Earth and Planetary Science Letters, March 15, 2013, Vol.366, p.17(10)
To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.epsl.2013.01.028 Byline: Florian Kurzweil (a)(b), Mark Claire (c), Christophe Thomazo (d), Marc Peters (e), Mark Hannington (f), Harald Strauss (a) Keywords: multiple sulfur isotopes; MIF-S; atmospheric oxygenation; late Archean; Great Oxidation Event Abstract: Mass-independently fractionated sulfur isotopes (MIF-S) provide strong evidence for an anoxic atmosphere during the Archean. Moreover, the temporal evolution of MIF-S shows increasing magnitudes between 2.7 and 2.5Ga until the start of the Great Oxidation Event (G.O.E.) at around 2.4Ga. The conclusion of a completely anoxic atmosphere up to the G.O.E. is in contrast to recent studies on redox-sensitive elements, which suggest slightly oxidizing conditions during continental weathering already several hundred million years prior to the G.O.E. In order to investigate this apparent inconsistency, we present multiple sulfur isotopes for 2.71Ga pyritic black shales derived from the Kidd Creek area, Ontario, Canada. These samples display high positive [DELTA].sup.33S values up to 3.8a[degrees] and the typical late Archean slope in [DELTA].sup.36S/[DELTA].sup.33S of -0.9. In contrast, the time period before (3.2-2.73Ga) is characterized by greatly attenuated MIF-S magnitudes and a slope in [DELTA].sup.36S/[DELTA].sup.33S of -1.5. We attribute the increase in [DELTA].sup.33S magnitude as well as the contemporaneous change in the slope of [DELTA].sup.36S/[DELTA].sup.33S to changes in the relative reaction rate of different MIF-S source reactions and changes in atmospheric sulfur exit channels. Both of these are dependent on atmospheric CH.sub.4:CO.sub.2 and O.sub.2 mixing ratios. We propose a distinct change in atmospheric composition at 2.7Ga resulting from increased fluxes of oxygen and methane as the best explanation for the observed Neoarchean MIF-S record. Our data and modeling results suggest that oxygenic photosynthesis was a major contributor to primary productivity 2.7 billion years ago. Author Affiliation: (a) Westfalische Wilhelms-Universitat Munster, Institut fur Geologie und Palaontologie, Munster, Germany (b) Eberhard Karls University of Tubingen, Institute of Geochemistry, Wilhelmstra[sz]e 56, 72076 Tubingen, Germany (c) School of Environmental Sciences, University of East Anglia, Norwich, Norfolk, United Kingdom (d) UMR/CNRS 5561 Biogeosciences, Universite de Bourgogne, Dijon, France (e) Laboratory of Resource Engineering and Remediation, Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Sciences, Beijing 100101, China (f) Department of Earth Sciences, University of Ottawa, Marion Hall, Ottawa ON, Canada Article History: Received 28 August 2012; Revised 11 January 2013; Accepted 21 January 2013 Article Note: (miscellaneous) Editor: B. Marty
Sulfur Compounds -- Analysis ; Sulfur -- Analysis ; Photosynthesis -- Analysis ; Plant Biochemistry -- Analysis
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