Formula omitted] in speleothem carbonates in Soreq Cave as an archive for hydro-climatic conditions

Abstract

δ18O in speleothem carbonates is a common archive for paleoclimate on land. Recently, it has been shown that triple oxygen isotopes in CaCO3 (given as Oexcess17=106[ln⁡(10−3δ17O+1)−0.528(ln⁡(10−3δ18O+1)]) record Oexcess17 of its parent water, thus providing additional paleo-hydrology information, primarily about relative humidity. The O17 fractionation between CaCO3 and water has been determined in biogenic and synthetic carbonates. In speleothems, however, this fractionation is expected to be modified by kinetic isotope effects associated with CO2 degassing from cave drip water. Here, we used Soreq Cave as a case study to examine this fractionation and the use of Oexcess17 in speleothems as an archive for paleo rainfall composition. We first characterized Oexcess17 in rainfall collected above the cave. The Oexcess17 value in amount weighted integrated rainfall was 49 per meg, consistent with the low relative humidity at the moisture source of the Eastern Mediterranean Sea. The measured cave water was slightly lower in Oexcess17 and higher in δ18O relative to rainfall, due to modifications of the infiltrating water by epikarst processes such as evaporation and mixing. Oexcess17 in CO2 extracted from CaCO3 of modern speleothems was compared to that in modern cave water, to calculate the carbonate-water fractionation slope (θ=ln17⁡α/ln18⁡α). The resulting θ in Soreq Cave was 0.5230±0.0002, consistent with that observed in other carbonates, indicating the lack of significant dis-equilibrium effects in speleothems Oexcess17. Hence, the common fractionation slope can be used together with a speleothem-specific α18 to reconstruct rainfall Oexcess17 from speleothem carbonates. Whereas speleothems from a variety of caves can be used through this approach to reconstruct Oexcess17 in paleo rainfall, the atmospheric processes that govern rainfall Oexcess17 are expected to be region-specific. Oexcess17 measurements in ancient Soreq speleothems are expected to reveal potential glacial-interglacial changes in moisture source conditions as well as possible changes in epikarst processes over time.