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PHYSICAL SCIENCES / GEOLOGY
Late Archean rise of aerobic microbial ecosystems

Jennifer L. Eigenbrode^*,,, and Katherine H. Freeman^*

*Department of Geosciences and Penn State Astrobiology Research Center, Pennsylvania State University, University Park, PA 16802; and Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC 20015

Communicated by John M. Hayes, Woods Hole Oceanographic Institution, Woods Hole, MA, August 30, 2006 (received for review January 18, 2006)

We report the ¹³C content of preserved organic carbon for a 150 million-year section of late Archean shallow and deepwater sediments of the Hamersley Province in Western Australia. We find a ¹³C enrichment of 10 in organic carbon of post-2.7-billion-year-old shallow-water carbonate rocks relative to deepwater sediments. The shallow-water organic-carbon ¹³C content has a 29 range in values (–57 to –28), and it contrasts with the less variable but strongly ¹³C-depleted (–40 to –45) organic carbon in deepwater sediments. The ¹³C enrichment likely represents microbial habitats not as strongly influenced by assimilation of methane or other ¹³C-depleted substrates. We propose that continued oxidation of shallow settings favored the expansion of aerobic ecosystems and respiring organisms, and, as a result, isotopic signatures of preserved organic carbon in shallow settings approached that of photosynthetic biomass. Facies analysis of published carbon-isotopic records indicates that the Hamersley shallow-water signal may be representative of a late Archean global signature and that it preceded a similar, but delayed, ¹³C enrichment of deepwater deposits. The data suggest that a global-scale expansion of oxygenated habitats accompanied the progression away from anaerobic ecosystems toward respiring microbial communities fueled by oxygenic photosynthesis before the oxygenation of the atmosphere after 2.45 billion years ago.

organic carbon | isotopes | methanotrophy | oxygen

The authors declare no conflict of interest.

To whom correspondence should be addressed at: Geophysical Laboratory, Carnegie Institution of Washington, 5251 Broad Branch Road NW, Washington, DC 20015. E-mail: jeigenbrode{at}ciw.edu

© 2006 by The National Academy of Sciences of the USA

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