Recently growing subaqueous flowstones: Occurrence, petrography, and growth conditions.

Subaqueous flowstones have been studied in two resurgence caves in Slovak Karst (southern Slovakia), namely Krásnohorská and Drienovská Caves. The flowstones litter streambeds within the distance of several hundred meters upstream from resurgences. They form laminated crusts within channels of the underground streams flowing through the caves. Layers composed of columnar sparry calcite alternate with those composed of microcrystalline calcite. The latter are enriched in fine, detrital, non-carbonate components, which resulted in crystallization of microcrystalline calcite instead of sparry calcite. The growth rate of the studied flowstones reaches 0.96 mg/cm 2 /day, which corresponds to ca. 0.3 mm per year. Seasonal observations conducted between November 2010 and September 2012 reveal that deposition of subaqueous flowstones in the studied caves significantly depends on the local hydrological conditions. During the periods of low flow, SI calc. values of underground stream increase and columnar sparry calcite grows. Conversely, during peak flow periods SI calc. values decrease and subaqueous flowstones are subjected to destruction, both by mechanical erosion and corrosion. The decreasing discharge, just after peak flows, causes deposition of fine-grained particles, which results in precipitation of microcrystalline layers. Subaqueous flowstones grow in complete darkness and in atmosphere with elevated CO 2 content, similarly to typical speleothems, but they are fed with turbulent stream water, similarly to calcareous tufas. Their growth rate is higher than those of ‘normal’ flowstones fed with a film of water seeping down; it falls within a range of growth rate of calcareous tufas, though near the lower limit of this range. Therefore, subaqueous flowstones represent an intermediate link between other speleothems and tufas deposited outside the cave environment. [ABSTRACT FROM AUTHOR]