Biogeochemical cycling of zinc and its isotopes in the Southern Ocean

• Published in: Geochimica et Cosmochimica Acta
• Main Authors: Zhao, Y., Vance, D., Abouchami, W., de Baar, H.J.W.
• Format: Article in Journal/Newspaper
• Language: English
• Published: 2014
• Subjects: Antarctic; Southern Ocean; Pacific; Greenwich; Antarc*; IPY; North Atlantic
• Online Access: http://imis.nioz.nl/imis.php?module=ref&refid=239841

We report Zn concentration and isotope data for seawater samples from the Atlantic sector of the Southern Ocean, collected during the IPY/GEOTRACES ANT-XXIV/III cruise along the Greenwich Zero Meridian. Data are reported for the full depth range of the water column at three stations, as well as a transect of surface samples, using a new analytical approach that is presented in detail here. Zn concentrations increase with depth, though due to proximity to upwelling sites, surface concentrations are not as low as in some parts of the ocean such as further northward into the Sub-Antarctic Zone. For two depth profiles south of the Polar Front Zone, the physical stratification of the upper water column is reflected in sudden near-surface changes in Zn concentration with depth. In contrast, beneath 100-300 m Zn concentrations barely change with depth. Zn isotopic data beneath 1000 m, for the Southern Ocean data presented here as well as published data from the North Atlantic and North Pacific, are strikingly homogeneous, with an average delta Zn-66 = +0.53 +/- 0.14 parts per thousand (2SD, 2SE = 0.03, n = 21). The surface Southern Oceanis more variable, with delta Zn-66 ranging from 0.07 parts per thousand to 0.80 parts per thousand. Between the two is a thin horizon at 40-80 m which, in the Southern Ocean as well as the North Atlantic and North Pacific, is characterised by distinctly light isotopic signatures, with delta Zn-66 about 0.3 parts per thousand lower than surface waters. Strong correlations between Si and Zn concentrations seen here and elsewhere, coupled to the lack of any systematic relationship between Si and Zn isotopes in the Southern Ocean, suggest that the removal of Zn associated with diatom opal involves little or no isotopic fractionation. Regeneration of this Zn also explains the homogeneous Zn isotopic composition of the global deep ocean so far sampled. However, the low Zn content of opal requires that deep ocean Zn does not directly come from the opal phase itself, but rather from associated organic material external to the diatom frustule during growth. Experimental data are consistent with little or no fractionation during incorporation of Zn into this material. On the other hand, the light zinc at 40-80 m is most consistent with the regeneration of an intra-cellular pool that both culturing experiments and field data suggest will be isotopically light. The data thus imply two processes by which Zn is taken up in the surface ocean, that these pools have very different regeneration lengthscales, and that physical mixing of the oceans cannot eradicate their isotopic signatures. Finally, the deep delta Zn-66 ocean value is significantly higher than the current best estimate of the input to the oceans. The most obvious candidate for the required light sink is the survival of some of the cellular Zn to be buried in sediment.