Bacterial sulfate reduction and salinity: two controls on dolomite precipitation in Lagoa Vermelha and Brejo do Espinho (Brazil)

The hydrological system of Lagoa Vermelha, a dolomite-precipitating lagoon in Brazil, was investigated using hydrogen and oxygen stable isotopic composition of the water collected during an annual cycle (1996-1997). These data demonstrated that dolomite formed in May-June during high saline conditions. High salinity apparently provides the ions and saturation state necessary for dolomite precipitation. Ion concentrations in the lagoon water indicated an identical timing of dolomite precipitation and demonstrated that dolomite formed at decreased sulfate concentrations. In Brejo do Espinho, a neighbouring lagoon, the ion concentrations in the water column revealed that dolomite precipitates throughout the year, most likely due to its higher salinity than Lagoa Vermelha during the measured period. In Lagoa Vermelha, high delta(34)S of pore water sulfate and high sulfide concentrations correlated with dolomitic horizons, demonstrating the association of bacterial sulfate reduction with dolomite formation. In Brejo do Espinho high delta(34)S of pore water sulfate and high sulfide concentrations occurred throughout the dolomitic sedimentary column. We conclude that elevated salinity and sulfate reduction are the main factors inducing dolomite precipitation in these lagoons, confirming the microbial dolomite formation theory. These results suggest that there may be other settings where sulfate-reducing bacteria induce dolomite precipitation under saline conditions, such as deep-sea sediments or sabkhas, and imply that microbial dolomite may significantly contribute to the sedimentary carbonate budget, particularly in the earliest Earth's history when anoxic conditions were more prevalent.