Geochemical evolution of a karst stream in Devils Icebox cave, Missouri, USA

A 3.7 km flowpath along the main stream channel in Devils Icebox Cave, Boone County Missouri, was sampled on 23 January, 23 March and 18 September 1994. In January 1994, the water was oversaturated with respect to both calcite and dolomite. and only minor compositional changes were observed along the flowpath. In March 1994, the water was oversaturated with respect to calcite but undersaturated with respect to dolomite. Using a mass-balance approach, the composition of the stream water at downstream locations was predicted by dissolution of dolomite ia maximum of 0.16 mmol s(-1)) and by a minor amount of calcite precipitation (a maximum of 0.03 mmol s(-1)). In September 1994, there were increases in the Mg, Ca, and total inorganic carbon (TIG) mass fluxes that were due to the dissolution of dolomite (SIdolomite < 0 where SI is saturation index) and calcite (SIcalcite < 0). The expected composition of the water at downstream locations predicted by a mass-balance approach matched the observed composition. When calcite and dolomite are dissolving, as on 18 September, the partial pressure of CO2 of the water should decrease downstream; however, we found an increase in the partial pressure of CO2, along the stream. The source of this additional CO2 is thought to be microbial degradation of bar guano. The decomposition of bat guano appeared to change the composition of the stream water during the period the bats are in the cave, and this change was reflected in the composition of the stream water collected in September 1994. Based on the length of the flowpath and on the average velocity of the water along the flowpath, the travel time of water in this karst stream is less than 4 days, The reactions that control the chemistry of the karst water must be those with equally short characteristic times: the dissolution of dolomite and calcite, CO2 exchange, and microbial degradation of organic matter.