Oxygen isotopes in tree rings are a good proxy for Amazon precipitation and El Niño-Southern Oscillation variability

Abstract

We present a unique proxy for the reconstruction of variation in precipitation over the Amazon: oxygen isotope ratios in annual rings in tropical cedar (Cedrela odorata). A century-long record from northern Bolivia shows that tree rings preserve the signal of oxygen isotopes in precipitation during the wet season, with weaker influences of temperature and vapor pressure. Tree ring δ 18 O correlates strongly with δ 18 O in precipitation from distant stations in the center and west of the basin, and with Andean ice core δ 18 O showing that the signal is coherent over large areas. The signal correlates most strongly with basin-wide precipitation and Amazon river discharge. We attribute the strength of this (negative) correlation mainly to the cumulative rainout processes of oxygen isotopes (Rayleigh distillation) in air parcels during westward transport across the basin. We further find a clear signature of the El Niño-Southern Oscillation (ENSO) in the record, with strong ENSO influences over recent decades, but weaker influence from 1925 to 1975 indicating decadal scale variation in the controls on the hydrological cycle. The record exhibits a significant increase in δ 18 O over the 20th century consistent with increases in Andean δ 18 O ice core and lake records, which we tentatively attribute to increased water vapor transport into the basin. Taking these data together, our record reveals a fresh path to diagnose and improve our understanding of variation and trends of the hydrological cycle of the world's largest river catchment.climate change | dendrochronology | plant physiology T he Amazon basin is a major center of atmospheric convection and precipitation (1), and at the same time the world's largest drainage basin. The Amazon's river discharge accounts for ∼17% of the annual global discharge to the oceans (2); its hydrological cycle is tightly linked with the carbon cycle of the Amazon rainforest (3), which itself is one of the largest terrestrial biomass carbon pools. Changes in the hydrological cycle of the Amazon may therefore significantly affect atmospheric dynamics and global climate by changing atmospheric CO 2 concentration. Two particularly severe droughts occurred within the last decade (4), and long-term river records of the Amazon show a steady increase in discharge over the last century (5), indicating that the system may start to undergo some changes. It has been suggested that these changes are a result of anthropogenic warming, but they may also be part of longer-term, natural climatic variability. To clarify such a globally important issue, long-term, accurate records of the hydrological cycle of the Amazon basin are needed. However, meteorological data are only reliable for the last 50-60 y (6), and annually resolved long-term proxies for the hydrological cycle from within the Amazon basin itself are very scarce (7).A commonly used diagnostic for strength and functioning of the water cycle is the isotopic composition of precipitation. Although variation of oxygen isotope ratios in precip...

Multiple tree-ring chronologies (ring width, δ13C and δ18O) reveal dry and rainy season signals of rainfall in Indonesia

“… 18 O studies on teak from India (Managave et al, 2010a;Managave et al, 2011) found both negative and positive correlations with the amount of rainy season rainfall at inter-and intra-annual scale. Studies in tropical regions of South-America have likewise reported negative correlation with the rainfall amount during rainy season (Ballantyne et al, 2010;Brienen et al, 2012), as have isotope studies in Southeast Asia (Sano et al, 2012;Xu et al, 2011;Zhu et al, 2012). However, none reported correlations with dry and rainy season rainfall as revealed here for Indonesian teak.…”

“…Plant physiological effects like leaf water enrichment due to transpiration (Barbour, 2007;Helle and Schleser, 2004b;McCarroll and Loader, 2004;Roden et al, 2000) do not seem to be particularly pronounced in  18 O records from tropical tree rings (e.g. Brienen et al, 2012). Variation of …”

Seasonal variations in the stable oxygen isotope ratio of wood cellulose reveal annual rings of trees in a Central Amazon terra firme forest

“…Brienen et al (2012) reported that "inter-annual" variations in δ 18 O wc of Cedrela odorata trees sampled in northern Bolivia were correlated strongly with those of δ 18 O prec measured at considerably distant sites (>1000 km) in the Amazon, including Manaus. In the case of E. coriacea, however, the seasonal δ 18 O wc patterns of the trees in Manaus were not similar to those in the other two study areas (EC-JU, EC-AT; Fig.…”

ENSO flavors in a tree-ring δ<sup>18</sup>O record of <i>Tectona grandis</i> from Indonesia

“…In tropical regions, oxygen isotope data from tree rings (δ 18 O TR ) are often more sensitive to precipitation than ring width (e.g., Brienen et al, 2012;Schollaen et al, 2013). δ 18 O TR data are primarily controlled by the isotopic composition of precipitation, i.e., the source water, and relative humidity (e.g., Barbour, 2007;McCarroll and Loader, 2004).…”