AN EVALUATION OF DIEL BIOGEOCHEMICAL CYCLES AND CALCITE PRECIPITATION IN A TUFA DEPOSITING STREAM

Diel cycles in temperature and chemistry of Spout Run, a karstic spring-fed stream in Clarke County, Virginia, were studied to determine biogeochemical factors that control calcite precipitation across a tufa waterfall. Calcite is observed to precipitate throughout the stream reach studied. Samples were collected above and below the waterfall at two-hour intervals across 48 hrs on June 26-28, 2012. Field measurements of pH, conductivity, water temperature, and light intensity were taken continuously, and water samples were analyzed for major ion chemistry and isotopic composition of dissolved inorganic carbon (δ¹³C-DIC). Preliminary results reveal that water temperature, calcite saturation index, and δ¹³C-DIC show clear diel cycles with peak values near midnight at both sites. The coincident cycles in δ¹³C-DIC values and temperature suggest that an increase in temperature caused a decrease in the solubility of CO₂, and therefore an increase in δ¹³C-DIC values due to outgassing of CO₂ and precipitation of calcite. Conversely, dissolved oxygen (DO) concentration is lowest at night and highest during the day, but is not exactly anti-phased with changes in the carbonate system, with a ~4-hour time lag between the DO and δ¹³C-DIC peaks. Calcite saturation index and excess partial pressure of CO₂ decreased as pH and δ¹³C-DIC increased slightly across the waterfall, suggesting that CO₂ outgassing and calcite precipitation occurred across the waterfall in addition to within the stream itself. We hypothesize that water temperature exerts a dominant control on diel variability of calcite saturation at this site as opposed to photosynthesis and respiration cycles, and that this signal is set within a pool located at the spring ~3 km upstream. Future work will focus on downstream transect sampling to test this hypothesis, and to investigate the possibility that biogeochemical processes within the pool at the spring create chemical changes that are carried downstream and are observed at the tufa waterfall after some time lag.