Field observations on the evolution of reticulate patterns in microbial mats in a modern siliciclastic coastal environment

Abstract

Reticulate patterns often found in siliciclastic rocks as wrinkle structures may have been associated with or the product of biological activity. They are also present in modern environments, and laboratory experiments have elucidated the role of filamentous cyanobacteria in their formation, thus considering these microbes putatively as ecosystem engineers. The present study traces the evolution of reticulate structures in situ in a modern siliciclastic coastal sedimentary basin for over a year, under different hydrodynamic conditions. The results give new insights on the parameters involved in the formation and preservation of these microbial structures. Field observations documented the development of millimeter-size microbial reticulate structures with specific geometries. They were found in ephemeral ponds starting from two-dimensional submerged laminated cyanobacterial mats, and ultimately created three-dimensional protruding tufts and pinnacles in junctional positions after their desiccation. Reticulate patterns were formed on top of microbial mats four days after a storm flooding and two days after seawater vacated the area under calm conditions, by virtue of the motility of filamentous cyanobacteria. Through their subsequent consolidation they can be maintained in the form of reticulate structures and tufts for extended periods (months). These structures were found sharing an area with deformation sedimentary structures such as microbial folds, roll-ups, and ripped mats, formed under high energy. Therefore, calm settings such as those created by a shallow-water lamina seem to be a requisite for the formation of reticulate structures in the microbial mats, but once they become established, the microbial mats withstand high-energy hydrodynamic regimes. Our observations of modern structures and the sequential in situ study of their evolution provide linking references to laboratory and rock-record microbial reticulates, aiding in paleoenvironmental reconstruction.