Universität zu Köln

Merschel, Gila, Bau, Michael, Schmidt, Katja, Muenker, Carsten and Dantas, Elton L. (2017). Hafnium and neodymium isotopes and REY distribution in the truly dissolved, nanoparticulate/colloidal and suspended loads of rivers in the Amazon Basin, Brazil. Geochim. Cosmochim. Acta, 213. S. 383 - 400. OXFORD: PERGAMON-ELSEVIER SCIENCE LTD. ISSN 1872-9533

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Abstract

Radiogenic isotopes in river sediments and river waters have been widely used in provenance studies, as these samples naturally integrate the geology/chemistry of the entire catchment. While the Hf and Nd isotope systems are coupled during igneous processes, they are decoupled during supergene processes at the Earth's surface, which is reflected by the isotope composition of riverine sediments. We present the first data for both Hf and Nd isotope compositions of the dissolved (0.2 mu m-filtrates rich in nanoparticles and colloids, NPCs) and the truly dissolved (1 kDa-ultrafiltrates) load of rivers. Hafnium and Nd isotope compositions and concentrations of the Rare Earths and Yttrium (REY) and Hf were determined for suspended particles (>0.2 mu m) as well as for the dissolved and the truly dissolved load of the Rio Solimoes, the Amazon's largest tributary draining the Andes, and of the Rio Negro, an organic NPC- and particle-rich river draining the rainforest of northern Amazonia. We also analyzed the Nd isotope compositions of suspended sediments and 0.2 mu m-filtered water samples from the Amazon River and its tributaries Rio Tapajos, Rio Xingu and Rio Jari. Our novel results clearly show that the decoupling of the Hf and Nd isotope systems is related to incongruent weathering processes on the continent, as this decoupling can already be observed in the different Hf and Nd pools, i.e. in the particulate, the NPC-dominated dissolved and the truly dissolved load of rivers. In the Rio Negro and Rio Solimoes, a strong particle size-dependent difference in Hf isotope composition is observed. Values of epsilon(Hf) become more radiogenic as filter poresize decreases, which can be related to the density- and size-dependent distribution of Hf-rich minerals, e.g. zircons, and their absence from the truly dissolved pool. In contrast, the Nd isotope composition of Amazonian river waters reflects that of their catchment geology. Tributaries draining the Precambrian Brazilian and Guyana shields show very unradiogenic epsilon(Nd) values of -19 to -25 in their dissolved load, whereas the Rio Solimoes draining the Andes yields a more radiogenic eNd signal of only -7. The dissolved Nd isotope composition of the Amazon is dominated by its Andean tributaries and averages at -8. Although Nd isotope compositions are thought to not being fractionated by Earth surface processes, significant differences of 1.3-1.9 epsilon-units can be observed between the dissolved and suspended loads in the Amazon River and its main tributary, the Rio Solimoes. In these rivers, the dissolved load is more radiogenic than the suspended sediment, which is likely due to incongruent weathering and related mineral sorting in the Andean headwaters. In contrast, the organic-rich and mineral-poor shield rivers do not show a difference between the truly dissolved, dissolved and suspended load, as the Nd in all these pools is controlled by surface- and solution-complexation and hence isotopically homogenized by continuous exchange and reequilibration. (C) 2017 Elsevier Ltd. All rights reserved.

Item Type:	Journal Article
Creators:	Creators Email ORCID ORCID Put Code Merschel, Gila UNSPECIFIED UNSPECIFIED UNSPECIFIED Bau, Michael UNSPECIFIED UNSPECIFIED UNSPECIFIED Schmidt, Katja UNSPECIFIED UNSPECIFIED UNSPECIFIED Muenker, Carsten UNSPECIFIED UNSPECIFIED UNSPECIFIED Dantas, Elton L. UNSPECIFIED UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-217735
DOI:	10.1016/j.gca.2017.07.006
Journal or Publication Title:	Geochim. Cosmochim. Acta
Volume:	213
Page Range:	S. 383 - 400
Date:	2017
Publisher:	PERGAMON-ELSEVIER SCIENCE LTD
Place of Publication:	OXFORD
ISSN:	1872-9533
Language:	English
Faculty:	Unspecified
Divisions:	Unspecified
Subjects:	no entry
Uncontrolled Keywords:	Keywords Language RARE-EARTH-ELEMENTS; MARINE-SEDIMENTS; TRACE-ELEMENTS; ANTHROPOGENIC GADOLINIUM; CRUSTAL EVOLUTION; ORGANIC-CARBON; WORLD RIVERS; ND ISOTOPES; LU-HF; WATERS Multiple languages Geochemistry & Geophysics Multiple languages
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/21773