Microbial carbonates are unique deposits formed by the direct or indirect action of benthic microbial communities. They witness early life since Precambrian times, and occur in a wide variety of environments - from marine to continental settings. Although their deposition forms a continuum through geological time scales, microbial carbonates dominate geological records when extreme environmental conditions prevail. Many studies have focused on the characterization of microbial carbonate fabrics and processes of microbial mediated carbonate mineralization in recent environments, but the presence of organic compounds in ancient microbialites remains often questionable. Moreover, microbial carbonates witness high spatial fabric heterogeneity at micro- and nanoscale and often the functional role of specific organic compounds in carbonate mineralization is not yet resolved. Additionally, diagenesis occurs at different spatial and temporal scales but mostly starts in nano- to micro-environments immediately after initial precipitation. This renders detangling diagenetic and environmental signals in microbial carbonates often difficult.
This study highlights the importance of sedimentary petrography, multi-scaled X-ray computed tomography (3D-visualization) and Raman spectroscopy combined with geochemistry and experimental approaches to unravel precipitation mechanisms, environmental conditions and early diagenesis of microbial mediated Ca-Mg carbonates in extreme environments. The unique presence of Holocene and Pleistocene microbialites in the Danakil Depression (Afar, Ethiopia) at varying preservation states, evidences (1) the co-occurrence of both biotically mediated and abiotic driven mineralization processes through space and (2) a continuum between primary precipitation and early diagenetic modification through time.