Microfabrics and organominerals as indicator of microbial dolomite in deep time: An example from the Mesoproterozoic of North China

Abstract

Both laboratory experiment and modern environment studies show that primary dolomite can precipitate at low temperature (≤60 ℃) only when microbes were involved. Vast dolostones in mid-Proterozoic have long been suspected to be the results of microbial mediation, but direct evidence is rare. To reveal their origin, an integrated study was conducted on the Mesoproterozoic Wumishan Formation (∼1.48 Ga) of North China using multiple techniques. The results show that the Wumishan dolostone contains abundant multiscale organominerals that are closely associated with fossilized remnants of extracellular polymeric substances (EPS) and putative bacteria fossils. These microfabrics are strikingly similar to those in dolomite precipitates produced in laboratory culture and modern sabkha environment, suggesting microbial origin. Nanoglobules (60–200 nm) preferentially attach to fossilized EPS filaments, and tend to merge into polyhedrons (3–8 μm) that in turn coalesce into microspheres (20–50 μm). EDS analysis revealed a successive decrease in carbon from nanoglobules to microspheres with the increases of Mg and Ca contents, implying an increased mineralization. Nanoglobules may have derived from EPS degradation during nucleation, and served as seeds for subsequent growth of carbonate polyhedrons. Microsphere, as a basic building block in dolostone, can transform into rhombohedral crystal via neomorphogenesis during early diagenesis. Petrographic and XRD analyses show that the dolostone is dominated by ordered dolomite and was largely formed in peritidal environment with thriving microbial community. Geochemical analyses of iodine species, redox sensitive elements, Ce anomalies, and C–O isotopes in the Wumishan dolostone suggested an environment predominated by anoxic to suboxic conditions, with low P but high Si contents in seawater.Our study offered direct evidence of microbial origin for the Mesoproterozoic dolostone, providing new insights into the “Dolomite Problem”. The massive development of microbial dolostone in Mesoproterozoic points to a specific ocean chemistry, where low oxygen and active bacterial metabolisms may have played crucial roles in precipitating dolomite. The co-existence of abundant microbial components and multiscale organominerals may be taken as textural evidence for microbial dolomites and signatures of life-environment interactions in deep time.