MODERN FRESH-WATER MICROBIALITES FROM KELLY LAKE, BRITISH-COLUMBIA, CANADA

Small stromatolites and thrombolites occur in Kelly Lake, British Colu mbia, Canada. Thrombolites appear as well-lithified, irregular calcite crusts on hard submerged surfaces, whereas poorly mineralized stromat olites exist on the thrombolite crusts as small laminated hemispherica l domes 1.0 to 2.0 cm in diameter and height. Microscopic examination of the thrombolitic crusts reveal the presence of many coccoid and few er small filamentous cyanobacteria. In contrast, large filamentous cya nobacteria are predominant in the stromatolitic domes. The inorganic c arbon and elemental content of the two different microbialites are sim ilar; however, the stromatolites contain. more organic carbon (0.5% dr y wt) than the thrombolites (0.2% dry wt). This implies that the produ ction rate of organic matter in the stromatolites is higher, relative to the calcification rate, than in the thrombolites. Stable carbon iso tope analyses show that the calcite precipitated within the microbiali tes is enriched in C-13 compared to the dissolved inorganic carbon (DI C) source. The enrichments are the result of photosynthetic C-12 fract ionation by the respective microbial communities. Calcite precipitated within the stromatolites is even more enriched in C-13 than that with in the thrombolites, corresponding to an. enhanced productivity level for the filamentous cyanobacteria in the stromatolites. These data ind icate that the degree of mineralization, isotopic fractionation, and m orphogenesis of modern microbialites are controlled to a large extent by relative rates of microbial growth and calcification.