Groundwater isotopic data as potential proxy for Holocene paleohydroclimatic and paleoecological models in NE Brazil
Holocene paleoclimatic patterns in NE Brazil are recognized to present singular characteristics when compared with the remaining tropical South-America. In particular, isotopic variations in speleothem calcite highlight that in contrast to the rest of tropical SA, NE Brazil experienced humid conditions during lower summer insolation, i.e. throughout the Early-Mid Holocene, and aridity when, as nowadays, summer insolation was high. In parallel, paleobotanical and palynological investigations suggest that these wetter conditions, also associated with colder climate, could have promoted the setting of ecological corridors between the current Amazonian (continental) and Atlantic (littoral) forests. In this context, this work aims at showing how groundwater isotopic data could be used as a complementary proxy to further explain these Holocene paleohydroclimatic and paleoecologic processes. By comparing δ18O, δ2H and d-excess of modern waters with Early-Mid Holocene groundwater in Recife (Pernambuco state, NE Brazil), differences in recharge patterns and moisture origin can be constrained. We find that Early-Mid Holocene waters present higher d-excess than the modern groundwater. Given that the Early-mid Holocene colder and wetter conditions (higher relative humidity) should lead to a reverse trend, i.e. lower d-excess, we hypothesize that the groundwater moisture sources was heavily recycled. Such a hypothesis would be consistent with the presence of a rainforest-type ecosystem, similar to the present Amazonian forest, in the currently arid NE Brazil. These observations highlight the potential added value of the groundwater isotopes proxy to discuss the interrelationships of paleohydrological and paleoecological patterns during the Early-Mid Holocene. These new proxies might allow the spatio-temporal extent of the above-mentioned ecological corridors to be discussed.