The isotope composition of inorganic germanium in seawater and deep sea sponges

• Published in: Geochimica et Cosmochimica Acta
• Main Authors: Guillermic, Maxence, Lalonde, Stefan, Hendry, Katharine, Rouxel, Olivier
• Other Authors: Laboratoire Géosciences Océan (LGO), Institut Français de Recherche pour l'Exploitation de la Mer - Brest (IFREMER Centre de Bretagne), Institut Français de Recherche pour l'Exploitation de la Mer (IFREMER)-Institut Français de Recherche pour l'Exploitation de la Mer (IFREMER)-Université de Bretagne Sud (UBS)-Université de Brest (UBO)-Centre National de la Recherche Scientifique (CNRS), School of Earth Sciences Bristol, University of Bristol Bristol, Unité de recherche Géosciences Marines (Ifremer) (GM), Institut Français de Recherche pour l'Exploitation de la Mer (IFREMER)
• Format: Article in Journal/Newspaper
• Language: English
• Published: HAL CCSD 2017
• Subjects: stable isotopes; germanium; silicon; sponges; Southern Ocean; chemical oceanography; [SDU.STU.PG]Sciences of the Universe [physics]/Earth Sciences/Paleontology; [SDU.STU.OC]Sciences of the Universe [physics]/Earth Sciences/Oceanography; [SDU.STU.TE]Sciences of the Universe [physics]/Earth Sciences/Tectonics; [SDU.STU.VO]Sciences of the Universe [physics]/Earth Sciences/Volcanology; Drake Passage; Pacific; Sars Seamount
• Online Access: https://hal.univ-brest.fr/hal-01919348; https://hal.univ-brest.fr/hal-01919348/document; https://hal.univ-brest.fr/hal-01919348/file/50529.pdf; https://doi.org/10.1016/j.gca.2017.06.011

International audience Although dissolved concentrations of germanium (Ge) and silicon (Si) in modern seawater are tightlycorrelated, uncertainties still exist in the modern marine Ge cycle. Germanium stable isotope systematics inmarine systems should provide additional constraints on marine Ge sources and sinks, however the lowconcentration of Ge in seawater presents an analytical challenge for isotopic measurement. Here, we presenta new method of pre-concentration of inorganic Ge from seawater which was applied to measure three Geisotope profiles in the Southern Ocean and deep seawater from the Atlantic and Pacific Oceans. Germaniumisotopic measurements were performed on Ge amounts as low as 2.6 ng using a double-spike approach anda hydride generation system coupled to a MC-ICP-MS. Germanium was co-precipitated with iron hydroxideand then purified through anion-exchange chromatography. Results for the deep (i.e. > 1000 m depth) PacificOcean off Hawaii (nearby Loihi Seamount) and the deep Atlantic off Bermuda (BATS station) showed nearlyidentical δ74/70Ge values at 3.19 ± 0.31 ‰ (2SD, n = 9) and 2.93 ± 0.10 ‰ (2SD, n = 2), respectively.Vertical distributions of Ge concentration and isotope composition in the deep Southern Ocean for waterdepth > 1300 m yielded an average δ74/70Ge = 3.13 ± 0.25 ‰ (2SD, n = 14) and Ge/Si = 0.80 ± 0.09μmol/mol (2SD, n = 12). Significant variations in δ74/70Ge, from 2.62 to 3.71 ‰, were measured in the first1000 m in one station of the Southern Ocean near Sars Seamount in the Drake Passage, with the heaviestvalues measured in surface waters. Isotope fractionation by diatoms during opal biomineralization mayexplain the enrichment in heavy isotopes for both Ge and Si in surface seawater. However, examination ofboth oceanographic parameters and δ74/70Ge values suggest also that water mass mixing and potentialcontribution of shelf-derived Ge also could contribute to the variations. Combining these results with new Geisotope data for deep-sea sponges sampled nearby allowed us to ...