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

Xin Yuan Zheng, Yves Plancherel, Mak A. Saito, Peter M. Scott, Gideon M. Henderson

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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.

Original languageEnglish (US)
Pages (from-to)217-237
Number of pages21
JournalGeochimica et Cosmochimica Acta
StatePublished - Mar 15 2016
Externally publishedYes

Bibliographical note

Funding Information:
We thank Abigail Noble for sub-sampling and shipping samples to Oxford, Chao Liu for advice on developing the codes for REE deconvolution, and Jie Yang for helping with sample preparation and stimulating discussions. X.-Y. Zheng was supported by the Clarendon Scholarship, the Exeter College Mandarin Scholarship from University Of Oxford, the Chinese Student Awards from the Great Britain–China Educational Trust (GBCET) and W Wing Yip and Brothers bursaries. Comments from Martin Frank, Stuart Robinson, two anonymous reviewers and the editor Silke Severmann have significantly improved the manuscript.

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