Rare earth elements and neodymium isotopes in world river sediments revisited

Type Article
Date 2015-12
Language English
Author(s) Bayon Germain1, 2, Toucanne SamuelORCID1, Skonieczny Charlotte1, Andre L.2, Bermell Sylvain1, Cheron Sandrine1, Dennielou BernardORCID1, Etoubleau Joel1, Freslon Nicolas1, 3, Gauchery T.1, Germain Yoan1, Jorry StephanORCID1, Menot G.4, Monin L.2, Ponzevera Emmanuel1, Rouget M. -L.3, 5, Tachikawa K.4, Barrat Jean-Alix3, 5
Affiliation(s) 1 : IFREMER, Unite Rech Geosci Marines, F-29280 Plouzane, France.
2 : Royal Museum Cent Afr, Dept Earth Sci, B-3080 Tervuren, Belgium.
3 : Univ Europeenne Bretagne, F-35000 Rennes, France.
4 : Univ Aix Marseille, CNRS, CEREGE, IRD,Coll France, F-13545 Aix En Provence 04, France.
5 : Univ Bretagne Occidentale, CNRS UMS 3113, Inst Univ Europeen Mer, F-29280 Plouzane, France.
Source Geochimica Et Cosmochimica Acta (0016-7037) (Pergamon-elsevier Science Ltd), 2015-12 , Vol. 170 , P. 17-38
DOI 10.1016/j.gca.2015.08.001
WOS© Times Cited 68
Abstract Over the past decades, rare earth elements (REE) and their radioactive isotopes have received tremendous attention in sedimentary geochemistry, as tracers for the geological history of the continental crust and provenance studies. In this study, we report on elemental concentrations and neodymium (Nd) isotopic compositions for a large number of sediments collected near the mouth of rivers worldwide, including some of the world’s major rivers. Sediments were leached for removal of non-detrital components, and both clay and silt fractions were retained for separate geochemical analyses. Our aim was to re-examine, at the scale of a large systematic survey, whether or not REE and Nd isotopes could be fractionated during Earth surface processes.

Our results confirmed earlier assumptions that river sediments do not generally exhibit any significant grain-size dependent Nd isotopic variability. Most sediments from rivers draining old cratonic areas, sedimentary systems and volcanic provinces displayed similar Nd isotopic signatures in both clay and silt fractions, with ΔεNd (clay-silt) < |1.| A subtle decoupling of Nd isotopes between clays and silts was identified however in a few major river systems (e.g. Nile, Mississippi, Fraser), with clays being systematically shifted towards more radiogenic values. This observation suggests that preferential weathering of volcanic and/or sedimentary rocks relative to more resistant lithologies may occur in river basins, possibly leading locally to Nd isotopic decoupling between different size fractions. Except for volcanogenic sediments, silt fractions generally displayed homogeneous REE concentrations, exhibiting relatively flat shale-normalized patterns. However, clay fractions were almost systematically characterized by a progressive enrichment from the heavy to the light REE and a positive europium (Eu) anomaly. In agreement with results from previous soil investigations, the observed REE fractionation between clays and silts is probably best explained by preferential alteration of feldspars and/or accessory mineral phases. Importantly, this finding clearly indicates that silicate weathering can lead to decoupling of REE between different grain-size fractions, with implications for sediment provenance studies.

Finally, we propose a set of values for a World River Average Clay (WRAC) and Average Silt (WRAS), which provide new estimates for the average composition of the weathered and eroded upper continental crust, respectively, and could be used for future comparison purposes.
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Bayon Germain, Toucanne Samuel, Skonieczny Charlotte, Andre L., Bermell Sylvain, Cheron Sandrine, Dennielou Bernard, Etoubleau Joel, Freslon Nicolas, Gauchery T., Germain Yoan, Jorry Stephan, Menot G., Monin L., Ponzevera Emmanuel, Rouget M. -L., Tachikawa K., Barrat Jean-Alix (2015). Rare earth elements and neodymium isotopes in world river sediments revisited. Geochimica Et Cosmochimica Acta, 170, 17-38. Publisher's official version : https://doi.org/10.1016/j.gca.2015.08.001 , Open Access version : https://archimer.ifremer.fr/doc/00276/38733/