Iron and Germanium isotope geochemistry in river and hydrothermal systems.

• Auteur / Autrice : Raphaëlle Escoube
• Direction : Olivier Donard
• Type : Thèse de doctorat
• Discipline(s) : Chimie analytique et environnement
• Date : Soutenance en 2011
• Etablissement(s) : Pau

Mots clés

Mots clés contrôlés

Océanographie

Biogéochimie

Géochimie isotopique

Résumé

Ce travail de thèse étudie les signatures isotopiques du Fer (Fe) et du Germanium (Ge) aux niveaux des interfaces océaniques. Les analyses du Fe dissous dans les rivières montrent que leur composition isotopique (56Fe) présente une forte hétérogénéité (pouvant atteindre 1‰ pour les rivières importantes du nord des Etats-Unis et de Sibérie et jusqu’à 3. 3‰ pour les eaux de marécages et d’étangs). De plus, il a été démontré que le processus de floculation n’induit pas de fractionnement isotopique. Il est donc possible d’assimiler l’apport de fer à l’océan par les rivières à leurs valeurs isotopiques. Au cours de ce même processus, la composition isotopique du fer particulaire, quant à elle, enregistre le mélange de matériels en suspension originaires (1) de la rivière, (2) des colloïdes nouvellement formés et (3) des sédiments côtiers. L’étude des fluides hydrothermaux de basses (Loihi, USA) et hautes (EPR 9-10°N) températures révèle un fractionnement des rapports Ge/Si et 74Ge par rapport à la roche basaltique, qui peut s’expliquer par la précipitation du quartz dans la zone de réaction en profondeur et/ou lors de la remontée du fluide. L’étude des dépôts hydrothermaux suggère que Ge fractionne jusqu’à -2. 74‰ lors de sa sorption sur les oxyhydroxydes de fer (Loihi) et jusqu’à -6. 28‰ aves les sulfures (EPR). La combinaison des rapports Ge/Si et 74Ge se révèle être un traceur utile pour comprendre les systèmes hydrothermaux et pourrait déboucher sur des applications en paléoenvironnement. Un bilan de masse préliminaire de 74Ge dans l’eau de mer confirme que la précipitation authigénique du Ge au niveau des sédiments marins représente un puits important de Ge.

During this thesis, Iron (Fe) and Germanium (Ge) isotope signatures at oceanic interfaces have been investigated. River/seawater and hydrothermal vent/seawater interfaces have been chosen for their high content of Ge and Fe in the source and for the high reactivity of those environments. Our study and previous analysis of dissolved iron in rivers suggest that river water exposes a large heterogeneity in 56Fe values (up to 1‰ for large rivers from North and South America and Siberia and up to 3. 3‰ for swamp or pond water). Since the input of dissolved iron from river water is generally controlled by flocculation processes that occur during estuarine mixing, it is important to investigate potential fractionation of Fe-isotopes during this process. A systematic study along a salinity profile of the North River estuary (US) demonstrated the absence of significant isotope fractionation during flocculation process. Therefore, Fe river input to the ocean can be assimilated to the river composition (0. 43‰ for North River iron composition input). For particulate iron, 56Fe estuary profile has been interpreted as reflecting the progressive dilution of (1) river bone particle with (2) precipitated colloidal Fe and (3) near shore sediment resuspension. Recent analysis of Fe isotope composition of dissolved Fe in open seawater reveals slightly heavy isotope composition at around 0. 2‰ which suggest significant Fe isotope fractionation in surface seawater relative to atmospheric source. Our river and estuary study highlighted several mechanisms by which positive 56Fe values can be produced in aqueous systems, which provide guidance for seawater studies. The study of Germanium isotope composition of natural waters requires new analytical methods, in particular using double-spike approach and on-line compound-specific isotope analysis. In this study, the NIST3120a elemental standard solution has been proposed as the new isotopic standard after calibration against other Ge standard solutions and major Earth reservoirs reported in previous studies. Germanium isotope and Ge/Si ratio systematics were investigated in low temperature hydrothermal vents from Loihi Seamount (Hawaii, USA) and results were compared to high-temperature vents from East Pacific Rise at 9-10°N. The study of hydrothermal fluids at Loihi and EPR highlights that Ge/Si and Ge are fractionated from the basaltic rock. The relative enrichment in Ge vs. Si together with Ge isotope fractionation in hydrothermal fluids is best explained by quartz precipitation in the reaction zone at depth and/or during fluid upflow. The study of microbial mats at Loihi seamount, composed essentially of Fe-oxyhydroxide with minor enrichment of amorphous silica and volcanic materials, suggest that Ge isotopes are fractionated upon sorption onto Fe-oxyhydroxide by up to -2. 74‰. At EPR, very light Ge isotope signatures of hydrothermal sulfide deposits were obtained with a maximum fraction factor of -6. 28‰ between sphalerite and fluid. Both results for Ge isotope fractionation in Fe-oxyhydroxide and sulfide are consistent with recent experimental and theoretical calculation. This study shows that combining Ge/Si and 74Ge systematics provide a useful tool to understand hydrothermal systems and suggest potential applications for paleoenvironmental studies. A Preliminary Ge isotopic mass balance in seawater has been also established and results confirm that authigenic Ge precipitation in marine sediments provides an important sink of Ge in seawater.