Biomarker evidence for arid intervals during the past ∼1,800 years in the central Andean highlands

Abstract

The late Holocene behavior of the South American Summer Monsoon (SASM) has been a focal point of studies of oxygen isotope ratios in Andean speleothems and other archives, as they primarily reflect large-scale atmospheric circulation over the South American lowlands rather than localized precipitation vs. evaporation. Consequently, the local effective moisture history throughout the Andes has remained poorly constrained. Here we present ∼1800-year plant wax hydrogen (δ²H_wax) and carbon (δ¹³C_wax) isotope records from Lake Chacacocha, southeastern Peru. δ²H_wax and δ¹³C_wax from terrestrial upland and wetland vegetation allow large-scale atmospheric conditions and local-scale effective moisture to be evaluated side-by-side. In agreement with δ¹⁸O-based proxies from the region, Chacacocha δ²H_wax shows that the SASM was strengthened during the Little Ice Age (LIA). However, our record shows that the SASM began this intensification around 1250 CE, well before the external radiative forcings of the LIA, suggesting that long-term shifts in SASM behavior can be catalyzed by internal drivers e.g., teleconnections with the tropical Pacific. Independent of SASM dynamics, Chacacocha δ¹³C_wax reveals two pronounced, centennial to multi-centennial arid intervals between ca. 450-900 CE and ca. 1640-1850 CE – the latter co-occurring with peak SASM intensity. LIA aridity aligns with nearby evidence for reduced net ice accumulation and glacial retreat from the LIA maximum extent around this time, implying that despite enhanced monsoon activity regionally, reduced effective moisture locally affected the glacial mass balance in southern Peru. Our results help reconcile conflicting interpretations of the SASM, glacial, and high-alpine environmental histories in the Central Andes over the past 1,800 years.