Does the low diversity of microfossils in Archean sedimentary rocks reflect a low diversity of microbial life in the Archean?

submitted by Mark Skidmore, Montana State University; Joel Thompson, Eckerd College; and Stanley Awramik, University of California, Santa Barbara

Why is this question important?

The diversity of life in the Archean is a key component in the history/development of life on Earth

What we know...

• Archean microfossils have been reported from hydrothermal massive sulfides (Rasmussen 2000) and glassy basalts (Furnes et al. 2004) however this page focuses on the sedimentary record.
• Preservation of biota is selective through Earth history favoring organisms with hard parts.
• Biota in the Archean is microbial and these microbes did not have hard parts.
• For microbial fossils in the Archean to be preserved requires silicification i.e. replacement of the microbe with silica (SiO₂) and producing a permineralized fossil"
• SiO₂ entombs the organics of the microbes thus leading to good preservation.
• Calcite (CaCO₃) is not as effective for long term preservation since it is readily soluble by carbonic acid in ground/surface waters and doesn't preserve the organics from degradation.
• Phototrophic organisms are commonly observed at the sediment water interface in shallow water environments in oceans and lakes.
• The cell envelope (plasma membrane outer membrane and sheath) in cyanobacteria is particularly resistant to decay and decomposition as observed in modern environments thus may be preferentially preserved in the rock record.
• Cyanobacteria may be either oxygenic or anoxygenic (sulfide utilizing) however it is not possible to distinguish a metabolism from the microfossil

How to link this topic to the classroom

This may be used in Historical Geology, Paleontology, or Geomicrobiology courses.

This question may be supplemented with images from thin sections from microbial fossils and modern analogous environments showing greater diversity, such as those found at Microbial Life - Educational Resources, and/or a video of microbial fossils (e.g. Gunflint chert)

References and other Resources

Knoll A. H. (1985) Exceptional preservation of photosynthetic organisms in silicified carbonates and silicified peats. Phil Trans Royal SocietyB vol 311 111-122.

Ferris F.G. Fyfe W. S. and Beveridge T.J. (1988) Metallic iron binding by Bacillus Subtilis implications for fossilization of microorganisms. Geology 16 149-152.

Jones B. Konhauser K. Renaut R.W. and Wheeler R.S. (2004) Microbial silicification in Iodine Pool Waimangu geothermal area North Island NZ: implications for recognition and identification of ancient silicified microbes. Journal of the Geological Society. V. 161 983-993

Benning L.G. Phoenix V.R. Yee N. and Konhauser K.O. 2004 The Dynamics of cyanobacterial silicification: An infrared micro-spectroscopic investigation. Geochimica et Cosmochimica Acta v. 68 p. 743-757.

Bartley J. (1996) Actualistic Taphonomy of Cyanobacteria:Implications for the Precambrian Fossil Record. Palaios 11 571-586.

Furnes H. Banerjee N.R. Muehlenbachs K. Staudigel H. de Wit M. (2004) Early Life Recorded in Archean Pillow Lavas. Science 304 578-581.

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