Microbialites of terminal Ediacaran in the Upper Yangtze Platform, China: From mesoscopic to nanoscale

Dolomitic microbialites were developed extensively during the Neoproterozoic and record widespread oxygenation of the oceans and the accompanying rise of animal life. Primary precipitation of dolomite cements has now been recognized in both Cryogenian and Ediacaran periods, although the origin of matrix dolomite is debated. The terminal Ediacaran Dengying Formation (551–541 Ma) in the Upper Yangtze Platform of China contains massive dolomite. Particularly delicate microbial structures are preserved in the second member of the formation (Z2dn2). This study aimed to elucidate the origin of matrix dolomite in these microbialites, the main mesoscale features of which are stromatolites, thrombolites and composite microbialites. The stromatolites typically have planar and low-relief planar–domal forms, with distinct laminated microstructure alternated with diffuse laminated microstructures. The thrombolites comprise a variety of mesoclot structures and cavities, which are rimmed by isopachous fringes of fibrous cement and filled by late sparry cement. The composite microbialites have clotted and laminated textures of various styles that are intermingled at the sub-centimeter scale. These microbialites are inferred to have been deposited in a subtidal to intertidal setting. Abundant nanoscale dolomite biomorphs were observed with scanning electron microscopy, including single nanoscale spheroids and aggregates of nanometer filaments with granulated, micro-rose shaped, micro-sized foliage-like, and trumpet-like forms, most of which are presumably of biogenic origin. These nanoscale microbial textures act as archives of ancient microbial activity that may have produced abundant extracellular polymeric substance, providing sites for the nucleation and maturation of dolomite. This process may have been facilitated by Mn–S redox cycling and resulted in precipitation of primary ferroan dolomite in the matrix.