Abstract
Speleothems have proven to be one of the most reliable terrestrial archives for palaeoclimate research. However, due to the complexity of karst systems, long-term monitoring and high-resolution analyses of the cave atmosphere and water geochemistry have become essential to better constrain the factors that control calcite growth and how geochemical palaeoclimate proxies are encoded into speleothems. While calcite precipitation incorporates the palaeoclimate signals into the speleothem fabric, certain conditions in caves can favour dissolution, which may form hiatuses or even destroy these signals. In extreme cases, in-cave dissolution by dripwater can form cup-shaped features (i.e., corrosion cups), which were the main focus of this study. The study site in Postojna Cave, Slovenia was investigated through cave climate monitoring and drip and cup water sampling, which took place during 2017–2021. We found that the cups are fed by low-calcium drips as the consequence of the thin rock overburden above the cave. Due to the specific configuration of the airflow pathways, the study site accumulates high levels of CO2 (>10,000 ppm), which shifts low-calcium dripwater into undersaturation. This causes dissolution on the rock surfaces and speleothems on the cave floor. The results of this study have broader significance in addressing the suitability of cave environments and speleothems used in paleoclimate research.
Highlights
Speleothems provide robust palaeoclimate archives in continental settings, because they can be absolutely dated using U-series methods and contain multiple palaeoclimate proxy records
In Pisani Passage of Postojna Cave, Slovenia, several hundred cup-shaped corrosion features and stalagmites affected by severe dissolution were found
We investigated the most likely variables affecting the formation of corrosion cups: host rock composition, hydrology, spatial and temporal distribution of pCO2 in cave air and the geochemical evolution of the water from the vadose zone as cave dripwaters and within the corrosion cups
Summary
Speleothems provide robust palaeoclimate archives in continental settings, because they can be absolutely dated using U-series methods and contain multiple palaeoclimate proxy records. Ventilation may be restricted, for example, in descending caves with only one entrance, which act as cold traps in the warm season and are only ventilated in the colder season when cold and dense outside air flows into the cave [11,12,18]. In such cases, a build-up of CO2 can limit or prevent calcite precipitation altogether and, in extreme cases, lead to dissolution. A lack of or reduced speleothem growth can be associated with colder climates and/or higher altitudes due to a thin soil zone that does not provide sufficient CO2 for the initial dissolution of the host rock [20]
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