Calibrating hydroclimate proxies in three modern southern Portugese speleothems

Stable isotopes archived in 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 other processes may complicate the target signal. Here we compare high-resolution (sampled every 20-40 μm) oxygen isotopes in three small stalagmites with modern tops collected from two caves Companheira (C-18-1) and Ibn Ammar (GIA-19-1, GIA-19-2). Both caves are located in southern Portugal and are ~2.3 km apart. Oxygen isotope data from C-18-1 and GIA-19-2 indicate that the stalagmites precipitated in equilibrium with cave drip water. Equilibrium precipitation allows for the assumption that the 𝛿18O values of the stalagmite accurately reflects the 𝛿18O values of local precipitation. Preliminary data from C-18-1 suggests precipitation 𝛿18O values are more impacted by changes in temperature and less impacted by the so-called "amount effect". All samples were dated using U/Th disequilibrium techniques and were found to have low U concentrations. Three distinct climate patterns in the Algarve region can be observed with the 𝛿18O record of C-18-1. High 𝛿18O values in the lowest section indicate regionally stable and warmer conditions. The region then began to experience generally 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. Data from both GIA-19-1 and GIA-19-2 show a significant decrease in 𝛿18O values in the years 1920 and 1980 which suggests that both stalagmites from Ibn Ammar are reflecting time periods of either colder or wetter climatic conditions during these years. The data from Companheira will be compared with the Ibn Ammar samples. 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. This work is coauthored by Diana Thacher and Alan Wanamaker from Iowa State, and Victor J. Polyak and Yemane Asmerom from the University of New Mexico.