Domed-rim microbial polygons and their preservation potential

Abstract

Microbially induced sedimentary structures (MISS) derive from the interaction between sediments and unicellular microorganisms and are of interest to the search for the earliest signs of life in ancient rocks, on Earth and on Mars. Here, we describe a type of MISS in the form of a polygonal structure characterised by a domed rim. Despite earlier suggestions of a biological origin, several aspects regarding their formation mechanism, the criteria to distinguish these structures from abiotic mud cracks, and their preservation potential in the geological record remain insufficiently constrained. To identify and define the distinctive features characterising this MISS, we have investigated the morphology and formation mechanism of polygonal microbial mats in two modern sabkha locations in Qatar. The sedimentological, mineralogical and geochemical analyses suggest that the studied polygons result from microbe-mineral interactions causing the stabilisation of detrital minerals (e.g. quartz, feldspar) and precipitation of micrite comprised of calcite and high-Mg calcite. The polygonal morphology is mainly the result of two co-occurring mechanisms: shrinkage, as a result of desiccation and microbial growth. A ∼1.5 cm-sized domed-rim has been identified as the key morphological feature that is exclusively present in and that allows for the recognition of, the polygons that form in association with a growing microbial mat. In cross-section, a domed-rim is comprised of millimetre-thick laminae showing angular relationships (e.g. overgrowth of older lamina sets) that, we argue, cannot be formed in the absence of growing biomass. Finally, through the study of a radiocarbon-dated ancient sabkha outcrop, it is shown that domed-rim microbial polygons can survive degradation and diagenesis, producing a mineral fossil structure that can potentially be preserved for billions of years.