Radium-228-derived ocean mixing and trace element inputs in the South Atlantic
Journal
Biogeosciences
Journal Volume
18
Journal Issue
5
Pages
1645-1671
Date Issued
2021
Author(s)
Abstract
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 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= 1:8 ± 1:4 (106cm2s-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 (107cm2s-1) and vertical mixing Kz= 1.0 1.7 cm2s-1in 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 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 (×103) nmol Co m-2d-1, 8 19 (×104) nmol Fe m-2d-1and 2.7 6.3 (×104) nmol Zn m-2d-1. Off-shelf fluxes from the Cape Basin margin are 4.3 6.2 (×103) nmol Co m-2d-1, 1.2 3.1 (×104) nmol Fe m-2d-1, and 0.9 1.2 (×104) nmol Zn m-2d-1. On average, at 40° S in the Atlantic, vertical mixing supplies 0.1 1.2 nmol Co m-2d-1, 6 9 nmol Fe m-2d-1, and 5 7 nmol Zn m-2 d-1to 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. ? 2021 EDP Sciences. All rights reserved.
Subjects
biological uptake
continental shelf
dust
eddy covariance
gyre
mixing
open ocean
radionuclide
radium isotope
trace element
transition zone
vertical mixing
water column
Argentine Basin
Atlantic Ocean
Atlantic Ocean (South)
Cape Basin
Southern Ocean
Argentinidae
SDGs
Type
journal article