Astrobiology Workshop, Macquarie University July 12-13 2001

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Rocks that Grow: Searching for Biotic Signatures in Rocks Produced by Microbes

Gregory Webb (School of Natural Resource Sciences - Queensland University of Technology)

Rocks produced largely by the biologically induced precipitation of carbonate minerals have been abundant throughout Earth history and include some of the oldest traces of life. Microbes and organic matter were capable of inducing carbonate precipitation long before organisms evolved the ability to control precipitation and thereby construct skeletons. The products of biologically induced precipitation include a vast array of microbialites, organomicrites, and automicrites. Unfortunately, owing to the wide variety of microbial and eukaryotic communities, dead organic matter, and processes that have been implicated in the formation of such deposits and the lack of direct biological control on the detailed morphology of individual precipitates, interpretation of the origin of certain ancient deposits is by no means clear. Hence, there is a need to identify "signatures" in biologically induced precipitates that can provide unequivocal evidence of their origin.

The gross structure and microstructure of some microbialites provide clues as to their origins. Biologically induced precipitation may form crystals on the surface of organic matter or within organic matter, either as a replacement of particular material, or in spaces within larger organic frameworks. Where carbonate minerals replace organic matter, recognizable biotic structures may be preserved, thereby leading to formation of calcimicrobes. However, where precipitation occurs on an organic surface, unconstrained crystal growth leads to crust morphologies that are indistinguishable from the products of abiotic precipitation. Where organic matter breaks down prior to complete precipitation of carbonates, individual silt- and clay-sized crystals may be released as discrete sediment that may be indistinguishable from the products of abiotic precipitation or bioerosion.

A variety of modern and Holocene microbialites have been documented in the Great Barrier Reef, Queensland Australia. High-Mg-calcite thrombolite crusts occur in cryptic cavities in Holocene reef framework. Initial precipitation occurred as a replacement of the basal mucus layer of a biofilm, but that carbonate subsequently provided nucleation sites for unconstrained precipitation of scalenohedra that have an abiotic appearance. Cavities in beachrock on Heron Island contain microstromatolites consisting of radiating aragonite crusts, irregularly distributed aragonite crystals and irregular high-Mg-calcite. Rare calcimicrobes occur, but most of the microstructures appear very similar to abiotic minerals. Hence, even in living microbialites, microstructure may not be independently indicative of a biotic origin.

Trace element geochemistry may provide signatures of biotic influence in carbonate precipitation, because a variety of microbes make use of specific cations in their metabolic pathways, thereby leading to concentration or fractionation of certain elements. Reefal microbialites from the Great Barrier Reef (Holocene), the Akiyoshi Reef of Japan (Permo-Carboniferous) and the Gamohaan Formation, South Africa (Late Archean) have been shown to concentrate rare earth elements in equilibrium with the seawater in which they grew. Although no independent microbial signature has been recognized in the REE distributions, the microbialites have much higher REE concentrations than co-occurring skeletal material and skeletal material commonly causes some degree of fractionation. Although microbialite REE seem to be in equilibrium with seawater, other elements are under investigation.