Desiccation Cracks: Microbially Induced Compared to Sterilized Sediment

Abstract

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 with a microbial mat, where grains are bound by bacterial filaments and mucous-like extracellular polymeric substances (EPS), responds differently to physical stresses resulting in sedimentary structures with distinct morphologies which can be identified in the geologic record as microbially induced. Studying MISS is important, as they provide insight into the oldest ecosystem on Earth, microbial mats, and can aid in paleoenvironmental reconstruction. A series of desiccation experiments were carried out to determine morphological differences between microbially induced desiccation cracks and those formed in identical, but sterilized siliciclastic sediment. Three sediment mixtures were used: (1) very fine-sized sand, (2) mixed silt/clay and (3) normally graded silt/clay. During a 30-day growth period, microorganisms proved most successful at colonizing the sand substratum, forming a continuous microbial mat, while only forming a loose attachment to the clay-rich sediment and becoming easily detached by accumulating metabolic gasses. Presence of the microbial mat only effected the top most layer of sediment during desiccation, up to 1 mm in the sand and up to 100 micrometers in the silt/clay as determined by SEM. The water rich mat and clumps of attached siliciclastic grains coiled upwards with desiccation, producing wide inter-crack regions and leaving rugged and pitted texture in the subsediment. Younger microbial mats and biofilms were not thick enough to curl upwards with drying. The data indicate that the appearance of coiled crack margins can be an accurate indicator of ancient microbial life especially when combined with other textures unique to mat growth. Desiccation of the sterilized sediment-only material, produced desiccation cracks with flat-laying, narrow crack margins and only the normally graded sediment produced concave-up crack polygons which can be mistaken for microbially induced. However, the concave up slabs were much thicker than the millimetre/micrometer-thin mat coils and did not show the same degree of curvature. The dry microbial curls remained intact with flooding and water agitation showing good preservation potential, while the abiotic concave-up crack margins quickly disappeared with reintroduction of water into the tank.

AAPG Datapages/Search and Discovery Article #90259 ©2016 AAPG Annual Convention and Exhibition, Calgary, Alberta, Canada, June 19-22, 2016