THE RELATIONSHIP BETWEEN STRATIGRAPHIC DISTRIBUTION OF MAGNESITES AND DEVELOPMENT OF CYANOBACTERIA

Abstract

A biochemogenic mechanism is proposed for the formation of magnesite. During photosynthesis, cyanobacteria and, sometimes, other autotrophs consumed CO2 dissolved in water; thus, they increased the pH value of the medium. High alkalinity promoted the precipitation of magnesian compounds that were subsequently transformed into magnesite. This process was also fostered by an elevated content of organic matter, which decayed during diagenesis and produced an alkaline environment stimulating precipitation of magnesian compounds. Thus, Mg precipitation is controlled by high pH values rather than by elevated salinity. The frequent increase of salinity in the basin suppresses highly organized heterotrophic organisms. If such organisms are absent, the ecological niche is occupied by the cyanobacterial communities and other microbial organisms.