EQUILIBRIUM-LINE-ALTITUDE RECONSTRUCTIONS FOR LGM- HOLOCENE PALEOGLACIERS IN THE HUAGURUNCHO MASSIF, EASTERN PERUVIAN ANDES

Alpine moraines provide a basis for estimating the equilibrium-line-altitude (ELA) of former glaciers, which, in turn, enables quantification of the magnitude and gradient of regional climatic change. Nevado Huaguruncho (5789 masl; 10°14’S, 76°03’W) in central Peru is one of the few presently glaciated peaks in the eastern cordillera of central Peru. Today, the flanks of the Huaguruncho massif are glaciated to a lower elevational limit of ~4700 masl; well-preserved downvalley moraines document the extent of glaciation from prior to the last local glacial maximum (LGM) to the Little Ice Age. Here we focus on Jaico Valley, which is located on the southeastern side of the massif, and is the sole drainage that contains pre-LGM moraines. Using handheld GPSs, and supplemented with data from georeferenced images and stereo pairs of vertical aerial photographs, we mapped moraines and reconstructed former glaciers. The glacial sequence in the Jaico Valley records former ice positions during the following intervals: the Little Ice Age (LIA; ~300 cal yr BP), Neoglacial, Late glacial and early Holocene (~9-12 ka), probable LGM, and during multiple pre-LGM intervals. Age constraints for moraines were obtained from ¹⁴C dates by accelerator mass spectrometry of basal organic material in moraine dammed lakes and wetlands, and from dating the exposure of erratics on moraine crests using the cosmogenic radionuclide ¹⁰Be.

The ELA of modern glaciers in the Huaguruncho massif is ~4800 masl, which is at least 100 m lower than that for modern glaciers in the western cordillera of central Peru. Using a toe-to-headwall altitude ratio of 0.7, we calculated the ELA of paleoglaciers in the Jaico Valley. Relative to the modern, ELA depression during the LIA was ~290m, and that for neoglacial, early Holocene, late glacial and LGM paleoglaciers was ~310, 350, 375, and 550m, respectively. The estimated LGM paleo-ELA depression is similar to that for LGM paleoglaciers in the western cordillera of Peru, but less than that estimated for LGM paleoglaciers in nearby regions of the eastern cordillera, where workers reported ELA depression of ~1km. This latter discrepancy may be due to the misidentification of LGM ice limits, or to significant and localized changes in precipitation during the LGM.