A sedimentary perspective from Lake Junín on monsoon strength and glaciation in the tropical Andes over multiple glacial cycles

The South American Summer Monsoon (SASM) sustains water resources from the Amazon lowlands to the high-elevation Andes. This system is influenced by low and high latitude climate forcing with impacts on precipitation across numerous spatiotemporal scales. Relatively little is known about long-term hydroclimate in the region, however, due to the limited number of well-dated, high-resolution paleoclimate records. This dissertation presents a multi-proxy reconstruction of changes in monsoon strength and glaciation in the tropical Peruvian Andes using sediment cores spanning ~700 ka from Lake Junin. Chapter 2 explores glacier history and precipitation-evaporation balance during the deglacial and Holocene period. Deglaciation occurred earlier in the tropical Andes compared to higher latitudes and was accompanied by lake level reductions. A delayed onset of wet conditions is observed during Heinrich Stadial 1, and the abrupt isotope excursion registered in many Andean records occurred earlier than previously thought. Unconformities indicate major drops in lake level during the early and mid-Holocene, consistent with a weaker SASM. The findings demonstrate that climatic changes at Junin were at times in and out of phase with high latitude regions. Chapter 3 examines the past 50 ka with sufficient chronologic precision to document Junin’s response to Dansgaard-Oeschger (D-O) cycles, the millennial-scale climatic fluctuations that characterized the last glacial. The results demonstrate rapid glacial retreat and reductions in lake level during D-O events, suggesting intense periods of aridity. This study is the first to reveal the magnitude of the climatic disruptions in the Andes and highlights the sensitivity of the region to Northern Hemisphere forcing. Chapter 4 explores two complete glacial/interglacial cycles during Marine Isotope Stages (MIS) 12-15 (~420-620 ka). High amplitude δ18O variability during MIS 15 follows precession, indicating greatly enhanced SASM intensity, whereas no precessional signal is apparent during the more arid MIS 13. MIS 14 shows moderate glacial advance and millennial-scale climatic instability, whereas major expansion of ice during MIS 12 contributed to more stable lake level. This study showcases the large range of past climatic variability in the tropical Andes at unprecedented resolution and demonstrates considerably different responses to climatic forcing among individual glacial and interglacial periods.