Rare earth elements (REEs) in the tropical South Atlantic and quantitative deconvolution of their non-conservative behavior

Zheng, Xin-Yuan and Plancherel, Yves and Saito, Mak A. and Scott, Peter M. and Henderson, Gideon M. (2016) Rare earth elements (REEs) in the tropical South Atlantic and quantitative deconvolution of their non-conservative behavior. Geochimica et Cosmochimica Acta, 177. pp. 217-237. ISSN 00167037 DOI https://doi.org/10.1016/j.gca.2016.01.018

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Abstract

This study presents new concentration measurements of dissolved rare earth elements (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 behavior 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 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.

Item Type:	Article
Uncontrolled Keywords:	NILAREP; IA70; weekly list
Subjects:	02 - Geodynamics, Geophysics and Tectonics
Divisions:	02 - Geodynamics, Geophysics and Tectonics 08 - Green Open Access
Journal or Publication Title:	Geochimica et Cosmochimica Acta
Volume:	177
Page Range:	pp. 217-237
Identification Number:	https://doi.org/10.1016/j.gca.2016.01.018
Depositing User:	Sarah Humbert
Date Deposited:	21 Apr 2016 10:12
Last Modified:	12 Sep 2019 09:38
URI:	http://eprints.esc.cam.ac.uk/id/eprint/3607

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