Abstracts

The Amazon hydrological cycle has intensified since approximately 1990, and yet long-term meteorological data from the region are limited, making it difficult to determine the cause of current variability. Proxy records can be used to reconstruct past climate, thus providing useful historical context for recent changes. This thesis focuses on the climate insights that can be gained from oxygen isotopes in tree rings (18OTR). The consistency of annual ring formation was tested first, as this is an important prerequisite for constructing a well-dated proxy record. Cedrela trees were found to form annual rings across most of the species natural range, but biannual rings in Suriname (Chapter 3). Next, new 18OTR records were developed from Cedrela and seven other tree species from northern Bolivia. 18OTR signals were shown to correlate between different species, and between sites large distances apart (<1000 km), indicating a large-scale environmental control on 18OTR (Chapter 4). Following this, atmospheric back-trajectory modelling and basin-scale vapour transport analysis were used to confirm that rainout of heavy isotopes during moisture transport across the continent is the primary control on interannual 18OTR signals in the western Amazon (Chapter 5). Finally, new Cedrela 18OTR chronologies from Ecuador and Bolivia were developed. These records show an increase in 18OTR from the early 1800s until approximately 1950, indicating a change in hydrological functioning over this period, with a reversal in the trend over the last 12 decades. The increase is most likely driven by a reduction in the fraction of incoming water vapour that rains out over the Amazon, which could be caused by a reduction in precipitation, or an increase in the volume of imported vapour. Overall, these results provide evidence for long-term changes in Amazon hydrology over the past 200 years, and make an important contribution to the field of tropical dendroclimatology.