What drives interannual variation in tree ring oxygen isotopes in the Amazon?

Oxygen isotope ratios in tree rings (delta O-18(TR)) from northern Bolivia record local precipitation delta O-18 and correlate strongly with Amazon basin-wide rainfall. While this is encouraging evidence that delta O-18(TR) can be used for paleoclimate reconstructions, it remains unclear whether variation in delta O-18(TR) is truly driven by within-basin processes, thus recording Amazon climate directly, or if the isotope signal may already be imprinted on incoming vapor, perhaps reflecting a pan-tropical climate signal. We use atmospheric back trajectories combined with satellite observations of precipitation, together with water vapor transport analysis to show that delta O-18(TR) in Bolivia are indeed controlled by basin-intrinsic processes, with rainout over the basin the most important factor. Furthermore, interannual variation in basin-wide precipitation and atmospheric circulation are both shown to affect delta O-18(TR). These findings suggest delta O-18(TR) can be reliably used to reconstruct Amazon precipitation and have implications for the interpretation of other paleoproxy records from the Amazon basin.