Description: Trace elements play important roles as micronutrients in modulating marine productivity in the global ocean. The South Atlantic around 40° S is a prominent region of high productivity and a transition zone between the nitrate-depleted Subtropical Gyre and the iron-limited Southern Ocean. However, the sources and fluxes of trace elements to this region remain unclear. In this study, the distribution of the naturally occurring radioisotope 228Ra in the water column of the South Atlantic (Cape Basin and Argentine Basin) has been investigated along a 40° S zonal transect to estimate ocean mixing and trace element supply to the surface ocean. Ra-228 profiles have been used to determine the horizontal and vertical mixing rates in the near-surface open ocean. In the Argentine Basin, horizontal mixing from the continental shelf to the open ocean shows an eddy diffusion of Kx = 1.7 ± 1.4 (106 cm2 s−1) and an integrated advection velocity w = 0.6 ± 0.3 cm s−1. In the Cape Basin, horizontal mixing is Kx = 2.7 ± 0.8 (107 cm2 s−1) and vertical mixing Kz = 1.0–1.5 cm2 s−1 in the upper 600 m layer. Three different approaches (228Ra-diffusion, 228Ra-advection and 228Ra/TE-ratio) have been applied to estimate the dissolved trace-element fluxes from shelf to open ocean. These approaches bracket the possible range of off-shelf fluxes from the Argentine margin to be: 3.8–22 (× 103) nmol Co m−2 d−1, 7.9–20 (× 104) nmol Fe m−2 d−1 and 2.7–6.5 (× 104) nmol Zn m−2 d−1. Off-shelf fluxes from the Cape margin are: 4.3–6.2 (× 103) nmol Co m−2 d−1, 1.2–3.1 (× 104) nmol Fe m−2 d−1 and 0.9–1.2 (× 104) nmol Zn m−2 d−1. On average, at 40° S in the Atlantic, vertical mixing supplies 0.4–1.2 nmol Co m−2 d−1, 3.6–11 nmol Fe m−2 d−1, and 13–16 nmol Zn m−2 d−1 to the euphotic zone. Compared with atmospheric dust and continental shelf inputs, vertical mixing is a more important source for supplying dissolved trace elements to the surface 40° S Atlantic. It is insufficient, however, to provide the trace elements removed by biological uptake. Other inputs (e.g. particulate, or from winter deep-mixing) are required to balance the trace element budgets in this region.

Description: We report the distributions and stoichiometry ofdissolved zinc (dZn) and cobalt (dCo) in sub-tropical andsub-Antarctic waters of the south-eastern Atlantic Oceanduring austral spring 2010 and summer 2011/2012. In sub-tropical surface waters, mixed-layer dZn and dCo con-centrations during early spring were 1.60±2.58 nM and30±11 pM, respectively, compared with summer values of0.14±0.08 nM and 24±6 pM. The elevated spring dZn con-centrations resulted from an apparent offshore transport ofelevated dZn at depths between 20–55 m, derived from theAgulhas Bank. In contrast, open-ocean sub-Antarctic surfacewaters displayed largely consistent inter-seasonal mixed-layer dZn and dCo concentrations of 0.10±0.07 nM and11±5 pM, respectively. Trace metal stoichiometry, calcu-lated from concentration inventories, suggests a greater over-all removal for dZn relative to dCo in the upper water columnof the south-eastern Atlantic, with inter-seasonally decreas-ing dZn/dCo inventory ratios of 19–5 and 13–7 mol mol−1for sub-tropical surface water and sub-Antarctic surface wa-ter, respectively. In this paper, we investigate how the sea-sonal influences of external input and phytoplankton succes-sion may relate to the distribution of dZn and dCo and varia-tion in dZn/dCo stoichiometry across these two distinct eco-logical regimes in the south-eastern Atlantic.