Constraining Holocene Hydrological Changes in the Carpathian–Balkan Region Using Speleothem δsup18/supO and Pollen-Based Temperature Reconstructions

<p> ere we present a speleothem isotope record (POM2) from Ascuns&abreve; Cave (Romania) that provides new data on past climate changes in the Carpathian&ndash;Balkan region from 8.2 ka until the present. This paper describes an approach to constrain the effect of temperature changes on calcite &delta; ¹⁸ O values in stalagmite POM2 over the course of the middle Holocene (6&ndash;4 ka), and across the 8.2 and 3.2 ka rapid climate change events. Independent pollen temperature reconstructions are used to this purpose. The approach combines the temperature-dependent isotope fractionation of rain water during condensation and fractionation resulting from calcite precipitation at the given cave temperature. The only prior assumptions are that pollen-derived average annual temperature reflects average cave temperature, and that pollen-derived coldest and warmest month temperatures reflect the range of condensation temperatures of rain above the cave site. This approach constrains a range of values between which speleothem &delta; ¹⁸ O changes should be found if controlled only by surface temperature variations at the cave site. Deviations of the change in &delta; ¹⁸ O _{c _spel} values from the calculated temperature-constrained range of change are interpreted towards large-scale variability of climate&ndash;hydrology. Following this approach, we show that an additional &sim;0.6&permil; enrichment of &delta; ¹⁸ O _c in the POM2 stalagmite was caused by changing hydrological patterns in SW Romania across the middle Holocene, most likely comprising local evaporation from the soil and an increase in Mediterranean moisture &delta; ¹⁸ O. Further, by extending the calculations to other speleothem records from around the entire Mediterranean basin, it appears that all eastern Mediterranean speleothems recorded a similar isotopic enrichment due to changing hydrology, whereas all changes recorded in speleothems from the western Mediterranean are fully explained by temperature variation alone. This highlights a different hydrological evolution between the two sides of the Mediterranean. Our results also demonstrate that during the 8.2 ka event, POM2 stable isotope data essentially fit the temperature-constrained isotopic variability. In the case of the 3.2 ka event, an additional climate-related hydrological factor is more evident. This implies a different rainfall pattern in the Southern Carpathian region during this event at the end of the Bronze Age.</p>