Rare earth elements (REEs) in the tropical South Atlantic and quantitative deconvolution of their non-conservative behavior
Rare earth elements (REEs) in the tropical South Atlantic and quantitative deconvolution of their non-conservative behavior
Date
2016-01-24
Authors
Zheng, Xin-Yuan
Plancherel, Yves
Saito, Mak A.
Scott, Peter M.
Henderson, Gideon M.
Plancherel, Yves
Saito, Mak A.
Scott, Peter M.
Henderson, Gideon M.
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Abstract
This study presents new concentration measurements of dissolved REEs (dREEs)
along a full-depth east-west section across the tropical South Atlantic (~12°S), and
uses these data to investigate the oceanic cycling of the REEs. Enrichment of dREEs,
associated with the redox cycling of Fe-Mn oxides, is observed in the oxygen
minimum zone (OMZ) off the African shelf. For deeper-waters, a multi-parameter
mixing model was developed to deconvolve the relative importance of physical
transport (i.e. water mass mixing) from biogeochemical controls on the dREE
distribution in the deep Atlantic. This approach enables chemical processes involved
in REE cycling, not apparent from the measurements alone, to be distinguished and
quantified. Results show that the measured dREE concentrations below ~1000 m are
dominantly controlled (>75%) by preformed REE concentrations resulting from water
mass mixing. This result indicates that the linear correlation between dREEs and
dissolved Si observed in Atlantic deep waters results from the dominantly
conservative behaviour of these tracers, rather than from similar chemical processes
influencing both dREEs and Si. Minor addition of dREEs (~10% of dNd and ~5% of
dYb) is observed in the deep (>~4000 m) Brazil Basin, resulting from either
remineralization of particles in-situ or along the flow path. Greater addition of dREEs
(up to 25% for dNd and 20% for dYb) is found at ~1500 m and below ~4000 m in the
Angola Basin near the African continental margin. Cerium anomalies suggest that
different sources are responsible for these dREE addition plumes. The 1500 m excess
is most likely attributed to dREE release from Fe oxides, whereas the 4000 m excess
may be due to remineralization of calcite. Higher particulate fluxes and a more
sluggish ocean circulation in the Angola Basin may explain why the dREE excesses in
this basin are significantly higher 45 than that observed in the Brazil Basin.
Hydrothermal venting over the mid-Atlantic ridge acts as a regional net sink for light
REEs, but has little influence on the net budget of heavy REEs. The combination of
dense REE measurements with water mass deconvolution is shown to provide
quantitative assessment of the relative roles of physical and biogeochemical processes
in the oceanic cycling of REEs.
Description
© The Author(s), 2016. This is the author's version of the work and is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Geochimica et Cosmochimica Acta 177 (2016): 217-237, doi:10.1016/j.gca.2016.01.018.