Stable Isotope Hydrology of Cave Groundwater and Its Relevance for Speleothem-Based Paleoenvironmental Reconstruction in Croatia

Speleothems deposited from cave drip waters retain, in their calcite lattice, isotopic records of past environmental changes. Among other proxies, δ18O is recognized as very useful for this purpose, but its accurate interpretation depends on understanding the relationship between precipitation and drip water δ18O, a relationship controlled by climatic settings. We analyzed water isotope data of 17 caves from different latitudes and altitudes in relatively small but diverse Croatian karst regions in order to distinguish the dominant influences. Drip water δ18O in colder caves generally shows a greater resemblance to the amount-weighted mean of precipitation δ18O compared to warmer sites, where evaporation plays an important role. However, during glacial periods, today’s ‘warm’ sites were cold, changing the cave characteristics and precipitation δ18O transmission patterns. Superimposed on these settings, each cave has site-specific features, such as morphology (descending or ascending passages), altitude and infiltration elevation, (micro) location (rain shadow or seaward orientation), aquifer architecture (responsible for the drip water homogenization) and cave atmosphere (governing equilibrium or kinetic fractionation). This necessitates an individual approach and thorough monitoring for best comprehension.

Stable isotope constraints on hydrostratigraphy and aquifer connectivity in the Table Mountain Group

The Table Mountain Group is a folded, faulted, quartzite-dominated sedimentary sequence, metamorphosed to lower greenschist facies, that forms steep mountains dominating the topography of the Western Cape and causing orographic rainfall in an otherwise semi-arid region. These quartzites are highly fractured to depths of kilometres and act as a complex aquifer system that supplies groundwater directly and indirectly, through baseflow, essential for sustaining the natural environment and human activity in the region. Hydrogen and oxygen isotope data for rain, rivers and groundwater (boreholes and springs) in the region give typical altitude effects of -1.8‰ δD/100 m and -0.33‰ δ18O/100 m, and a very strong continental effect of -30‰ δD/100 km and -4.7‰ δ18O/100 km. This allows for application of stable isotopes as natural hydrological tracers. Groundwater at several locations had stable isotope compositions different from ambient rainfall, but similar to rainfall at high altitudes in adjacent mountains, indicating recharge at high altitude. The groundwater flow is through the Skurweberg Aquifer, here defined as all three formations of the Nardouw Subgroup. Observations on the Peninsula Aquifer suggest a very well mixed aquifer, due to extensive fracturing. Potential exists to delineate groundwater protection zones, detect overabstraction and understand aquifer connectivity better by applying stable isotope hydrology to the Table Mountain Group.