Seasonal changes in silica deposition in hot spring systems

Abstract

Monthly monitoring of water chemistry at Octopus Spring, Yellowstone National Park, reveals no changes in the chemical composition over this eight-month study. Differences in silica deposition are attributed to flow dynamics and seasonal air temperatures. Colder air temperatures in winter promote visible accumulation of siliceous sinter probably as a result of decreased solubility. Periodic drying promotes accumulation and subsequent dehydration of the silica. This combination of intermittent supply and periodic dehydration may account for the laminated siliceous deposits observed in the geological record. The processes described herein are important in the siliceous preservation of microbial fossils but the following observations underscore the difficulty in preserving microbial fabrics in the geological record. Pools populated by cyanobacteria are bounded by margins built of silicified microbial mats. Silicification of the pool margins occurs through an evaporative process called wicking; silica-rich water is transported upward through the microbial mats by capillary action, depositing silica at the surface of the mat as water evaporates. The upper surface of such wicking deposits is extremely resistant to brecciation but the underlying support is highly friable and would be difficult to preserve intact. In winter months, temporary isolation of some pools from active flow causes fragile silica deposition around biological and other debris. Film-coated, gas bubbles on the surfaces of microbial mats are observed during all seasons and probably are filled with oxygen released during photosynthesis, although this has not been confirmed. They become coated with silica during winter months presumably because of increased cooling possibly forming thin, discontinuous silica sheets as observed in the geological record.