Light attenuation as a control for microbiogeomorphic features: Implications for coastal cave speleogenesis

Abstract

San Salvador (Bahamas) is a carbonate island with dozens of flank margin caves formed in the phreatic zone by fresh seawater mixing within the freshwater lens. These caves have no direct connection with the sea, and form at or close to the tidally influenced fluctuating water table. After sea-level fall, in their subaerial parts caves are enlarged mainly by rock dissolution and by erosion close to the water level, condensation-corrosion and breakdown processes. For understanding the geomorphological features observed in these caves and how they are related to light attenuation, we investigated three sampling sites in the tidally influenced zone of Lighthouse Cave, which has been re-invaded by seawater during the Holocene sea-level highstand. A freshwater lens no longer exists within or adjacent to the cave. Rock samples were collected above and below the internal lake shores close to the entrance, and in the twilight and dark zones of this cave. Light and electron microscopy examinations were conducted for detecting microbial cells, as well as bioconstruction and bioweathering features. In addition, a high precision laser scanner was used for characterising sample microtopography. Our data showed that the microtopography and geomorphology of the lake shore samples (cave entrance) are dominated by bioweathering, whereas the samples of the twilight and dark zones are controlled by a combination of both bioweathering and bioconstructive processes depending on light availability. Bioconstructive structures, such as semi-planar lamination, at the fluctuating water level of the Lighthouse Cave show that dissolution due to water mixing of sea and freshwater in the Holocene is no longer the most important speleogenetic process. We propose that the geomorphological evolution is strongly influenced by the degree of rock diagenesis more than the initial mechanism of speleogenesis.