P/E Balance and Glaciation in the Tropical Andes during Marine Isotope Stages 13-15

Many records of past interglacial periods come from locations in the mid- to high-latitudes or the ocean, and the wide range of results highlight the major differences among interglacials owing to variable configurations of global climate forcings such as greenhouse gases, ice sheet volume, and insolation. Less is known about the response of the tropics because fewer available terrestrial records span multiple glacial/interglacial cycles. Here we document changes in sedimentology and stable isotopes from authigenic lacustrine calcite during MIS 13-15 from Lake Junín, a semi-closed basin at 4100 m a.s.l. in the tropical Andes of central Peru. The P/E balance of Lake Junín is sensitive both to changes in evaporation and variations in the strength of the South American Summer Monsoon (SASM). MIS 15 is characterized by consistently elevated carbonate content and large amplitude variations in δ¹⁸O and δ¹³C that appear to follow precession. The range of δ¹⁸O values spans nearly 15 per mil, ~3x larger than that observed during the Holocene, suggesting greatly enhanced SASM intensity and/or duration. The positive δ¹⁸O excursions during MIS 15 occur more abruptly than would be predicted by precessional forcing alone and may result from evaporative enrichment of the lake water, whereas the subsequent transitions to negative values are more gradual. Despite highly enriched δ¹⁸O values during the precessional minima of MIS 15, Total Organic Content (TOC) remains at moderate levels suggesting relatively higher lake level compared to the following glacial period based on our facies model. During MIS 14, XRF counts of erosional elements, such as titanium, indicate enhanced glacial activity. Fluctuations in TOC demonstrate that large millennial-scale variations in P/E balance and lake level dominated this glacial interval, similar to the pattern observed at Junín during MIS 3 in association with D/O events. Highly variable TOC continued into MIS 13 and was accompanied by lower amplitude δ¹⁸O variations, suggesting weaker regional monsoon intensity. This is consistent with findings from several paleorecords that document unusually strong Northern Hemisphere monsoons during MIS 13, which is surprising given the cooler temperatures relative to other interglacials inferred from atmospheric CO₂ and CH₄ concentrations.