Hydroclimate and ENSO Variability Recorded by Oxygen Isotopes From Tree Rings in the South American Altiplano

Hydroclimate variability in tropical South America is strongly regulated by the South American Summer Monsoon (SASM). However, past precipitation changes are poorly constrained due to limited observations and high-resolution paleoproxies. We found that summer precipitation and the El Nino-Southern Oscillation (ENSO) variability are well registered in tree-ring stable oxygen isotopes (delta O-18(TR)) of Polylepis tarapacana in the Chilean and Bolivian Altiplano in the Central Andes (18-22 degrees S, similar to 4,500 m a.s.l.) with the northern forests having the strongest climate signal. More enriched delta O-18(TR) values were found at the southern sites likely due to the increasing aridity toward the southwest of the Altiplano. The climate signal of P. tarapacana delta O-18(TR) is the combined result of moisture transported from the Amazon Basin, modulated by the SASM, ENSO, and local evaporation, and emerges as a novel tree-ring climate proxy for the southern tropical Andes. Plain Language Summary Understanding past climatic changes in the Central Andes in tropical South America is of great importance to contextualize current hydroclimatic conditions. Here, we present the first P. tarapacana tree-ring stable oxygen isotope (delta O-18(TR)) chronologies and analyze their value as environmental records for this region. Locally known as quelioa, P. tarapacana grows in the South American Altiplano from 16 degrees S to 23 degrees S at very high elevations (up to 5,100 m a.s.l), making it the highest elevation tree species worldwide. We analyze P. tarapacana delta O-18(TR) from 1950 to present and find that it registers precipitation changes in the Altiplano and the El Nino - Southern Oscillation (ENSO). We suggest that delta O-18(TR) is likely affected by soil evaporation and leaf transpiration due to the high solar radiation and aridity in the Altiplano, leading to an enrichment in delta O-18(TR) values with a more pronounced effect at the more arid sites. P tarapacana delta O-18(TR) reflects the atmospheric processes transporting moisture to the Altiplano and the influence of local evaporation. Our findings are relevant for generating robust hydroclimate reconstructions in the Central Andes to improve circulation models and provide better management of water resources in tropical South America.

Automated summary

The climate variability in tropical South America is regulated by the South American Summer Monsoon (SASM). In this study, stable oxygen isotopes from tree rings were used to analyze past precipitation changes and El Nino-Southern Oscillation (ENSO) variability in the Chilean and Bolivian Altiplano. The results showed that the tree rings can effectively register summer precipitation and ENSO variability, making it a novel climate proxy for the southern tropical Andes.