Cyanobacterial-algal crusts from Late Ediacaran paleosols of the East European Craton

The top surface of the Ediacaran flood basalts of West Ukraine (Volyn), extruded during the break-up of Rodinia supercontinent, is weathered into several meters thick paleosol, covered by late Ediacaran (ca. 550 Ma) sediments. The paleosol grades from unaltered basaltic material into saprock and finally clay-dominated saprolite with a minor content of the organic matter. The upper parts are dominated by kaolinite and hematite. SEM images and SEM-EDS analyses revealed in one of the paleosol profiles a horizon with significant amount of morphologically recognizable organic structures, mineralized by silicates and iron oxides. HF-etching of the bulk rock samples revealed lithic fragments covered irregularly by a biocrust of colonial coccoid cyanobacteria associated with irregularly distributed unicellular algae. The cyanobacteria are preserved as mineralized common mucilage sheaths (glycocalyx), which reproduce the original shape typical of sheaths of modern coccoidal cyanobacterial biofilms, whereas the heavily mineralized algae show features characteristic of modern chlorococcaleans. The preservation of morphologically distinguishable remnants of cyanobacterial biofilms and aggregates of unicellular algae in terrestrial weathering deposits is a rare preservational phenomenon requiring rapid post-mortem permineralization of the dead microbiota. One of the main factors facilitating such a unique fossilization was supposedly a copious excretion of extracellular polymeric substances (EPS) by the biocrust forming biota in which early mineral nucleation occurred. The colonization of the Late Ediacaran land by the presumably desiccation-resistant cyanobacteria-dominated crust communities associated with algae was of importance for creating fertile soils and a soil-forming microbial ecosystem that later allowed a successful land invasion by the ancestors of vascular plants.