This study presents the first comprehensive assessment of the Petersen Ice Shelf and the Petersen Bay epishelf lake, and examines their current characteristics and changes to their structure between 1959 and 2012. The surface of the Petersen Ice Shelf is characterized by a rolling topography of ridges and troughs, which is balanced by a rolling basal topography, with thicker ice under the surface ridges and thinner ice under the surface troughs. Based on thickness measurements collected in 2011 and area measurements from August 2012, the Petersen Ice Shelf has a surface area of 19.32 km2 and a mean thickness of 29 m, with the greatest thicknesses (>100 m) occurring at the fronts of tributary glaciers feeding into the ice shelf. The tributary glaciers along the northern coast of Petersen Bay contributed an estimated area-averaged 7.89 to 13.55 cm yr-1 of ice to the ice shelf between 2011 and 2012. This input is counteracted by a mean surface ablation of 1.30 m yr-1 between 2011 and 2012, suggesting strongly negative current mass balance conditions on the ice shelf. The Petersen Ice Shelf remained relatively stable until 2005 when the first break-up in recent history occurred, removing >8 km2 of ice shelf surface area. This break-up led to the drainage of the epishelf lake once the ice shelf separated from the southern coast, providing a conduit through which the freshwater from the lake escaped. More break-ups occurred in summers 2008, 2011 and 2012, which resulted in a >31.2 km2 loss in surface area (~63% of June 2005 area). While ephemeral regions of freshwater have occurred along the southern coast of Petersen Bay since 2005 (with areas ranging from 0.32-0.53 km2), open water events and a channel along the southern coast have prevented the epishelf lake from reforming. Based on these past and present observations it is unlikely that Petersen Ice Shelf will continue to persist long into the future.

This study presents a comprehensive overview of the current state of the Milne Ice Shelf and how it has changed over the last 59 years. The 205 ±1 km2 ice shelf experienced a 28% (82 ±0.8 km2) reduction in area between 1950 – 2009, and a 20% (2.5 ±0.9km3 water equivalent (w.e.)) reduction in volume between 1981 – 2008/2009, suggesting a long-term state of negative mass balance. Comparison of mean annual specific mass balances (up to -0.34 m w.e. yr-1) with surface mass balance measurements for the nearby Ward Hunt Ice Shelf suggest that basal melt is a key contributor to total ice shelf thinning. The development and expansion of new and existing surface cracks, as well as ice-marginal and epishelf lake development, indicate significant ice shelf weakening. Over the next few decades it is likely that the Milne Ice Shelf will continue to deteriorate.

This study presents the first comprehensive assessment of the Petersen Ice Shelf and the Petersen Bay epishelf lake, and examines their current characteristics and changes to their structure between 1959 and 2012. The surface of the Petersen Ice Shelf is characterized by a rolling topography of ridges and troughs, which is balanced by a rolling basal topography, with thicker ice under the surface ridges and thinner ice under the surface troughs. Based on thickness measurements collected in 2011 and area measurements from August 2012, the Petersen Ice Shelf has a surface area of 19.32 km2 and a mean thickness of 29 m, with the greatest thicknesses (>100 m) occurring at the fronts of tributary glaciers feeding into the ice shelf. The tributary glaciers along the northern coast of Petersen Bay contributed an estimated area-averaged 7.89 to 13.55 cm yr-1 of ice to the ice shelf between 2011 and 2012. This input is counteracted by a mean surface ablation of 1.30 m yr-1 between 2011 and 2012, suggesting strongly negative current mass balance conditions on the ice shelf. The Petersen Ice Shelf remained relatively stable until 2005 when the first break-up in recent history occurred, removing >8 km2 of ice shelf surface area. This break-up led to the drainage of the epishelf lake once the ice shelf separated from the southern coast, providing a conduit through which the freshwater from the lake escaped. More break-ups occurred in summers 2008, 2011 and 2012, which resulted in a >31.2 km2 loss in surface area (~63% of June 2005 area). While ephemeral regions of freshwater have occurred along the southern coast of Petersen Bay since 2005 (with areas ranging from 0.32-0.53 km2), open water events and a channel along the southern coast have prevented the epishelf lake from reforming. Based on these past and present observations it is unlikely that Petersen Ice Shelf will continue to persist long into the future.

This thesis presents a 40Ar/39Ar geochronological analyses of biotite from thin ductile shear zones in Paleoproterozoic granulite-facies gneisses from the Ellesmere-Devon crystalline terrane, Nunavut, Canada. The gneisses are part of the Paleoproterozoic Thelon tectonic zone. U-Pb dates of zircon show that the gneisses have magmatic protolith ages ranging from 2007–1958 Ma. The quartzofeldspathic gneisses in southeast Ellesmere Island display centimeter-scale E- to NE-striking sinistral and dextral mylonite zones offsetting pegmatitic dikes that are the last stage of ductile deformation of the basement rocks. Samples were taken from nearshore outcrops at Hayes Fiord, Pim Island, NE of the Leffert Glacier and NW of Cape Isabella. Biotite clusters replace orthopyroxene as the result of post-granulite facies metamorphism in the gneisses. Biotite in mylonitic and ultramylonitic fabrics is found as flattened clusters and also as individual crystals defining shear bands related to mylonitization. Eight samples were dated, including biotite from five mylonites, one deformed pegmatite, one tonalite and muscovite from a pegmatite. Major element X-ray maps demonstrate that the biotite is chemically homogenous. Backscattered electron images and electron dispersive spectroscopy via scanning electron microscopy confirm that biotite lacks intercrystalline layering with other K phases. Step-heating analysis of mica at the University of Vermont yielded Paleoproterozoic 40Ar/39Ar ages. The apparent age spectra form plateau ages in all but one mylonite sample. Biotite from a protomylonite was 2051 ± 26 Ma, older than the protolith ages obtained from U-Pb zircon geochronology, and most likely indicates excess Ar. Pegmatitic muscovite was 1977 ± 35 Ma. Biotite dates range from 1874 ± 13 Ma to 1838 ± 14 Ma for the five mylonites without excess Ar. Biotite dated from ductile shear zones signals the latest deformation in the basement, which was active as early as 1887 Ma.

No description available.

Thèse ou mémoire avec insertion d'articles / L'Arctique se réchauffe rapidement, et les lacs de ces régions sont typiquement décrits comme étant très sensibles aux variations environnementales. Cependant, les données physicochimiques sous forme de profils ou de séries temporelles sont rares, en particulier dans les régions les plus nordiques, et peu d'attention a été accordée à la période sous glace. Cette thèse se compose de trois chapitres visant à mieux comprendre la dynamique des lacs de l'Arctique face aux changements climatiques, en se penchant sur la dynamique de l'oxygène et les processus biogéochimiques impliqués. Le premier chapitre se penche sur quatre lacs côtiers de l'île d'Ellesmere. Des profils physicochimiques ont été mesurés dans ces lacs avant la fonte printanière sur trois années consécutives. Malgré leur proximité géographique, ces lacs présentaient des différences majeures en termes de propriétés limnologiques. Les lacs moins profonds, et situés plus proches de l'océan étaient pauvres en oxygène, tandis que les lacs plus profonds et plus éloignés étaient mieux oxygénés. En association avec la déplétion en oxygène dans les lacs peu profonds, des gradients verticaux prononcés en ions, métaux et éléments nutritifs ont été observés, indiquant des processus microbiens anaérobies importants. Ces résultats suggéraient que les caractéristiques biogéochimiques étaient davantage influencées par la morphologie du lac que par leur position le long du gradient océan-intérieur. Le deuxième chapitre est une revue systématique des connaissances sur la dynamique de l'oxygène dissous dans les lacs arctiques. Des données de 167 sites sur 76 ans, incluant 40 sites avec des séries temporelles ont été recueillies pour réaliser une méta-analyse. Les résultats ont montré que les lacs peu profonds situés au sud du gradient latitudinal étaient plus susceptibles d'avoir des concentrations d'oxygène faibles, et que les lacs méromictiques étaient situés vers le nord du gradient. Un modèle conceptuel a été développé pour expliquer les mécanismes derrière les variations de la dynamique d'oxygène en réponse aux pressions climatiques. Cette étude met en évidence la nécessité d'une surveillance continue de l'oxygène dissous dans les lacs arctiques, et suggère des efforts de modélisation pour mieux comprendre les répercussions des changements climatiques. Le troisième chapitre se penche sur la dynamique de l'oxygène et de la température dans un lac du Haut-Arctique sur trois cycles annuels complets. Les variations interannuelles ont révélé que ce lac est sensible aux changements de nature climatiques. Les étés plus chauds ont entraîné des concentrations d'oxygène élevées dans les eaux libres de glace et une stratification thermique, tandis que l'été le plus froid a provoqué une anoxie persistante dans les eaux de fond. Ces résultats indiquent que de légères variations climatiques peuvent entraîner des trajectoires différentes pour les lacs de l'Extrême-Arctique, avec des conséquences majeures pour les processus biogéochimiques. Cela souligne également l'importance de ces écosystèmes en tant qu'indicateurs des changements environnementaux globaux. Dans son ensemble, cette thèse montre que les lacs du Haut-Arctique sont sensibles aux changements climatiques et souligne l'importance de surveiller de près ces écosystèmes en transition. Ce projet fournit également des recommandations pour de futures études et met en évidence la nécessité de mieux comprendre les processus limnologiques dans l'Arctique pour prédire les répercussions des changements climatiques. / The Arctic is warming rapidly, and lakes in these regions are typically very sensitive to environmental variations. However, physicochemical data in the form of profiles or time series are rare, particularly in the northernmost regions, and little attention has been paid to the ice-covered period. This thesis consists of three chapters aiming for a better understanding of the dynamics of Arctic lakes in the face of climate change, with a focus on oxygen dynamics and the biogeochemical processes involved. In the first chapter, we studied four coastal lakes on Ellesmere Island. Physicochemical profiles were measured in these lakes before spring melt over three consecutive years. Despite their proximity to each other, these lakes presented major differences in terms of limnological properties. Shallower lakes located closer to the ocean were oxygen depleted, while deeper and more inland lakes were well oxygenated. In association with oxygen depletion in shallow lakes, pronounced vertical gradients in ions, metals and nutrients were observed, indicating significant anaerobic microbial processes. These results suggested that biogeochemical characteristics were more influenced by lake morphology than by their position along the ocean-inland gradient. The second chapter is a systematic review of knowledge about oxygen dynamics in Arctic lakes. Data from 167 sites over 76 years, including 40 sites with time series, were collected to perform a meta-analysis. The results showed that shallow lakes located south of the latitudinal gradient were more likely to have low oxygen concentrations, and that meromictic lakes were located toward the north of the gradient. A conceptual model was developed to explain the mechanisms behind variations in oxygen dynamics in response to climatic pressures. This study highlights the need for continued monitoring of dissolved oxygen in Arctic lakes, and suggests modelling efforts to better understand the impacts of climate change. The third chapter looks at the dynamics of oxygen and temperature in a High Arctic lake over three complete annual cycles. Interannual variations revealed that this lake is sensitive to climatic variables. Warmer summers led to elevated oxygen concentrations in ice-free waters and thermal stratification, while the colder summer caused persistent anoxia in bottom waters. These results indicate that small climatic variations can lead to different trajectories for High Arctic lakes, with major consequences for biogeochemical processes. They also highlight the importance of these ecosystems as indicators of global environmental changes. This thesis shows that High Arctic lakes are sensitive to climate change and highlights the importance of closely monitoring these transitioning ecosystems. This project also provides recommendations for future studies and highlights the need to better understand limnological processes in the Arctic to predict the impacts of climate change.