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For the past few decades, many researchers have sought to understand how tropical hydroclimate responds to climate change via lakes, marine sediments, and speleothems records. Speleothem δ18O records throughout South America have shown that regional rainfall responds to Northern Hemisphere forcing on the millennial scale. Areas under the influence of the South Atlantic Convergence Zone (SACZ) have also shown a close relationship with local insolation on longer timescales. However, apart from the Cruz et al. (2007) record in Southern Brazil, long-term speleothem records throughout the continent have relied primarily on stable oxygen isotopes and are therefore limited to describing large-scale regional variability in rainfall. As such, many areas in South America still lack long-term records of local hydroclimate, which is critical to understanding how different components of the monsoon system respond to orbital and millennial-scale climate change. One proxy that has gained more attention in recent years is trace metal-to-calcium ratios (TM/Ca). Sr, Mg, and Ba to Ca ratios in speleothems are known in certain situations to respond to the degree of Prior Calcite Precipitation (PCP) above a drip site, a phenomenon directly tied to local aridity. In this study, we have obtained high-resolution TM/Ca measurements to pair with stable isotopes from samples spanning 23 to 66 ka from Huagapo Cave in the Peruvian Andes (11.27°S; 75.79°W). TM/Ca ratios in these samples are strongly correlated (R2>0.89), making them suitable for use as PCP proxies. We see that decreases in δ18O during Heinrich events are accompanied by a drop in TM/Ca. The period defined by the MIS 4/3 transition is accompanied by a simultaneous increase in TM/Ca and δ18O. TM/Ca and δ18O negatively correlate with local insolation for the entire record. Interestingly, the Paraíso Cave record from the Amazon Basin shows no correlation between regional or local hydroclimate and insolation during the last glacial period. The discrepancy between the two records and the close relationship between TM/Ca, δ18O, and local insolation in Huagapo samples, may call for a revised interpretation of Andes speleothem δ18O variability, which was originally thought to reflect rainout over the Amazon Basin. 

The biodiverse montane forests of the tropical Andes are today frequently disturbed by rainfall-driven mass movements which occur mostly during extreme El Ni~no events. Over the coming decades these events are projected to double under the 1.5  C global warming scenario. The consequent increased rainfall and mass movement events likely present an elevated risk to millions of people living in the Andes. However, the impact of more frequent rainfall extremes remains unclear due to a lack of studies that directly link past changes in El Ni~no-Southern Oscillation (ENSO) frequency to forest and landscape disturbance patterns. Here, we present the first Holocene palaeoecological record from Laguna Pallcacocha, southern Ecuador, a key site for El Ni~no reconstructions. We demonstrate that for the past 10,000 years plant taxa indicative of recolonization e such as Alnus acuminata e covary with El Ni~no-induced flood layers in the lake. An amplified forest disturbance pattern is observed in the late Holocene, suggesting enhanced slope instability following deforestation. The temporal pattern is not explained by tree line fluctuations or human impact, while the latter does amplify the impact of ENSO on landscape disturbance. Spatial correlations between modern ENSO and precipitation are consistent with a regional comparison of Holocene records of landscape disturbance. Our results indicate that climate extremes, such as those associated with future intensification of El Ni~no, combined with ongoing land use change will increase the frequency of mass movements elevating risks for millions of people in the Andes.