Calcimicrobes in Cambrian microbialites (Shandong, North China) and comparison with experimentally produced biomineralization precipitates

Microbialites can be important oil and gas reservoirs because of their high initial porosity and they may well be source rocks too. However, the formation mechanisms of calcimicrobes in microbialites are still not fully understood, and further research is needed. Herein, microbialites (thrombolites, dendrolites, and oncolites) from the Cambrian successions of Shandong Province, China, are examined for their microbial fabrics and comparisons are made with the results of laboratory biomineralization experiments using the modern cyanobacterium Synechocystis sp. PCC6803 and the halophilic Bacillus licheniformis SRB2. The thrombolites show mosaic, meshed, and spotted textures, each with a different microbial clot morphology, and Epiphyton is the dominant calcimicrobe. Dendrolitic buildups consist of a lower gray core dominated by the chambered Renalcis, and an upper orange part dominated by dendritic Epiphyton , suggesting a transition of the microbial community across the color change, related to an increase in iron content. Oncolites occur in beds up to 10-cm-thick with spherical to ellipsoidal oncoids containing Girvanella developed around a bioclast nucleus. Scanning electron microscopy observations distinguish the calcimicrobes with their rough, peloidal surface morphology from adjacent cement; aggregated spheroidal and or filamentous calcimicrobes can be recognized, the latter with hollow centers in some cases. The results of the laboratory biomineralization experiments show that the two modern microbes can induce the precipitation of carbonate minerals, mostly of calcite. These biominerals are commonly granular and have a complex microscopic structure including spherical and dumbbell shapes. The microbes are commonly encrusted with calcite and after decomposition of the bacteria, hollow mineral crusts are produced; these are the result of the nucleation and growth of nano-minerals on the extracellular polymeric substances (EPS) present around the microbes. The bio-precipitates induced by modern microbes in the laboratory experiments are similar in many respects to the calcimicrobes in the Cambrian microbialites; biomineralization processes clearly play an important role in the formation of carbonate minerals and lithification of microbial mats.