INFLUENCE OF MICROCLIMATE ON STALACTITE MORPHOLOGY: EVIDENCE FROM CAVE ENTRANCES

The morphology and petrology of stalactites are generally thought to be determined by the properties of water from which they are precipitated. However, specific microclimatic conditions in which they form can also fundamentally affect the properties of stalactites. This is not so apparent inside the enclosed atmosphere of caves, where the microclimate is all but constant, but is the primary controlling factor in settings characterized by microclimatic variations, notably cave entrances.

We have monitored temperature, humidity and light intensity patterns in two caves on Guam by data loggers and found that the microclimatic regimes dramatically change along transects from cave entrances to cave interiors. The general patterns show distinct daily oscillations in the entrance area, their gradual buffering toward the interior of the cave, and nearly constant conditions at the back. Following microclimatic measurements, we have sampled actively growing stalactites along the same transects and found that they are morphologically highly variable. Their macromorphology, porosity, crystal size and fabric are directly related to each sample’s position in the cave and its local microclimate. Stalactites growing at the dripline are soft, porous, microcrystalline and organic-rich deposits. Entering the twilight zone, as temperature and humidity oscillations stabilize, illumination declines and relative humidity increases, the stalactites become progressively denser and more organized. They form an uninterrupted morphological sequence from calcareous tufa stalactites in the most exposed locations to classic speleothems deeper inside caves. Since particular microfabrics can be related to specific microclimatic parameters, these observations promise to be a useful tool in paleoenvironmental interpretation.