Radium-228-derived ocean mixing and trace element inputs in the South Atlantic

Hsieh, Yu-Te; Geibert, Walter; Woodward, E. Malcolm S.; Wyatt, Neil J.; Lohan, Maeve C.; Achterberg, Eric P.; Henderson, Gideon M.

Trace elements (TEs) play important roles as micronutrients in modulating marine productivity in the global ocean. The South Atlantic around 40inline-formula 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 inline-formula228Ra in the water column of the South Atlantic (Cape Basin and Argentine Basin) has been investigated along a 40inline-formula 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 inline-formula M4inlinescrollmathml K x = normal 1.8 ± normal 1.4 69pt12ptsvg-formulamathimg5b3549c8a00c9456492d0ff0f458c984 bg-18-1645-2021-ie00001.svg69pt12ptbg-18-1645-2021-ie00001.png (10inline-formula6 cminline-formula2 sinline-formula−1) and an integrated advection velocity inline-formula M8inlinescrollmathml w = normal 0.6 ± normal 0.3 65pt10ptsvg-formulamathimgf6b2d4fc9d40ed1ae0f758ddbbc79007 bg-18-1645-2021-ie00002.svg65pt10ptbg-18-1645-2021-ie00002.png  cm sinline-formula−1. In the Cape Basin, horizontal mixing is inline-formula M10inlinescrollmathml K x = normal 2.7 ± normal 0.8 69pt12ptsvg-formulamathimge21a8b9f16f78a2eb0517d91ba672b15 bg-18-1645-2021-ie00003.svg69pt12ptbg-18-1645-2021-ie00003.png (10inline-formula7 cminline-formula2 sinline-formula−1) and vertical mixing inline-formulaKzinline-formula= 1.0–1.7 cminline-formula2 sinline-formula−1 in the upper 600 m layer. Three different approaches (inline-formula228Ra diffusion, inline-formula228Ra advection, and inline-formula M20inlinescrollmathml chem normal 228 normal Ra / normal TE 50pt15ptsvg-formulamathimga2f5eea8f7d173659b78099a3a8047d1 bg-18-1645-2021-ie00004.svg50pt15ptbg-18-1645-2021-ie00004.png ratio) have been applied to estimate the dissolved trace element fluxes from the shelf to the open ocean. These approaches bracket the possible range of off-shelf fluxes from the Argentine Basin margin to be 4–21 (inline-formula×103) nmol Co minline-formula−2 dinline-formula−1, 8–19 (inline-formula×104) nmol Fe minline-formula−2 dinline-formula−1 and 2.7–6.3 (inline-formula×104) nmol Zn minline-formula−2 dinline-formula−1. Off-shelf fluxes from the Cape Basin margin are 4.3–6.2 (inline-formula×103) nmol Co minline-formula−2 dinline-formula−1, 1.2–3.1 (inline-formula×104) nmol Fe minline-formula−2 dinline-formula−1, and 0.9–1.2 (inline-formula×104) nmol Zn minline-formula−2 dinline-formula−1. On average, at 40inline-formula S in the Atlantic, vertical mixing supplies 0.1–1.2 nmol Co minline-formula−2 dinline-formula−1, 6–9 nmol Fe minline-formula−2 dinline-formula−1, and 5–7 nmol Zn minline-formula−2 dinline-formula−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 40inline-formula S Atlantic transect. It is insufficient, however, to provide the trace elements removed by biological uptake, particularly for Fe. Other inputs (e.g. particulate or from winter deep mixing) are required to balance the trace element budgets in this region.

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Hsieh, Yu-Te / Geibert, Walter / Woodward, E. Malcolm S. / et al: Radium-228-derived ocean mixing and trace element inputs in the South Atlantic. 2021. Copernicus Publications.

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