A glacial geochronology of Cordillera Pariacaca, central Peru

Glacial dynamics in the Outer Tropics reflects a complex interplay of variations in precipitation and temperature driven by large scale ocean-atmospheric dynamics. Cosmogenic dating of moraines, palaeoglacier reconstruction and comparison to climate proxies adds to our understanding of palaeoglacier dynamics in relation to these forcings and can help assess future changes of tropical glaciers in response to ongoing climatic changes. Here we present 12 cosmogenic 10Be exposure ages from three moraines of the Mullucocha Valley in the previously unresearched Cordillera Pariacaca, Peru. These moraines tightly constrain three phases of glaciation in the central Peruvian Andes at 28.0 ka, 15.5 ka and 11.6 ka, attributed to the Last Local Glacial Maximum, Older Dryas and Late Pleistocene-Early Holocene transition, respectively. They add new evidence for the synchronous behaviour of Central Peruvian glaciers relative to the wider Outer Tropics, from the Lateglacial to Early Holocene. Following comparison with regional moraine chronologies, we suggest an early Last Local Glacial Maximum (LLGM) in Central Peru which occurred prior to the global LGM. A two-phase glacial maximum in the Outer Tropics at 28 ka and ∼22 ka is hypothesised. An undated moraine nested inside the LLGM moraine in the Mullucocha Valley is theorised to be representative of the younger 22 ka LLGM phase, closer to the timing of global LGM. The 15.5 ka moraine adds to the growing evidence of stabilisation during the Older Dryas, when cold northern hemisphere conditions enhanced precipitation in the Outer Tropics. The 11.6 ka moraine could be connected to the very Early Holocene or late Younger Dryas and is exemplary of a time of widespread moraine formation across the Outer Tropics. It represents a period of glacier stillstand within the wider post-LGM retreat, reflecting periods of cooler, wetter conditions within the overall warming trend. This study provides new insights into the glacial and climate history of the central Peruvian Andes and highlights the need for updated palaeoclimate reconstruction methods.