Universität zu Köln

Kurzweil, Florian, Wille, Martin ORCID: 0000-0003-1083-4730, Gantert, Niklas, Beukes, Nicolas J. and Schoenberg, Ronny (2016). Manganese oxide shuttling in pre-GOE oceans - evidence from molybdenum and iron isotopes. Earth Planet. Sci. Lett., 452. S. 69 - 79. AMSTERDAM: ELSEVIER SCIENCE BV. ISSN 1385-013X

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Abstract

The local occurrence of oxygen-rich shallow marine water environments has been suggested to significantly predate atmospheric oxygenation, which occurred during the Great Oxidation Event (GOE) ca. 2.4 billion years ago. However, the potential influence of such 'oxygen oases' on the mobility, distribution and isotopic composition of redox sensitive elements remains poorly understood. Here, we provide new molybdenum and iron isotopic data from shallow marine carbonate and silicate iron formations of the Koegas Subgroup, South Africa, that confirm local ocean redox stratification prior to the GOE. Mn concentrations correlate negatively with both delta Mo-98 and delta Fe-56 values, which highlights the substantial role of particulate manganese for the cycling of Mo and Fe in the Paleoproterozoic oceans. Based on these trends we propose that pore water molybdate was recharged (1) by the diffusional transport of seawater molybdate with high delta Mo-98 and (2) by the re-liberation of adsorbed molybdate with low delta Mo-98 during Mn oxide dissolution within the sediment. The relative contribution of isotopically light Mo is highest close to a Mn chemocline, where the flux of Mn oxides is largest, causing the negative correlation of Mn concentrations and delta Mo-98 values in the Koegas sediments. The negative correlation between delta Fe-56 values and Mn concentrations is likely related to Fe isotope fractionation during Fe(II) oxidation by Mn oxides, resulting in lower delta Fe-56 values in the uppermost water column close to a Mn chemocline. We argue that the preservation of these signals within Paleoproterozoic sediments implies the existence of vertically extended chemoclines with a smoother gradient, probably as a result of low atmospheric oxygen concentrations. Furthermore, we suggest that abiotic oxidation of Fe(II) by a Mn oxide particle shuttle might have promoted the deposition of the Koegas iron formations. (C) 2016 Elsevier B.V. All rights reserved.

Item Type:	Journal Article
Creators:	Creators Email ORCID ORCID Put Code Kurzweil, Florian UNSPECIFIED UNSPECIFIED UNSPECIFIED Wille, Martin UNSPECIFIED orcid.org/0000-0003-1083-4730 UNSPECIFIED Gantert, Niklas UNSPECIFIED UNSPECIFIED UNSPECIFIED Beukes, Nicolas J. UNSPECIFIED UNSPECIFIED UNSPECIFIED Schoenberg, Ronny UNSPECIFIED UNSPECIFIED UNSPECIFIED
URN:	urn:nbn:de:hbz:38-258370
DOI:	10.1016/j.epsl.2016.07.013
Journal or Publication Title:	Earth Planet. Sci. Lett.
Volume:	452
Page Range:	S. 69 - 79
Date:	2016
Publisher:	ELSEVIER SCIENCE BV
Place of Publication:	AMSTERDAM
ISSN:	1385-013X
Language:	English
Faculty:	Unspecified
Divisions:	Unspecified
Subjects:	no entry
Uncontrolled Keywords:	Keywords Language BILLION YEARS AGO; OXYGENIC PHOTOSYNTHESIS; ANCIENT EARTH; FE; FRACTIONATION; OXIDATION; OXYGENATION; SEDIMENTS; REDOX; RISE Multiple languages Geochemistry & Geophysics Multiple languages
Refereed:	Yes
URI:	http://kups.ub.uni-koeln.de/id/eprint/25837