Etude des échanges côte-large au moyen des isotopes du radium : cas de la fertilisation en fer au large des îles Crozet et Kerguelen (Océan Austral)

The Southern Ocean is known to be the largest High-Nutrient, Low-Chlorophyll region of the global ocean. While nutrient concentrations (nitrate, phosphate, silicate) are high, the phytoplankton development is paradoxically limited mostly because of the low dissolved iron concentrations of the Southern Ocean waters (Martin_iron_1990). The Crozet and Kerguelen Archipelagos, located in the Indian sector of the Southern Ocean, constitute two major topographic obstacles to the eastward-flowing Antarctic Circumpolar Current. The interaction of the current with the sediments of the shallow Crozet and Kerguelen plateaus contributes to the supply of iron downstream of these islands, thus leading to large phytoplankton blooms in these regions (Blain et al. 2007, Pollard et al. 2007). These phytoplankton blooms constitute unique open-air laboratories to study the response of the ecosystems and the impact on biogeochemical cycles to natural iron fertilization. This PhD thesis was done in the framework of the KEOPS-2 project. Radium isotopes (223Ra, 224Ra, 226Ra and 228Ra), that are powerful tools to study the ocean circulation and mixing, are the main tools used here. The four natural occurring isotopes display half-lives ranging from a few days to thousands of years and are produced by the decay of particle-bound thorium isotopes in sediments. They are delivered to the open ocean by diffusion and advection processes where they behave as conservative tracers in such a way that the water body keeps the signature of its contact with the sediments modulated by the half-lives of the radium isotopes. Therefore, we used Ra isotopes to (i) investigate the origin and the dispersion of the sediment-derived inputs - including iron - and (ii) to estimate the timescales of the transfer of surface waters between the shelf and offshore waters. We compared the Ra dataset with data acquired using physical tools (surface drifters and Lagrangian model derived from altimetry). Firstly, the use of three independent methods - including geochemical and physical methods - in the Crozet region allowed us to validate each method. Secondly, we show that the Crozet Island phytoplankton plume is fed by two different flows of water that interacted with either the western plateau or the eastern plateau. Thirdly, this physical-geochemical coupled approach was also used in the Kerguelen region and helped us to constrain the origin of the iron fertilization in that area. The observation of 223Ra and 224Ra in surface waters east of the Kerguelen Islands, south of the polar front (PF), indicates that these waters have recently interacted with shallow sediments. The spatial variability observed in the 223Ra and 224Ra distribution in surface waters south of the PF suggests that the input of waters and associated chemical elements across or via the PF - potentially driven by wind stress or eddies - act as sporadic pulses that may highly vary in both space and time. This pathway may thus constitute a mechanism that contributes to fertilizing the phytoplankton bloom with iron and other micronutrients east of the Kerguelen Islands. This finding also suggest that the PF may not act as a strong barrier for surface waters and associated chemical elements, a finding that may also apply for other frontal systems of the world's ocean. Finally, we investigated the 226Ra and barium (Ba) distributions offshore from the Crozet and Kerguelen islands, with the aim to provide additional constraints on the circulation patterns in this area. In particular, we observed temporal changes in the dissolved 226Ra/Ba ratios. Among potential hypothesis, one can invoke (i) changes in the circulation patterns or (ii) the impact of biological processes on the dissolved Ra and Ba concentrations.L'Océan Austral est connu pour être la plus vaste zone "High-Nutrient, Low-Chlorophyll" de l'océan mondial. Bien que les concentrations en nutriments (nitrates, phosphates, silicates) soient élevées, le développement du phytoplancton est paradoxalement limité principalement par les faibles concentrations en fer (Martin et al. 1990). Les archipels de Crozet et des Kerguelen, situés dans le secteur Indien de l'océan Austral, constituent deux obstacles topographiques importants à l'écoulement vers l'Est du Courant Circumpolaire Antarctique. L'interaction du courant avec les sédiments des plateaux peu profonds alimente en fer les eaux en aval de ces îles, générant ainsi d'importants blooms phytoplanctoniques (Blain et al. 2007, Pollard et al. 2007). Ceux-ci constituent des laboratoires à ciel ouvert uniques pour étudier la réponse des écosystèmes et l'impact de la fertilisation naturelle en fer sur les cycles biogéochimiques. Cette thèse s'inscrit sans le cadre du projet KEOPS-2. Les isotopes du radium (223Ra, 224Ra, 226Ra et 228Ra), qui constituent de puissants outils pour étudier la circulation océanique et le mélange, sont les principaux outils utilisés ici. Les quatre isotopes du radium ont des périodes radioactives allant de quelques jours à plus d'un millier d'années et sont produits par la décroissance radioactive du thorium dans le sédiment. Ils sont apportés à l'océan par des processus de diffusion et d'advection où ils se comportent comme des traceurs conservatifs de telle manière que la masse d'eau garde la signature de son contact avec les sédiments modulée par la période radioactive des isotopes du radium. Par conséquent, les isotopes du radium ont été utilisés pour (i) tracer l'origine et la dispersion des éléments chimiques - y compris le fer - libérés par les sédiments et (ii) estimer les échelles de temps du transit des eaux de surface depuis les plateaux continentaux vers le large. Les informations acquises avec les isotopes du radium ont été comparées aux informations issues d'outils physiques (flotteurs dérivant de surface et modèle Lagrangien dérivé de l'altimétrie). Premièrement, la comparaison de ces trois méthodes indépendantes - géochimiques et physiques - dans la région de Crozet a permis de valider leur utilisation. Deuxièmement, nous avons montré que le panache de phytoplancton associé aux îles Crozet est alimenté par deux sources différentes d'eau qui ont interagi avec soit le plateau ouest soit le plateau est. Troisièmement, cette approche couplée physique-géochimique a également été utilisée dans la région des Kerguelen et a aidé à contraindre l'origine de la fertilisation en fer dans cette zone. L'observation d'activités significatives de 223Ra et 224Ra dans les eaux de surface à l'est des îles Kerguelen indique que ces eaux ont récemment interagi avec des sédiments peu profonds. La variabilité spatiale de ces activités en surface au sud du Front Polaire (PF) suggère que le passage des eaux et des éléments chimiques à travers ou via le PF peut varier à la fois spatialement et temporellement. Cette voie constituerait donc un mécanisme de fertilisation (en fer et autres micronutriments) du bloom phytoplanctonique qui se développe au large des îles Kerguelen. Ces résultats indiquent que le PF n'agirait donc pas comme une barrière physique aussi forte qu'on le pensait, pour les masses d'eau et les éléments chimiques. Ces conclusions pourraient également s'appliquer à d'autres systèmes de fronts de l'océan mondial. Finalement, j'ai compilé les distributions de 226Ra et de baryum dissous (Ba) au large des îles Crozet et Kerguelen dans le but de fournir des contraintes supplémentaires sur la circulation locale. En particulier, des variations temporelles des rapports 226Ra/Ba dans la phase dissoute ont été observées. Parmi les hypothèses potentielles, on peut évoquer (i) des changements de la circulation ou (ii) un impact des processus biologiques sur les concentrations de Ra et Ba de la phase dissoute

Unexpected source of Fukushima-derived radiocesium to the coastal ocean of Japan

Author Posting. © The Author(s), 2017. This is the author's version of the work. It is posted here under a nonexclusive, irrevocable, paid-up, worldwide license granted to WHOI. It is made available for personal use, not for redistribution. The definitive version was published in Proceedings of the National Academy of Sciences of the United States of America 114 (2017): 11092-11096, doi:10.1073/pnas.1708659114.There are 440 operational nuclear reactors in the world, with approximately half situated along the coastline. This includes the Fukushima Dai-ichi Nuclear Power Plant (FDNPP), which experienced multiple reactor meltdowns in March 2011 followed by the release of radioactivity to the marine environment. While surface inputs to the ocean via atmospheric deposition and rivers are usually well monitored after a nuclear accident, no study has focused on subterranean pathways. During our study period, we found the highest cesium-137 (137Cs) levels (up to 23,000 Bq m-3) outside of the FDNPP site not in the ocean, rivers or potable groundwater, but in groundwater beneath sand beaches over tens of kilometers away from the FDNPP. Here, we present evidence of a previously unknown, ongoing source of Fukushima-derived 137Cs to the coastal ocean. We postulate that these beach sands were contaminated in 2011 through wave and tide driven exchange and sorption of highly radioactive Cs from seawater. Subsequent desorption of 137Cs and fluid exchange from the beach sands was quantified using naturally occurring radium isotopes. This estimated ocean 137Cs source (0.6 TBq y-1) is of similar magnitude as the ongoing releases of 137Cs from the FDNPP site for 2013-2016, as well as the input of Fukushima-derived dissolved 137Cs via rivers. Though this ongoing source is not at present a public health issue for Japan, the release of Cs of this type and scale needs to be considered in NPP monitoring and scenarios involving future accidents.V. Sanial was supported by a Postdoctoral Scholarship from the Center for Marine and Environmental Radioactivity (CMER). Funding for this work was provided by the Gordon and Betty Moore Foundation, the Deerbrook Charitable Trust, as well as the EC 7th Framework project COMET-FRAME (COordination and iMplementation of a pan-Europe instrumenT for radioecology) and KAKENHI (24110008)

Study Of The Phytoplankton Plume Dynamics Off The Crozet Islands (Southern Ocean): A Geochemical-Physical Coupled Approach

2169-9291The Crozet Archipelago, in the Indian sector of the Southern Ocean, constitutes one of the few physical barriers to the Antarctic Circumpolar Current. Interaction of the currents with the sediments deposited on the margins of these islands contributes to the supply of chemical elements--including iron and other micro-nutrients--to offshore high-nutrient, low-chlorophyll (HNLC) waters. This natural fertilization sustains a phytoplankton bloom that was studied in the framework of the KEOPS-2 project. In this work, we investigated the time scales of the surface water transport between the Crozet Island shelves and the offshore waters, a transport that contributes iron to the phytoplankton bloom. We report shelf-water contact ages determined using geochemical tracers (radium isotopes) and physical data based on in situ drifter data and outputs of a model based on altimetric Lagrangian surface currents. The apparent ages of surface waters determined using the three independent methods are in relatively good agreement with each other. Our results provide constraints on the time scales of the transport between the shelf and offshore waters near the Crozet Islands and highlight the key role played by horizontal transport in natural iron fertilization and in defining the extension of the chlorophyll plume in this HNLC region of the Southern Ocean

Functioning of Coastal River-Dominated Ecosystems and Implications for Oil Spill Response: From Observations to Mechanisms and Models

Coastal river-dominated oceans are physically complex, biologically productive, and intimately connected to human socioeconomic activity. The Deepwater Horizon blowout and subsequent advection of oil into coastal waters of the northern Gulf of Mexico (nGOM) highlighted the complex linkages among oceanographic processes within this river-dominated system and knowledge gaps about it that resulted in imprecise information on both oil transport and ecosystem consequences. The interdisciplinary research program implemented through the CONsortium for oil exposure pathways in COastal River-Dominated Ecosystems (CONCORDE) is designed to identify and quantitatively assess key physical, biological, and geochemical processes acting in the nGOM, in order to provide the foundation for implementation of a synthesis model (coupled circulation and biogeochemistry) of the nGOM shelf system that can ultimately aid in prediction of oil spill transport and impacts. CONCORDE field and modeling efforts in 2015–2016 focused on defining the influence of freshwater input from river plumes in the nGOM. In situ observations, combined with field-deployed and simulated drifters, show considerable variability in the spatial extent of freshwater influence that is related to wind direction and strength. Increased primary production and particle abundance (a proxy for secondary production) was observed during the spring when nGOM shelf waters were becoming stratified. Zooplankton and marine snow displayed intense vertical and horizontal patchiness during all seasons, often aggregating near the halocline. Simulations of a neutrally buoyant tracer released offshore of the Mississippi Bight showed surface advection of low tracer concentrations onto the inner shelf under high river discharge, high stratification, and variable wind conditions compared to almost no advection onto the inner shelf under low discharge, negligible stratification, and generally northeasterly winds. The interconnectedness of environmental variables and biological activity indicate that multiple factors can affect the transport of oil and the resulting ecological impacts. The process-oriented understanding provided by CONCORDE is necessary to predict ecosystem-level impacts of oil spills, and these results are applicable to other river-dominated coastal systems worldwide that often support oil extraction activities

Tracking the chemical elements derived from sediments to the open ocean using Ra isotopes : the case study of the Crozet and Kerguelen Islands (Southern Ocean)

L'Océan Austral est connu pour être la plus vaste zone "High-Nutrient, Low-Chlorophyll" de l'océan mondial. Bien que les concentrations en nutriments (nitrates, phosphates, silicates) soient élevées, le développement du phytoplancton est paradoxalement limité principalement par les faibles concentrations en fer (Martin et al. 1990). Les archipels de Crozet et des Kerguelen, situés dans le secteur Indien de l'océan Austral, constituent deux obstacles topographiques importants à l'écoulement vers l'Est du Courant Circumpolaire Antarctique. L'interaction du courant avec les sédiments des plateaux peu profonds alimente en fer les eaux en aval de ces îles, générant ainsi d'importants blooms phytoplanctoniques (Blain et al. 2007, Pollard et al. 2007). Ceux-ci constituent des laboratoires à ciel ouvert uniques pour étudier la réponse des écosystèmes et l'impact de la fertilisation naturelle en fer sur les cycles biogéochimiques. Cette thèse s'inscrit sans le cadre du projet KEOPS-2. Les isotopes du radium (223Ra, 224Ra, 226Ra et 228Ra), qui constituent de puissants outils pour étudier la circulation océanique et le mélange, sont les principaux outils utilisés ici. Les quatre isotopes du radium ont des périodes radioactives allant de quelques jours à plus d'un millier d'années et sont produits par la décroissance radioactive du thorium dans le sédiment. Ils sont apportés à l'océan par des processus de diffusion et d'advection où ils se comportent comme des traceurs conservatifs de telle manière que la masse d'eau garde la signature de son contact avec les sédiments modulée par la période radioactive des isotopes du radium. Par conséquent, les isotopes du radium ont été utilisés pour (i) tracer l'origine et la dispersion des éléments chimiques - y compris le fer - libérés par les sédiments et (ii) estimer les échelles de temps du transit des eaux de surface depuis les plateaux continentaux vers le large. Les informations acquises avec les isotopes du radium ont été comparées aux informations issues d'outils physiques (flotteurs dérivant de surface et modèle Lagrangien dérivé de l'altimétrie). Premièrement, la comparaison de ces trois méthodes indépendantes - géochimiques et physiques - dans la région de Crozet a permis de valider leur utilisation. Deuxièmement, nous avons montré que le panache de phytoplancton associé aux îles Crozet est alimenté par deux sources différentes d'eau qui ont interagi avec soit le plateau ouest soit le plateau est. Troisièmement, cette approche couplée physique-géochimique a également été utilisée dans la région des Kerguelen et a aidé à contraindre l'origine de la fertilisation en fer dans cette zone. L'observation d'activités significatives de 223Ra et 224Ra dans les eaux de surface à l'est des îles Kerguelen indique que ces eaux ont récemment interagi avec des sédiments peu profonds. La variabilité spatiale de ces activités en surface au sud du Front Polaire (PF) suggère que le passage des eaux et des éléments chimiques à travers ou via le PF peut varier à la fois spatialement et temporellement. Cette voie constituerait donc un mécanisme de fertilisation (en fer et autres micronutriments) du bloom phytoplanctonique qui se développe au large des îles Kerguelen. Ces résultats indiquent que le PF n'agirait donc pas comme une barrière physique aussi forte qu'on le pensait, pour les masses d'eau et les éléments chimiques. Ces conclusions pourraient également s'appliquer à d'autres systèmes de fronts de l'océan mondial. Finalement, j'ai compilé les distributions de 226Ra et de baryum dissous (Ba) au large des îles Crozet et Kerguelen dans le but de fournir des contraintes supplémentaires sur la circulation locale. En particulier, des variations temporelles des rapports 226Ra/Ba dans la phase dissoute ont été observées. Parmi les hypothèses potentielles, on peut évoquer (i) des changements de la circulation ou (ii) un impact des processus biologiques sur les concentrations de Ra et Ba de la phase dissoute.The Southern Ocean is known to be the largest High-Nutrient, Low-Chlorophyll region of the global ocean. While nutrient concentrations (nitrate, phosphate, silicate) are high, the phytoplankton development is paradoxically limited mostly because of the low dissolved iron concentrations of the Southern Ocean waters (Martin_iron_1990). The Crozet and Kerguelen Archipelagos, located in the Indian sector of the Southern Ocean, constitute two major topographic obstacles to the eastward-flowing Antarctic Circumpolar Current. The interaction of the current with the sediments of the shallow Crozet and Kerguelen plateaus contributes to the supply of iron downstream of these islands, thus leading to large phytoplankton blooms in these regions (Blain et al. 2007, Pollard et al. 2007). These phytoplankton blooms constitute unique open-air laboratories to study the response of the ecosystems and the impact on biogeochemical cycles to natural iron fertilization. This PhD thesis was done in the framework of the KEOPS-2 project. Radium isotopes (223Ra, 224Ra, 226Ra and 228Ra), that are powerful tools to study the ocean circulation and mixing, are the main tools used here. The four natural occurring isotopes display half-lives ranging from a few days to thousands of years and are produced by the decay of particle-bound thorium isotopes in sediments. They are delivered to the open ocean by diffusion and advection processes where they behave as conservative tracers in such a way that the water body keeps the signature of its contact with the sediments modulated by the half-lives of the radium isotopes. Therefore, we used Ra isotopes to (i) investigate the origin and the dispersion of the sediment-derived inputs - including iron - and (ii) to estimate the timescales of the transfer of surface waters between the shelf and offshore waters. We compared the Ra dataset with data acquired using physical tools (surface drifters and Lagrangian model derived from altimetry). Firstly, the use of three independent methods - including geochemical and physical methods - in the Crozet region allowed us to validate each method. Secondly, we show that the Crozet Island phytoplankton plume is fed by two different flows of water that interacted with either the western plateau or the eastern plateau. Thirdly, this physical-geochemical coupled approach was also used in the Kerguelen region and helped us to constrain the origin of the iron fertilization in that area. The observation of 223Ra and 224Ra in surface waters east of the Kerguelen Islands, south of the polar front (PF), indicates that these waters have recently interacted with shallow sediments. The spatial variability observed in the 223Ra and 224Ra distribution in surface waters south of the PF suggests that the input of waters and associated chemical elements across or via the PF - potentially driven by wind stress or eddies - act as sporadic pulses that may highly vary in both space and time. This pathway may thus constitute a mechanism that contributes to fertilizing the phytoplankton bloom with iron and other micronutrients east of the Kerguelen Islands. This finding also suggest that the PF may not act as a strong barrier for surface waters and associated chemical elements, a finding that may also apply for other frontal systems of the world's ocean. Finally, we investigated the 226Ra and barium (Ba) distributions offshore from the Crozet and Kerguelen islands, with the aim to provide additional constraints on the circulation patterns in this area. In particular, we observed temporal changes in the dissolved 226Ra/Ba ratios. Among potential hypothesis, one can invoke (i) changes in the circulation patterns or (ii) the impact of biological processes on the dissolved Ra and Ba concentrations

Tracking the chemical elements derived from sediments to the open ocean using Ra isotopes : the case study of the Crozet and Kerguelen Islands (Southern Ocean)

L'Océan Austral est connu pour être la plus vaste zone "High-Nutrient, Low-Chlorophyll" de l'océan mondial. Bien que les concentrations en nutriments (nitrates, phosphates, silicates) soient élevées, le développement du phytoplancton est paradoxalement limité principalement par les faibles concentrations en fer (Martin et al. 1990). Les archipels de Crozet et des Kerguelen, situés dans le secteur Indien de l'océan Austral, constituent deux obstacles topographiques importants à l'écoulement vers l'Est du Courant Circumpolaire Antarctique. L'interaction du courant avec les sédiments des plateaux peu profonds alimente en fer les eaux en aval de ces îles, générant ainsi d'importants blooms phytoplanctoniques (Blain et al. 2007, Pollard et al. 2007). Ceux-ci constituent des laboratoires à ciel ouvert uniques pour étudier la réponse des écosystèmes et l'impact de la fertilisation naturelle en fer sur les cycles biogéochimiques. Cette thèse s'inscrit sans le cadre du projet KEOPS-2. Les isotopes du radium (223Ra, 224Ra, 226Ra et 228Ra), qui constituent de puissants outils pour étudier la circulation océanique et le mélange, sont les principaux outils utilisés ici. Les quatre isotopes du radium ont des périodes radioactives allant de quelques jours à plus d'un millier d'années et sont produits par la décroissance radioactive du thorium dans le sédiment. Ils sont apportés à l'océan par des processus de diffusion et d'advection où ils se comportent comme des traceurs conservatifs de telle manière que la masse d'eau garde la signature de son contact avec les sédiments modulée par la période radioactive des isotopes du radium. Par conséquent, les isotopes du radium ont été utilisés pour (i) tracer l'origine et la dispersion des éléments chimiques - y compris le fer - libérés par les sédiments et (ii) estimer les échelles de temps du transit des eaux de surface depuis les plateaux continentaux vers le large. Les informations acquises avec les isotopes du radium ont été comparées aux informations issues d'outils physiques (flotteurs dérivant de surface et modèle Lagrangien dérivé de l'altimétrie). Premièrement, la comparaison de ces trois méthodes indépendantes - géochimiques et physiques - dans la région de Crozet a permis de valider leur utilisation. Deuxièmement, nous avons montré que le panache de phytoplancton associé aux îles Crozet est alimenté par deux sources différentes d'eau qui ont interagi avec soit le plateau ouest soit le plateau est. Troisièmement, cette approche couplée physique-géochimique a également été utilisée dans la région des Kerguelen et a aidé à contraindre l'origine de la fertilisation en fer dans cette zone. L'observation d'activités significatives de 223Ra et 224Ra dans les eaux de surface à l'est des îles Kerguelen indique que ces eaux ont récemment interagi avec des sédiments peu profonds. La variabilité spatiale de ces activités en surface au sud du Front Polaire (PF) suggère que le passage des eaux et des éléments chimiques à travers ou via le PF peut varier à la fois spatialement et temporellement. Cette voie constituerait donc un mécanisme de fertilisation (en fer et autres micronutriments) du bloom phytoplanctonique qui se développe au large des îles Kerguelen. Ces résultats indiquent que le PF n'agirait donc pas comme une barrière physique aussi forte qu'on le pensait, pour les masses d'eau et les éléments chimiques. Ces conclusions pourraient également s'appliquer à d'autres systèmes de fronts de l'océan mondial. Finalement, j'ai compilé les distributions de 226Ra et de baryum dissous (Ba) au large des îles Crozet et Kerguelen dans le but de fournir des contraintes supplémentaires sur la circulation locale. En particulier, des variations temporelles des rapports 226Ra/Ba dans la phase dissoute ont été observées. Parmi les hypothèses potentielles, on peut évoquer (i) des changements de la circulation ou (ii) un impact des processus biologiques sur les concentrations de Ra et Ba de la phase dissoute.The Southern Ocean is known to be the largest High-Nutrient, Low-Chlorophyll region of the global ocean. While nutrient concentrations (nitrate, phosphate, silicate) are high, the phytoplankton development is paradoxically limited mostly because of the low dissolved iron concentrations of the Southern Ocean waters (Martin_iron_1990). The Crozet and Kerguelen Archipelagos, located in the Indian sector of the Southern Ocean, constitute two major topographic obstacles to the eastward-flowing Antarctic Circumpolar Current. The interaction of the current with the sediments of the shallow Crozet and Kerguelen plateaus contributes to the supply of iron downstream of these islands, thus leading to large phytoplankton blooms in these regions (Blain et al. 2007, Pollard et al. 2007). These phytoplankton blooms constitute unique open-air laboratories to study the response of the ecosystems and the impact on biogeochemical cycles to natural iron fertilization. This PhD thesis was done in the framework of the KEOPS-2 project. Radium isotopes (223Ra, 224Ra, 226Ra and 228Ra), that are powerful tools to study the ocean circulation and mixing, are the main tools used here. The four natural occurring isotopes display half-lives ranging from a few days to thousands of years and are produced by the decay of particle-bound thorium isotopes in sediments. They are delivered to the open ocean by diffusion and advection processes where they behave as conservative tracers in such a way that the water body keeps the signature of its contact with the sediments modulated by the half-lives of the radium isotopes. Therefore, we used Ra isotopes to (i) investigate the origin and the dispersion of the sediment-derived inputs - including iron - and (ii) to estimate the timescales of the transfer of surface waters between the shelf and offshore waters. We compared the Ra dataset with data acquired using physical tools (surface drifters and Lagrangian model derived from altimetry). Firstly, the use of three independent methods - including geochemical and physical methods - in the Crozet region allowed us to validate each method. Secondly, we show that the Crozet Island phytoplankton plume is fed by two different flows of water that interacted with either the western plateau or the eastern plateau. Thirdly, this physical-geochemical coupled approach was also used in the Kerguelen region and helped us to constrain the origin of the iron fertilization in that area. The observation of 223Ra and 224Ra in surface waters east of the Kerguelen Islands, south of the polar front (PF), indicates that these waters have recently interacted with shallow sediments. The spatial variability observed in the 223Ra and 224Ra distribution in surface waters south of the PF suggests that the input of waters and associated chemical elements across or via the PF - potentially driven by wind stress or eddies - act as sporadic pulses that may highly vary in both space and time. This pathway may thus constitute a mechanism that contributes to fertilizing the phytoplankton bloom with iron and other micronutrients east of the Kerguelen Islands. This finding also suggest that the PF may not act as a strong barrier for surface waters and associated chemical elements, a finding that may also apply for other frontal systems of the world's ocean. Finally, we investigated the 226Ra and barium (Ba) distributions offshore from the Crozet and Kerguelen islands, with the aim to provide additional constraints on the circulation patterns in this area. In particular, we observed temporal changes in the dissolved 226Ra/Ba ratios. Among potential hypothesis, one can invoke (i) changes in the circulation patterns or (ii) the impact of biological processes on the dissolved Ra and Ba concentrations

Does a Bottom-Up Mechanism Promote Hypoxia In the Mississippi Bight?

The Mississippi Bight, east of the Mississippi River, is a complex coastal ecosystem that, like the better-known Louisiana Shelf to the west, experiences seasonal bottom water hypoxia. However, input of allochthonous nutrients from the Mississippi River to the Mississippi Bight appears to be limited, begging the question of what drives seasonal hypoxia in this system. Prior research has suggested submarine groundwater discharge (SGD) could be an overlooked component of the Mississippi Bight biogeochemical system. We thus examined the hypothesis that SGD provides a “bottom up” driver for seasonal hypoxia in this area. We used a multi-tracer approach based on known SGD indicators (dissolved Ra, Ba, Si, methane) to: i) demonstrate the presence of SGD as a constituent contributor to Bight bottom waters, ii) constrain the SGD flux of macronutrients, and, iii) investigate the hypoxia-SGD linkage. We found excess SGD tracers in saline bottom waters relative to surface waters, suggesting a bottom source. Examination of other sources for the constituent enrichments besides SGD (e.g., rivers, produced waters from oil wells) appear inadequate to close the bottom water chemical mass balances. Additionally, inverse correlations between DO and SGD indicators in bottom waters support a common mechanism supplying dissolved Ra, Ba, and Si, and decreasing DO concentrations in these waters. Two different approaches to modeling the bottom water Ra distribution both suggest a seepage rate of ~0.055 m3 m−2 d−1, in line with previous estimates in similar systems. Our more complex model, involving four mass balances, suggests that as much as 10–20% of the bottom water in the Bight circulates through the underlying permeable sediments on a time scale of ~10 days. This circulated water emerges as SGD with completely altered chemistry. More specifically, SGD appears in some cases to be the dominant contributor of nutrients to Bight bottom waters. Additionally, the potential oxygen demand of reduced species within SGD likely contributes significantly to the development of seasonal hypoxia in Bight bottom waters. Further work is needed to better resolve sources of nutrients and additional reduced species within the Mississippi Bight SGD as well as the variability and pathways of this supply. Nonetheless, the bottom-up influence of SGD on the Mississippi Bight appears to be a significant and overlooked aspect of this system. We suggest that such a bottom-up influence may be a generally important feature of coastal ecosystems

Extent of Mississippi River Water in the Mississippi Bight and Louisana Shelf Based On Water Isotopes

The northern Gulf of Mexico is a complex and very productive coastal river-dominated system that receives freshwater from numerous rivers including the Mississippi River. The dynamics of coastal ecosystems in the northern Gulf of Mexico are greatly influenced by the freshwater discharge but also by the high nutrient loads carried by the Mississippi River that lead to the seasonal development of one of the largest coastal hypoxic areas. Constraining the origin and fate of the freshwater inputs in the northern Gulf of Mexico will help increase understanding the physical and biogeochemical processes occurring in this region. Here, we focus on investigating the extent of the Mississippi River plume on both sides of the Mississippi River Delta: to the east in the Mississippi Bight, and to the west over the Louisiana Shelf. We determined the water isotopic signature (δ18O and δD) along with salinity of the different river plumes and performed a river mixing model on the coastal waters. Our findings provide useful information to better understand the functioning of the northern Gulf of Mexico ecosystem. In particular, the development of hypoxia is often attributed to the nutrient load of the Mississippi River, yet the Mississippi River seemed to have a limited influence on the Mississippi Bight. That is, the dominant source of freshwater in the Bight was supplied by local Mississippi/Alabama rivers. Furthermore, the water isotope mixing model showed that the source of freshwater to the Louisiana Shelf was dominated by the Atchafalaya River in summer, and by the Mississippi River during non-summer seasons. This pattern is consistent with the general shelf circulation that reverses in summer, but could not have been shown solely by the use of salinity

Comparison of methods to determine extraction efficiencies of Ra isotopes and 227Ac from large volume seawater samples

International audienceRadium isotopes, other than 226Ra, and 227Ac are typically present at low activities in the open ocean. The analysis of these isotopes thus requires the collection of large volumes of seawater and high sensitivity, low background instruments. To obtain the required large volumes (hundreds to thousands of liters), these radionuclides are typically preconcentrated on cartridge-style filters impregnated with MnO2 (Mn-cartridges) deployed on in-situ pumps. This technique, however, requires the determination of the extraction efficiency of the Mn-cartridges for the radionuclides of interest. For Ra isotopes, we used two methods to estimate the extraction efficiency of these Mn-cartridges at two stations on the South-West Indian Ridge in the Southern Ocean (GEOTRACES GS02). Method (1) compares the 226Ra activities recovered on the Mn-cartridges versus the activities determined in Mn-fibers, through which seawater was passed at a flow rate < 1 L min−1 to quantitatively sorb Ra (Mn-fiber method) while method (2) combines the 226Ra activities determined from two Mn-cartridges placed in series on in-situ pumps (A-B method). The second method is also applied to determine the 227Ac extraction efficiency. We find a relatively wide-range of Ra and 227Ac extraction efficiencies across the dataset (from 44.8% to 99.6% for Ra, and from 23.7% to 77.5% for 227Ac). Overall, the yield of 227Ac extraction is lower than that of Ra (mean value of 49.3 ± 19.0% for 227Ac, n = 10, mean value of 79.2 ± 10.3% for Ra, n = 13, using the Mn-fiber method; and a mean value of 63.9 ± 12.5%, n = 11 using the A-B method). Our dataset suggests that the Ra extraction efficiencies using either the A-B method or the Mn-fiber method are in relatively good agreement. Consequently, the 223Raex, 224Raex and 228Ra activities determined from the Mn-cartridges by applying the two Ra extraction yields are similar. We also show that the 227Ac extraction efficiency can be estimated from the Ra extraction efficiency allowing the use of a single Mn-cartridge. Finally, we recommend to determine the Ra and 227Ac extraction efficiencies in each individual Mn-cartridge, rather than applying a single extraction efficiency to all the Mn-cartridges, since a significant variability in the extraction efficiencies was observed between the different Mn-cartridges

Reproduire la circulation thermohaline à échelle réduite et comprendre son rôle dans le climat

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