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<p>Trace elements (TEs) play important roles as micronutrients in modulating marine productivity in the global ocean. The South Atlantic around 40<span class="inline-formula">^∘</span> 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 <span class="inline-formula">²²⁸</span>Ra in the water column of the South Atlantic (Cape Basin and Argentine Basin) has been investigated along a 40<span class="inline-formula">^∘</span> 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 <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M4" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi>K</mi><mi>x</mi></msub><mo>=</mo><mn mathvariant="normal">1.8</mn><mo>±</mo><mn mathvariant="normal">1.4</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="69pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="5b3549c8a00c9456492d0ff0f458c984"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-1645-2021-ie00001.svg" width="69pt" height="12pt" src="bg-18-1645-2021-ie00001.png"/></svg:svg></span></span> (10<span class="inline-formula">⁶</span> cm<span class="inline-formula">²</span> s<span class="inline-formula">⁻¹</span>) and an integrated advection velocity <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M8" display="inline" overflow="scroll" dspmath="mathml"><mrow><mi>w</mi><mo>=</mo><mn mathvariant="normal">0.6</mn><mo>±</mo><mn mathvariant="normal">0.3</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="65pt" height="10pt" class="svg-formula" dspmath="mathimg" md5hash="f6b2d4fc9d40ed1ae0f758ddbbc79007"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-1645-2021-ie00002.svg" width="65pt" height="10pt" src="bg-18-1645-2021-ie00002.png"/></svg:svg></span></span> cm s<span class="inline-formula">⁻¹</span>. In the Cape Basin, horizontal mixing is <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M10" display="inline" overflow="scroll" dspmath="mathml"><mrow><msub><mi>K</mi><mi>x</mi></msub><mo>=</mo><mn mathvariant="normal">2.7</mn><mo>±</mo><mn mathvariant="normal">0.8</mn></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="69pt" height="12pt" class="svg-formula" dspmath="mathimg" md5hash="e21a8b9f16f78a2eb0517d91ba672b15"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-1645-2021-ie00003.svg" width="69pt" height="12pt" src="bg-18-1645-2021-ie00003.png"/></svg:svg></span></span> (10<span class="inline-formula">⁷</span> cm<span class="inline-formula">²</span> s<span class="inline-formula">⁻¹</span>) and vertical mixing <span class="inline-formula"><i>K</i>_<i>z</i></span> <span class="inline-formula">=</span> 1.0–1.7 cm<span class="inline-formula">²</span> s<span class="inline-formula">⁻¹</span> in the upper 600 m layer. Three different approaches (<span class="inline-formula">²²⁸</span>Ra diffusion, <span class="inline-formula">²²⁸</span>Ra advection, and <span class="inline-formula"><math xmlns="http://www.w3.org/1998/Math/MathML" id="M20" display="inline" overflow="scroll" dspmath="mathml"><mrow class="chem"><msup><mi/><mn mathvariant="normal">228</mn></msup><mi mathvariant="normal">Ra</mi><mo>/</mo><mi mathvariant="normal">TE</mi></mrow></math><span><svg:svg xmlns:svg="http://www.w3.org/2000/svg" width="50pt" height="15pt" class="svg-formula" dspmath="mathimg" md5hash="a2f5eea8f7d173659b78099a3a8047d1"><svg:image xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="bg-18-1645-2021-ie00004.svg" width="50pt" height="15pt" src="bg-18-1645-2021-ie00004.png"/></svg:svg></span></span> 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 (<span class="inline-formula">×10³</span>) nmol Co m<span class="inline-formula">⁻²</span> d<span class="inline-formula">⁻¹</span>, 8–19 (<span class="inline-formula">×10⁴</span>) nmol Fe m<span class="inline-formula">⁻²</span> d<span class="inline-formula">⁻¹</span> and 2.7–6.3 (<span class="inline-formula">×10⁴</span>) nmol Zn m<span class="inline-formula">⁻²</span> d<span class="inline-formula">⁻¹</span>. Off-shelf fluxes from the Cape Basin margin are 4.3–6.2 (<span class="inline-formula">×10³</span>) nmol Co m<span class="inline-formula">⁻²</span> d<span class="inline-formula">⁻¹</span>, 1.2–3.1 (<span class="inline-formula">×10⁴</span>) nmol Fe m<span class="inline-formula">⁻²</span> d<span class="inline-formula">⁻¹</span>, and 0.9–1.2 (<span class="inline-formula">×10⁴</span>) nmol Zn m<span class="inline-formula">⁻²</span> d<span class="inline-formula">⁻¹</span>. On average, at 40<span class="inline-formula">^∘</span> S in the Atlantic, vertical mixing supplies 0.1–1.2 nmol Co m<span class="inline-formula">⁻²</span> d<span class="inline-formula">⁻¹</span>, 6–9 nmol Fe m<span class="inline-formula">⁻²</span> d<span class="inline-formula">⁻¹</span>, and 5–7 nmol Zn m<span class="inline-formula">⁻²</span> d<span class="inline-formula">⁻¹</span> 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<span class="inline-formula">^∘</span> 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.</p>