Tracking organomineralization processes from living microbial mats to fossil microbialites

Abstract

Geneses of microbialites and, more precisely, lithification of microbial mats have been studiedin dierent settings to improve the recognition of biogenicity in the fossil record. Living microbial matsand fossil microbialites associated with older paleoshorelines have been studied in the continentalMaquinchao Basin in southernmost South America. Here, we investigate carbonate crusts from aformer pond where active mineralizing microbial mats have been previously studied. Petrographicobservations revealed the presence of abundant erect and nonerect microfilaments and molds withdiameters varying from 6 to 8 micrometers. Additionally, smaller pores and organic matter (OM)remains have been identified in areas containing less filaments and being dominated by carbonate.A Mg, Al and Si-rich phase has also been identified in the carbonate matrix associated with thedominant micritic calcite. Moreover, mineralized sheaths contain mixed carbonate (calcite) withMg, Al and Si, where the latter elements are associated with authigenic clays. The presence ofmineralized sheaths further attests to biologically induced processes during the uptake of CO2 byphotosynthetic microorganisms. Additionally, the high density of the micritic phase supports thesubsequent mineralization by nonphotosynthetic microorganisms and/or physicochemical processes,such as evaporation. Since the micritic filament microstructure of these recent crusts is very similarto that observed in fossil microbialites, they can be used to bridge the gap between living mats andfossil buildups.