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.

Description: © The Author(s), 2016. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Philosophical Transactions of the Royal Society A 374 (2016): 20160076, doi:10.1098/rsta.2016.0076.

Description: Continental shelves and shelf seas play a central role in the global carbon cycle. However, their importance with respect to trace element and isotope (TEI) inputs to ocean basins is less well understood. Here, we present major findings on shelf TEI biogeochemistry from the GEOTRACES programme as well as a proof of concept for a new method to estimate shelf TEI fluxes. The case studies focus on advances in our understanding of TEI cycling in the Arctic, transformations within a major river estuary (Amazon), shelf sediment micronutrient fluxes and basin-scale estimates of submarine groundwater discharge. The proposed shelf flux tracer is 228-radium (T1/2 = 5.75 yr), which is continuously supplied to the shelf from coastal aquifers, sediment porewater exchange and rivers. Model-derived shelf 228Ra fluxes are combined with TEI/ 228Ra ratios to quantify ocean TEI fluxes from the western North Atlantic margin. The results from this new approach agree well with previous estimates for shelf Co, Fe, Mn and Zn inputs and exceed published estimates of atmospheric deposition by factors of approximately 3–23. Lastly, recommendations are made for additional GEOTRACES process studies and coastal margin-focused section cruises that will help refine the model and provide better insight on the mechanisms driving shelf-derived TEI fluxes to the ocean.

Description: This paper would not have been possible without the financial support of a number of national funding agencies (US NSF OCE-1458305 to M.A.C.; US NSF OCE-0963026 to P.J.L.; Korea NRF-2013R1A1A1058203 to E.Y.K.; U.K. NERC NE/G016267/1 to M.C.L. and A.M.; U.K. NERC NE/K009532/1 to W.B.H.)