Characterization of the Depositional Environment of Phreatic Overgrowths on Speleothems in the Littoral Caves of Mallorca (Spain): a Physical, Geochemical, and Stable Isotopic Study.

摘要

Phreatic overgrowths on speleothems (POS) are one of many sea-level proxies available to Quaternary geologists in Mallorca; these carbonate encrustations form at the air-water interface in cave passages flooded with brackish water. POS are ideal for reconstruction of western Mediterranean sea level because they are widespread in Mallorca's caves, can be precisely dated by U-series methods, constrain sea-level stands to sub-meter elevation, and are well preserved and accessible in the subterranean environment.;This research investigates the POS depositional environment, which is relatively understudied compared to the other proxies used for sea-level reconstructions. This disparity has led to assumptions on many aspects of the POS precipitation. Further, POS are typically composed of calcite, but sometimes the metastable polymorph aragonite is present instead. Two caves were studied because of the presence of a modern POS horizon of aragonite and calcite: Cova des Pas de Vallgornera (Vallgornera) and Coves del Drac (Drac), respectively.;High-resolution air and water physical parameters were collected for the first time, along with monthly water samples for stable isotope and elemental analysis. This 16-month record was supplemented with detailed geochemical studies throughout the project, including water-column profiles and CO 2 sampling campaigns.;The water level in both caves preserves the semi-diurnal Mediterranean Sea tide signal, with a lag of approximately four hours. The fluctuation in both caves is slightly attenuated, and the direct effects of barometric pressure and precipitation could not be discerned from the primary control of tidal pumping. Calculations based on salinity and isotope analysis show that less than 20% of the solution in each cave is seawater.;Degassing of CO2 from the cave water to air was documented at both locations, with sporadic calcium carbonate supersaturation. These conditions are strongly dependent on annual cave ventilation, which becomes active during winter when cold, dense tropospheric air sinks into the subsurface. In addition to seasonal thermo-circulation, fluctuating water level displaces cave air and likely initiates tropospheric exchange throughout the year. This process primarily occurs through fissures in thin overlying bedrock at Vallgornera and through the large entrance in Drac.;Higher elemental ratios (Mg:Ca, Sr:Ca, Mg:Sr) known to enhance aragonite precipitation or inhibit calcite precipitation were recorded in Vallgornera's water. A linear correlation with salinity was not observed, so higher ratios in Vallgornera must be contributed from differences in lithology, bedrock weathering intensity, or nearby rising thermal waters.;In summary, this research confirms POS strengths as sea-level proxies from geochemical and hydrological perspectives. These carbonate encrustations are precipitated at the air-water interface, which fluctuates as an attenuated expression of Mediterranean Sea tide. Degassing of CO2 from the cave water to air, which promotes calcium carbonate supersaturation, is the major control on POS deposition, and is facilitated by winter ventilation and likely water-level fluctuations. CO2 degassing (and theoretically precipitation of POS) in isotopic equilibrium is possible in caves with restricted ventilation (small/sealed entrances, small passages). Aragonite may be precipitated instead of calcite because of local modifications to the geochemical system from bedrock weathering or contributions from deep groundwater.