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The composition of the atmosphere has changed substantially over Earth's history, with important implications for past climate. A number of case studies will be presented which employ coupled climate model simulations to assess the strength of these chemical feedbacks on the climate. The eruption of Mount Samalas in 1257 led to the largest stratospheric volcanic injection of aerosol precursor gases in the Common Era, however climate model simulations of the last millennium typically overestimate the resulting climatic cooling when compared with tree-ring proxy records. A novel configuration of the Met Office UM-UKCA climate model is presented which couples an atmosphere-ocean general circulation model to a rigorous treatment of the relevant atmospheric chemistry and microphysical aerosol processes. This permits the climate response to a particular stratospheric injection of reactive volatile gases to be quantified and for the first time to date applied to a historical volcanic eruption. This model configuration compares favourably to observational data for simulations of the 1991Mount Pinatubo eruption. Results from an ensemble of model simulations are presented, with different assumptions about the sulfur dioxide and halogen loadings based on a recent geochemical reconstruction. These show a muted climate response, in reasonable agreement with tree ring records. Emissions of halogenated compounds lead to an increase in the sulfur dioxide lifetime, widespread ozone depletion and a prolonged climatic cooling. Strong increases in incident ultraviolet radiation at Earth's surface also occur. Oxygen levels may have varied fromas little as 10% to as high as 35% in the Phanerozoic (541Ma - Present). An increase in atmospheric oxygen increases atmospheric mass which leads to a reduction in incident shortwave radiation at Earth's surface due to Rayleigh scattering. However, this is offset by an increase in the pressure broadening of greenhouse gas absorption lines. Dynamical feedbacks also lead to increased meridional heat transport, warming polar regions and cooling tropical regions. An increase in oxygen content using the HadCM3-BL and HadGEM3-AO climate models leads to a global mean surface air temperature increase for a pre-industrial Holocene base case, in agreement with idealised 1D and 2D modeling studies. Case studies from past climates are investigated using HadCM3-BL which show that in the warmest climates, increasing oxygen may lead to a temperature decrease, as the equilibrium climate sensitivity is lower. For the Maastrichtian (72.1 - 66.0Ma), increasing oxygen content leads to a better agreement with proxy reconstructions of surface temperature at that time irrespective of the carbon dioxide content. There is considerable uncertainty in the timing of the rise in atmospheric oxygen content from values around 1% in the Neoproterozoic (1000 Ma - 541 Ma) to the 10- 35% values inferred in the Phanerozoic with respect to two global glaciation episodes (717-635Ma). Results of simulations with HadCM3-BL which investigate the impact of oxygen content on the Neoproterozoic Snowball Earth glaciations are presented. These demonstrate that a smaller reduction in carbon dioxide content is required to initiate a Snowball Earth at low oxygen content. Geological evidence suggests the presence of a basaltic large igneous province before the Sturtian Snowball Earth episode. This could have caused episodes of paced explosive volcanism, injecting sulfate aerosol precursors into the stratosphere. Results of simulations to investigate the impact of different volcanic aerosol emission scenarios are presented. 500 Tg SO2 is investigated with a range of aerosol sizes. For aerosol size distributions consistent with the aerosol evolution in the aftermath of the Mount Pinatubo eruption, the Earth enters a Snowball Earth in between 30 and 80 years. Using a larger size of aerosols, consistent with a larger eruption, does not lead to a Snowball Earth. These simulations show that changes to the chemical composition of the atmosphere, whether reactive gases or bulk chemical composition may have played an important role in the past climate of Earth.

Water filters and sediment trap samples were collected weekly from late May to early September 2009 from four lakes of the Lofoten archipelago, northwestern Norway, and were used to explore the applicability of the alkenone unsaturation index (UK37) for temperature reconstruction in limnic systems in the area. For the first time, we observed the occurrence of long-chain alkenones (LCAs) within the water columns of lakes in this region. Water filters from two of the four studied lakes contained measurable concentrations of alkenones that were restricted to spring turnover and disappeared with the onset of summer stratification. These results indicate that alkenones in the lake sediment of these lakes reflect biological production and temperature during lake mixing, taking place in late spring to early summer. Measurements from sediment trap material collected over the sampling season combined with water temperature measurements from automated data loggers provide an in situ calibration of the alkenone paleothermometer (Temperature = 33.0 x UK37 + 22.8; N=10; R2=0.95). Notably, this calibration reveals a UK37 sensitivity to temperature (i.e., the slope of the relationship) that is very similar to previous calibrations reported from both marine and lacustrine environments. LCAs can therefore serve as the first quantitative proxy for reconstructing past temperature variability from the Lofoten Islands. Based on this temperature calibration, a high-resolution temperature record was reconstructed over the past millennium, which shows unprecedented lake surface temperature warming during the past decades.

L’objectif principal de cette thèse a été d’établir une histoire précise des interactions sociétés-environnement durant le dernier millénaire dans deux zones de moyenne montagne, territoires particulièrement sensibles aux aléas naturels ainsi qu’aux évènements politiques, économiques et sociaux à travers un jeu de données multiples issues de différentes disciplines. L’étude de deux séquences sédimentaires lacustres de sites jurassien et alpin, grâce à l’outil palynologique - grains de pollen, spores et microfossiles non-polliniques -, particulièrement développé ici avec la haute-résolution, croisé avec l’étude des données historiques et archivistiques nous a permis, en plus de fournir un corpus complémentaire, de tester une approche comparative (données polliniques vs données cadastrales). Au lac de Remoray (Doubs, massif jurassien, 850 m d’altitude), l’étude a précisé le schéma du processus d’occupation de cette partie de la haute vallée du Doubs. Les données du haut Moyen Âge illustrent une fois de plus que l’idée qu’un désert forestier précédant l’arrivée des moines défricheurs est à nuancer. L’installation des communautés religieuses durant les XIème-XIIIème siècles, et l’influence des pouvoirs seigneuriaux sur cette zone stratégique, semblent toutefois l’étape clef dans la mise en place d’un peuplement durable. Les activités économiques se diversifient et prennent leur essor durant les siècles suivants bien que certaines périodes soient marquées de crises socio-politiques et traversées par les effets, plus ou moins ressentis, du Petit Âge Glaciaire. Il faudra attendre la transition XIXème-XXème siècle pour que les paysages sylvo-pastoraux que nous connaissons actuellement se mettent en place. Situé un peu plus au sud (Savoie, massif de la Chartreuse), notre second site d’étude trouve son origine dans ce qui donne à la montagne une autre identité : les risques naturels. Le site est le témoignage d’une zone dévastée en 1248 de notre ère par un immense glissement de terrain suite à la chute d’une partie du Mont Granier (1933 m d’altitude). Cet évènement, privant une partie de la vallée de cinq paroisses et d’un millier d’habitants, a engendré rapidement un nouveau territoire, aussi bien topographique, végétal que socio-économique. L’observation particulière des sédiments du lac Saint-André formé post-éboulement a offert l’occasion de suivre pas à pas cette reconquête végétale et humaine, appuyé par les approches géologiques, géographiques, archéologiques et historiques qui animent la recherche de façon récurrente. Les résultats obtenus témoignent d’une période de recolonisation végétale suivie d’une reconquête agro-pastorale relativement rapide, avec comme particularité la mise en place d’un territoire viticole. L’enregistrement sédimentaire particulier de ce lac a permis de suivre, en plus des indices d’une agriculture vivrière variée jusqu’au début du XXème siècle, le témoignage pollinique particulier de la vigne qui deviendra petit à petit une monoculture. / The main objective of this thesis was to establish a precise story of the interaction between societies and environment during the last millennium, in two zones of middle mountain area, which are particularly sensitive to the natural hazards as well as to the political, economic and social events, through a set of multiple data. The high-resolution study of two lacustrine sediment cores in two Jura and alpine sites, thanks to the palynological tool - pollen grains, spores and non-pollen palynomorphs - and the study of the historical and archival data allowed us, besides supplying a complementary corpus, to test a comparative approach (pollen vs cadastral data). At Lake Remoray (Doubs, Jura massif, 850 m asl.), the study specified the settlement process. The data of the Early Middle Ages, illustrate one more time that the idea that a forest desert preceding the arrival of the monks land-clearers is to be revised. The installation of religious communities during the XI-XIIIth centuries and the influence of the seigniorial powers in this strategic zone seem however the key stage in the implementation of a sustainable settlement. Economic activities diversify and take their development during the next centuries although certain periods are marked with sociopolitical crises and cross effects, more or less felt, of Little Ice Age. It will be necessary to wait for the XIX-XXth century transition to see the development of the recent silvo-pastoral landscape. Our second site of study finds its origin in what gives to the mountain another identity: the natural risks. The site is the testimony of a zone destroyed in 1248 AD by an immense landslide further to the fall of a part of the Mount Granier (Savoy, massif of the Chartreuse ; 1933 m asl.). This event, depriving the valley of five parishes and a thousand inhabitants quickly engendered a new territory, in terms of topographic, vegetal as well as socioeconomic aspect. The particular observation Lake Saint André lacustrine sequence, formed post-collapse, offered the opportunity to follow step by step this vegetal and human recovery, supported by the geological, geographical, archaeological and historic approaches which liven up the research in a recurring way. The results show of a period of vegetal recolonisation followed by a relatively fast agro-pastoral recovery with, as peculiarity, the implementation of a wine-making territory. The sedimentary recording of this lake allowed following, besides the indications of a varied food-producing farming till the beginning of the XXth century, the particular pollen testimony of the vineyard which will gradually become a monoculture.