Intra-annual stable isotopes in the tree rings of Hymenaea courbaril as a proxy for hydroclimate variations in southern Amazonia

The hydrology of Amazonia is changing due to climate and land-use changes, especially in the southern region, which has warmed and dried faster than other tropical regions. Yet there are no long-term hydrological records to put these changes in a historical perspective. Here we investigate the use of tree-ring carbon (δ¹³C) and oxygen isotopes (δ¹⁸O) to assess the seasonal variation in climate for the southern Amazonia basin. We analysed the intra-annual variation of δ¹³C and δ¹⁸O in 10 segments of each tree ring from 2013 to 2017 from individuals of Hymenaea courbaril, a long-lived and widespread neotropical tree species. We find strong seasonal patterns of tree-ring δ¹³C supporting previous observations of annual growth rhythms for this species. The intra-annual variation in δ¹⁸O shows that the lowest values generally occur just after the middle point of ring formation, corresponding to the peak rainy season. We find strong correlations between the δ¹⁸O in the middle of the growth ring and vapour pressure deficit (r = 0.92, P = 0.02) and precipitation (r = −0.93, P = 0.02). We further find associations between the oxygen isotopic series and the discharge of the Araguaia basin’s main rivers during the rainy period. Our results show that these δ¹⁸O records are sensitive to fluctuations in rainfall and humidity, and thus to river discharge in the region. Longer reconstructions of based on tree-ring δ¹⁸O of Hymenaea courbaril could provide a novel proxy to assess past hydrological changes.