First full depth profiles of zinc isotopes in the ocean, thanks to IPY/GEOTRACES cruise (GIPY5)

Three major and original features are deduced from the first three full depth profiles of zinc measured by Zhao and co-workers (2014; see reference below) in the Southern Ocean:

• below 1000 m, the comparison of the results with North Atlantic and Pacific data reveals that the oceanic zinc (Zn) isotopic composition appears to be homogeneous (δ66Zn = +0.53 ± 0.14 per mil (2SE = 0.03, n = 21)).
• oceanic Zn isotopic composition is more variable in the upper 1000 m (δ66Zn values are more variable); these new Zn isotope data are consistent with a scenario whereby Zn removal from the surface ocean occurs via two processes: a dominant one that does not involve an isotopic fractionation (incorporation of Zn into organic matter associated with only diatom frustules, a type of phytoplankton) and a lesser one that preferentially removes the light isotope (metabolic uptake into the cells of all phytoplankton).
• a mass balance calculation is proposed to explain the homogeneous Zn isotopic composition of the deep ocean. The δ66Zn value is slightly heavier than all the possible external sources (~+0.35 per mil). Thus, an isotopically light sink is required but not identified yet. The author’s working hypothesis is that the burial of isotopically light Zn in cellular organic matter could represent the light sink from the oceanic dissolved pool.

Figure: Zinc (Zn) isotopic data for IPY GEOTRACES samples from the Southern Ocean (green), plotted with data from the same laboratory for the GEOTRACES BATS intercalibration site in the Atlantic (blue, Boyle et al., 2012) and for the SAFe sample at 1000m in the Pacific (red). There is variability in Zn isotopes at depths shallower than about 500m, and a sample from the sediment-water interface in one depth profile at 67°S is anomalous, but in between all seawater samples yet published have a mean δ66Zn of 0.53 per mil, with a spread of only 0.06 per mil (± 2 standard errors of the mean).

Reference:

Zhao, Y., Vance, D., Abouchami, W., & de Baar, H. J. W. (2014). Biogeochemical cycling of zinc and its isotopes in the Southern Ocean. Geochimica et Cosmochimica Acta, 125, 653–672. doi:10.1016/j.gca.2013.07.045. Click here to access the paper.

Boyle, E. A., John, S., Abouchami, W., Adkins, J. F., Echegoyen-Sanz, Y., Ellwood, M., Flegal, A. R., Fornace, K., Gallon, C., Galer, S. (2012). GEOTRACES IC1 (BATS) contamination-prone trace element isotopes Cd, Fe, Pb, Zn, Cu, and Mo intercalibration. Limnology and Oceanography: Methods, 10, 653–665. doi: 10.4319/lom.2012.10.653. Click here to access the paper.