Vanadium and its isotope composition of river water and seawater

Autor(en)

Stephan Schuth, Annika Brüske, Simon V. Hohl, Shao-Yong Jiang, Ann-Katrin Meinhardt, Daniel D. Gregory, Sebastian Viehmann, Stefan Weyer

Abstrakt

Investigation of redox variations in recent and paleo-oceans has been of particular scientific interest to elucidate the rise and variations of the atmospheric oxygen level by analyses of isotopic signatures of redox-sensitive elements like Fe, Mo, and U. Vanadium is another redox-sensitive metal that has become the target of stable isotope research during the last decade. During the last decade, research of the oceanic V cycle revealed a rather complex interplay of riverine V as a major V source to the oceans on one side with V deposition in sediments and at hydrothermal vents as major sinks on the other. The balance between these major V pools is sensitive to the ocean water oxygen level and chemistry. However, the data set of stable V isotope signatures of seawater is still very small, but indicates already subtle variation of the V isotope signatures in the marine environment. However, the V isotopes of marine sediments and particularly the riverine V isotope composition of dissolved and particulate V, i.e. the major source of V in modern marine environments, has not been constrained at all so far. In this study, we present a new method for efficient V separation from seawater that allows multiple analyses of the V isotope composition of a single sample. To separate V from large amounts (volume ≥2 L) of seawater samples, we employ the Bio-Rad® Chelex-100 resin and conventional cation and anion resins to yield a high V recovery of ≥90% from an UV-irradiated sample. Non-irradiated samples were marked by lower V recovery rates of ca. 75%, which was also observed in earlier studies. Further tests however revealed that even such reduced V yields do not incur significant V isotope fractionation within analytical uncertainty. Our δ

⁵¹V

_AA value of +0.27‰ ±0.14 (2s.d., n = 3) for the NASS-6 seawater reference solution perfectly matched earlier results. In addition, seawater collected in the Wadden Sea at the German North Sea coast is marked by a δ

⁵¹V

_AA signature of around +0.02‰ ±0.19 (2s.d., n = 17), which is slightly lower than those of the great oceans, and may be related to an influx of river water, bioactivity, or a tide-induced V mobilization. To characterize the V isotope composition of the major V source to the oceans, we determined for the first time V isotope signatures of 13 selected rivers (dissolved and particulate fractions of source water, tributary rivers, and the Yangtze River) in the Yangtze River Basin, China. A large variation of dissolved V (ca. 0.07 to 6.0 μg/L) and particulate-bound V (ca. 0.03 to 17 μg/L) was found for the sample suite. The obtained δ

⁵¹V

_AA values of the dissolved V pool span a range of −0.76‰ (±0.18; 2s.d.) to −0.10‰ (±0.22, 2s.d.), whereas particulate-bound V extends to lower δ

⁵¹V signatures between −2.13‰ (±0.30, 2s.d.) and −0.11‰ (±0.11, 2s.d.). Notably, dissolved V from the river sources and small tributaries scatters between ca. −0.4‰ to −0.7‰, and agrees well with the predicted average δ

⁵¹V

_AA value of −0.6‰ ±0.3 for continental run-off by Wu et al. (2019). For the lower Yangtze River, however, the dissolved δ

⁵¹V

_AA signatures increase from the Three-Gorges Dam towards the estuary from −0.76‰ to −0.10‰, suggesting V isotope fractionation due to adsorption to abundant particulate Fe oxides, but may also reflect an input of anthropogenic V. The low δ

⁵¹V

_AA of particulate V largely follow this trend, and thus indicate ongoing V isotope fractionation during riverine V transport to the ocean. Our first results of stable V isotope investigation of river waters show that V isotope signatures can indeed carry their host rock signature, but are also sensitive to adsorption-driven fractionation in oxidized environments. The latter strongly depends, as predicted from earlier theoretical calculations, on the presence of particulate Fe-(oxyhydr)oxides and highlights gradual V isotope fractionation during riverine V transport to the ocean.

Organisation(en)

Institut für Geologie

Externe Organisation(en)

China University of Geosciences, Gottfried Wilhelm Leibniz Universität Hannover, Nanjing University, Tongji University, Carl von Ossietzky Universität Oldenburg, University of Toronto

Journal

Chemical Geology

Band

528

Anzahl der Seiten

16

ISSN

0009-2541

DOI

https://doi.org/10.1016/j.chemgeo.2019.07.036

Publikationsdatum

2019

Peer-reviewed

Ja

ÖFOS 2012

105105 Geochemie

Schlagwörter

ASJC Scopus Sachgebiete

Geochemistry and Petrology, Geology

Sustainable Development Goals

SDG 14 – Leben unter Wasser

Link zum Portal

https://ucrisportal.univie.ac.at/de/publications/d623b5ea-a36f-4bcb-a42f-eada3b564982