CALCITE RAFTS - FIELD INVESTIGATION OF SEDIMENTARY TRANSGRESSIVE INFILL CAVE SEQUENCES AS A NEW PALEO-SEA LEVEL PROXY

Paleo sea level records for low-latitude sites based on coral or mangrove deposits have uncertainties of 1-10+ m, due to the depth range over which coral species grow, and the compaction and decay of mangrove peat. Polygenetic tiers of caves are documented along the Caribbean coast of the Yucatan Peninsula, and the speleogenesis is tied to past sea levels. Cave infill sediments include sequences of carbonate rafts, with several meters of accumulation in places. The rafts precipitate on the water table due to off-gassing of CO₂. Once sedimented, the raft deposits are well preserved due to their stable mineral form, resistance to hydrological reworking, and limited effects of physical and biological processes inside the caves. The flat toped raft sequences accumulate to within 0.1-1 m of the water table, which is constrained to have a small offset from sea level due to the extreme permeability in this post-Paleozoic karst system where the hydraulic gradient is 10^-5 (cm:km) and tidal fluctuations extend to 10 km inland.

Rafts were observed to reach visible size in 72 hours, and over four months cover 80% of .25 m² experimental berms. Time lapse photography revealed a water level dependence on raft formation. Air ventilation near the water table is greatly reduced when water level reaches the lowest level of cave ceiling elevation, occluding air flow even where the passage is not fully flooded. The cave air pCO₂ rises, decreasing the off-gassing of CO₂ from the water, and inhibiting raft formation. While the greatest raft formation rate was observed in quiescent waters, floating rafts were transported intact particularly at higher water levels when flow at the water table was greatest. Sedimentation within each site was noted to be spatially heterogeneous within passages. Manual push cores from flat toped raft banks revealed intact stratigraphy, with variation in raft textures, size, and organic content. Raft sedimentation rates are on the order of 1 cm/100 years based on accumulation rates in traps, and radiometric dating down core. These results support the potential of calcite rafts deposits as a valuable new expanded record for low latitude carbonate coastlines, particularly with the elevations of flat toped infill sequences sedimented near the modern and paleo water table level reflecting sea level transgressions.