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 indica te that the Cordillera Oriental, a part of the eastern Andes in northe rn 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 s tage 2). This glaciation was the most extensive late Quaternary glacia tion in the area. Radiocarbon-dated sedimentological changes in six se diment cores from three lakes and one bog reveal that the last deglaci ation is marked by a two-stepped transition from rhythmically laminate d inorganic sediment to massive, organic-rich sediment. The first step of this transition commenced before 12.1 +/- 0.2 ka (+/- 1 sigma) and probably by ca. 13.5 ka. Glaciers covered less than 50% of their last glacial maximum area by 12.1 +/- 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 +/- 0.6 and 10.3 +/- 0.4 ka. This may reflect an episode of incr eased stream discharge and glacier expansion during which glaciers reo ccupied cirques above approximately 3,800 m. This glacier expansion ad ds to widespread evidence of a late-glacial readvance in the tropical Andes. All cirques have been ice-free since between 10.3 approximately 0.4 and 9.7 +/- 0.5 ka.