Precipitation of Phreatic Overgrowths at the Water Table of Meteoric-Marine Mixing Zones in Coastal Cave Systems: A Useful Tool in Sea Level Change Reconstruction

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Precipitation of carbonates within coastal karst caves at the present-day water table has been extensively documented in Mallorca and Sardinia. These depositions frequently occur either as calcite rafts floating at the brackish waters surface or, more commonly, as horizontal bulbous belts of calcite or aragonite overgrowths that develop over the walls of the caves or whatever suitable support is available (stalactites, stalagmites, columns) in correspondence to current sea level. The water table in the coastal caves is at about the same elevation as sea level and undergoes analogous daily fluctuations. Therefore, the presentday precipitates, known as Phreatic Overgrowths on Speleothems (POS), mark the current position of sea level and provide an excellent analogue for interpreting past bands of precipitates now located above or below modern sea level. With the aim of testing the strength of POS as indicators of present and past sea levels, some overgrowths, now located at current water table in Mallorca, have been dated by U-series methods. The results indicate that the phreatic carbonate precipitation took place sometimes between ~2,800 and ~600 years BP. The discovery of a drowned prehistoric construction, at a depth of 1 m below current sea level in a cave from the same area, and its temporal chronological attribution to about 3,700 - 3,000 years BP seem to be in good agreement with the obtained age of calcite precipitation at the current water table. The geographical distribution of POS is restricted to some very specific geochemical environments, always related to coastal caves characterized by low-range tidal fluctuations.

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Proceedings 15th International Congress of Speleology, v. 1, p. 554-560