The timing of the last deglaciation in Cordillera Oriental, northern Peru, based on glacial geology and lake sedimentology

The degree of soil development on moraines and radiocarbon ages indicate that the Cordillera Oriental, a part of the eastern Andes in northern Peru (7 degrees 30'-7 degrees 55'S; 77 degrees 20'-77 degrees 35'W), was last glaciated during the last glacial maximum (marine isotope stage 2). This glaciation was the most extensive late Quaternary glaciation in the area. Radiocarbon-dated sedimentological changes in six sediment cores from three lakes and one bog reveal that the last deglaciation is marked by a two-stepped transition from rhythmically laminated inorganic sediment to massive, organic-rich sediment. The first step of this transition commenced before 12.1 + or - 0.2 ka (+ or - 1 Sigma) and probably by ca. 13.5 ka. Glaciers covered less than 50% of their last glacial maximum area by 12.1 + or - 0.6 ka, and cirques west of the main divide were probably ice-free by this time. A return to rhythmically laminated inorganic sediment in the lakes and the bog occurred between 12.1 + or - 0.6 and 10.3 + or - 0.4 ka. This may reflect an episode of increased stream discharge and glacier expansion during which glaciers reoccupied cirques above approximately 3,800 m. This glacier expansion adds to widespread evidence of a late-glacial readvance in the tropical Andes. All cirques have been ice-free since between 10.3 + or - 0.4 and 9.7 + or - 0.5 ka.