Modern freshwater microbialites from Kelly Lake, British Columbia, Canada

Small stromatolites and thrombolites occur in Kelly Lake, British Columbia, Canada. Thrombolites appear as welllithified, irregular calcite crusts on hard submerged surfaces, whereas poorly mineralized stromatolites exist on the thrombolite crusts as small laminated hemispherical domes 1.0 to 2.0 cm in diameter and height. Microscopic examination of the thrombolitic crusts reveal the presence of many coccoid and fewer small filamentous cyanobacteria. In contrast, large filamentous cyanobacteria are predominant in the stromatolitic domes. The inorganic carbon and elemental content of the two different microbialites are similar; however, the stromatolites contain more organic carbon (0.5% dry wt) than the thrombolites (0.2% dry wt). This implies that the production rate of organic matter in the stromatolites is higher, relative to the calcification rate, than in the thrombolites. Stable carbon isotope analyses show that the calcite precipitated within the microbialites is enriched in 13C compared to the dissolved inorganic carbon (DIC) source. The enrichments are the result of photosynthetic 12C fractionation by the respective microbial communities. Calcite precipitated within the stromatolites is even more enriched in 13C than that within the thrombolites, corresponding to an enhanced productivity level for the filamentous cyanobacteria in the stromatolites. These data indicate that the degree of mineralization, isotopic fractionation, and morphogenesis of modern microbialites are controlled to a large extent by relative rates of microbial growth and calcification.