2009年03月21日

Reassessing the first appearance of eukaryotes and cyanobacteria

The evolution of oxygenic photosynthesis had a profound impact on the Earth's surface chemistry, leading to sharp rise in atomo-spheric oxygen between 2.45 and 2.32 billion years (Gyr) ago and the onset of extreme ice ages.(Beekker,A. at al. Dating the rise of atmospheric oxygen. nature427,117-120/2004)(Canfield,D.E The early history of atmospheric oxygen annu. Rev. Earth Planet. Sci. 33,1-36/2005)(Kopp,R.E., Kirschvink, J.L., Hilburn,I.A. &Nash,C.Z. The Paleoproterozoic snowball Earth : a climate disaster triggered by the evolution of oxygenic photosynthesis.Proc. Natl Acad. Sci. USA 102,11131-11136/2005)

The oldest widely accepted evidace for oxygenic photosynthesis has come from hydorocarbons extracted from 〜2.7-Gyr-old shales in the Pilbara Craton, Australia,whicth contain traces of biomarkers (molecular fossils) indicative of eukaryotes and suggestive of oxygen-producing cyanobacteria.(brocks,J.J., Logan,G.A., Buick,R. & Summons,R.E. Archean molecular fossils and the early rise of eukaryotes. Science 285,1033-1036/1999)(Brocks,J.J., Logan,G.A., Buick,R. & Summons,R.E. Composition and sungeneity of molecular fossils from the 2.38 to 2.45 billion-year-old Mount Bruce Supergroup, Pilbara Craton, Western Australia. Geochim. Cosmochim. Acta 67,4289-4319-/2003)(Brocks,J.J., Buick,R., Summons, R. E. & G. A. A reconstruction of Archean biological diversity based on molecular fossils from the 2.78 to 2.45 billion-year-old Mount Bruce Supergroup, Hamersley Basin, Western Australia. Geochim. Cosmochim. Acta 67,4321-4335/2003)(Simmons,R.E., Jahnke,L.L., Hope,J.M. & Logan,G.A. 2-Methylhopanoids as biomarkers for cyanobacterial oxygenic photosynthesis. Nature 400,554-557/1999)

The soluble hydrocarbons were interpreted to be indigenous and syngentic despite metamorphic alteration and extreme enrichment (10-20‰) of ^13~C relative to bulk sedimentary organic matter.(Brocks,J.J., Logan,G.A., Buick,R. & Summons,R.E. Composition and sungeneity of molecular fossils from the 2.38 to 2.45 billion-year-old Mount Bruce Supergroup, Pilbara Craton, Western Australia. Geochim. Cosmochim. Acta 67,4289-4319-/2003)(Heyes, J.M., Kaplan,I.R> & Wedeking,K.W. in Earth's Earliest Biosphere : Its Oligin and Evolution / Ed. Schopf,J.W / 93-134 / Princeton Iniv. Press,1983)

Here we present micrometre-scale, in situ ^13~C/^12~C measurements of pyrobitumen (thermally altered petroleum) and kerogen from these metamorphosed shales, including samples that priginally yielded biomarkers.

Our results show that both kerogen and pyrobitumen are stotongly depleted in ^13~C, indicating that indigenous petroleum is 10-20‰ lighter than the extracted hydrocarbons.(brocks,J.J., Logan,G.A., Buick,R. & Summons,R.E. Composition and sungeneity of molecular fossils from the 2.38 to 2.45 billion-year-old Mount Bruce Supergroup, Pilbara Craton, Western Australia. Geochim. Cosmochim. Acta 67,4289-4319-/2003)

These ersults are inconsistent with an indigenous origin for the biomarkers.

Whatever their origin, the biomarkers must have entered the rock after peak metamorphism 〜2.2Gyr ago and thusdo not provide evidence for the existence of eukaryotes and cyanobacteria in the Archaean eon.(Rasmussen,B., Fletcher,I.R. & Sheppard,S. Isotopic dating of the migration of a low-grade metamorphic front during orogensis. Geology 33,773-776/2005)

The old est fossil evidence for eukaryotes and cyanobacteria therefore reverts to 1.78-1.68Gyr ago and 〜2.15Gyr ago, respectively(Knoll,A.H., Javaux, E.J., Hewitt,D & Cohen, P. Eukarryyotic organisms in Proterrozoic oceans. Phil. Trans R. Soc. B 361,1023-1038/2006)(Hofmann,H.J. Precambrian microflora, Belcher Lslands, Canada:Significance and systematics. J.Paleontol, 50,1040-1073/1976)

Our results eliminate the evidence for oxygenic photosynthesis 〜2.7Gyr ago and exclude previous biomarker evidence for a long delay (〜300 million years) between the appearance of oxygen-producing cyanobacteria and tha rise in atmospheric axygen 2.45-2.32Gyr ago.(Beekker,A. at al. Dating the rise of atmospheric oxygen. nature427,117-120/2004)


### DataBace ###
nature Vol.445 1007-1148 Issue no.7216 23 October 2008
Letter p.1101 / Reassessing the first appearance of eukaryotes and cyanobacteria / B Rasmussen et al. (Cartin Univercity of technology)


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