Climate Variability reconstructed from La Cueva Chica speleothems for the last 13 ka BP: implications for Megafauna in Southern Patagonia, Chile.

Investigating palaeoclimate records is of major importance for evaluating the impact of past forcing factors on the evolution of ecosystems, megafauna and human dispersal, especially in Southern Patagonia where few records are available. We report on a 40 cm long flowstone core S6, and fragments of flowstone and a stalagmite from Cueva Chica. The samples were radiometrically dated (U-Th & ¹⁴C) to construct age-depth models for the proxy profiles (δ¹³C, δ¹⁸O, and chemical composition). The speleothem proxy data are further informed by both petrographic analysis of the flowstone, and monitoring data. The main objectives of this work are to: i) reconstruct past climate variations using geochemical analyses conducted on the speleothems, and ii) assess the palaeoclimatic context of megafauna extinction in the area. The flowstone core S6 grew discontinuously from ~13 ka to ~1 ka with several possible hiatuses at ~10 ka BP, from 5.7 to 3.0 ka BP, and 2.5 to 1.8 ka BP (interpolated ages). Sample S8 grew from 6.8 to 5.8 ka BP and after 1.2 ka BP. Stable isotopes analyzed at sub-centennial resolution show a 3‰ range for δ¹⁸O, and more than 14‰ for δ¹³C, and the isotope ratios covary along the entire record. These changes are likely caused by kinetic fractionation and prior calcite precipitation (PCP), controlled mostly by changes in moisture availability. The sensitivity of the proxies to hydrological changes and PCP is further tested with indicators using μXRF element data. The multiproxy record from Cueva Chica suggests a wet phase from ~13 to 9 ka BP, likely related to strong westerlies in the Southern Hemisphere, preceded by a short dry/cold spell at ~13 ka BP. This early Holocene wet phase was followed by a colder/drier period from 8.5 to 5.8 ka BP, likely related to weaker westerlies, especially during the mid-Holocene. High precipitation and strong westerlies prevailed from 3.0 to 2.5 ka BP and in Medieval times. Our paleoclimate record implies that the presence of extensive megafauna, the development of Nothofagus forest and human arrival, all occurred during a climatically favorable wet/warm period ca. 13 to 9 ka BP, after the Antarctic Cold Reversal. However, the deterioration of the vegetation cover at the Cerro Benitez coinciding with high δ¹³C values excursions was initiated after ca. 11 ka BP. Previous studies suggest an extinction of major megafauna species (e.g., Mylodon, Smilodon, Panthera onca mesembrina) during this wet/warm period. Such climate-driven changes likely reduced the open ecosystem environment and may have led to local decline of herbivore populations. Later cooling/drying after ca. 9 ka may have contributed to the disappearance of megafauna and other large mamals (e.g., Hippidion Saldiasi).