Metalliferous Biosignatures for Deep Subsurface Microbial Activity

John Parnell, Connor Brolly, Sam Spinks, Stephen Bowden

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Abstract

The interaction of microbes and metals is widely assumed to have occurred in surface or very shallow subsurface environments. However new evidence suggests that much microbial activity occurs in the deep subsurface. Fluvial, lacustrine and aeolian ‘red beds’ contain widespread centimetre-scale reduction spheroids in which a pale reduced spheroid in otherwise red rocks contains a metalliferous core. Most of the reduction of Fe (III) in sediments is caused by Fe (III) reducing bacteria. They have the potential to reduce a range of metals and metalloids, including V, Cu, Mo, U and Se, by substituting them for Fe (III) as electron acceptors, which are all elements common in reduction spheroids. The spheroidal morphology indicates that they were formed at depth, after compaction, which is consistent with a microbial formation. Given that the consequences of Fe (III) reduction have a visual expression, they are potential biosignatures during exploration of the terrestrial and extraterrestrial geological record. There is debate about the energy available from Fe (III) reduction on Mars, but the abundance of iron in Martian soils makes it one of the most valuable prospects for life there. Entrapment of the microbes themselves as fossils is possible, but a more realistic target during the exploration of Mars would be the colour contrasts reflecting selective reduction or oxidation. This can be achieved by analysing quartz grains across a reduction spheroid using Raman spectroscopy, which demonstrates its suitability for life detection in subsurface environments. Microbial action is the most suitable explanation for the formation of reduction spheroids and may act as metalliferous biosignatures for deep subsurface microbial activity.
Original languageEnglish
Pages (from-to)107-118
Number of pages12
JournalOrigins of Life and Evolution of the Biosphere
Volume46
Issue number1
Early online date16 Sep 2015
DOIs
Publication statusPublished - Mar 2016
EventThe 14th European Astrobiology Conference - Edinburgh, United Kingdom
Duration: 13 Oct 201416 Oct 2014

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Keywords

  • deep subsurface
  • deep biosphere
  • reduction spheroid
  • iron-reducing bacteria
  • metalliferous biosignature
  • selenium
  • raman spectroscopy

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