A laboratory model of abiotic peloid formation

Peloids are rounded grains of micritic calcite whose origin has been attributed to various biological and abiotic mechanisms. To constrain abiotic parameters that favour the formation of peloids, we precipitated calcite crusts in the absence of microorganisms. Clotted opaque fabrics that formed during the initial stages of the experiment consisted of similar to 10 mu M peloids, while compact clear sparitic crusts precipitated in subsequent stages. The increasing supersaturation of the solution in time is responsible for this morphological succession. Initially, peloids form by the radial growth of spar crystals around a small number of nuclei. As the supersaturation increases, more spar crystals nucleate and aggregate nonradially into compact crusts. Rounded clotted precipitates are a consequence of the growth in suspension and geopetal settling, and isopachous crusts grow in the absence of these processes. Although peloids are commonly assumed to have a microbial origin, our results show that very similar morphologies can be created by purely abiotic mechanisms. Thus, the biological origin of rounded micritic calcite grains in the rock record must be verified against the abiotic null-hypothesis in each specific case.