Le sujet de ma thèse traite les changements paléoenvironnementaux et paléoclimatiques qui ont affecté les écosystèmes du Moyen Atlas marocain depuis la dernière période glaciaire, centrée autour de 25,000 cal BP. Les trois séquences sédimentaires étudiées dans cette thèse ont été prélevées le long d'un transect Nord-Sud dans le Moyen Atlas. Cette étude est basée sur une approche multi-bioindicateurs intégrant la palynologie, la géochimie élémentaire et la granulométrie afin de comprendre la dynamique des écosystèmes passés et de mettre en évidence les impacts anthropiques sur ces écosystèmes et leurs bassins versants. Les résultats obtenus montrent que durant la dernière période glaciaire et jusqu’au début de l’holocène, une végétation steppique a dominé les paysages du Moyen Atlas en réponse à des conditions climatiques froides et arides. Cependant, des populations de cèdres, de chênes et de pins ont persisté durant la période glaciaire dans des micro-refuges. Bien que le début de l’holocène révèle une expansion des chênes et des pins (principaux composants des écosystèmes forestiers méditerranéens), la steppe (composée d'armoise, de chénopodiacées et de graminées) a persisté jusqu’à 6.5 ka cal BP. Ces écosystèmes tolérant à la sécheresse reflètent un début de l’holocène chaud mais avec une quantité de précipitations annuelles assez réduite pour permettre l'expansion des forêts tel que nous l'observons en Europe et d’autres régions de la Méditerranée. Ce n'est qu'à partir de 6.5 ka cal BP que les forêts de cèdre s’installent aux altitudes des sites étudiés (autour de 1600m) et que des plantes aquatiques prolifèrent dans les marais ou nous avons effectué les sondages. Nous observons ensuite une nette régression du couvert arboré à partir de 4.5 ka cal BP, notamment les chênes décidus, et une expansion des taxons herbacés. Ceci est le résultat d’une tendance vers des conditions plus arides témoignant d'une installation du climat méditerranéen tel que nous le connaissons aujourd'hui. Le début des changements environnementaux liés à l'impact humain, à travers les données palynologiques et géochimiques, n'est enregistré qu’à partir de ca. 1500 cal BP. Les résultats obtenus dans cette thèse sont en accord avec ceux d'autres études paléoenvironnementales menées en Méditerranée autant pour la période glaciaire que pour le début des conditions arides à partir de 5.5 ka cal BP. / This thesis discusses the paleoenvironmental and paleoclimatic changes that have affected the ecosystems of the Moroccan Middle Atlas since the last glacial period, and focuses around 25000 cal BP. The three sedimentary sequences that have been studied in this thesis were taken along a North-South transect in the Middle Atlas. This study is based on a multi-bio-indicators approach, including palynology, geochemical elements and particle size, in order to better understand the dynamic of previous ecosystems and to highlight the human activities on these ecosystems and on their catchment areas.The results obtained show that during the last glacial period, and until the beginning of the Holocene, steppe vegetation was predominant in the Middle Atlas, in reaction to cold and arid climate conditions. However, populations of cedars, oaks and pine trees remained in micro-refugia. Although the beginning of the Holocene reveals an expansion of oaks and pines trees (the main components of Mediterranean forests ecosystems), the steppe (which is composed of Artemisia, Chenopodiaceae and grasses) persisted till 6.5 ka cal BP. These drought-tolerant ecosystems reflect a warm beginning of the Holocene, with an amount of precipitation that was low enough to allow the expansion of forests similar to those we can observe in Europe and other areas of the Mediterranean. It’s only since 6.5 ka cal BP that cedars forests have started to grow at the altitudes of the sites we studied (around 1600m), and that aquatic plants have proliferated in the marshes where we took our samples. We can then observe a sharp decline of tree cover, including deciduous oaks, starting 4.5 ka cal BP and an expansion of herbaceous taxa. These are the consequences of a tendency to conditions that were more arid, attesting the installation of the Mediterranean climate as we know it today. The first environmental changes linked to human impact have only been recorded, through palynology and biochemical data, from ca. 1500 cal BP. The results obtained in this thesis are in line with those obtained in other paleoenvironmental studies conducted in the Mediterranean area, for the glacial period as well as for the beginning of arid conditions starting 5.5 ka cal BP.

Palynology, the study of organic microfossils, is an important tool for improving our understanding of past environments and landscapes. Palynology provides a wealth of information from which climatic and environmental conditions can be inferred. However, inferred climatic and environmental conditions are often open to interpretation. Assumptions made about past climate conditions from pollen assemblages often rely on qualitative understanding of modern-day vegetation distributions, rather than empirical relationships. Historic anthropogenic impact on the environment must also be inferred, and assessments made as to whether vegetation changes are a result of climate change or human impact. This study seeks to address some of the questions that arise through the interpretation of pollen assemblages, by establishing empirical relationships between the geochemistry of modern pollen and climate or environmental controls. It focuses on the pollen of the climatically sensitive montane conifer Cedrus atlantica, which is distributed across the mountains of Morocco and Algeria. The study investigates aspects of modern pollen geochemistry and morphology and finds a strong relationship between the stable carbon isotope composition of modern pollen and mean annual precipitation (r2 = 0.54, p <0.001) and summer precipitation (r2 = 0.63, p <0.0001). Furthermore, a stronger relationship exists with aridity measured using the self-calibrating Palmer Drought Severity Index (r2 = 0.86, p <0.0001), suggesting that the stable carbon isotope composition of Cedrus atlantica pollen is influenced by environmental moisture availability. The study also finds there is an increased abundance of ultraviolet absorbing compounds (UACs) in modern Cedrus atlantica pollen with increasing summer UV-B flux. This relationship was evident with samples growing in their native range (r2 = 0.84, p <0.0001), but not with samples from outside this range (r2 = 0.00, p = 0.99), suggesting a possible genetic influence. Lastly, the study finds that grain size of Cedrus atlantica pollen is highly variable within and between samples, and we rule out climatic control on pollen grain size. These results suggest that quantitative relationships can be established between the geochemistry of Cedrus atlantica pollen and environmental and climatic influences. Stable carbon isotope analysis of fossil pollen could be used as a proxy for reconstruction of summer moisture availability, while analysis of UACs in fossil pollen could be used as a proxy for the reconstruction of summer UV-B flux. These proxies will enhance our understanding of climatic and environmental change in Northwest Africa and will complement existing palynological techniques for environmental and climate reconstruction. Palynology, the study of organic microfossils, is an important tool for improving our understanding of past environments and landscapes. Palynology provides a wealth of information from which climatic and environmental conditions can be inferred. However, inferred climatic and environmental conditions are often open to interpretation. Assumptions made about past climate conditions from pollen assemblages often rely on qualitative understanding of modern-day vegetation distributions, rather than empirical relationships. Historic anthropogenic impact on the environment must also be inferred, and assessments made as to whether vegetation changes are a result of climate change or human impact. This study seeks to address some of the questions that arise through the interpretation of pollen assemblages, by establishing empirical relationships between the geochemistry of modern pollen and climate or environmental controls. It focuses on the pollen of the climatically sensitive montane conifer Cedrus atlantica, which is distributed across the mountains of Morocco and Algeria. The study investigates aspects of modern pollen geochemistry and morphology and finds a strong relationship between the stable carbon isotope composition of modern pollen and mean annual precipitation (r2 = 0.54, p <0.001) and summer precipitation (r2 = 0.63, p <0.0001). Furthermore, a stronger relationship exists with aridity measured using the self-calibrating Palmer Drought Severity Index (r2 = 0.86, p <0.0001), suggesting that the stable carbon isotope composition of Cedrus atlantica pollen is influenced by environmental moisture availability. The study also finds there is an increased abundance of ultraviolet absorbing compounds (UACs) in modern Cedrus atlantica pollen with increasing summer UV-B flux. This relationship was evident with samples growing in their native range (r2 = 0.84, p <0.0001), but not with samples from outside this range (r2 = 0.00, p = 0.99), suggesting a possible genetic influence. Lastly, the study finds that grain size of Cedrus atlantica pollen is highly variable within and between samples, and we rule out climatic control on pollen grain size. These results suggest that quantitative relationships can be established between the geochemistry of Cedrus atlantica pollen and environmental and climatic influences. Stable carbon isotope analysis of fossil pollen could be used as a proxy for reconstruction of summer moisture availability, while analysis of UACs in fossil pollen could be used as a proxy for the reconstruction of summer UV-B flux. These proxies will enhance our understanding of climatic and environmental change in Northwest Africa and will complement existing palynological techniques for environmental and climate reconstruction.

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