Abstracts Earth and Environmental Sciences

The Role and Significance of Authigenic Magnesium Silicates in the Organomineralisation of Microbialites in the Yalgorup Lakes, Western Australia

by Robert Victor Burne

This work re-examines samples of modern microbialites collected in the 1980s from Lakes Clifton and Preston, two of the Yalgorup Lakes in southwest Western Australia. Lake Clifton contains the first convincing modern examples of thrombolites to have been recognized. It was known that aragonite mineralisation took place in these structures within near-surface biofilms. New research reveals that stevensite, a Mg-rich trioctahedral smectite, is the principal primary phase that establishes the initial structural rigidity of Lake Clifton thrombolites. Aragonite microcrystals then grow within the stevensite matrix. In adjacent Lake Preston, lithified, centimetre-scale, coniform structures occur that are similar to pinnacle-like microbial mats that grow intermittently in a small pond adjacent to the lake. Microstructures within the lithified cones confirm their microbial origin, but they have undergone four phases of mineralization; an amorphous Mg silicate phase (of smectite-like composition); some areas of Mg silicate were then partially transformed into authigenic serpentine (chrysotile and/or lizardite); aggregates of aragonite microcrystals then overprinted much of the fabric; and finally high-magnesium calcite grew as void fills and rims, as well as overprinting some of the remaining areas of the Mg silicate phase. It is concluded that syngenetic and early diagenetic carbonate mineralisation of microbialites may effectively obscure all traces of the original microbial communities, leaving only faint evidence for their organo-sedimentary origin. Secondary carbonate mineralisation of microbialites may thus eliminate the evidence of primary organomineralisation. Many published examples of apparently abiogenic but microbialite-like carbonates should be re-examined for traces of early silicate mineralisation. The discovery of microbial permineralisation of modern microbialites by Mg silicates in Lakes Clifton and Preston raises the possibility that phyllosilicates could have contributed to the early structural rigidity of some Proterozoic Stromatolites such as Conophyton. Reexamination of the literature on Conophyton tends supports this hypothesis. Finally, the significance of this research is considered in relation to ; clarifying the role of Mg silicates in microbialite organomineralisation; examining the evidence that apparently abiotic crystalline carbonate may be of secondary origin; understanding the nature of the supposed “microbialites” in Cretaceous pre-salt sequences of the proto-Atlantic rift; assessing the continuing relevance of the “microbialite” concept; and clarifying evidence for the recognition of the earliest signs of life on Earth.