The Oceanic Biogeochemical Cycle of Zinc and Its Isotopes

Zinc (Zn) is the most abundant trace metal in the phytoplankton that dominate vertical carbon export in the ocean, the diatoms. But the strong relationship between the vertical distributions of Zn and the silicon (Si) that makes up the opal hard parts of diatoms represents a long-standing puzzle. Zn is overwhelmingly co-located with phosphate in the organic matter of diatom cells, not with Si in opal, and is regenerated with phosphate in the upper ocean, not with Si in the deep. The resolution of this apparent paradox is key both to an understanding of the global oceanic cycling of Zn, and to the rates and mechanisms by which biologically-assimilated trace metals are returned to the photic zone. Here, we show that oceanic dissolved Zn exhibits significant isotopic variation in the upper ocean that is consistent with vertical cycling. However, we suggest that the isotopically homogeneous global deep ocean Zn pool is largely sourced from the Southern Ocean. This leads to a new view of the global oceanic cycling of this important trace metal, one that is consistent with the unique physiology of Southern Ocean diatoms, the coupling of Zn and Si in the global deep ocean, and the emerging paradigm for global ocean nutrient dynamics. Our data and interpretation imply a small Zn pool that is biologically cycled in the upper ocean, but is to a great extent decoupled from the much larger Southern-Ocean-dominated deep ocean pool.