Oxygen isotopes in tree rings are an excellent archive of precipitation dynamics in the tropical Amazon region. The precise determination of the ratios of stable oxygen isotopes (18O/16O) proves to be a new parameter for detecting the dynamics of the water cycle in tropical rain forest areas. It can therefore replace the classic climate observables such as tree ring width or wood density, which are unsuitable for high-quality reconstructions of climate conditions in tropical areas. These are the findings of a group of researchers from the GFZ German Research Centre for Geosciences, the Universities of Leeds (United Kingdom) and Utrecht (Netherlands), and the Institut de Recherche pour le Développement (IRD, Peru), published in the new online edition of the Proceedings of the National Academy of Sciences (PNAS).
The researchers studied tree rings of the tropical tree species Cedrela odorata from Bolivia and found that they preserve the isotopic composition of rainwater in the Amazon. As the variation in oxygen isotopes is strongly determined by the amount of rainfall over the Amazon basin, it provides a valuable historical archive of rainfall in the past. This now paves the way for a better understanding of long term hydrological patterns.
The Amazon region, which is 17 times larger than Germany, plays a central role in the global climate, but so far little is known about its climatic history. This is mainly because at present only a few precisely datable and temporally highly resolved climate archives are available from these regions. Dr. Gerhard Helle from the GFZ German Research Centre for Geosciences explains: "With this new method, we have discovered an extremely powerful tool to look into the past, which allows us to better understand the magnitude of natural variability of the climate system. Climate models vary widely in their predictions for the Amazon, and we still do not know whether the Amazon will become wetter or dryer in a warmer world. The surprise for us was that the isotopic analysis of only eight trees from a single location in the area of the upper basin of the Amazon not only provided information about the local rainfall conditions, but also about the entire catchment area of the Amazon." About one-fifth of the global land area precipitation falls in the Amazon basin, which is drained by the world's largest river into the ocean. The oxygen isotope values of the tree rings showed a strong relationship with the water level fluctuations in the Amazon and therefore provide valuable information on the amount of rain that is transported into the Southern Atlantic.
The examined trees, each approximately 150 years old, even delivered clear evidence of extreme events. For example, the very low water level of the Amazon produced by the climate phenomenon El Nino in 1926 clearly stands out in the isotope time series of the tree rings. Although the current data record is relatively short, it shows an increase in oxygen isotope values across the 20th century, which is accompanied by the observation of a slight increase in the discharge rates of the Amazon. "Both can in all likelihood be attributed to an intensification of the hydrological cycle", analyzes Gerd Helle, and looks to the future: "To be sure of this, we have to investigate further at additional locations in the Amazon basin. We have developed our current procedure to a novel method of a combined isotopic analysis of oxygen and carbon in tree rings, which with its high temporal resolution is well suited for this."
Brienen, R.J.W., Helle, G., Pons, T.L., Guyot, J.L., Gloor, M. :„ Oxygen isotopes in tree rings are a good proxy for Amazon precipitation and El Nino Southern Oscillation", Proceedings of the National Academy of Sciences of the United States of America; www.pnas.org/cgi/doi/10.1073/pnas.1205977109
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