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

• Published in: Biogeosciences
• Main Authors: Hsieh, Yu-Te, Geibert, Walter, Woodward, E. Malcolm S., Wyatt, Neil J., Lohan, Maeve C., Achterberg, Eric P., Henderson, Gideon M.
• Format: Text
• Language: English
• Published: 2021
• Subjects: Argentine; Southern Ocean
• Online Access: https://doi.org/10.5194/bg-18-1645-2021; https://bg.copernicus.org/articles/18/1645/2021/

Trace elements (TEs) 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 228 Ra 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 <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> <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> (10 6 cm 2 s −1 ) and an integrated advection velocity <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> <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> cm s −1 . In the Cape Basin, horizontal mixing is <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> <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> (10 7 cm 2 s −1 ) and vertical mixing K z = 1.0–1.7 cm 2 s −1 in the upper 600 m layer. Three different approaches ( 228 Ra diffusion, 228 Ra advection, and <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> <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> 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 ( ×10 3 ) nmol Co m −2 d −1 , 8–19 ( ×10 4 ) nmol Fe m −2 d −1 and 2.7–6.3 ( ×10 4 ) nmol Zn m −2 d −1 . Off-shelf fluxes from the Cape Basin margin are 4.3–6.2 ( ×10 3 ) nmol Co m −2 d −1 , 1.2–3.1 ( ×10 4 ) nmol Fe m −2 d −1 , and 0.9–1.2 ( ×10 4 ) nmol Zn m −2 d −1 . On average, at 40 ∘ S in the Atlantic, vertical mixing supplies 0.1–1.2 nmol Co m −2 d −1 , 6–9 nmol Fe m −2 d −1 , and 5–7 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 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.