Experimental modelling of beachrock cementation under microbial influence

A laboratory model has been developed, which reproduces natural conditions of beachrock formation. These conditions include the porosity of beach sand, tidal movements (alternation of phreatic and vadose conditions), temperature, evaporation, illumination, and water composition (seawater, with a control of the following parameters: salinity, Ca, Mg, ΣCO₂, pH, CaCO₃ saturation). The conditions of two beachrocks from the Red Sea (Egypt) and Shark Bay (Australia) have been simulated, and the influence of the micro-organism population (undetermined) sampled in these beachrocks was observed for 3–4 months. In the experiments inoculated with micro-organisms, biomineralization produced characteristic crystals of various forms from 1 to 20 μm in size. The precipitates resemble the micritic cements of natural beachrocks. On the contrary, precipitation was much less developed in the control experiments under nearly sterile conditions. These results confirm the importance of microbial activity during beachrock formation. The first micritic cement, which seems to control cementation and produce the initial consolidation, is formed under a strong microbial influence in or near a biofilm covering the grain surface. The presence of this micritic cement is necessary for the development of the subsequent prismatic cement, which probably results from an abiotic precipitation during higher fluid circulation