Universität zu Köln

Roue, Lucile, Kurzweil, Florian, Wille, Martin, Wegwerth, Antje, Dellwig, Olaf ORCID: 0000-0002-0028-8812, Munker, Carsten ORCID: 0000-0001-6406-559X and Schoenberg, Ronny (2021). Stable W and Mo isotopic evidence for increasing redox-potentials from the Paleoarchean towards the Paleoproterozoic deep ocean. Geochim. Cosmochim. Acta, 309. S. 366 - 388. OXFORD: PERGAMON-ELSEVIER SCIENCE LTD. ISSN 1872-9533

Full text not available from this repository.

Abstract

The scavenging of dissolved trace metals such as chromium (Cr), molybdenum (Mo) and tungsten (W) and their authigenic enrichment in sedimentary archives mainly occur by particle shuttling that cause resolvable isotope fractionation. Because their scavenging is also dependent on the local marine redox potential and the overall marine chemical environment, the stable isotope composition of these elements in Archean sediments is widely used as paleoredox proxy. Tungsten, a new element in this tool box, is dissolved as the oxyanion tungstate (WO42-) at extremely low marine redox potentials and can thus help to reconstruct the earliest changes in the marine redox state. We tested the applicability of stable W isotopes as a new paleoredox proxy of the atmosphere-ocean system. We analyzed the delta W-186/184 of Precambrian igneous rocks to investigate the detrital background delta W-186/184 signature. This Precambrian igneous inventory (PII) shows delta W-186/184 values between -0.007 and +0.097%o, identical to the range of modern igneous crustal rocks, which indicates that the average Earth's crust delta W-186/184 remained constant over billions of years. Furthermore, we present delta W-186/184 values of euxinic sediments from the Black Sea as a modern sedimentary analog, which we then compare with delta W-186/184 data of Archean and Proterozoic black shales (3.47-2.3 Ga). Modern Black Sea sapropels reveal crustal-like delta W-186/184 values between +0.050 and +0.071%o suggesting limited authigenic enrichment of W in euxinic environments. Similarly, post-Great Oxidation Event shales (2.3 Ga) show crustal- or PII-like delta W-186/184 values indicating the deposition in euxinic or oxic environment. However, the Archean shale sample suites show elevated W concentrations and fractionated delta W-186/184 values up to +0.246%o, which we attribute to authigenic enrichment of heavy seawater W in a ferruginous setting. Thus, black shales delta W-186/184 values can help to distinguish between anoxic-ferruginous and euxinic depositional environments. Furthermore, we combine previously published Cr and Mo isotope data with new Cr and W isotope data to quantify the evolution of the marine redox state during the Archean. By combining the Eh-pH stability fields of W and Mo with the delta W-186/184 and delta Mo-98/95 values of Precambrian sediments, we suggest a 3-step evolution of the Precambrian ocean. (1) From 3.47 to 3.0 Ga, the Eh of a dominantly ferruginous ocean was between -0.4 V and -0.25 V, enabling the persistence of soluble WO42- but not MoO42- and thus only the authigenic enrichment of isotopically fractionated W. (2) Starting from 3.0 Ga, the Eh of some shallow-marine environments increased above -0.25 V, as indicated by delta Mo-98/95 values above the detrital background in shallow-marine sediments, while the Eh of the deep ocean still remained below -0.25 V indicated by the lack of authigenic Mo enrichment and fractionated delta Mo-98/95 values in deep-sea shales. (3) After 2.7 Ga elevated delta Mo-98/95 and delta W-186/184 values in deep-marine sediments suggest that the Eh of the ferruginous open-ocean now also increased above -0.25 V. We therefore suggest that the combination of delta Mo-98/95 and delta W-186/184 values of shales is a very promising tool to investigate earliest changes in marine redox conditions during the Archean. (C) 2021 Elsevier Ltd. All rights reserved.

Item Type:	Journal Article
Creators:	Creators Email ORCID ORCID Put Code Roue, Lucile UNSPECIFIED UNSPECIFIED UNSPECIFIED Kurzweil, Florian UNSPECIFIED UNSPECIFIED UNSPECIFIED Wille, Martin UNSPECIFIED UNSPECIFIED UNSPECIFIED Wegwerth, Antje UNSPECIFIED UNSPECIFIED UNSPECIFIED Dellwig, Olaf UNSPECIFIED orcid.org/0000-0002-0028-8812 UNSPECIFIED Munker, Carsten UNSPECIFIED orcid.org/0000-0001-6406-559X UNSPECIFIED Schoenberg, Ronny UNSPECIFIED UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-591083
DOI:	10.1016/j.gca.2021.05.013
Journal or Publication Title:	Geochim. Cosmochim. Acta
Volume:	309
Page Range:	S. 366 - 388
Date:	2021
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 BARBERTON GREENSTONE-BELT; MASS-DEPENDENT VARIATIONS; ATMOSPHERIC OXYGEN LEVELS; UPPER CONTINENTAL-CRUST; MOLYBDENUM ISOTOPE; GREAT OXIDATION; SULFIDIC WATERS; BLACK SHALES; DOUBLE-SPIKE; CHEMICAL-COMPOSITION Multiple languages Geochemistry & Geophysics Multiple languages
URI:	http://kups.ub.uni-koeln.de/id/eprint/59108