A 2,300-year-long annually resolved record of the South American summer monsoon from the Peruvian Andes

Abstract

Decadal and centennial mean state changes in South American summer monsoon (SASM) precipitation during the last 2,300 years are detailed using an annually resolved authigenic calcite record of precipitation δ 18 O from a varved lake in the Central Peruvian Andes. This unique sediment record shows that δ 18 O peaked during the Medieval Climate Anomaly (MCA) from A.D. 900 to 1100, providing evidence that the SASM weakened considerably during this period. Minimum δ 18 O values occurred during the Little Ice Age (LIA) between A.D. 1400 and 1820, reflecting a prolonged intensification of the SASM that was regionally synchronous. After the LIA, δ 18 O increased rapidly, particularly during the current warm period (CWP; A.D. 1900 to present), indicating a return to reduced SASM precipitation that was more abrupt and sustained than the onset of the MCA. Diminished SASM precipitation during the MCA and CWP tracks reconstructed Northern Hemisphere and North Atlantic warming and a northward displacement of the Intertropical Convergence Zone (ITCZ) over the Atlantic, and likely the Pacific. Intensified SASM precipitation during the LIA follows reconstructed Northern Hemisphere and North Atlantic cooling, El Niño-like warming in the Pacific, and a southward displacement of the ITCZ over both oceans. These results suggest that SASM mean state changes are sensitive to ITCZ variability as mediated by Western Hemisphere tropical sea surface temperatures, particularly in the Atlantic. Continued Northern Hemisphere and North Atlantic warming may therefore help perpetuate the recent reductions in SASM precipitation that characterize the last 100 years, which would negatively impact Andean water resources.

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

“…1). Among the station records, the correlation is strongest with annual values of oxygen isotopes in precipitation (δ 18 O prec ) from Manaus (r = 0.86, n = 15). Generally variation in δ 18 O tr correlated most strongly with δ 18 O in precipitation during the wet season (December-May), which corresponds to the main period of growth for this drought-deciduous species (SI Appendix, Fig.…”

“…Observations of the isotopic composition of meteoric precipitation over South America show that isotopic composition is indeed closely associated with the amount of precipitation along the air parcel path (10), which suggests that oxygen isotopes in precipitation are predominantly a proxy for the large-scale atmospheric component of the basin-wide hydrological cycle (9,14). However, the interpretation of results from paleoclimatic studies of oxygen isotopes in ice cores (15)(16)(17), lake sediments (18), and speleothems (19,20) remains disputed, and proxies have been interpreted variously as indicators of temperature (17), basin-wide rainfall (18)(19)(20)(21), or a combination of both (22). Despite numerous oxygen paleorecords from the Andes, there are no such proxies from within the Amazon basin itself.…”

“…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.…”

“…As tree roots take-up soil water without fractionation of the heavier H 2 18 O (24,25), at least part of the isotopic signature in tree rings should reflect variation in precipitation δ 18 O. To test this hypothesis, we correlated δ 18 O tr with isotopic ratios in meteoric precipitation (data; Global Network of Isotopes in Precipitation; see SI Appendix), which shows indeed significant relations with various stations across the Amazon Basin (Fig. 1).…”

“…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.…”