Le cycle biogéochimique du silicium dans l’Océan Austral par les approches isotopiques

Southern Ocean biogeochemistry plays a crucial role on global marine primary production by impacting the nutrient availability even in low latitude surface water. Variations in the silicon (Si) cycle are large and its coupling to other nutrient biogeochemical cycles is still not well understood in this ocean. Results of two different isotopic approaches suggested that a strong silicon pump was quickly initiated in spring by a switch from regenerated to new biogenic silica production. The seasonal evolution of natural Si isotopic composition (δ30Si) was mainly driven by the balance between the “dissolution to production” and “Si-supply to Si-uptake” ratios that decoupled the isotopic dynamics of particulate and dissolved Si-pools (DSi and BSi, repectively). We also used δ30Si measurements to track seasonal flows of BSi to the deep sea with. These results confirmed that the δ30Si is well preserved during particles settling. The seasonal evolution of δ30Si signal allows for the first time to quantify important features about the processes controlling the diatoms’ productivity and its fate, such as mixing events that bring nutrient in the ML or the seasonal evolution of particles sinking velocities. These insights confirm that the δ30Si of DSi and BSi, combined to isotopic technics to measure Si fluxes in the ML, are promising tools to improve our understanding on the Si-biogeochemical cycle and provide new constraints for application to biogeochemical models.La biogéochimie de l’Océan austral joue un rôle crucial dans la régulation de la production primaire marine globale en contrôlant la disponibilité des nutriments dans les eaux de surface des basses latitudes. Les variations du cycle du silicium (Si) sont nombreuses et son couplage avec les autres éléments n’est pas encore bien compris dans cet océan. Les résultats issus de deux approches isotopiques différentes suggèrent qu’une pompe de Si active est rapidement initiée au printemps par la transition d’un mode de production de silice biogénique régénéré à une production dite « nouvelle ». L’évolution saisonnière de la composition isotopique naturelle du Si (δ30Si) est principalement contrôlée par l’équilibre entre les rapports « dissolution/production » et « Si-supply/Si-uptake » qui découplent la dynamique isotopique des réservoirs de Si dissout et particulaire (respectivement DSi et BSi). Nous avons également utilisé les mesures de δ30Si pour retracer les flux saisonniers de BSi vers l’océan profond. Ces résultats confirment que le δ30Si n’est pas altéré durant la sédimentation des particules. L’évolution saisonnière du δ30Si a permis de quantifier pour la première fois certains processus contrôlant la production des diatomées et leur devenir, tels que les évènements de mélange alimentant la ML en nutriments, ou l’évolution saisonnière de la vitesse de sédimentation des particules. Ces résultats confirment que le δ30Si du DSi et de la BSi, combinés aux techniques isotopiques de mesure des flux dans la ML, sont des outils prometteurs dans l’amélioration de nos connaissances du cycle du Si et apportent des informations nouvelles à intégrer aux modèles biogéochimiques

The biogeochemical silicon cycle in the Southern Ocean tracked by isotopic approaches

La biogéochimie de l’Océan austral joue un rôle crucial dans la régulation de la production primaire marine globale en contrôlant la disponibilité des nutriments dans les eaux de surface des basses latitudes. Les variations du cycle du silicium (Si) sont nombreuses et son couplage avec les autres éléments n’est pas encore bien compris dans cet océan. Les résultats issus de deux approches isotopiques différentes suggèrent qu’une pompe de Si active est rapidement initiée au printemps par la transition d’un mode de production de silice biogénique régénéré à une production dite « nouvelle ». L’évolution saisonnière de la composition isotopique naturelle du Si (δ30Si) est principalement contrôlée par l’équilibre entre les rapports « dissolution/production » et « Si-supply/Si-uptake » qui découplent la dynamique isotopique des réservoirs de Si dissout et particulaire (respectivement DSi et BSi). Nous avons également utilisé les mesures de δ30Si pour retracer les flux saisonniers de BSi vers l’océan profond. Ces résultats confirment que le δ30Si n’est pas altéré durant la sédimentation des particules. L’évolution saisonnière du δ30Si a permis de quantifier pour la première fois certains processus contrôlant la production des diatomées et leur devenir, tels que les évènements de mélange alimentant la ML en nutriments, ou l’évolution saisonnière de la vitesse de sédimentation des particules. Ces résultats confirment que le δ30Si du DSi et de la BSi, combinés aux techniques isotopiques de mesure des flux dans la ML, sont des outils prometteurs dans l’amélioration de nos connaissances du cycle du Si et apportent des informations nouvelles à intégrer aux modèles biogéochimiques.Southern Ocean biogeochemistry plays a crucial role on global marine primary production by impacting the nutrient availability even in low latitude surface water. Variations in the silicon (Si) cycle are large and its coupling to other nutrient biogeochemical cycles is still not well understood in this ocean. Results of two different isotopic approaches suggested that a strong silicon pump was quickly initiated in spring by a switch from regenerated to new biogenic silica production. The seasonal evolution of natural Si isotopic composition (δ30Si) was mainly driven by the balance between the “dissolution to production” and “Si-supply to Si-uptake” ratios that decoupled the isotopic dynamics of particulate and dissolved Si-pools (DSi and BSi, repectively). We also used δ30Si measurements to track seasonal flows of BSi to the deep sea with. These results confirmed that the δ30Si is well preserved during particles settling. The seasonal evolution of δ30Si signal allows for the first time to quantify important features about the processes controlling the diatoms’ productivity and its fate, such as mixing events that bring nutrient in the ML or the seasonal evolution of particles sinking velocities. These insights confirm that the δ30Si of DSi and BSi, combined to isotopic technics to measure Si fluxes in the ML, are promising tools to improve our understanding on the Si-biogeochemical cycle and provide new constraints for application to biogeochemical models

The Antarctic Silicon Trap

The Southern Ocean, the ocean encircling Antarctica, has been described by explorers as cold, empty, and dangerous. Despite this, it is a paradise for tiny algae called diatoms that are crucial players in the regulation of our climate. Why are these tiny organisms so happy in this cold and far away ocean? Diatoms have a solid shell made of a glass-like material called silica, so they need to find silicon in surface waters to build it. The Southern Ocean is the perfect place for diatoms because it is full of silicon compared to the other oceans. This is due to a special phenomenon called the silicon pump, which makes the Southern Ocean a giant trap for silicon. In this article, we point out the central role of the Southern Ocean in the regulation of Earth’s climate and how it controls the distribution of silicon and the wellbeing of diatoms in Antarctic waters

The forensic of diatoms: exploring different modelling approaches to reconstruct seasonal Si cycling from deep sediment trap data

info:eu-repo/semantics/nonPublishe

Diatoms Like It Light! How Diatom Eating Habits Help Us Understand the Past Ocean

Have you ever wondered if today’s oceans were different millions of years ago? Well, a group of small algae called diatoms can help us to find this out. Diatoms build a strong glass skeleton, like a shell, which can last for thousands and even millions of years after their deaths. To build their glass skeletons, diatoms take up silicon from the seawater, similar to us eating food to build our bodies. Diatoms preferentially use one type of silicon in their menu, leaving behind the type they do not like. Researchers can track this eating habit by measuring the proportion of the two types of silicon stored within diatoms. Using this silica-print like a fingerprint, scientists can investigate what the surface ocean was like, how much diatoms were eating silicon, and how these organisms have affected Earth’s past climate

New Insights Into Processes Controlling the δ³⁰Si of Sinking Diatoms: A Seasonally Resolved Box Model Approach

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Unveiling the Si cycle using isotopes in an iron-fertilized zone of the Southern Ocean: from mixed-layer supply to export

International audienceA massive diatom bloom forms annually in the surface waters of the naturally iron-fertilized Kerguelen Plateau (Southern Ocean). In this study, silicon isotopic signatures (δ30Si) of silicic acid (DSi) and suspended bio-genic silica (BSi) were investigated through the whole water column with unprecedented spatial resolution, during the KEOPS-2 experiment (spring 2011). We used δ30Si measurements to track the sources of silicon that fuelled the bloom, and investigated the seasonal evolution of the Si bio-geochemical cycle in the iron-fertilized area. We compared the results from stations with various degrees of iron enrichment and bloom conditions to an HNLC reference station. Dissolved and particulate δ30 Si signatures were highly variable in the upper 500 m, reflecting the effect of intense silicon utilization in spring, while they were quite homogeneous in deeper waters. The Si isotopic and mass balance identified a unique Winter Water (WW) Si source for the iron-fertilized area that originated from southeast of the Kerguelen Plateau and spread northward. When the WW reached a retroflection of the Polar Front (PF), the δ30Si composition of the silicic acid pool became progressively heavier. This would result from sequential diapycnal and isopycnal mixings between the initial WW and ML water masses, highlighting the strong circulation of surface waters that defined this zone. When comparing the results from the two KEOPS expeditions , the relationship between DSi depletion, BSi production , and their isotopic composition appears decoupled in the iron-fertilized area. This seasonal decoupling could help to explain the low apparent fractionation factor observed in the ML at the end of summer. Taking into account these considerations , we refined the seasonal net BSi production in the ML of the iron-fertilized area to 3.0 ± 0.3 mol Si m−2 yr−1 , which was exclusively sustained by surface water phytoplankton populations. These insights confirm that the isotopic composition of dissolved and particulate silicon is a promising tool to improve our understanding of the Si biogeochemical cycle since the isotopic and mass balance allows resolution of processes in the Si cycle (i.e. uptake, dissolution , mixing)

Biogeochemical composition of settling particles in the Southern Ocean and processes controlling seasonal variations of production and export

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Seasonal variation, origin and fate of settling diatoms in the Southern Ocean tracked by silicon isotope record in deep sediment traps

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New Constraints on the Physical and Biological Controls on the Silicon Isotopic Composition of the Arctic Ocean

The silicon isotope composition of silicic acid, delta Si-30(OH)(4), in the deep Arctic Ocean is anomalously heavy compared to all other deep ocean basins. To further evaluate the mechanisms leading to this condition, delta Si-30(OH)(4) was examined on US GEOTRACES section GN01 from the Bering Strait to the North Pole. Isotope values in the polar mixed layer showed a strong influence of the transpolar drift. Drift waters contained relatively high [Si(OH)(4)] with heavy delta Si-30(OH)(4) consistent with the high silicate of riverine source waters and strong biological Si(OH)(4) consumption on the Eurasian shelves. The maximum in silicic acid concentration, [Si(OH)(4)], within the double halocline of the Canada Basin formed a local minimum in delta Si-30(OH)(4) that extended across the Canada Basin, reflecting the high-[Si(OH)(4)] Pacific source waters and benthic inputs of Si(OH)(4) in the Chukchi Sea. delta Si-30(OH)(4) became lighter with the increase in [Si(OH)(4)] in intermediate and deep waters; however, both Canada Basin deep water and Eurasian Basin deep water were heavier than deep waters from other ocean basins. A preliminary isotope budget incorporating all available Arctic delta Si-30(OH)(4) data confirms the importance of isotopically heavy inflows in creating the anomalous deep Arctic Si isotope signature, but also reveals a surprising similarity in the isotopic composition of the major inflows compared to outflows across the main gateways connecting the Arctic with the Pacific and the Atlantic. This similarity implies a major role of biological productivity and opal burial in removing light isotopes entering the Arctic Ocean from rivers.ISSN:2296-774