COMPARING RECONSTRUCTED PLEISTOCENE EQUILIBRIUM-LINE ALTITUDES FOR THE TROPICAL ANDES OF CENTRAL PERU

The equilibrium line altitude (ELA) of a glacier is the altitude of the line along which annual mass gain equals mass loss. Change in ELA (ΔELA) can be interpreted as a proxy for climate change. The terminus-to-headwall-area ratio (THAR) and accumulation-area ratio (AAR) methods can be used to reconstruct paleoglacier ELAs. THAR estimates are easy to calculate from map data, but do not incorporate hypsometric variability between valleys. AAR values for ELA derived from digital elevation models (DEMs) may be a more robust way to estimate paleo-ELAs because they take into account the range of elevations at which mass changes can occur.  We present a comparison of paleo-ELA reconstructions for valleys in the tropical Andes based on both THAR and AAR methods. Valleys bordering the Junin Plain (11° S 76° W, 4000 masl) in the eastern cordillera of the Peruvian Andes are presently ice-free but were glaciated repeatedly during the Pleistocene. Cosmogenic dating (¹⁰Be) of boulders on moraines in three west-facing Junin valleys indicates that low-relief end moraines located near the midpoints of the valleys were deposited during the last glacial maximum (LGM) in the region, whereas large lateral moraines at the lower ends of the valleys predate the last glacial cycle. We are using a combination of 90-m shuttle radar topography mission (SRTM) data, DEMs derived from 15-m Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) data, and 1:25,000 topographic maps to reconstruct ELAs in the field area. Preliminary results indicate that AAR estimates of ELA tend to be lower than THAR estimates for the same glacier, but that ELAs of the LGM glaciers and those of the largest glaciers to occupy the Junin valleys are not markedly different from each other, regardless of the method used in their calculation. Our Junin ΔELA estimates can be compared with those derived from similar two-method reconstructions of late-glacial ELAs for the Cordillera Huayhuash (10°15’ S 77° W), which showed that AAR (=0.7) estimates of ELAs were lower than THAR (=0.45) estimates by ~300 m on the west side of the range and ~100-150 m on the east side. We address the variation of ELAs at these sites and the implications for regional correlations of ELA reconstruction and ELA-based paleoclimates.