LACUSTRINE MICROBIALITES FROM AN ACTIVE COLD WATER SPRING MOUND: BRISCO, BRITISH COLUMBIA, CANADA

Two small lakes (fed by subaqueous spring vents) are perched on a large cold water spring mound located near the town of Brisco, British Columbia. The core of the spring mound is primarily composed of relic bryophyte tufa that is 7 m thick near the spring vents and gradually thins as it extends southeast about 600 m. Both lakes are slightly alkaline (pH 7.3 and 7.6), have similar summer temperatures (11.5 and 12.5°C), similar conductivities (summer: 897 and 893 µS/cm) and similar major ion concentrations. The water in both lakes is supersaturated with respect to calcite. These similarities suggest a common source for the water that discharges into the two lakes.

Although the water chemistry of the two lakes is similar, only one of the lakes contains large lacustrine microbialites (bioherms up to 4 m in diameter and 0.4 m thick). The microbialites are found as isolated bioherms on the floor of the lake or as thick encrustations of submerged logs and branches. The microbialites can be characterized as having a calcified, laminated, lobate crust (about 1 cm thick) that overlies large (1-10 cm diameter), irregularly shaped, internal cavities. At the base of the mounds, the lamination is obliterated by bioturbation (by bryophyte rhizoids and other organisms) and by diagenetic processes. Charophytes, bryophytes, diatoms, cyanobacteria, ostracods, mollusks, testate amoeba and caddisfly larvae are found in and around the microbialites.

One major difference between the two lakes is that the upper lake is steep-sided and deep (9.8 m) and the lower lake (containing the microbialite bioherms) has a large, flat-bottomed, shallow (1.5 m deep) littoral zone. Since many factors that might favor carbonate precipitation are similar in the two lakes (such as water chemistry and outgassing), microbialite formation in the lower lake seems to be differentially favored by calcite precipitation mechanisms associated with improved photosynthesis in the shallow lake.