest fragmentation, current interpretations based on pollen data attribute the "rainforest crisis" to climate change toward a drier, more seasonal climate. A rigorous test of this hypothesis, however, requires climate proxies independent of vegetation changes. Here we resolve this controversy through a continuous 10,500-y record of both vegetation and hydrological changes from Lake Barombi in Southwest Cameroon based on changes in carbon and hydrogen isotope compositions of plant waxes. δ13C-inferred vegetation changes confirm a prominent and abrupt appearance of C4 plants in the Lake Barombi catchment, at 2,600 calendar years before AD 1950 (cal y BP), followed by an equally sudden return to rainforest vegetation at 2,020 cal y BP. δD values from the same plant wax compounds, however, show no simultaneous hydrological change. Based on the combination of these data with a comprehensive regional archaeological database we provide evidence that humans triggered the rainforest fragmentation 2,600 y ago. Our findings suggest that technological developments, including agricultural practices and iron metallurgy, possibly related to the large-scale Bantu expansion, significantly impacted the ecosystems before the Common Era.

International audience; In palaeoclimate reconstructions, the combination of proxy records measured in different climate archives is challenging because of the uncertainties associated with each proxy, but it can also help reduce some of these uncertainties. Here, we present a novel approach to combine speleothem and tree ring proxies for a drought reconstruction of the last 640 years: a fluid inclusion δ 18 O record from a stalagmite from Villars Cave (southwest France) and a tree ring cellulose δ 18 O record of Quercus spp. from the nearby Angoulême area. The δ 18 O of the fluid inclusions is taken as an estimate of the δ 18 O of the trees' source water. Then, the cellulose and source water δ 18 O are used to calculate the leaf water isotopic enrichment, as well as relative humidity, which is the dominant controlling factor of this enrichment. The reconstructed long-term trends in relative humidity differ from a previously published reconstruction of moisture variability based on the tree ring record alone. Further measurements will be necessary to support either reconstruction. Nevertheless, this investigation demonstrates the great potential for combining isotope proxies from speleothems and tree rings to reconstruct both the low-and high-frequency variability of drought.; En paléoclimatologie, la comparaison entre les différentes archives est complexe à cause des incertitudes associées à chaque indicateur ; elle peut cependant aussi aider à réduire les incertitudes. Nous présentons ici une nouvelle approche pour combiner les indicateurs paléoclimatiques issus des spéléothèmes et des cernes d'arbre pour reconstituer les sécheresses au cours des derniers 640 ans : le δ 18 O des inclusions fluides d'une stalagmite de la grotte de Villars (sud-ouest de la France) et le δ 18 O de la cellulose des cernes de chênes (Quercus spp.) proches d'Angoulême. Le δ 18 O des inclusions fluides est considéré comme le δ 18 O de l'eau souterraine qui alimente les arbres. Puis le δ 18 O de la cellulose et celui de l'eau souterraine sont utilisés pour calculer l'enrichis-sement isotopique de l'eau de la feuille ainsi que l'humidité relative qui est le facteur dominant contrôlant cet enrichissement. La variation de l'humidité relative reconstituée diffère des reconstitutions publiées basées uniquement sur les cernes d'arbre suggérant la nécessité de faire d'autres reconstitutions pour mieux comprendre ces différences. Ce travail montre tout l'intérêt de comparer deux archives pour tenter de reconstituer les périodes de sécheresse passées.

The transition between the last glacial maximum and the actual interstadial (from 20 to 12 ka before present) is characterized by short and abrupt climate reorganizations at different scales. Over the Bolivian Altiplano, giant palaeo-lakes wax and wane within 2000 years. The glaciers dynamics is controlled by the climate, making the deposits from the former glaciers one of the few climatic archives available on this arid plateau. The exposure age method enables to reconstruct the history of former glaciers. This thesis brings new tools for the calculation of these ages. It also presents new ages coupled with palaeo-climate reconstructions. These results indicates that the palaeo-lake formations are concomitant with mean coolings of 2.5 to 4.2°C compared to present and with an enhanced moist transport from the East, crossing the Bolivian Oriental Cordillera; La transition entre le dernier épisode glaciaire et l'actuel interglaciaire (de 20 000 à 10 000 ans avant le présent) est caractérisée par de courtes et abruptes réorganisations du climat à différentes échelles. Sur l'Altiplano bolivien des paléolacs géants se développent et disparaissent en moins de 2000 ans. La dynamique des glaciers étant contrôlée par le climat, les dépôts laissés par les anciens glaciers constituent une des rares archives climatiques disponibles sur ce haut plateau aride. La méthode des âges d'exposition permet de reconstituer l'histoire des anciens glaciers. Cette thèse apporte de nouveaux outils méthodologiques pour le calcul de ces âges. Elle présente également de nouveaux âges couplés à des reconstructions climatiques. Ces résultats indiquent que la formation des paléolacs est concomitante de refroidissements moyens compris entre 2.5 et 4.5 C par rapport au présent et d'un apport accru d'humidité depuis l'Est, franchissant la Cordillère Orientale bolivienne

The transition between the last glacial maximum and the actual interstadial (from 20 to 12 ka before present) is characterized by short and abrupt climate reorganizations at different scales. Over the Bolivian Altiplano, giant palaeo-lakes wax and wane within 2000 years. The glaciers dynamics is controlled by the climate, making the deposits from the former glaciers one of the few climatic archives available on this arid plateau. The exposure age method enables to reconstruct the history of former glaciers. This thesis brings new tools for the calculation of these ages. It also presents new ages coupled with palaeo-climate reconstructions. These results indicates that the palaeo-lake formations are concomitant with mean coolings of 2.5 to 4.2°C compared to present and with an enhanced moist transport from the East, crossing the Bolivian Oriental Cordillera, La transition entre le dernier épisode glaciaire et l'actuel interglaciaire (de 20 000 à 10 000 ans avant le présent) est caractérisée par de courtes et abruptes réorganisations du climat à différentes échelles. Sur l'Altiplano bolivien des paléolacs géants se développent et disparaissent en moins de 2000 ans. La dynamique des glaciers étant contrôlée par le climat, les dépôts laissés par les anciens glaciers constituent une des rares archives climatiques disponibles sur ce haut plateau aride. La méthode des âges d'exposition permet de reconstituer l'histoire des anciens glaciers. Cette thèse apporte de nouveaux outils méthodologiques pour le calcul de ces âges. Elle présente également de nouveaux âges couplés à des reconstructions climatiques. Ces résultats indiquent que la formation des paléolacs est concomitante de refroidissements moyens compris entre 2.5 et 4.5 C par rapport au présent et d'un apport accru d'humidité depuis l'Est, franchissant la Cordillère Orientale bolivienne

The Southern Hemisphere, and particularly southern South America, are very under-represented in global climate reconstructions due to a lack of robust paleoclimatic data. My thesis was therefore focused on the evaluation of the tree-ring archive, which is widespread along the Andes, as a paleoclimatic tracer. I studied the regional response to external climate forcing of two native species (Fitzroya cupressoides and Nothofagus pumilio) in northern Patagonia (41°S, Argentina), by analysing their different physical tree-ring parameters. In this perspective, I analysed the temporal variations of the width and of the oxygen and carbon isotopic compositions of the cellulose of their rings and related them to local and large-scale climate variations. Two major results of my thesis have emerged: I highlighted (1) non-linear growth responses over time related to shifts in climate regimes and, (2) the potential of cellulose delta18O and delta13C to record summer-autumn temperature variations over a large area in the mid latitudes of South America (35°-55°S). As their variations are strongly linked to temperature the isotopic tools can be used to reconstruct chronologies of temperature but also of climate modes such as the Southern Annular Mode.; L’Hémisphère Sud, et plus précisément le sud de l’Amérique du Sud, sont très sous-représentés dans les reconstitutions climatiques globales du fait d’un manque de données paléoclimatiques robustes. Mes travaux de thèse ont donc porté sur l’évaluation de l’archive naturelle cernes d’arbre, abondante le long de la Cordillère des Andes, comme traceur paléoclimatique. Je me suis ainsi appliquée à étudier la réponse régionale de deux espèces d’arbre (Fitzroya cupressoides et Nothofagus pumilio) natives du nord de la Patagonie (41°S, Argentine) aux forçages climatiques externes en analysant leurs différents paramètres physiques. Pour cela, j’ai commencé par déterminer l’influence du climat local et de grande échelle sur leur croissance en étudiant les variations temporelles de leurs largeurs de cerne, puis j’ai essayé d’extraire le signal climatique enregistré dans la composition isotopique de l’oxygène et du carbone de la cellulose de ces cernes. Deux résultats majeurs de ma thèse ont émergé: j’ai mis en évidence (1) des réponses non-linéaires de la croissance au cours du temps liées aux changements de régimes climatiques et, (2) le fort potentiel du delta18O et delta13C de la cellulose pour enregistrer les variations de température d’été-automne sur une large région aux latitudes moyennes de l’Amérique du Sud (35°-55°S). Du fait des relations fortes qui les lient aux températures, les outils isotopiques peuvent être utilisés pour reconstituer des chronologies de température mais aussi des modes climatiques tels que l’Oscillation Antarctique.

Two well-dated delta 18O-speleothem records from Shatuca cave, situated on the northeastern flank of the Peruvian Andes (1960 m asl) were used to reconstruct high-resolution changes in precipitation during the Holocene in the South American Summer Monsoon region (SASM). The records show that precipitation increased gradually throughout the Holocene in parallel with the austral summer insolation trend modulated by the precession cycle. Additionally the Shatuca speleothem record shows several hydroclimatic changes on both longer- and shorter-term time scales, some of which have not been described in previous paleoclimatic reconstructions from the Andean region. Such climate episodes, marked by negative excursions in the Shatuca delta 18O record were logged at 9.7-9.5, 9.2, 8.4, 8.1, 5.0, 4.1, 3.5, 3.0, 2.5, 2.1 and 1.5 ka b2k, and related to abrupt multi-decadal events in the SASM. Some of these events were likely associated with changes in sea surface temperatures (SST) during Bond events in the North Atlantic region. On longer time scales, the low delta 18O values reported between 5.1-5.0, 3.5-3.0 and 1.5 ka b2k were contemporaneous with periods of increased sediment influx at Lake Pallcacocha in the Andes of Ecuador, suggesting that the late Holocene intensification of the monsoon recorded at Shatuca site may also have affected high altitudes of the equatorial Andes further north. Numerous episodes of low SASM intensity (dry events) were recorded by the Shatuca record during the Holocene, in particular at 10.2, 9.8, 9.3, 6.5, 5.1, 4.9, 2.5 and 2.3 ka b2k, some of them were synchronous with dry periods in previous Andean records.

A paleoreconstruction of the length and intensity of the rainy season over western Africa has been recently proposed, using analysis of fossil mollusk shells from the Saloum delta region, in western Senegal. In order to evaluate the significance of local long-term reconstructions of precipitations from paleoclimate proxies, and to better characterize the spatial homogeneity of rainfall distribution in northern Africa, we analyze here the spatial representativeness of rainfall in this region, from seasonal to decadal timescales. The spatial coherence of winter episodic rainfall events is relatively low and limited to surrounding countries. On the other hand, the summer rainfall, associated with the West African Monsoon, shows extended spatial coherence. At seasonal timescales, local rainfall over the Saloum is significantly correlated with rainfall in the whole western half of the Sahel. At interannual and longer timescales, the spatial coherence extends as far as the Red Sea, covering the full Sahel region. This spatial coherence is mainly associated to the zonal extension of the Inter Tropical Convergence Zone (ITCZ). Coherently, summer rainfalls appear to be driven by Sea Surface Temperature (SST) anomalies mainly in the Pacific, the Indian Ocean, the Mediterranean basin, and the North Pacific. A more detailed analysis shows that consistency of the spatial rainfall coherence is reduced during the onset season of the West African Monsoon.

The contribution of landslides to the Quaternary evolution of relief is poorly documented in arid contexts. In southern Peru and northern Chile, several massive landslides disrupt the arid western Andean front. The Chuquibamba landslide, located in southern Peru, belongs to this set of large landslides. In this area, the Incapuquio fault system captures the intermittent drainage network and localizes rotational landslides. Seismic activity is significant in this region with recurrent Mw 9 subduction earthquakes; however, none of the latest seismic events have triggered a major landslide. New terrestrial cosmogenic dating of the Chuquibamba landslide provides evidence that the last major gravitational mobilization of these rotational landslide deposits occurred at ~ 102 ka, during the Ouki wet climatic event identified on the Altiplano between 120 and 98 ka. Our results suggest that wet events in the arid and fractured context of the Andean forearc induced these giant debris flows. Finally, our study highlights the role of tectonics and climate on (i) the localization of large Andean landslides in the Western Cordillera and on (ii) the long-term mass transfer to the trench along the arid Andean front.

Diploastrea heliopora forms dense, robust, dome-shaped coral colonies throughout the reef ecosystems of the tropical Pacific and Indian Oceans. This slow-growing (2 to 6 mm/yr) coral has the potential to yield continuous paleoclimate records spanning up to 1000 yr for the warmest waters on Earth, the Indo-Pacific Warm Pool, and has a long fossil history as a single recognizable species. Despite the potential of Diploastrea to be an important new paleoclimate archive, little is known about the systematics of geochemical tracers incorporated into its skeleton. To fill this knowledge gap, we compared skeletal δ18O signatures in live Diploastrea and Porites colonies from Southwest Lagoon, near Amédée Lighthouse, New Caledonia (at the southern latitudinal limit for Diploastrea) and Alor, Indonesia (in the core area of the Indo-Pacific Warm Pool). We designed a new microsampling technique to minimize smoothing and distortion of the isotopic records due to the complex calyx architecture and slow growth of Diploastrea. High-resolution isotope profiles from the septal portion of the Diploastrea corallite are attenuated, whereas those restricted to the central columella are similar in quality to those extracted from the well-established Porites coral archive. The δ18O-temperature relationship for the columellar portion of Diploastrea (−0.18‰/°C) is in good agreement with that derived for a nearby Porites (−0.19‰/°C; Quinn et al., 1996), on the basis of comparison with an in situ sea surface temperature record from Amédée Lighthouse. There is a measurable difference of 0.3 ± 0.1‰ between the kinetic/biological disequilibrium offsets from seawater δ18O composition for Diploastrea and Porites. Despite this offset in mean δ18O, Diploastrea accurately records the regional differences in mean temperature and salinity between New Caledonia and the Indo-Pacific Warm Pool. Additional tests show that Diploastrea records El Niño–Southern Oscillation (ENSO) interannual variability in sea surface temperature and salinity across the southwestern Pacific, indicating that it should yield dependable paleo-ENSO records. Based on these results, we propose that D. heliopora has the potential to provide an important new coral archive of tropical paleoclimate.

The cause of the fast rise in atmospheric CO2 concentration seen in ice core data during the last glacial termination is not fully understood. Most studies indicate the deep Southern ocean as a possible source of this carbon released to the atmosphere. In this study, the role of high carbon soils associated with permafrost is considered. We found that changes in summer insolation in high northern latitudes resulted in a permafrost thaw, that then released carbon to the atmosphere, causing the atmospheric CO2 to rise. This mechanism would have been active during all insolation cycles. The extent of ice sheets on the land was found to be a strong controller for the permafrost-carbon response. The deep Southern ocean still had an important role to play for atmospheric CO2 concentrations. The permafrost-carbon mechanism significantly changed the carbon cycle response to climatic changes. It also improved the match between model output and data.; La cause de l'augmentation rapide de CO2 atmosphérique observée sur les données des carottes de glace au cours de la dernière terminaison glaciaire n'est pas entièrement comprise. La plupart des études indiquent l'océan australe profond comme une source de carbone dans l'atmosphère. Dans cette étude, le rôle des sols riches en carbone associés au pergélisol est étudier. Nous avons trouvé que les changements de l'insolation d'été dans les hautes latitudes ont abouti à un dégel du pergélisol, que puis relâché carbone dans l'atmosphère, provoquant le CO2 atmosphérique à la hausse. Ce mécanisme aurait été actif durant tous les cycles d'insolation. L'étendue des calottes glaciaires sur la terre a été trouvé comme contrôleur important pour la réponse pergélisol-carbone. L'océan australe profond avait encore un rôle important à jouer. Le mécanisme pergélisol carbone a modifié fortement la réponse du cycle du carbone. Il a également amélioré le correspondance entre la modèle et les données.

The characterization of natural climate variability is important in order to understand the climate response to natural forcings and to identify anthropogenic influences. The aim of this thesis is to reconstruct climate changes in the southwest of France, a region which is characterised by recurrent drought periods, where high resolution proxy records of the last millennia were lacking.The reconstruction is based on multiple proxies from two continental archives: speleothems and tree rings. Their combination can make use of the strengths of each archive while compensating their weaknesses. There are two principal objectives: first, to gain a better understanding of the climatic and non-climatic influences on each proxy; and second, to reconstruct drought periods in the past.The oxygen isotopic composition (δ18O) of speleothem fluid inclusions and tree ring cellulose is controlled to a large extent by the δ18O of precipitation, which can serve as a tracer of the atmospheric circulation. In order to interpret these proxies in terms of climate, it is necessary to understand how the climate signal becomes recorded in the proxy, and which processes modify the original signal during the formation of the archive.Measurements of δ18O in precipitation, cave drip water, and fluid inclusions in modern speleothem samples from Villars Cave demonstrated that the isotopic composition of cave drip water corresponds to the pluri-annual average precipitation. The speleothem fluid inclusions, in turn, preserve the isotopic composition of the drip water. Based on this calibration, it is possible to reconstruct drip water isotope variability using fluid inclusions in a more than 2000 year old stalagmite, which has been dated by laminae counting, as well as U-Th and 14C measurements. Changes in the cave environment, e.g. the vegetation cover, are indicated by other proxies from the same stalagmite (stable isotopes in calcite and trace element concentrations), but these changes do not seem to impact the fluid inclusion δ18O significantly.The isotopic composition of tree ring cellulose from Quercus spp. in the study area is strongly influenced by climate conditions during the summer. However, non-climatic influences on the isotopic composition of cellulose are identified. They are linked to the age of the trees and to site hydrology, and must be accounted for in the sampling and analytical procedures. Crossdated cores from living trees and timber wood in historic buildings near Angoulême are used to build an annually resolved chronology of cellulose δ18O. Significant correlations with meteorological data enable a calibration and a reconstruction of drought periods since 1360 AD.Lastly, this thesis explores a novel approach of integrating oxygen isotope records from speleothem fluid inclusions and tree ring cellulose from closely located sites to reconstruct both high- and low-frequency variability of droughts in the past.; La caractérisation de la variabilité naturelle du climat est importante pour comprendre la réponse de celui-ci aux forçages naturels et pour identifier les influences anthropiques. L'objectif de cette thèse est de reconstituer les changements climatiques dans le sud-ouest de la France, une région qui se caractérise par des périodes de sécheresse récurrentes, où les reconstitutions millénaires basées sur des « proxies » à haute résolution font défaut.La reconstruction est basée sur plusieurs proxies dans deux archives continentales : les spéléothèmes et les cernes d'arbre. Leur combinaison permet de tirer avantage de leurs points forts tout en compensant leurs faiblesses. Il y a deux objectifs principaux : d’une part de mieux comprendre les influences climatiques et non climatiques qui s’exercent sur chaque proxy ; et, d'autre part, de reconstituer les périodes de sécheresse dans le passé.La composition isotopique en oxygène (δ18O) des inclusions fluides des spéléothèmes et de la cellulose des cernes d’arbre est contrôlée dans une large mesure par le δ18O des précipitations, qui peut servir de traceur de la circulation atmosphérique. Pour interpréter ces proxies en termes de climat, il est nécessaire de comprendre comment le signal climatique est enregistré dans le proxy, et quels processus modifient ce signal lors de la formation de l'archive.Les mesures de δ18O dans les précipitations, l'eau d’infiltration, et les inclusions fluides dans des échantillons de spéléothèmes modernes de la Grotte de Villars démontrent que la composition isotopique de l'eau d’infiltration dans la grotte correspond à une moyenne pluriannuelle des précipitations, et que les inclusions fluides des spéléothèmes préservent la composition isotopique de cette eau d’infiltration. Sur la base de cette calibration, il est possible de reconstituer les variations de la composition isotopique de l'eau d’infiltration à partir de celles des inclusions fluides d’une stalagmite vieille de plus de 2000 ans, datée par comptage de lamines, et par des mesures U-Th et 14C. Des changements dans l'environnement de la grotte, par exemple du couvert végétal, sont indiqués par d'autres proxies de la même stalagmite (les isotopes stables dans la calcite et les concentrations d'éléments traces), mais ces changements n'ont pas d'incidence significative sur le δ18O des inclusions fluides.La composition isotopique de la cellulose des cernes d'arbre de Quercus spp. dans la zone d'étude est fortement influencée par les conditions climatiques estivales. Cependant, des influences non-climatiques sur la composition isotopique de la cellulose sont identifiées ; elles sont liées à l'âge de l'arbre et à l'hydrologie du site, et doivent être prises en compte dans l'échantillonnage et dans les procédures analytiques. Grace à l’inter-datation de carottes d’arbres vivants et de poutre de bâtiments historiques des environs d'Angoulême, une chronologie de δ18O dans la cellulose à résolution annuelle a été établie. Des corrélations significatives avec les données météorologiques permettent une calibration des données isotopiques et une reconstruction des périodes de sécheresse depuis 1360 AD.Enfin, cette thèse explore une nouvelle approche intégrant les enregistrements isotopiques de l'oxygène dans les inclusions fluides des spéléothèmes et dans la cellulose des cernes d’arbre de sites proches pour reconstruire la variabilité à la fois haute et basse fréquence des sécheresses dans le passé.

The characterization of natural climate variability is important in order to understand the climate response to natural forcings and to identify anthropogenic influences. The aim of this thesis is to reconstruct climate changes in the southwest of France, a region which is characterised by recurrent drought periods, where high resolution proxy records of the last millennia were lacking.The reconstruction is based on multiple proxies from two continental archives: speleothems and tree rings. Their combination can make use of the strengths of each archive while compensating their weaknesses. There are two principal objectives: first, to gain a better understanding of the climatic and non-climatic influences on each proxy; and second, to reconstruct drought periods in the past.The oxygen isotopic composition (δ18O) of speleothem fluid inclusions and tree ring cellulose is controlled to a large extent by the δ18O of precipitation, which can serve as a tracer of the atmospheric circulation. In order to interpret these proxies in terms of climate, it is necessary to understand how the climate signal becomes recorded in the proxy, and which processes modify the original signal during the formation of the archive.Measurements of δ18O in precipitation, cave drip water, and fluid inclusions in modern speleothem samples from Villars Cave demonstrated that the isotopic composition of cave drip water corresponds to the pluri-annual average precipitation. The speleothem fluid inclusions, in turn, preserve the isotopic composition of the drip water. Based on this calibration, it is possible to reconstruct drip water isotope variability using fluid inclusions in a more than 2000 year old stalagmite, which has been dated by laminae counting, as well as U-Th and 14C measurements. Changes in the cave environment, e.g. the vegetation cover, are indicated by other proxies from the same stalagmite (stable isotopes in calcite and trace element concentrations), but these changes do not seem to impact the fluid inclusion δ18O significantly.The isotopic composition of tree ring cellulose from Quercus spp. in the study area is strongly influenced by climate conditions during the summer. However, non-climatic influences on the isotopic composition of cellulose are identified. They are linked to the age of the trees and to site hydrology, and must be accounted for in the sampling and analytical procedures. Crossdated cores from living trees and timber wood in historic buildings near Angoulême are used to build an annually resolved chronology of cellulose δ18O. Significant correlations with meteorological data enable a calibration and a reconstruction of drought periods since 1360 AD.Lastly, this thesis explores a novel approach of integrating oxygen isotope records from speleothem fluid inclusions and tree ring cellulose from closely located sites to reconstruct both high- and low-frequency variability of droughts in the past.; La caractérisation de la variabilité naturelle du climat est importante pour comprendre la réponse de celui-ci aux forçages naturels et pour identifier les influences anthropiques. L'objectif de cette thèse est de reconstituer les changements climatiques dans le sud-ouest de la France, une région qui se caractérise par des périodes de sécheresse récurrentes, où les reconstitutions millénaires basées sur des « proxies » à haute résolution font défaut.La reconstruction est basée sur plusieurs proxies dans deux archives continentales : les spéléothèmes et les cernes d'arbre. Leur combinaison permet de tirer avantage de leurs points forts tout en compensant leurs faiblesses. Il y a deux objectifs principaux : d’une part de mieux comprendre les influences climatiques et non climatiques qui s’exercent sur chaque proxy ; et, d'autre part, de reconstituer les périodes de sécheresse dans le passé.La composition isotopique en oxygène (δ18O) des inclusions fluides des spéléothèmes et de la cellulose des cernes d’arbre est contrôlée dans une large mesure par le δ18O des précipitations, qui peut servir de traceur de la circulation atmosphérique. Pour interpréter ces proxies en termes de climat, il est nécessaire de comprendre comment le signal climatique est enregistré dans le proxy, et quels processus modifient ce signal lors de la formation de l'archive.Les mesures de δ18O dans les précipitations, l'eau d’infiltration, et les inclusions fluides dans des échantillons de spéléothèmes modernes de la Grotte de Villars démontrent que la composition isotopique de l'eau d’infiltration dans la grotte correspond à une moyenne pluriannuelle des précipitations, et que les inclusions fluides des spéléothèmes préservent la composition isotopique de cette eau d’infiltration. Sur la base de cette calibration, il est possible de reconstituer les variations de la composition isotopique de l'eau d’infiltration à partir de celles des inclusions fluides d’une stalagmite vieille de plus de 2000 ans, datée par comptage de lamines, et par des mesures U-Th et 14C. Des changements dans l'environnement de la grotte, par exemple du couvert végétal, sont indiqués par d'autres proxies de la même stalagmite (les isotopes stables dans la calcite et les concentrations d'éléments traces), mais ces changements n'ont pas d'incidence significative sur le δ18O des inclusions fluides.La composition isotopique de la cellulose des cernes d'arbre de Quercus spp. dans la zone d'étude est fortement influencée par les conditions climatiques estivales. Cependant, des influences non-climatiques sur la composition isotopique de la cellulose sont identifiées ; elles sont liées à l'âge de l'arbre et à l'hydrologie du site, et doivent être prises en compte dans l'échantillonnage et dans les procédures analytiques. Grace à l’inter-datation de carottes d’arbres vivants et de poutre de bâtiments historiques des environs d'Angoulême, une chronologie de δ18O dans la cellulose à résolution annuelle a été établie. Des corrélations significatives avec les données météorologiques permettent une calibration des données isotopiques et une reconstruction des périodes de sécheresse depuis 1360 AD.Enfin, cette thèse explore une nouvelle approche intégrant les enregistrements isotopiques de l'oxygène dans les inclusions fluides des spéléothèmes et dans la cellulose des cernes d’arbre de sites proches pour reconstruire la variabilité à la fois haute et basse fréquence des sécheresses dans le passé.

The Kura-Araxes culture fl ourished in the South Caucasus and neighbouring regions during the 4th– 3rd millennia BC. More than in previous periods, this period saw a greater concentration of populations in the vast volcanic highlands of this region. Palaeoecological records suggest that deciduous forests reached their maximum extent at the time, triggered by changes in temperature and precipitation. In this contribution, the environmental conditions that accompanied the Kura-Araxes culture are reconstructed from palaeoecological data from the region, placing settlement patterns in context and providing insights into human– environment interactions during this period of social and environmental change., La culture Kura-Araxe s’est épanouie dans le sud du Caucase et les régions voisines au cours des 4e et 3e millénaires. De façon plus marquée que dans les périodes précédentes, cette époque a connu une concentration de populations sur les hautes terres de la région, qui couvrent un vaste territoire. Les données paléoécologiques suggèrent que les forêts à feuilles caduques ont atteint leur extension maximale à cette époque, par suite de changements de températures et dans les précipitations. Dans cet article, les conditions environnementales qui ont prévalu pendant la culture Kura-Araxe sont reconstituées à partir des données paléoécologiques de la région, en replaçant les différents modes de peuplement dans leur contexte et en donnant un aperçu des interactions homme-environnement pendant cette période de mutation environnementale et sociétale., Connor Simon E., Kvavadze Eliso. Environmental context of the Kura-Araxes culture. In: Paléorient, 2014, vol. 40, n°2. The Kura-Araxes culture from the Caucasus to Iran, Anatolia and the Levant: Between unity and diversity. pp. 11-22.

The Lapita colonization,which occurred in the late Holocene, is one of the most remarkable prehistorical human colonizations. To explore the possible influence of El Niño–Southern Oscillation (ENSO) on this event, bulk oxygen (δ18Oshell) and carbon (δ13Cshell) stable isotope records were obtained from eight fossil Tridacna sp. and Hippopus hippopus giant clams, unearthed from Lapita archeological sites of New Caledonia and Vanuatu. These giant clamswere dated ca. 3.8–2.3 ka BP. These δ18Oshell and δ13Cshell records were used as proxies for combined sea surface temperature and salinity and precipitation. In addition, geochemical records were obtained from modern conspecifics from New Caledonia to create a baseline against which fossil giant clam records could be compared. The isotopic records revealed the occurrence of two distinct climate states in New Caledonia ca. 3.2–2.3 ka BP: one climate state was characterized by climatic conditions similar to those observed today and the second was comparable to warmer and wetter conditions similar to Vanuatu's modern climate. Considering that previous paleo-climate reconstructions in the West Pacific did not show a shift of the mean climatic state and that they revealed aweak centennial climate variability, our results suggest that the climatic mean state has been alternating between these two states at a decadal or an inter-annual frequency. This strong climate variability recorded in the giant clam shells may reflect an increase in the ENSO variability, supporting the hypothesis of an ENSO-forced Lapita colonization as suggested by Anderson et al. (2006).