Amazon Climate Reconstruction Using Growth Rates and Stable Isotopes of Tree Ring Cellulose from the Madre De Dios Basin, Peru

The Amazon basin is a center of deep atmospheric convection and thus acts as a major engine for global atmospheric circulation. From this basin, one fifth of the world's freshwater flux is discharged into the Atlantic and nearly two-thirds of the global rain forest resides herein. Yet despite its significance, little is known about past Amazon climate variability and the response of the forest ecosystem to climate.

Here, I attempt to reconstruct the paleoclimate history of a portion of the Amazon basin using both tree ring growth and the carbon and oxygen isotopes of tree ring cellulose from the Madre de Dios department of Southeastern Peru. Bomb 14C dating identifies annual rings in tropical species Cedrela odorata and Dipteryx micrantha. A ring width chronology spanning 189 years (1817-2006) is developed for Cedrela odorata and 5 trees of Dipteryx micrantha are utilized for isotope reconstruction. The oldest tree used in the isotope reconstructions has an error-adjusted age of about 473 years (1533-2006). Using the species Cedrela odorata, Ceiba pentandra, Hymenaea courbaril, Myroxylon balsamum, and Tabebuia serratifolia, I develop 5 tree ring chronologies using relative ring width measurements and chart the growth behaviors of over 40 trees. Ring width chronologies from tree species Cedrela odorata and Ceiba pentandra show a significant correlation with wet season precipitation (r = 0.42, and 0.37, respectively, p<0.05). The ring width chronology developed from the species Hymenaea courbaril is significantly correlated (r = 0.68, p<0.05) with January river discharge. Correlations between wet season precipitation and ring growth in Cedrela odorata are used to identify extreme wet and dry events. Nine historic droughts of the 20th century are identified in the C. odorata record. An increase in the frequency of extreme events (mean recurrence interval = 5-6 years) is observed in the 20th century and both Atlantic and Pacific sea surface temperature (SST) forcing mechanisms are implicated. The chronology shows a moderate correlation with both ENSO and tropical North Atlantic SST anomalies, suggesting that both basins play a role in precipitation variability over tropical South America.

Carbon and oxygen isotopic measurements (proxies of moisture stress and precipitation amount, respectively), in tree ring cellulose from 5 Dipteryx micrantha trees are used to reconstruct an error adjusted 473 year long record of precipitation variability. Because an error correction factor is applied to the chronologies of the trees of this species, assessment of annual-scale variability is precluded. Instead lower frequency trends are examined. No long term trends are identified in the oxygen isotopic records from individual trees. The carbon isotopic records of all five individual trees track the depletion of atmospheric δ13C during the 20th century due to the anthropogenic input of fossil fuel CO₂ (The Suess effect). Relatively large variability in the oxygen isotopic records between trees suggests that site-specific and tree-specific conditions dominate this signal. Carbon isotopic records reveal a better correlation between records from multiple trees (r = 0.47, p<0.01) suggesting that a common climate signal is more robustly recorded by the δ13C of these trees. At interannual frequencies (5 year), both carbon and oxygen isotopic records correlate significantly with wet season precipitation (r = -0.50, and -0.55 respectively, p<0.05). Spectral analysis reveals dominant 8-10 year and 3-5 year periodicities in both the carbon and oxygen isotopic records of individual trees. The oldest tree examined reveals a shift from this 8-10 year periodicity during the early part of the record to a lower frequency (20-24 year) variability during the last century. The lower frequency variability identified in the records is associated with both the Pacific Decadal Oscillation and the decadal and multidecadal variability observed in the tropical North Atlantic.

Collectively, these data show that tropical tree ring growth and isotopic composition in the southwestern Amazon basin are precipitation dependent and these measures can be exploited to reconstruct a hydrologic history for this region.