High-resolution variability of the South American summer monsoon over the last seven millennia: Insights from a speleothem record from the central Peruvian Andes

Stable oxygen isotope (δ¹⁸O) measurements of two speleothems, collected from Huagapo Cave in the central Peruvian Andes and with overlapping age from 1.1 to 1.4ka, characterize tropical South American climate variability over the last 7150 years. In the study region, precipitation δ¹⁸O (δ¹⁸O_p) is inversely correlated to rainfall amount upstream in the Amazon Basin and the intensity of convection associated with the South American summer monsoon (SASM). Speleothem long-axis profiles yield an average age resolution of five years and permit investigation of climate over orbital to decadal timescales. Variations in the isotopic composition of Huagapo Cave calcite (δ¹⁸O_c) are in good agreement with several precipitation proxy records from ice cores, speleothems, and lake sediments from the central Peruvian Andes. From the mid-Holocene to today, δ¹⁸O_c, a proxy for δ¹⁸O_p, tracks changes in local insolation and exhibits a ~2‰ decrease. In the Late Holocene, Huagapo Cave δ¹⁸O_c is characterized by two periods of significant decline in SASM intensity (up to 1.5‰ increase in δ¹⁸O_c) even when insolation is reaching a local maximum and the SASM would be expected to intensify. These millennial-scale reductions in SASM intensity could in part be influenced by a reduction in the zonal SST gradient of the Pacific Ocean, favoring El Niño-like development.