Solid phase transformations through Loire Estuary in relations to chemical and biological environment

Before reaching the ocean, riverine particles from the Loire watershed undergo important chemical transformations in the estuary. This study aims to identify and characterize main processes involving iron (and to a lesser extent Al, Mn, PO4 3- and rare earths elements). Suspended particulate matter data show that 15% of their iron is lost through the estuary. Reductive iron dissolution during deposition periods is likely to explain most of this loss. However, estuarine sediments are characterized by high spatial heterogeneity due to macrofaunal activities. To overcome this difficulty, new methodologies of high resolution and two-dimensional analyses and calculation of pore water, sediment and benthic meiofauna were developed. These developments enable to estimate the impact of macrofaunal activity to 80% of dissolved iron export from sediment towards water column, and a total flux of 0.5 mmol m-2 d-1. Such kinetics requires a 10-year of residence time of particles within the sediment in order to match to observed iron loss on suspended particles. This time is much longer than the mean particle residence time in the estuary. Therefore, sediment reactivity over a larger surface that stretches to the external mudflats has to be taken into account to explain transformation of solid iron through the estuary.Avant d’atteindre l’océan, les particules drainées par la Loire subissent d’importantes transformations chimiques dans l’estuaire. L’objectif de cette étude est d’identifier et de caractériser les processus majeurs de transformation impliquant le fer (et dans une moindre mesure Al, Mn, PO4 3- et les terres rares). L’analyse de particules en suspension a montré une diminution de 15% du fer contenu dans les particules traversant l’estuaire. La dissolution réductive des oxydes de fer durant leurs périodes de dépôt apparait comme l’une des voies réactionnelles principales. Cependant les dépôts sont caractérisés par une forte hétérogénéité spatiale due aux activités de la macrofaune benthique. Pour surmonter cette difficulté, de nouvelles méthodes de calcul, d’analyses à haute résolution et en deux dimensions de la composition de l’eau interstitielle, des particules déposées et de la méiofaune benthique ont été développées. Ainsi, nous montrons que l’activité de la macrofaune semble responsable de 80% du flux de fer dissous du sédiment vers la colonne d’eau, estimé à 0,5 mmol.m-2.d-1. Ces vitesses de réaction requièrent un temps de résidence dans le sédiment d’une dizaine d’années pour expliquer la perte visible de fer dans les particules en suspension, temps largement supérieur au temps de résidence moyen dans l’estuaire. Il apparait donc nécessaire de prendre en compte la réactivité des dépôts dans une plus large zone qui s’étendrait jusqu’aux vasières externes pour rendre compte des transformations du fer solide à travers l’estuaire

Multiscale analysis of living benthic foraminiferal heterogeneity: Ecological advances from an intertidal mudflat (Loire estuary, France)

International audienceAn unprecedented sampling effort on the Loire estuary allowed a multi scale approach to identify parameters controlling density variations of benthic foraminifera. Indeed, the distances between the samples analysed for this study vary from 1 cm to hundreds of kilometres. To catch this range of distance variations, a model called Scale Variance Analysis was build describing the participation of each scale to the total observed variance. The SVA model requires, for each scale, the stability of relative variance. A comparison with the Moran's Index and experimental variogram is proposed showing coherent conclusions with the SVA analysis. The analysis shows that in order to maximize information on foraminiferal density variation, sampling campaigns should be designed with stations distant from few meters to 1 km, with a particular focus on the hectometre scale. A range of scale too rarely investigated in the community of benthic foraminifera ecology. Next, based on two intertidal mudflat stations separated of few hundred meters, the present study shows that for Ammonia tepida, the scale dependant preponderant parameters is the Chl a concentration in the top first centimetre. Contrastingly, the indicators of food quality such as the lability index and the oxygen penetration depth do not seem to affect A. tepida densities. This high quantity, low quality diet is interpreted as an opportunistic behaviour that is indirectly confirmed by a kinetic approach. This approach compares the deep infaunal microhabitat density with the shallow infaunal microhabitat density. The identical ratio indicates quick saturation of the available resources

Manganese, iron and phosphorus cycling in an estuarine mudflat sediment, Loire, France

International audienceThe sampling of surface sediment fromtwo sites of amudflat of the Loire Estuary during four contrasting seasonshas led to new information about geochemical cycling under transient diagenesis fuelled by flood deposition.Based on stocks of reactive iron-oxides andmanganese-oxides (ascorbate-extracted) and pore water concentrations,the progressive evolution of flood deposits is described. Three major steps are observed: at first, there is nomanganese, iron and phosphorus release into pore waterwithin the flood-deposited layer. Then, during a periodof approximately 1 month, Mn oxides are consumed while the dissolved Mn concentration increases. Simultaneously,the Fe oxide-rich layer from flood deposition prevents (or at least limits) phosphorus release intoporewater as shown by the increasing P/Fe ratio of the ascorbate extractions. During spring and summer, Fe oxidesare reductively dissolved until complete depletion results. This period also corresponds to the saturation ofFe oxides by phosphorus and probably maximum P release to the water column. The site located closer to theshore showed higher density of benthic faunas leading to more intense bioirrigation. The importance of bioturbationon the year scale for biogeochemical processes is discussed according to both bioirrigation and biomixingprocesses

Two-dimensional ammonium distribution in sediment pore waters using a new colorimetric diffusive equilibration in thin-film technique

This study presents a new gel based technique to describe the pore water ammonium distribution through the sediment-water interface in two dimensions at a millimeter scale. The technique is an adaptation of the classical colorimetric method based on the Berthelot's reaction. After the thin film of the gel probe was equilibrated by diffusion either in standard solutions or in pore waters, a colorimetric reagent gel was set on the gel probe, allowing development of the characteristic green color. A flatbed scanner and subsequent densitometry image analysis allowed to determine the concentration distribution of ammonium. The gel probe was tested in the laboratory for two media, deionized water and seawater, within the range 0–3000 μM in NH4+. Detection limit is about 20 μM and accuracy about ±25 μM. The field validation was realized in a tidal mudflat of the French Atlantic coast by comparison with conventional pore water extraction and colorimetric analysis. The large range of concentrations and its applicability in continental and marine sediments suggest a wide range of applications of the technique for a reasonable cost. Keywords: Biogeochemistry, High-resolution, Porewater, DET, Densitometr

2D Image Quantification of Microbial Iron Chelators (Siderophores) Using Diffusive Equilibrium in Thin Films Method

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Multiple regression analysis to assess the contamination with metals and metalloids in surface sediments (Aveiro Lagoon, Portugal)

International audienceAn innovative multiple regression analysis was used to evaluate metal/metalloid contamination in the surface sediments of a coastal lagoon. The concentrations of metals/metalloids were represented as a function of geochemical characteristics of the sediments (fine fraction, concentrations of organic carbon, Ca, Al, Mn) and distances between sampling points. The effect of distances on the concentrations were negligible for Li, Co, Ni, Ba, V, Cr, and only geochemical variables specific for each element explained its spatial variation. The concentrations of As, Cu, Zn and Pb were influenced by both geochemical and geographical distance variables, the latter representing the anthropogenic influence and the extent of transport of contaminants away from the upstream source. Enrichment of the sediment with Ba, As, Co, Cr and V was determined mainly by enrichment with Mn. The proposed approach is supplementary to the traditional utilization of enrichment factors, and is better suited for systems with anthropogenic influence

Simultaneous 2D Imaging of Dissolved Iron and Reactive Phosphorus in Sediment Porewaters by Thin-Film and Hyperspectral Methods

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Upside down sulphate dynamics in a saline inland lake

International audienceThe sulphur cycle has a key role on the fate of nutrients through its several interconnected reactions. Although sulphur cycling in aquatic ecosystems has been thoroughly studied since the early 70's, its characterisation in saline endorheic lakes still deserves further exploration. Gallocanta Lake (NE Spain) is an ephemeral saline inland lake whose main sulphate source is found on the lake bed minerals and leads to dissolved sulphate concentrations higher than those of seawater. An integrative study including geochemical and isotopic characterization of surface water, porewater and sediment has been performed to address how sulphur cycling is constrained by the geological background. In freshwater and marine environments, sulphate concentration decreases with depth are commonly associated with bacterial sulphate reduction (BSR). However, in Gallocanta Lake sulphate concentrations in porewater increase from 60 mM at the water-sediment interface to 230 mM at 25 cm depth. This extreme increase could be caused by dissolution of the sulphate rich mineral epsomite (MgSO 4 •7H 2 O). Sulphur isotopic data was used to validate this hypothesis and demonstrate the occurrence of BSR near the water-sediment interface. This dynamic prevents methane production and release from the anoxic sediment, which is advantageous in the current context of global warming. These results underline that geological context should be considered in future biogeochemical studies of inland lakes with higher potential availability of electron acceptors in the lake bed compared to the water column

Fe-catalyzed sulfide oxidation in hydrothermal plumes is a source of reactive oxygen species to the ocean

Historically, the production of reactive oxygen species (ROS) in the ocean has been attributed to photochemical and biochemical reactions. However, hydrothermal vents emit globally significant inventories of reduced Fe and S species that should react rapidly with oxygen in bottom water and serve as a heretofore unmeasured source of ROS. Here, we show that the Fe-catalyzed oxidation of reduced sulfur species in hydrothermal vent plumes in the deep oceans supported the abiotic formation of ROS at concentrations 20 to 100 times higher than the average for photoproduced ROS in surface waters. ROS (measured as hydrogen peroxide) were determined in hydrothermal plumes and seeps during a series of Alvin dives at the North East Pacific Rise. Hydrogen peroxide inventories in emerging plumes were maintained at levels proportional to the oxygen introduced by mixing with bottom water. Fenton chemistry predicts the production of hydroxyl radical under plume conditions through the reaction of hydrogen peroxide with the abundant reduced Fe in hydrothermal plumes. A model of the hydroxyl radical fate under plume conditions supports the role of plume ROS in the alteration of refractory organic molecules in seawater. The ocean's volume circulates through hydrothermal plumes on timescales similar to the age of refractory dissolved organic carbon. Thus, plume-generated ROS can initiate reactions that may affect global ocean carbon inventories