No description available.

Antarctic mass balance is mainly controlled by surface mass balance (SMB, i.e. the net effect of precipitations at the surface of the ice sheet) and ice discharge at its margins, mostly through ice shelves. These floating ice bodies made from ice flowing from the continent to the ocean are buttressed by ice rises (elevation of the sea floor on which ice shelf re-grounds) such as the Derwael Ice Rise (DIR) in Dronning Maud Land (DML). In addition to this role important to consider in the future contribution of Antarctica to sea level rise, ice rises are also “climate dipsticks” helping to reconstruct the climate of the past centuries to millennia at high resolution. Due to their coastal location, they witness the changes happening there more rapidly than inland. Furthermore, their internal stratigraphy forms arches that allow to assess their stability, to date their own formation and therefore, in some cases, to constrain the past extension of the ice sheet at the scale of several millennia. As part of the IceCon project :Constraining ice mass changes in Antarctica, this thesis aimed to drill a 120 m ice core (named IC12 for the IceCon project, 2012) at the divide of the DIR and perform physico-chemical analyses to study its density and its internal annual layering with the aim of reconstructing SMB of the last two centuries. We also recorded a virtual image of the borehole using an optical televiewer (OPTV) to assess the ability of this instrument to reconstruct a density profile and measure vertical strain rates when the logging is repeated in the same borehole after a sufficient period of time (here, 2 years).The results show a general increase in snow accumulation rates (SMB) of 30-40% during the 20th century, particularly marked during the last 20-50 years. SMB variability is governed to a large extent by atmospheric circulation and to a lesser extent by variations in sea ice cover. The vertical velocity profile measured from repeat borehole OPTV was applied to refine SMB correction and the results fall in the error range of the corrections made using a model previously developed to study the DIR’s stability. This thesis also contributed to characterizing the spatial variability of SMB across the DIR by dating internal reflection horizons (IRHs), former surfaces of the DIR buried under subsequent snow layers and detected using radio-echo-sounding, and by measuring the density profile of IC12. SMB is found to be 2.5 times higher on the upwind slope than on the downwind slope due to the orographic effect. This pattern is regularly observed on ice rises in DML and stresses the importance of adopting a sufficient spatial resolution (5 km) in climate models.Finally, the technical developments allowing to rapidly reconstruct a density profile from the OPTV image of a borehole contributed to improving our knowledge of two features of Antarctic ice shelves, namely melt ponds, influencing surface mass balance and subglacial channels, influencing basal mass balance. Specifically, the results show that density is 5 % higher in surface trenches associated with subglacial channels, and that ice below melt ponds can reach the density of bubble-free ice due to melting and refreezing processes, with implications on ice shelf viscosity. / Doctorat en Sciences / info:eu-repo/semantics/nonPublished

No description available.

Reconstructed snow accumulation rate observations from five new firn cores show a statistically significant negative trend in accumulation rates over the past four decades across the central West Antarctic ice sheet. A negative temporal trend in accumulation is unexpected in light of rising surface temperatures and simulations run by GCMs. Both the magnitude of the mean accumulation rates and the range of interannual variability observed in the new records compares favorably to older records, suggesting that the new accumulation rate records may serve as a regional proxy for recent temporal trends in West Antarctic accumulation rates. The observed negative trend is likely the result of Southern Hemisphere high-latitude internal atmospheric dynamics, dominated by changes in the austral fall season. The well-documented positive trend in the Southern Annular Mode causes a low pressure center to form over the Amundsen Sea, which in turn produces lower accumulation rates across the western portion of the West Antarctic ice sheet. The new accumulation rate records are compared to several models/reanalyses to test the skill of simulated accumulation rate predictions. While the models/reanalyses and the new observations agree well in both mean and variability, the simulated records do not capture the full negative trend observed in the reconstructed records.

No description available.

Esta dissertação investiga o registro de conteúdo iônico do testemunho de gelo monte Johns (79°55'28"S; 94°23'18"O) obtido na Antártica Ocidental, contribuindo para o adensamento espacial de informações glacioquímicas empregadas em estudos paleoclimáticos. As concentrações de Na+ (16,6 ± 28,2 μg L-1), K+ (1,3 ± 3,7 μg L-1), Mg2+ (3,7 ± 3,7 μg L-1), Ca2+ (5,4 ± 3,4 μg L-1), Cl- (33,3 ± 43,7 μg L-1), SO42- (25,9 ± 17,7 μg L-1), NO3- (50,8 ± 18,5 μg L-1) e H3CSO3- (7,1 ± 5,4 μg L-1) foram determinadas por cromatografia iônica em 2.164 amostras para o período 1882–2008. A variabilidade sazonal de NO3- e, especialmente do nssSO42-, em antifase com Na+, possibilitou a datação pela contagem de ciclos anuais ao longo do período estudado. A identificação dos sinais vulcânicos do Krakatoa (1883), Agung (1963) e Pinatubo/Hudson (1991) foi usada para determinar horizontes de referência (datação absoluta). Eventos de aporte significativo de aerossóis foram identificados e agrupados considerando o conteúdo iônico, proveniência e estação do ano. A avaliação da proveniência dos íons e do balanço iônico mostra a origem da carga iônica: 36% é oriunda de aerossóis de sal marinho, 13% de poeira mineral, 17% de atividade biogênica marinha e 34% de produtos da reatividade química na atmosfera. É observada uma leve redução nas concentrações de Mg2+ (-0,04 μg L-1 ano-1) e K+ (-0,01 μg L-1 ano-1), e mais forte para NO3- (-0,17 μg L-1 ano-1), no período 1909–2008. A média anual da acumulação líquida de neve foi 0,21 ± 0,04 m eq. H2O no período 1882–2008 sem apresentar tendência significante. A acumulação mostra alta correspondência com dados anuais de reanálise climática, com coeficiente de correlação cruzada de +0,42 (α < 0,05) para o período 1979–2008. / This dissertation investigates the ionic content record of the mount Johns ice core (79°55'28"S; 94°23'18"W) recovered from the West Antarctica, contributing to the spatial densification of glaciochemical information used in paleoclimatic studies. The concentrations of Na+ (16.6 ± 28.2 μg L-1), K+ (1.3 ± 3.7 μg L-1), Mg2+ (3.7 ± 3.7 μg L-1), Ca2+ (5.4 ± 3.4 μg L-1), Cl- (33.3 ± 43.7 μg L-1), SO42- (25.9 ± 17.7 μg L-1), NO3- (50.8 ± 18.5 μg L-1) e H3CSO3- (7.1 ± 5.4 μg L-1) were determined by ion chromatography on 2,164 samples for the period 1882–2008. The NO3- seasonal variability and, especially, of the nssSO42- in antiphase with the Na+, allowed the counting of annual cycles over the studied period. The identification of Krakatau (1883), Agung (1963) and Pinatubo / Hudson (1991) volcanic signals was used to determine reference horizons (absolute dating). Significant aerosols events were identified and grouped considering the ionic content, provenance and season of the year. The evaluation of the ions provenance and of the ionic balance points to ionic content origin: 36% come from sea salt aerosols, 13% from mineral dust, 17% originate from marine biogenic activity and 34% are chemical reactivity products in the atmosphere. It is observed a slight reduction in the Mg2+ (-0.04 μg L-1 y-1) and K+ (-0.01 μg L-1 y-1) concentrations, and stronger in the NO3- (-0.17 μg L-1 y-1), during the 1909–2008 period. The annual mean net accumulation rate averaged 0.21 ± 0.04 m w.e. y-1 in the period 1882–2008 did not show any significant trend and shows high correspondence with climatic reanalysis data in years with data overlap, with a cross-correlation coefficient of +0.42 (α < 0.05) for the period 1979–2008.

Au cours des dernières décennies, l'étude des paléoenvironnements de l’Hemisphere Sud s’est développée avec les informations fournies par l'analyse des carottes de glace polaires et plus récemment à partir des carottes de glace prélevées dans les Andes. Bien que les zones tropicales et subtropicales de l'Amérique du Sud jouent un rôle majeur dans la chimie atmosphérique globale en raison de l’existence de réservoirs importants, les échanges entre l’équateur et les hautes latitudes sud restent encore peu documentés. Même si d’autres archives apportent des informations sur le climat passé, il existe peu de données de la composition atmosphérique des moyennes latitudes de l’Hémisphère Sud, aussi bien en termes de variabilité naturelle que d’impact anthropique.Ce travail est basé sur l'analyse d'une carotte de glace de 122 m extraite d’un site d'altitude en Patagonie (Monte San Valentin 46º 35' S, 73º19' W, 3723 m). Outre l’établissement de profils à haute résolution des ions minéraux et organiques obtenus par chromatographie ionique, nous avons réalisé une étude des précipitations susceptibles d'influencer le site, fournissant ainsi une base pour l'interprétation de l'archive de glace.Ce site se caractérise par l’enregistrement d’événements à caractère marin et continental. Nous avons démontré que, sur ce glacier, les contributions marines primaires et secondaires ne sont pas synchrones et qu’elles sont très probablement sousreprésentées dans le profil en raison de processus d'accumulation. Contrairement au sel de mer, une partie importante des aérosols biogéniques marins est associée à un apport influencé par des contributions continentales diluées. L’apport continental est complexe : il est composé d’événements ponctuels de combustion, de retombées volcaniques et d’émissions par les sols. Il correspond à presque la moitié de la quantité totale de sulfate déposé. Cette contribution importante et omniprésente de sulfate est associée au nitrate et au chlorure, mais mieux cerner son origine nécessiterait des recherches plus poussées. La comparaison de notre archive glaciochimique avec des éruptions volcaniques datées et le début de la colonisation en Patagonie nous a permis de proposer quelques repères pour la datation de la carotte, qui reste à ce jour encore incertaine. Enfin, aucune tendance anthropique claire n’a été décelée à partir de l’évolution du bruit de fond des espèces continentales, excepté l'ammonium, lui pouvant être mis en relation avec l’utilisation croissante d'engrais après les années 1960. / Although tropical and subtropical South America play an important role in atmospheric global and hemispheric chemistry due to the coexistence of big reservoirs with large-scale atmospheric patterns, exchanges between tropics and higher latitudes have remained poorly documented. During the last few decades, the study of past climate and atmospheric composition has benefited greatly by information provided by ice core analysis. In the Southern Hemisphere, a large amount of data is now available from polar ice cores and more recently from ice cores recovered in the Andes. Despite a few studies on climate reconstruction, there is a lack of information on past atmospheric composition in Southern mid-latitudes, in terms of both natural variability and anthropogenic impact. This work is based on the analysis of a 122 m ice core extracted from a high altitude site in the Northern Patagonian Icefield (Monte San Valentin 46º 35'S, 73º19' W, summit at 4032 m.a.s.l., drilling plateau at 3723 m.a.s.l.). Besides high-resolution measurements of mineral and organic ions, a study of the precipitation events likely to influence the site was conducted in order to provide a basis for the interpretation of the ice archive. The selected site registers precipitation events of marine and continental origin. We have demonstrated that marine primary and biogenic contributions are not synchronous and they are very likely underrepresented along the ice archive due to the effect of accumulation processes. Contrary to sea salt, a significant part of marine biogenic aerosol is associated with a complex and diluted continental fingerprint. The continental imprint was studied in terms of background trends and major events involving combustion processes, urban pollution, volcanic and soil emissions. This set of mixed sources is responsible for almost half of the total sulfate input. This important and ubiquitous sulfate concentration is related to that of nitrate and chloride and its understanding in terms of sources and transport needs further research. A comparison of our records with documented volcanic eruptions and Patagonian settlement periods allowed us to propose a few dating horizons, although an accurate dating of the entire core has not been possible so far. Background trends of continental species do not show any trend that can be clearly associated with an anthropogenic influence, except for ammonium, which increases likely in response to fertilizer consumption after ca. 1960.

Esta dissertação investiga o registro de conteúdo iônico do testemunho de gelo monte Johns (79°55'28"S; 94°23'18"O) obtido na Antártica Ocidental, contribuindo para o adensamento espacial de informações glacioquímicas empregadas em estudos paleoclimáticos. As concentrações de Na+ (16,6 ± 28,2 μg L-1), K+ (1,3 ± 3,7 μg L-1), Mg2+ (3,7 ± 3,7 μg L-1), Ca2+ (5,4 ± 3,4 μg L-1), Cl- (33,3 ± 43,7 μg L-1), SO42- (25,9 ± 17,7 μg L-1), NO3- (50,8 ± 18,5 μg L-1) e H3CSO3- (7,1 ± 5,4 μg L-1) foram determinadas por cromatografia iônica em 2.164 amostras para o período 1882–2008. A variabilidade sazonal de NO3- e, especialmente do nssSO42-, em antifase com Na+, possibilitou a datação pela contagem de ciclos anuais ao longo do período estudado. A identificação dos sinais vulcânicos do Krakatoa (1883), Agung (1963) e Pinatubo/Hudson (1991) foi usada para determinar horizontes de referência (datação absoluta). Eventos de aporte significativo de aerossóis foram identificados e agrupados considerando o conteúdo iônico, proveniência e estação do ano. A avaliação da proveniência dos íons e do balanço iônico mostra a origem da carga iônica: 36% é oriunda de aerossóis de sal marinho, 13% de poeira mineral, 17% de atividade biogênica marinha e 34% de produtos da reatividade química na atmosfera. É observada uma leve redução nas concentrações de Mg2+ (-0,04 μg L-1 ano-1) e K+ (-0,01 μg L-1 ano-1), e mais forte para NO3- (-0,17 μg L-1 ano-1), no período 1909–2008. A média anual da acumulação líquida de neve foi 0,21 ± 0,04 m eq. H2O no período 1882–2008 sem apresentar tendência significante. A acumulação mostra alta correspondência com dados anuais de reanálise climática, com coeficiente de correlação cruzada de +0,42 (α < 0,05) para o período 1979–2008. / This dissertation investigates the ionic content record of the mount Johns ice core (79°55'28"S; 94°23'18"W) recovered from the West Antarctica, contributing to the spatial densification of glaciochemical information used in paleoclimatic studies. The concentrations of Na+ (16.6 ± 28.2 μg L-1), K+ (1.3 ± 3.7 μg L-1), Mg2+ (3.7 ± 3.7 μg L-1), Ca2+ (5.4 ± 3.4 μg L-1), Cl- (33.3 ± 43.7 μg L-1), SO42- (25.9 ± 17.7 μg L-1), NO3- (50.8 ± 18.5 μg L-1) e H3CSO3- (7.1 ± 5.4 μg L-1) were determined by ion chromatography on 2,164 samples for the period 1882–2008. The NO3- seasonal variability and, especially, of the nssSO42- in antiphase with the Na+, allowed the counting of annual cycles over the studied period. The identification of Krakatau (1883), Agung (1963) and Pinatubo / Hudson (1991) volcanic signals was used to determine reference horizons (absolute dating). Significant aerosols events were identified and grouped considering the ionic content, provenance and season of the year. The evaluation of the ions provenance and of the ionic balance points to ionic content origin: 36% come from sea salt aerosols, 13% from mineral dust, 17% originate from marine biogenic activity and 34% are chemical reactivity products in the atmosphere. It is observed a slight reduction in the Mg2+ (-0.04 μg L-1 y-1) and K+ (-0.01 μg L-1 y-1) concentrations, and stronger in the NO3- (-0.17 μg L-1 y-1), during the 1909–2008 period. The annual mean net accumulation rate averaged 0.21 ± 0.04 m w.e. y-1 in the period 1882–2008 did not show any significant trend and shows high correspondence with climatic reanalysis data in years with data overlap, with a cross-correlation coefficient of +0.42 (α < 0.05) for the period 1979–2008.

Le carbone-suie, ou "black carbon" (BC), est considéré comme le deuxième plus grand contributeur d'origine anthropique au changement climatique, après le dioxyde de carbone, en raison de son importante capacité à absorber la lumière (Bond et al., 2013). Malgré son influence sur le système climatique, les observations pluriannuelles des teneurs atmosphériques de BC demeurent rares, particulièrement en Europe. Cette étude propose une reconstruction nouvelle de la variabilité passée du BC atmosphérique à partir de l'analyse d'une carotte de glace forée au glacier de l'Elbrus (ELB), situé dans le Caucase en Russie. Un analyseur de carbone-suie SP2 (Single Particle Soot Photometer) a été exploité pour l'analyse du BC réfractaire (rBC) sur 153m de carotte de glace couvrant les derniers 140 ans. Un nouveau système analytique couplant un nébuliseur APEX-Q et le SP2, intégré au sein d'un système d'Analyse en Flux Continu, a été développée et a permis d'obtenir un enregistrement unique quasi-continu du rBC. La glace de l'Elbrus révèle une augmentation marquée des teneurs en rBC entre les années 1870 et 1980, suivie d'une diminution des concentrations jusqu'en 2000 environ. Au cours de la dernière décennie, les concentrations en rBC demeurent stables. Parallèlement, une augmentation des niveaux de fond jusqu'en 1980 indique clairement un impact non négligeable des émissions anthropiques sur la charge en BC atmosphérique à des échelles spatiales très larges, et notamment dans la troposphère libre. Une étude fine des inventaires d'émissions de BC disponibles et la modélisation FLEXPART viennent confirmer que l'augmentation forte de rBC depuis 1920 est principalement due aux émissions anthropiques de BC. Une analyse du gradient est-ouest des dépôts de rBC à haute altitude au cours de la dernière décennie a été réalisée, en comparant l'archive récente (10 ans) de l'Elbrus avec deux autres enregistrements provenant du Col du Dôme (CDD, France) et du Colle Gnifetti (CG, Italie). Les concentrations à l'Elbrus se révèlent deux à trois fois supérieures à celles des autres sites, une différence liée à l'intensité plus forte des sources en Europe de l'Est et confirmée par la modélisation FLEXPART. Les combustions de biomasse se trouvent identifiées comme la principale cause de variabilité interannuelle à l'Elbrus en été. Sur les derniers 10 ans, une diminution statistiquement significative des concentrations en rBC dans la neige est détectée au CDD, un résultat qui s'oppose aux observations réalisées à l'Elbrus où une tendance à l'augmentation au cours des périodes estivales est identifiée. Ces tendances, aussi bien CDD et ELB, sont relativement cohérentes avec les inventaires d'émissions de BC anthropiques disponibles. Ces enregistrements nouveaux et continus de rBC dans plusieurs carottes de glace européennes viennent faire progresser notre compréhension des évolutions passées des sources et de la charge atmosphérique du BC en Europe. Ces enregistrements peuvent participer à une meilleure évaluation de l'efficacité des politiques passées et actuelles de réduction des émissions de BC, ainsi qu'à une amélioration des inventaires d'émission. / Black carbon (BC) is considered as the second largest man-made contributor to global warming after carbon dioxide due to its highly light-absorbing ability (Bond et al., 2013). Despite its climatic role, multi-year observations of ambient BC concentrations are scarce, in particular over Europe. In this thesis, we reconstructed past variability of atmospheric BC using an ice core from the Elbrus glacier (ELB), Caucasus in Russia. We have used a single particle soot photometer (SP2) to retrieve refractive BC (rBC) along 153 m of ice core covering the last 140 years. We have developed and validated a novel analytical system coupling of APEX-Q nebulization to SP2 in a Continuous-flow Analysis system to derive a unique quasi-continuous record of rBC. Results reveal a substantial rBC increase since 1870s lasting until 1980, followed by a decrease until ~2000. In the last decade, rBC concentrations remained constant. In parallel, an increase in background concentration until 1980 clearly highlights that anthropogenic BC emissions have substantially affected the atmospheric BC loading on a very large spatial scale, particularly in the free troposphere. A comparison with the estimated BC reaching the ELB site using a BC emission inventory/FLEXPART modeling confirms that the strong rBC increase since around 1920 is mainly due to anthropogenic BC emissions. Analysis of the East-West gradient of rBC deposited in snow over the last decade is investigated comparing the recent archive (10-year) at ELB with two other records from Col du Dôme (CDD), France, and Colle Gnifetti (CG), Italy. Concentrations at ELB are 2-3 times higher than at other sites, which is linked to source intensity in the Eastern part of Europe, as confirmed by the FLEXPART modelling. Biomass burning is identified as a main cause of inter-annual variability at ELB during summer time. Over the last decade, a statistically significant reduction of rBC concentration in snow is found at CDD, opposite to what is found at ELB with an increasing trend observed for summer periods. These trends are also fairly consistent with anthropogenic BC emissions inventories. Availability of continuous records of rBC in European ice cores improved our understanding of past evolution of atmospheric BC over Europe. They can be used to assess efficiency of past and current emission reduction policies and improve emission inventories.

Esta dissertação investiga o registro de conteúdo iônico do testemunho de gelo monte Johns (79°55'28"S; 94°23'18"O) obtido na Antártica Ocidental, contribuindo para o adensamento espacial de informações glacioquímicas empregadas em estudos paleoclimáticos. As concentrações de Na+ (16,6 ± 28,2 μg L-1), K+ (1,3 ± 3,7 μg L-1), Mg2+ (3,7 ± 3,7 μg L-1), Ca2+ (5,4 ± 3,4 μg L-1), Cl- (33,3 ± 43,7 μg L-1), SO42- (25,9 ± 17,7 μg L-1), NO3- (50,8 ± 18,5 μg L-1) e H3CSO3- (7,1 ± 5,4 μg L-1) foram determinadas por cromatografia iônica em 2.164 amostras para o período 1882–2008. A variabilidade sazonal de NO3- e, especialmente do nssSO42-, em antifase com Na+, possibilitou a datação pela contagem de ciclos anuais ao longo do período estudado. A identificação dos sinais vulcânicos do Krakatoa (1883), Agung (1963) e Pinatubo/Hudson (1991) foi usada para determinar horizontes de referência (datação absoluta). Eventos de aporte significativo de aerossóis foram identificados e agrupados considerando o conteúdo iônico, proveniência e estação do ano. A avaliação da proveniência dos íons e do balanço iônico mostra a origem da carga iônica: 36% é oriunda de aerossóis de sal marinho, 13% de poeira mineral, 17% de atividade biogênica marinha e 34% de produtos da reatividade química na atmosfera. É observada uma leve redução nas concentrações de Mg2+ (-0,04 μg L-1 ano-1) e K+ (-0,01 μg L-1 ano-1), e mais forte para NO3- (-0,17 μg L-1 ano-1), no período 1909–2008. A média anual da acumulação líquida de neve foi 0,21 ± 0,04 m eq. H2O no período 1882–2008 sem apresentar tendência significante. A acumulação mostra alta correspondência com dados anuais de reanálise climática, com coeficiente de correlação cruzada de +0,42 (α < 0,05) para o período 1979–2008. / This dissertation investigates the ionic content record of the mount Johns ice core (79°55'28"S; 94°23'18"W) recovered from the West Antarctica, contributing to the spatial densification of glaciochemical information used in paleoclimatic studies. The concentrations of Na+ (16.6 ± 28.2 μg L-1), K+ (1.3 ± 3.7 μg L-1), Mg2+ (3.7 ± 3.7 μg L-1), Ca2+ (5.4 ± 3.4 μg L-1), Cl- (33.3 ± 43.7 μg L-1), SO42- (25.9 ± 17.7 μg L-1), NO3- (50.8 ± 18.5 μg L-1) e H3CSO3- (7.1 ± 5.4 μg L-1) were determined by ion chromatography on 2,164 samples for the period 1882–2008. The NO3- seasonal variability and, especially, of the nssSO42- in antiphase with the Na+, allowed the counting of annual cycles over the studied period. The identification of Krakatau (1883), Agung (1963) and Pinatubo / Hudson (1991) volcanic signals was used to determine reference horizons (absolute dating). Significant aerosols events were identified and grouped considering the ionic content, provenance and season of the year. The evaluation of the ions provenance and of the ionic balance points to ionic content origin: 36% come from sea salt aerosols, 13% from mineral dust, 17% originate from marine biogenic activity and 34% are chemical reactivity products in the atmosphere. It is observed a slight reduction in the Mg2+ (-0.04 μg L-1 y-1) and K+ (-0.01 μg L-1 y-1) concentrations, and stronger in the NO3- (-0.17 μg L-1 y-1), during the 1909–2008 period. The annual mean net accumulation rate averaged 0.21 ± 0.04 m w.e. y-1 in the period 1882–2008 did not show any significant trend and shows high correspondence with climatic reanalysis data in years with data overlap, with a cross-correlation coefficient of +0.42 (α < 0.05) for the period 1979–2008.