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

Trace elements (1Ls) play important roles as micronutrients in modulating marine productivity in the global ocean. The South Atlantic around 40 degrees 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 degrees 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 K-x = 1.8 +/- 1.4 (10(6) cm(2) s(-1)) and an integrated advection velocity w = 0.6 +/- 0.3 cm s(-1). In the Cape Basin, horizontal mixing is K-x = 2.7 +/- 0.8 (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 (Ra-228 diffusion, Ra-228 advection, and Ra-228/TE 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 ( x 10(3)) nmol Co m(-2) d(-1), 8-19 ( x 10(4)) nmol Fe m(-2) d(-1) and 2.7-6.3 ( x 10(4)) nmol Zn m(-2) d(-1). Off-shelf fluxes from the Cape Basin margin are 4.3-6.2 (x10(3))nmol Co m(-2) d(-1), 1.2-3.1 (x10(4))nmolFe m(-2) d(-1), and 0.9-1.2 (x10(4)) nmolZn m(-2) d(-1). On average, at 40 degrees 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 degrees 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.

Automated summary

This study investigated the distribution of the naturally occurring radioisotope 228Ra in the water column of the South Atlantic to estimate ocean mixing and trace element supply to the surface ocean at 40 degrees S. Results showed that vertical mixing is an important source of dissolved trace elements in the surface ocean, but additional inputs are needed to balance the element budgets, especially for iron.