ACTUALISTIC EXPERIMENTS IN MISS FORMATION

Microbially Induced Sedimentary Structures (MISS) represent a novel group of primary sedimentary structures that are produced by microbial and algal interactions with sedimentary substrates. Sediment covered and bound by microorganisms and mucous-like extracellular polymeric substances (EPS) responds differently to physical processes, resulting in structures with distinct morphologies which can be identified in the rock record as microbially induced. To determine how sterile versus microbially stabilized sediment of size < ¼ mm respond to the physical process of desiccation, two tanks with identical sediment mixtures, one sterile and the other colonized by a 35 day old microbial mat, were subjected to dehydration which led to formation of desiccation cracks. The microbial mat covered substrate curled upwards along crack margins and produced concave-up polygonal features. This morphology of microbially induced desiccation cracks was different from the overall planar cracking of the sterile sediment. During growth, the microbial mat produced interesting hemispherical features which were interpreted to be gas domes formed as the result of metabolic gases trapped underneath the mat fabric. As such, we suggest that curled crack margins and collapsed or intact gas domes are preserved in the rock record and can be used to identify microbial activity in the past.