Oxygen isotopes in Amazon tree rings as indicators of change in the hydrological cycle

The Amazon is the largest catchment in the world, discharging ca. 17% of global freshwater, and plays an important role in the global water and carbon cycles. Recent decades have seen an increase in floods of the Amazon river, but also increases in dry season severity and length. Instrumental long-term climate data to assess the magnitude of these changes are relatively scarce. Tree-ring based climate reconstructions may help improving past climate records from this vast region and put these changes in historical perspective.

While standard tree ring widths chronologies in the tropics are generally weakly related to climate, tree ring d¹⁸O records from Cedrela odorata in Bolivia are a proven proxy for Amazon basin-wide rainfall, and thus Amazon river discharge. However, these “terra firme” trees grow during the wet season and thus do not provide information on the dry season. Here we present a new proxy for dry season precipitation from Amazonian floodplain trees of Macrolobium acaciifolium from the western Amazon. As this species grows during the non-flooded period, the dry season, their tree ring d¹⁸O records should pertain variation of dry season precipitation d¹⁸O. A comparions of tree ring d¹⁸O from floodplain and terra firme trees show opposing trends since the 1970s, indicating increases in wet season precipitation and decreases in dry season precipitation. These records are consistent with recent trends in peaks and troughs of Amazon river levels, and provide support of a recent intensification of the Amazon hydrological cycle. We conclude that tree-ring d¹⁸O records are an important tool for tropical climate reconstructions, and even allow climate reconstructions with seasonal resolution. In addition, as signals arise from variation in (meteorological) precipitation d¹⁸O, tree ring d¹⁸O chronologies do not need detrending, and show highly synchronized patterns even over very large scales, allowing rigorous cross-dating between species and sites, and facilitating further development in this vast region