CALCITE RAFTS - RAPID DEPOSITION OF 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. Within the extensive coastal cave systems along the Caribbean coast of the Yucatan Peninsula, the polygenetic flooded caves contain sequences of calcite rafts. The rafts form 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. Near the modern water table, flat toped sequences are “catching up” to the base of the water table placing them within 0.1-1 m of the water table. In turn, the water table in the eastern Yucatan coastal aquifer lies very close to sea level due to the extreme permeability in this post-Paleozoic karst system; the hydraulic gradient is 10^-5, and tidal fluctuations extend to 10 km inland.

Field experiments in cave sites near the Caribbean coast provide observations on the mineralogy and physical nature of calcite rafts, sedimentation processes, and formation and sedimentation rates. Under some conditions, rafts form over days to hours; they may reach visible size in 72 hours, and over three months cover 80% of 1 m² experimental berms. 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 revealing 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 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.