Mo-Cr isotope evidence for a reducing Archean atmosphere in 3.46-2.76Ga black shales from the Pilbara, Western Australia

Martin Wille, Oliver Nebel, Martin Van Kranendonk, Ronny Schoenberg, Ilka C. Kleinhanns, Michael J. Ellwood

Research output: Contribution to journalArticle

29 Citations (Scopus)

Abstract

The distribution of redox-sensitive elements and their stable isotope variations in marine sediments has been employed to track the possible oxygenation of the atmosphere at, and before, the Great Oxidation Event (GOE; 2.4-2.2. Ga). Contrasting datasets have been used to advocate for and against the rise of free oxygen in the atmosphere prior to 2.4. Ga based on various geochemical tracers, and evidence for a partially oxidized environment remains elusive.Herein, we present stable isotope datasets for Mo and Cr, in conjunction with major and trace elements from four black shale horizons spanning a time interval from 3.46 to 2.76. Ga in order to ascertain atmospheric oxygen levels through the Archean. The Mo, Cr and U elemental signatures within all sedimentary units are dominated by continental input. Both Mo isotopic values and Mo and U elemental signatures suggest anoxic conditions with no indication of redox cycling of these elements. This contradicts previous interpretations of an oxidized atmosphere based on published sulfur isotopic data (Ohmoto et al., 2006) and oxidized hematite occurrences at ~. 3.45. Ga (Hoashi et al., 2009).Cr concentration patterns within the sample suites are controlled by continental provenance composition rather than by an authigenic Cr seawater contribution. This interpretation is supported by the relatively homogeneous Cr isotopic compositions across all sample suites and is consistent with high temperature controlled Cr isotopic ratios. Identification of redox cycling processes within detrital dominated sediments with Cr isotopes is likely to be less sensitive compared to Mo isotopes, due to the much smaller authigenic/crustal enrichment factors for Cr.

Original languageEnglish
Pages (from-to)68-76
Number of pages9
JournalChemical Geology
Volume340
DOIs
Publication statusPublished - Feb 4 2013
Externally publishedYes

Fingerprint

Isotopes
Archean
isotope
atmosphere
stable isotope
oxygen
Sediments
black shale
oxygenation
Oxygen
isotopic ratio
anoxic conditions
hematite
marine sediment
provenance
Oxygenation
Trace Elements
isotopic composition
Shale
Chemical analysis

Keywords

  • Archean
  • Atmospheric oxygen
  • Chromium isotopes
  • Molybdenum isotopes

ASJC Scopus subject areas

  • Geology
  • Geochemistry and Petrology

Cite this

Mo-Cr isotope evidence for a reducing Archean atmosphere in 3.46-2.76Ga black shales from the Pilbara, Western Australia. / Wille, Martin; Nebel, Oliver; Van Kranendonk, Martin; Schoenberg, Ronny; Kleinhanns, Ilka C.; Ellwood, Michael J.

In: Chemical Geology, Vol. 340, 04.02.2013, p. 68-76.

Research output: Contribution to journalArticle

Wille, Martin ; Nebel, Oliver ; Van Kranendonk, Martin ; Schoenberg, Ronny ; Kleinhanns, Ilka C. ; Ellwood, Michael J. / Mo-Cr isotope evidence for a reducing Archean atmosphere in 3.46-2.76Ga black shales from the Pilbara, Western Australia. In: Chemical Geology. 2013 ; Vol. 340. pp. 68-76.
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