Late Pleistocene equilibrium‐line reconstructions in the northern Peruvian Andes

Abstract

Equilibrium‐line‐altitude (ELA) reconstructions using the toe‐to‐headwall‐altitude ratio method for paleoglaciers in the Cordilleras Blanca and Oriental, northern Peruvian Andes (7–10°S; 77°20′–77°35'W), indicate that ELAs during the last glacial maximum (LGM; marine isotope stage 2) were c. 4300 m in the Cordillera Blanca, c. 3900–3600 m on the west side of the Cordillera Oriental, and c. 3200 m on the east (Amazon Basin) side of the Cordillera Oriental. Comparison with estimated modern ELAs and glaciation thresholds indicate that ELA depression ranged from c. 700 m in the Cordillera Blanca to c. 1200 m on the east side of the Cordillera Oriental. This augments data from many mountain ranges in middle‐ and low‐latitude regions that indicate that ELAs during the LGM were depressed by c. 1000 m. Published palynological evidence for drier conditions during the LGM in the tropical Andes suggests that ELA depression of this amount involved a temperature reduction (> 5–6°C) that greatly exceeded the tropical sea‐surface temperature depression estimates of CLIMAP (< 2°C). The west to east increase in ELA depression during the LGM indicates that the steep modern precipitation gradients may have been even steeper during the LGM.