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Meteorites are rocks that fell from space. These unique extraterrestrial samples on Earth provide crucial information for understanding the origin and evolution of our Solar System. Antarctica is the world’s most prolific site for collecting meteorites, with more than 60% of all ∼ 80,000 meteorites ever found on Earth being collected there. Antarctic meteorites are found in areas where ice is exposed, in contrast to most of the continent’s surface which is covered by snow. During Antarctic meteorite recovery missions, these typically dark meteorites are easily found while lying on the surface of the visually contrasting light-blue colored ice. Moreover, the number of meteorites in these blue ice areas can be anomalously high. This concentration of meteorites is related to the sublimation of ice layers and the flow of the ice. Meteorite finds from Antarctica were always fairly dependent on serendipity, with experts selecting sites to visit based on their experience and a limited amount of imagery and maps. In this thesis, I perform continent wide data-driven analyses to understand the potential of blue ice areas for the recovery of meteorites and project their persistence in a warming climate. The search for Antarctic meteorites can be approached with data as there is wealth of observations available from the Antarctic continent, including meteorite finds and satellite products. Moreover, techniques to analyze large amounts of data efficiently are rapidly evolving, of which machine learning the most prominent. Hence, by combining diverse satellite observations in a deep learning framework, I detected blue ice areas. Meteorites are found on blue ice, but not all blue ice areas act as meteorite trap. Therefore, I selected indirect observations of the meteorite concentrating mechanism to understand and predict the presence of meteorites. One of these observations entailed the surface temperature. Consequently, I used projections of future surface temperatures to estimat, Doctorat en Sciences, info:eu-repo/semantics/nonPublished

N2 fixation in low-latitude surface waters dominates the input of fixed nitrogen (N) to the global ocean, sustaining ocean fertility. In the Caribbean Sea, higher foraminifera-bound (FB-)delta 15N indicates a decline in N2 fixation during ice ages, but its cause and broader implications are unclear. Here, we report three additional Atlantic FB-delta 15N records, from the subtropical North and South Atlantic gyres (MSM58-50 and DSDP Site 516) and the equatorial Atlantic (ODP Site 662). Similar glacial and interglacial delta 15N in the equatorial Atlantic suggests a stable delta 15N for the nitrate below the gyre thermoclines. The North Atlantic record shows a FB-delta 15N rise during the ice ages, resembling a previously published FB-delta 15N record from the South China Sea. The commonality among the FB-delta 15N records is that they resemble sea level-driven variation in regional shelf area, with high FB-delta 15N (inferred reduction in N2 fixation) during periods of low shelf area. The South China Sea shows the largest delta 15N signal, the subtropical North Atlantic shows less, and the South Atlantic shows the least, the same ordering as the ice age reductions in continental shelf area in the different regions. Reduced shelf sedimentary denitrification would have increased the nitrogen-to-phosphorus ratio of the nutrient supply to open ocean surface waters, leading to decreased N2 fixation and thus higher gyre thermocline nitrate delta 15N, explaining the higher FB-delta 15N of peak ice ages. These observations identify shelf sediment denitrification as an important regional driver of modern N2 fixation and imply strong basin-scale coupling of fixed nitrogen losses and inputs. Nitrogen fixation plays the crucial role in the ocean of supplying bioavailable nitrogen (N), a major nutrient for phytoplankton growth. Variations in nitrogen fixation over time can, therefore, significantly impact ocean productivity and, consequently, carbon sequestration in the ocean int

Centrée au Pôle Sud, l’Antarctique constitue la plus grande étendue de glace continentale sur Terre,représentant le plus important réservoir d’eau douce. Cette masse de glace est constamment en mouvement.La glace se déforme sous l’action de la gravité, et glisse sur le substract rocheux sur lequel ellerepose. Depuis des années, l’Antarctique perd de la masse de glace en raison du réchauffement climatique.Les prédictions concernant l’évolution de sa masse de glace et sa contribution potentielle au niveau marinsont très incertaines. Une des principales sources d’incertitude réside dans comment exactement la glaceglisse-t-elle à la base? Le glissement basal inclut le glissement stricto sensu de la glace sur le lit rocheuxdans le cas d’un lit rigide, ainsi que la déformation du lit composé de sédiments dans le cas d’un lit déformable.Dans certaines zones, comme les ice streams, le glissement basal est le processus majoritaire dudéplacement de la glace. Les ice streams drainant la grande majorité de la calotte, il semble essentiel demieux comprendre le processus responsable de leur mouvement. Mathématiquement, le glissement basalest représenté par une loi de glissement reliant la vitesse du glacier à la contrainte exercée par celui-ci surle lit. Cette loi peut prendre plusieurs formes en fonction des phénomènes qu’elle englobe. L’eau sousglaciairejoue un rôle clef dans le glissement basal en lubrifiant le lit rocheux et en réduisant la résistance dusol à la déformation. Cette eau provient de la fonte de la glace à la base et s’organise en différents types desystèmes de drainage classés comme inefficaces ou efficaces. Les systèmes inefficaces se caractérisent parun faible flux en écoulement diffus, tandis que les systèmes efficaces présentent un fort flux en écoulementconcentré dans des conduits. Le premier objectif de cette thèse est de lier l’hydrologie sous-glaciaire auglissement basal à l’aide du modèle numérique Kori-ULB. Pour ce faire, nous évaluons quatre méthodesd, Centered at the South Pole, Antarctica constitutes the largest expanse of continental ice on Earth, representingthe most significant freshwater stock. This mass of ice is in constant motion, deforming underthe influence of gravity and sliding over the bedrock on which it is lying. Antarctica has been losing icemass for years due to global warming. Predictions regarding the evolution of its ice mass and its potentialcontribution to sea level rise are highly uncertain. One of the main sources of uncertainty lies in understandingexactly how the ice slides at the base. Basal sliding includes the sliding of the ice over the bedrockin the case of a hard bed, as well as the deformation of the bed composed of sediments in the case of adeformable and soft bed. In certain areas, such as ice streams, basal sliding is the predominant processin ice movement. As ice streams drain the majority of the ice sheet, it is essential to better understandthe process responsible for their motion. Mathematically, basal sliding is represented by a sliding law relatingthe glacier’s velocity to the stress it exerts on the bed. This law can take various forms dependingon the phenomena it encompasses. Subglacial water plays a key role in basal sliding by lubricating thebedrock and reducing the till’s resistance to deformation. This water comes from the melting of ice at thebase and organizes into different types of drainage systems classified as inefficient or efficient. Inefficientsystems are characterized by low-flow diffuse drainage, while efficient systems exhibit high-flow concentrateddrainage in conduits. The first objective of this thesis is to link subglacial hydrology to basal slidingusing the Kori-ULB numerical ice sheet model. To achieve this, we evaluate four methods developed inthe literature across the entire Antarctic ice sheet and compare them to simulations that do not considersubglacial hydrology. These methods differ in the representation of subglacial water: seawater pressure, Doctorat en Sciences, info:eu-repo/semantics/nonPublished

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Provenance records from sediments deposited offshore of the West Antarctic Ice Sheet (WAIS) can help identify past major ice retreat, thus constraining ice‐sheet models projecting future sea‐level rise. Interpretations from such records are, however, hampered by the ice obscuring Antarctica's geology. Here, we explore central West Antarctica's subglacial geology using basal debris from within the Byrd ice core, drilled to the bed in 1968. Sand grain microtextures and a high kaolinite content (∼38–42%) reveal the debris consists predominantly of eroded sedimentary detritus, likely deposited initially in a warm, pre‐Oligocene, subaerial environment. Detrital hornblende 40 Ar/ 39 Ar ages suggest proximal late Cenozoic subglacial volcanism. The debris has a distinct provenance signature, with: common Permian‐Early Jurassic mineral grains; absent early Ross Orogeny grains; a high kaolinite content; and high 143 Nd/ 144 Nd and low 87 Sr/ 86 Sr ratios. Detecting this “fingerprint” in Antarctic sedimentary records could imply major WAIS retreat, revealing the WAIS's sensitivity to future warming., info:eu-repo/semantics/published

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As a key biogeochemical pathway in the marine nitrogen cycle, nitrification (ammonia oxidation and nitrite oxidation) converts the most reduced form of nitrogen – ammonium–ammonia (NH4+–NH3) – into the oxidized species nitrite (NO2-) and nitrate (NO3-). In the ocean, these processes are mainly performed by ammonia-oxidizing archaea (AOA) and bacteria (AOB) and nitrite-oxidizing bacteria (NOB). By transforming nitrogen speciation and providing substrates for nitrogen removal, nitrification affects microbial community structure; marine productivity (including chemoautotrophic carbon fixation); and the production of a powerful greenhouse gas, nitrous oxide (N2O). Nitrification is hypothesized to be regulated by temperature, oxygen, light, substrate concentration, substrate flux, pH and other environmental factors. Although the number of field observations from various oceanic regions has increased considerably over the last few decades, a global synthesis is lacking, and understanding how environmental factors control nitrification remains elusive. Therefore, we have compiled a database of nitrification rates and nitrifier abundance in the global ocean from published literature and unpublished datasets. This database includes 2393 and 1006 measurements of ammonia oxidation and nitrite oxidation rates and 2242 and 631 quantifications of ammonia oxidizers and nitrite oxidizers, respectively. This community effort confirms and enhances our understanding of the spatial distribution of nitrification and nitrifiers and their corresponding drivers such as the important role of substrate concentration in controlling nitrification rates and nitrifier abundance. Some conundrums are also revealed, including the inconsistent observations of light limitation and high rates of nitrite oxidation reported from anoxic waters. This database can be used to constrain the distribution of marine nitrification, to evaluate and improve biogeochemical models of nitrification, and to, info:eu-repo/semantics/published

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The cyclic growth and decay of continental ice sheets can be reconstructed from the history of global sea level. Sea level is relatively well constrained for the Last Glacial Maximum (LGM, 26,500 to 19,000 y ago, 26.5 to 19 ka) and the ensuing deglaciation. However, sea-level estimates for the period of ice-sheet growth before the LGM vary by > 60 m, an uncertainty comparable to the sea-level equivalent of the contemporary Antarctic Ice Sheet. Here, we constrain sea level prior to the LGM by reconstructing the flooding history of the shallow Bering Strait since 46 ka. Using a geochemical proxy of Pacific nutrient input to the Arctic Ocean, we find that the Bering Strait was flooded from the beginning of our records at 46 ka until 35.7 - 2.4 + 3.3 ka. To match this flooding history, our sea-level model requires an ice history in which over 50% of the LGM’s global peak ice volume grew after 46 ka. This finding implies that global ice volume and climate were not linearly coupled during the last ice age, with implications for the controls on each. Moreover, our results shorten the time window between the opening of the Bering Land Bridge and the arrival of humans in the Americas., SCOPUS: ar.j, info:eu-repo/semantics/published

As a key biogeochemical pathway in the marine nitrogen cycle, nitrification (ammonia oxidation and nitrite oxidation) converts the most reduced form of nitrogen-ammonium-Ammonia (NH4+-NH3)-into the oxidized species nitrite (NO2-) and nitrate (NO3-). In the ocean, these processes are mainly performed by ammonia-oxidizing archaea (AOA) and bacteria (AOB) and nitrite-oxidizing bacteria (NOB). By transforming nitrogen speciation and providing substrates for nitrogen removal, nitrification affects microbial community structure; marine productivity (including chemoautotrophic carbon fixation); and the production of a powerful greenhouse gas, nitrous oxide (N2O). Nitrification is hypothesized to be regulated by temperature, oxygen, light, substrate concentration, substrate flux, pH and other environmental factors. Although the number of field observations from various oceanic regions has increased considerably over the last few decades, a global synthesis is lacking, and understanding how environmental factors control nitrification remains elusive. Therefore, we have compiled a database of nitrification rates and nitrifier abundance in the global ocean from published literature and unpublished datasets. This database includes 2393 and 1006 measurements of ammonia oxidation and nitrite oxidation rates and 2242 and 631 quantifications of ammonia oxidizers and nitrite oxidizers, respectively. This community effort confirms and enhances our understanding of the spatial distribution of nitrification and nitrifiers and their corresponding drivers such as the important role of substrate concentration in controlling nitrification rates and nitrifier abundance. Some conundrums are also revealed, including the inconsistent observations of light limitation and high rates of nitrite oxidation reported from anoxic waters. This database can be used to constrain the distribution of marine nitrification, to evaluate and improve biogeochemical models of nitrification, and to quantify the i

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Fossil-bound organic material holds great potential for the reconstruction of past changes in nitrogen (N) cycling. Here, with a series of laboratory experiments, we assess the potential effect of oxidative degradation, fossil dissolution, and thermal alteration on the fossil-bound N isotopic composition of different fossil types, including deep and shallow water scleractinian corals, foraminifera, diatoms and tooth enamel. Our experiments show that exposure to different oxidizing reagents does not significantly affect the N isotopic composition or N content of any of the fossil types analyzed, demonstrating that organic matter is well protected from changes in the surrounding environment by the mineral matrix. In addition, we show that partial dissolution (of up to 70%–90%) of fossil aragonite, calcite, opal, or enamel matrixes has a negligible effect on the N isotopic composition and N content of the fossils. These results suggest that the isotopic composition of fossil-bound organic material is relatively uniform, and also that N exposed during dissolution is lost without significant isotopic discrimination. Finally, our heating experiments show negligible changes in the N isotopic composition and N content of all fossil types at 100°C. At 200°C and hotter, any N loss and associated nitrogen isotope changes appear to be directly linked to the sensitivity of the mineral matrix to thermal stress, which depends on the biomineral type. These results suggest that, so long as high temperature does not compromise the mineral structure, the biomineral matrix acts as a closed system with respect to N, and the N isotopic composition of the fossil remains unchanged., SCOPUS: ar.j, info:eu-repo/semantics/published

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The Antarctic ice sheet (AIS) is the largest and yet the most uncertain potential contributor to future sea-level rise. While recent satellite observations have highlighted that the AIS is currently losing mass at an accelerating rate, projections of the future evolution of the ice sheet in a warming climate remain highly uncertain. As future sea-level rise is probably one of the biggest threats imposed on us by climate change, predicting it with the lowest possible uncertainty is of capital societal importance. Uncertainties in the future evolution of the AIS can be explained, notably, by the fact that the ice sheet is capable of abrupt and self-sustained changes associated with several positive feedback mechanisms, especially in its marine areas, i.e. where the ice lies on bedrock below sea level. This is the case for most of the West Antarctic ice sheet (WAIS) as well as for some basins of the East Antarctic ice sheet (EAIS). The interactions between the ice sheet and its surrounding environment (namely the ocean, the atmosphere, and the solid Earth) have been shown to strongly influence its stability, more particularly by triggering or dampening the instabilities threatening the ice sheet. Despite the uncertainties, recent studies suggest that the WAIS will lose mass in the future and eventually (partially) collapse. The uncertainties pertain to when, and to whether the weak Earth structure beneath that area of the ice sheet may be a stabilising factor, as a rapid bedrock uplift in response to ice mass loss has been shown to delay or even limit mass loss. The fate of the EAIS is less clear. A pending question is: will the EAIS lose or gain mass in the future? More specifically, will the grounding line retreat in its marine basins, and if so, can the associated mass loss be compensated by sufficient mass gain due to increased snow accumulation in the interior of the ice sheet?In this thesis, we contribute to clarifying and providing new insights to these question, La calotte glaciaire Antarctique est le plus gros contributeur potentiel à l'élévation future du niveau marin global, mais aussi le plus incertain. De plus en plus d’observations satellitaires mettent en évidence une actuelle perte de masse de l'Antarctique, et ce à un rythme accéléré. Malgré cela, les projections de l'évolution future de la calotte Antarcique en réponse aux changements climatiques actuels et à venir restent très incertaines. Pourtant, être en mesure de prédire avec la plus faible incertitude possible l'élévation future du niveau de la mer est d'une importance sociétale capitale. Les incertitudes sur l'évolution future de l'Antarctique s'expliquent notamment par le fait que la calotte glaciaire est capable de changements brusques associés à plusieurs mécanismes de rétroactions positives, et ce particulièrement dans les régions où la glace repose sur un socle rocheux situé sous le niveau de la mer. C'est le cas pour la majorité de l'Antarctique de l’Ouest ainsi que dans certains bassins de l’Antarctique de l’Est. Il a été démontré que les interactions entre la calotte glaciaire et son environnement (à savoir l'océan, l'atmosphère et le lit rocheux sous-jacent) influencent fortement sa stabilité, notamment en déclenchant ou en atténuant les instabilités qui la menacent. Malgré les incertitudes, des études récentes suggèrent que, à l'avenir, l’Antarctique de l’Ouest perdra de la masse et finira par (au moins partiellement) s'effondrer. Les incertitudes concernent dès lors le moment de cet effondrement, mais également une potentielle stabilisation de la calotte par le lit rocheux (qui repose sur une région du manteau terrestre particulièrement peu visqueuse) situé sous cette région occidentale. En effet, il a été démontré qu'un rebond rapide du lit rocheux pouvait ralentir voire limiter la perte de masse de la calotte. Le sort de l'Antarctique de l’Est est moins clair. Une des questions en suspens est la suivante :l'Antarctique de l’Est perdra-t-elle ou, Doctorat en Sciences, info:eu-repo/semantics/nonPublished