Rolf Warthmann

Calibration of the δ18O paleothermometer for dolomite precipitated in microbial cultures and natural environments

Decades of various and numerous isotopic studies to interpret the environmental conditions of dolomite formation proved to be inconclusive because the temperature-dependent oxygen isotope fractionation factor between dolomite and the solution from which it precipitated could not be determined experimentally at low temperatures. With the discovery of bacteria that mediate the precipitation of dolomite, it is now possible to overcome kinetic barriers and precipitate dolomite under controlled temperature conditions in culture experiments. Herein we report on the results of microbial experiments that have enabled us to calibrate the dolomite-water oxygen isotope fractionation factor and provide a paleothermometer to evaluate conditions of ancient dolomite formation. The temperature (T) dependence of the fractionation is defined by the equation: 1000 In α d o l o m i t e - w a t e r = 2.73 X 10 6 T - 2 + 0.26.

Microbes produce nanobacteria-like structures, avoiding cell entombment

Microsedimentary structures referred to as nanobacteria-like particles were described from modern carbonate environments, where they form in close spatial association with sulfate-reducing bacteria (SRB). However, the exact mechanism of their formation, as well as their paleontological significance, remains controversial. Here we report on an investigation of microbe-mineral interactions in experimentally produced carbonate globules. The experiments were carried out under anoxic conditions at 30 °C with Desulfovibrio brasiliensis, a SRB known to mediate dolomite formation. We observed that extracellular polymeric substances (EPS) secreted by the microbial community play a key role in the mineralization process. Nanobacteria-like particles represent the early stage of carbonate nucleation within the EPS, which progressively evolve to larger globules displaying a grainy texture. We excluded the possibilities that these structures are fossils of nanobacteria, dissolution surfaces, or artifacts created during sample preparation. D. brasiliensis cells are predominantly located outside of the EPS aggregates where mineral growth takes place. As a result, they remain mobile and are rarely entombed within the mineral. This self-preservation behavior may not be limited to D. brasiliensis. Other microbes may produce, or may have produced during the geological past, biogenic minerals through a similar process. Mineralization within EPS explains why microbial relics are not necessarily present in biogenic carbonates.

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 δ34S 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 δ34S 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 Earths history when anoxic conditions were more prevalent.