Proglacial lake sediment records of Holocene climate change in the western Cordillera of Peru

Sediment records from proglacial lakes between 9 and 10°S in the western Cordillera of the Peruvian Andes document the waxing and waning of alpine glaciers since the end of the Lateglacial stage. These records from the southern tropical Andes provide supporting evidence that the early Holocene (between 12 and 8 ka) was relatively warm and dry, and the middle Holocene (between 8 and 4 ka) was marked by a shift to cooler, and possibly wetter conditions in certain regions, leading to glacial advances. Although there were multiple periods of brief ice advances that interrupted the overall trend, glaciers in multiple valleys generally retreated from ∼4.0 ka through the Medieval Climate Anomaly (1.0-0.7 ka). This late Holocene pattern of ice retreat occurred during a period when lake level studies, and both lacustrine and speleothem stable isotopic records indicate wetter conditions relative to the middle Holocene, suggesting that higher temperatures contributed to the pattern of ice retreat. Following this period of glacial retreat, multiple proxy records suggest that the start of the Little Ice Age (∼0.6-0.1 ka) was a colder and wetter time throughout much of the tropical Andes. There appear to be two primary synoptic-scale climatic controls on temperature and precipitation linked to insolation dynamics that drive changes in ice cover in the southern tropical Andes during the Holocene: 1) the strength of the South America Summer Monsoon, which is linked to Northern Hemisphere temperatures and the mean position of the Intertropical Convergence Zone over the Atlantic, and 2) sea surface temperature distributions in the tropical Pacific Ocean and its influence on atmospheric temperature, precipitation and circulation patterns.