Seasonal cycling of zinc and cobalt in the south-eastern Atlantic along the GEOTRACES GA10 section

Abstract

We report the distributions and stoichiometry of dissolved zinc (dZn) and cobalt (dCo) in sub-tropical and sub-Antarctic waters of the south-eastern Atlantic Ocean during austral spring 2010 and summer 2011/2012. In sub-tropical surface waters, mixed-layer dZn and dCo concentrations during early spring were 1.60 ± 2.58 nM and 30 ± 11 pM, respectively, compared with summer values of 0.14 ± 0.08 nM and 24 ± 6 pM. The elevated spring dZn concentrations resulted from an apparent offshore transport of elevated dZn at depths between 20–55 m, derived from the Agulhas Bank. In contrast, open-ocean sub-Antarctic surface waters displayed largely consistent inter-seasonal mixed-layer dZn and dCo concentrations of 0.10 ± 0.07 nM and 11 ± 5 pM, respectively. Trace metal stoichiometry, calculated from concentration inventories, suggests a greater overall removal for dZn relative to dCo in the upper water column of the south-eastern Atlantic, with inter-seasonally decreasing dZn / dCo inventory ratios of 19–5 and 13–7 mol mol−1 for sub-tropical surface water and sub-Antarctic surface water, respectively. In this paper, we investigate how the seasonal influences of external input and phytoplankton succession may relate to the distribution of dZn and dCo and variation in dZn / dCo stoichiometry across these two distinct ecological regimes in the south-eastern Atlantic.