Stable isotope study of cave percolation waters in subtropical Brazil: Implications for paleoclimate inferences from speleothems

Interannual monthly variations in δ18O of rainfall reflect primarily regional changes in moisture source contribution related to seasonal shifts in atmospheric circulation from a more monsoonal regime in summer (negative values of δ18O) to a more extratropical regime in winter (positive values of δ18O). Variations in groundwater δ18O indicate that the climatic signal of recent rainfall events is rapidly transmitted through the relatively deep karst aquifer to the cave drip waters, regardless of location of collection in the cave. In addition, the data also suggest that water replenishment in the system is triggered by the increase in hydraulic head during periods when recharge exceeds the storage capacity of the soil and epikarst reservoirs. Significant perturbations in the groundwater composition, characterized by more positive values of δ18O, are probably connected to an increased Atlantic moisture contribution associated with extratropical precipitation. This implies that the δ18O of speleothems from caves in this region may be a suitable proxy for studying tropical-extratropical interactions over South America, a feature that is intrinsically related to the global atmospheric circulation.