Can We Detect Changes in Amazon Forest Structure Using Measurements of the Isotopic Composition of Precipitation?

Large-scale (>500 km) spatial gradients of precipitation oxygen isotope ratios (delta O-18(p)) hold information about the hydrological cycle. They result from the interplay between rainout and evapotranspiration along air-parcel paths, but these counteracting effects are difficult to disentangle, complicating quantification of the effect of land cover change on delta O-18(p). We show that disentangling can qualitatively be achieved using climate model simulations with a land-derived precipitation tracer for tropical South America. We then either vary land cover as observed since 1870 or replace Amazon forests with bare land to determine the resulting signals. Our results indicate that effects of historically changing land cover on annual mean delta O-18 isotope-ratio gradients are small and unlikely detectable, although there is a noticeable signal during the dry season. Furthermore, the effect of changes in water recycling on Amazon delta O-18(p) in paleo-records may have been overestimated and need reinterpretation.