Monitoring in the Barać and Lower Cerovačka caves (Croatia) as a basis for the characterization of the climatological and hydrological processes that control speleothem formation

Abstract

Determining the processes and conditions that control and affect the precipitation of carbonates in cave environments is essential to correctly interpret speleothem-based proxies and reconstruct past climate and environmental changes. Therefore, two cave systems rich in speleothems and that are located at different distances from the Adriatic Sea were selected to conduct cave monitoring during 2013–2014: 1) the Barać caves (Lower and Upper) situated in the inner part of Croatia; 2) the Lower Cerovačka Cave located on the northern slope of the southeastern part of Velebit Mountain, closer to the Adriatic coast. The monitoring included microclimate measurements, analyses of the elemental and stable isotope composition of drip water and precipitation, as well as stable isotope and 14C measurement of modern calcite precipitates formed on light bulbs.The stable isotope composition of drip waters in the Barać caves shows systematically lower values than those found in amount-weighted annual precipitation, which indicates that the source of the infiltrating water dominantly derives from winter precipitation. In contrast, the δD and δ18O values of the drip waters in the Lower Cerovačka Cave show similar or more positive values compared with those in the amount-weighted annual precipitation, which suggests more contribution from summer precipitation. All of the studied drip sites (except one) were characterized by stable δD and δ18O values during the whole monitoring period, which implies that the karst water above these sites is well mixed and the residence time of the groundwater is more than one year. Thus, it is likely that the speleothems from these sites could preserve the multiannual isotopic changes of precipitation. The deuterium-excess values of the drip waters and precipitation show systematically higher values than 10‰ suggesting significant contribution from moisture originated in the Mediterranean Basin. Although temperature time series do not cover a whole year, it seems that the Lower and Upper Barać caves display different calcite-water fractionation. These findings emphasize the importance of measurements on the carbonate precipitates from each site, even within the same cave system, before using speleothems for past climate reconstruction.