MICROBIAL MEDIATION OF MODERN DOLOMITE PRECIPITATION AND DIAGENESIS UNDER ANOXIC CONDITIONS (LAGOA-VERMELHA, RIO-DE-JANEIRO, BRAZIL)

Dolomite, despite its thermodynamic stability and abundance in the anc ient rock record, is rarely found forming in Holocene environments. Th is enigma is frequently called the Dolomite Problem. The recent discov ery of modern dolomite formation in Lagoa Vermelha, a shallow-water is olated coastal lagoon east of Rio de Janeiro, Brazil, provides a new e nvironment to investigate the factors promoting dolomite precipitation under earth surface conditions. Lagoa Vermelha serves as a natural la boratory in which the dolomite formation process was studied using an integrated hydrologic, geochemical, and sedimentological approach. The results of this study indicate that Ca-dolomite precipitation occurs under anoxic hypersaline conditions within a black sludge layer direct ly overlying the water/sediment interface. With deposition, the dolomi te undergoes an ''ageing'' process, whereby increased ordering of the crystal structure occurs, Both the initial precipitation and subsequen t early diagenesis are strongly mediated by microbial activity. In fac t, using sulfate-reducing bacteria cultured from Lagoa Vermelha sample s, a highly ordered dolomite has been produced in the laboratory at lo w temperatures. These experimental results combined with the study of the natural environment mandate that a microbial factor be added to th e list of factors capable of causing dolomite precipitation. Consideri ng the Lagoa Vermelha system, we propose a new actualistic model for d olomite formation, which we call the microbial dolomite model.