An estimate of the Germanium isotopic composition of the Ocean.

Ge is a trace element in seawater whose biogeochemistry is dominated by its Si-like behaviour. Its residence time is poorly constrained but could be close to the mixing time of the ocean. In addition, hydrothermal vents are enriched in Ge (relative to Si) and this excess has been witnessed in the water column. Moreover, Si isotopic variations have been reported in the ocean, related to the precipitation of biogenic opal, while the Si residence time is slightly higher than the Ge residence time. Therefore, variations in the isotopic composition of dissolved Ge in the ocean are expected provided that at least one of the major input or output of Ge has a different isotopic composition. Given the low Ge concentration (around 40 picomol/kg) and the state-of-the art analytical facilities, a direct measurement of the isotopic composition of the seawater is barely conceivable. The major input of Ge into the ocean are the rivers and the hydrothermal vents, while the removal of Ge occurs through the precipitation of biogenic opal and the early diagenesis of passive margins. The mechanism of the later is, however, not well established but could be related to the precipitation of Fe-oxyhydroxide. So the measurement of marine authigenic minerals, biogenic silica and the comparison with an estimate of the bulk silicate Earth (BSE) composition will give some constraints on the Germanium isotopic composition of the ocean. A new technique for the precise and accurate determination of Ge stable isotope compositions has been developed and applied to silicate, sulfide, and biogenic material. The analyses were performed using a continuous flow hydride generation system coupled to a Nu Instrument MC-ICPMS. Samples have been purified through anion and cation exchange resins to separate Ge from matrix elements and potential interferences. Deep sea clays have a similar isotopic composition that MORBs or granites, suggesting that isotopic composition of the dissolved Ge in rivers might not be very distinct from the BSE composition. On the other hand, modern deep-sea sponges have \delta⁷⁴Ge values clustered at 1.7\permil (standardised to BSE) while the \delta⁷⁴Ge values of modern diatoms are more variables and can be as high as 2.5\permil. Modern authigenic clays (glauconite) are also enriched in heavy isotopes by 2\permil. In addition, the Ge isotopic fractionation during Ge sorption on goethite has experimentally determined to be +1.5\permil. Therefore, we already have 3 reasons to believe that the Germanium isotopic composition of the ocean is enriched in heavy isotopes (relative to BSE). The exact amplitude of the isotopic enrichment factor will be determined on cultured diatoms and will be associated to the isotopic composition of diatoms from core-top from the Southern Ocean to give a first estimate of the isotopic difference between the ocean and the BSE.