DEGLACIAL HYDROLOGIC CONDITIONS IN TWO UNDERWATER CAVES FLOODED BY THE WESTERN FLORIDIAN AQUIFER

Thick sediments have accumulated in many inland phreatic caves in the Floridian Ocala limestone (USA), but the paleoenvironmental value of these sediments remains unknown. Based on basic principles of sedimentation, one would hypothesize that phreatic cave sediments could document long-term hydrodynamics of the local aquifer. To test this hypothesis, several sediment cores (< 2 m length) were collected from two caves: Hole in the Wall Cave and Twin Caves in Marianna, Florida (USA). These caves are located ~300 m apart on a riverine-like reservoir that is supplied by Jackson Blue Spring, a category 4 spring discharging greater than 75 million gallons of water daily. The subsurface stratigraphy in both caves captures snapshots of the deglacial evolution of the Western Floridian aquifer, but radiocarbon dating indicates that the temporal continuity of the paleoenvironmental records is hampered by a punctuated sediment supply and budget since the late Pleistocene. Fine-grained iron and manganese oxide and oxyhydroxide minerals dominated sedimentation in Hole in the Wall Cave, as constrained by x-ray fluorescence and x-ray diffraction. After 5600 Calibrated years Before Present (Cal yrs BP), however, deposition switches to fine-grained organic matter, perhaps marking the onset of modern hydrologic conditions. In contrast, coarser grained sediment along with Fe and Mn-oxide minerals suggests varying water velocities flowed through Twin Caves through time. However, a notable exception is a 40 cm unit of detrital organic matter deposited in ~100 years at 13,900 Cal yrs BP. This event is coincident with the regional shift towards a warmer and wetter climate at ~14,000 years ago according to a detailed pollen reconstruction from Camel Lake (60 km southwest), perhaps associated with the Bølling-Allerød warming event. Despite the limitations at these sites, the results do suggest that paleohydrogeologic reconstructions could be completed from phreatic inland caves using more continuous sedimentary records.