Biogenicity inferred from microbialite geochemistry

Abstract

Microbes utilise and/or concentrate diverse metal cations, whose detection may become a potent tool for reconstructing microbial processes and, in particular, for establishing the genesis of ancient carbonate rocks that were produced by microbes. Such rocks, termed microbialites, consist of trapped and bound sediment and, importantly, carbonate minerals precipitated as accidental byproducts of metabolic or decay processes within biofilms. Where trace metals are predictably incorporated into microbialites, they may reflect biofilm processes and allow interpretation of preserved carbonates. Holocene (about 5-6,000 years old) microbialites that formed in reef cavities in the Great Barrier Reef (GBR) faithfully incorporated high concentrations of rare earth elements and yttrium proportional to their abundance in shallow seawater. Ancient microbialites display similar behaviour.