Submitted
2005 to Astrobiology
Establishing
Biogenicity Criteria for Endolithic Microborings on Early Earth and Beyond
Nicola McLoughlin1*, Martin D.
Brasier1, Randall S. Perry1,2, David Wacey1
and Owen R. Green1.
1Earth Sciences, Oxford University, Parks Road, Oxford,
OX1 3PR, United Kingdom; 2Planetary Science Institute, 6920
Roosevelt Way NE 177, Seattle, WA 98115 USA;
* corresponding author: nicolam@earth.ox.ac.uk, tel. +44 1865 272069, fax. +44 1865 272072.
Micron sized cavities created by the actions of
rock-etching
microorganisms known as
euendoliths are explored as a biosignature for life on early Earth and perhaps
Mars. Rock dwelling organism can tolerate extreme environmental stresses and
are excellent candidates for the colonisation of early Earth and planetary
surfaces. We review the fossil record of euendoliths in both sedimentary and
volcanic rocks and formulate the following criteria for establishing their
biogenicity: firstly, a geological setting which demonstrates a primary
syngenetic origin for the microtubes and was viable for life; secondly,
uniquely biogenic microtube morphologies and behaviours distinct from abiotic
mimics, particularly ambient inclusion trails; thirdly, elemental and isotopic
evidence suggestive of biological processing. We use these criteria and the
fossil record of terrestrial endoliths to identify potential environments on
Mars where endolithic microborings may exist: aqueous carbonate deposits, volcanic, hydrothermal
and impact glasses.