CALIBRATING HYDROCLIMATE PROXIES IN TWO SOUTHERN PORTUGUESE SPELEOTHEMS

Stable isotopes archived in calcium carbonate cave deposits such as stalagmites have been widely used to reconstruct past climates. However, various processes can influence the isotope signal recorded in speleothems, potentially obscuring climate signals. Stalagmite oxygen isotopes can record the 𝛿¹⁸O values of regional precipitation and hence hydroclimate, but processes such as soil and karst water evaporation, mixing, and seasonally-biased carbonate precipitation may complicate the target signal. Here we compare high-resolution (sampled every 20 μm) oxygen isotopes in two small stalagmites with modern tops from Companheira and Ibn Ammar caves in southern Portugal (~2.3 km apart). Oxygen isotope data from the Companheira sample indicate that the stalagmite precipitated in equilibrium with cave drip water. Equilibrium between calcite and drip water allows for the assumption that the isotopic signature of the stalagmite accurately reflects the isotopic signature of local precipitation. Preliminary data suggests precipitation 𝛿¹⁸O values are more impacted by changes in temperature and less impacted by the so-called “amount effect”. Both samples were dated using U/Th disequilibrium techniques and were found to have low U concentrations. The Companheira sample returned a date of 721 ± 396 years BP at 12 mm from the top, while the Ibn Ammar sample had better date constraints and returned a date of 340 ± 44 years BP at 11 mm from the top. The Companheira stalagmite was progressively milled every 20 μm over the upper 4 mm. Three distinct climate patterns in the Algarve region can be observed with the Companheira 𝛿¹⁸O record. High 𝛿¹⁸O values in the lowest section indicate regionally stable and warmer conditions. The region then began to experience general colder and more variable climatic conditions, represented by a decrease in 𝛿¹⁸O values. These colder climatic conditions ended toward the top of the stalagmite when temperatures gradually increased until the present. The data from Companheira will be compared with the Ibn Ammar sample. Further development of an age model from the Companheira sample, through laminae counting and comparison with the Ibn Ammar sample, will allow for a more comprehensive evaluation of dynamical changes in the regional climate system through time.