Testing novel isotope approaches to reconstruct past precipitation regimes in the Amazon

The South American summer monsoon brings vast amounts of precipitation to the Amazon basin, providing an important lifeline to its forest and livelihoods. It remains uncertain however how climate change and increasing CO2 levels will affect Amazonian precipitation and forests. As climate models vary widely in their predictions and as our understanding of Amazon climate responses is still limited, significant insight in the Amazon hydrological cycle can be gained from reconstructions of climate responses in the past.

One approach to achieve this is by using stable isotopes in precipitation. Naturally water contains different abundances of light and heavy isotopes. As water moves through the hydrological cycle the heavy isotope is preferentially rained out. Isotope ratios in precipitation are therefore a useful recorder of the amount of rainfall. Various natural archives of isotopes in precipitation, like tree rings and speleothems (cave carbonate formations), have been used to reconstruct past climate, but each of these methodologies have their shortcomings. Tree rings are of high time resolution but cover short periods and are affected also by plant physiological processes. In contrast, speleothems cover thousands of years but time resolution is poor. Thus, combining speleothem records and tree ring records permits more faithful reconstructions of precipitation and provides also information about tree responses to changes in CO2.

In this proposal, we will build a new partnership between UK and Brazilian scientists working on two important natural proxies: isotopes in tree rings and speleothems to pursue the following primary aims

  1. to test new - but not yet proven - methods to separate isotope variation in tree rings due to leaf evaporation from the precipitation isotope signal
  2. to organise a workshop to assemble a network of scientists working on paleoclimate in the Amazon and adjacent regions, with the aim to improve and spatially integrate Amazon climate reconstructions.

The ultimate aim is to gain a better understanding of past and possibly future variation in Amazon precipitation associated with monsoon regime over South America. We will proceed as follows. We first use new tree ring records to measure isotope ratios at specific positions within the cellulose molecule, and, in parallel, in specific wood constituents (e.g., lignin). We then compare these signals with water isotope records of the actual rainfall from the study site covering past seven to eight years to determine what positions or which constituents most accurately record rainfall signals versus leaf evaporation signals. This work will profit from unique precipitation isotope records (at two sites) and high-resolution speleothem records (at one site) which have been and are still being collected by the participating labs over the last years.

The work will also include new tree ring data collections and real-time monitoring of tree water use and growth dynamics. The proposed analytical procedures are highly advanced and will benefit from unique specific facilities of the UK isotope labs to measure compound specific mass spectrometry and the use Nuclear Magnetic Resonance spectrometry. The assembled team makes maximum use of the synergies between the groups' expertise and facilities.

The UK team has worked for several years on isotopes in tree rings proving these methods can be used to reconstruct Amazon precipitation, while the Brazilian team has successfully used speleothems to reconstruct South American climate over long time scales (past thousands of years). The proposed pilot project will open the possibility to reconstruct more faithfully past precipitation patterns of the Amazon but also to reconstruct the history of leaf enrichment in trees and thus tree responses to a high CO2 world. These are ideal topics for future collaboration.