Comparing reconstructed Pleistocene equilibrium‐line altitudes in the tropical Andes of central Peru

Abstract

AbstractGlacier equilibrium‐line altitude (ELA), and the difference between modern and palaeo‐ELA can be interpreted as a proxy for climate change. One issue in ELA reconstruction is that different methods of ΔELA reconstruction may produce a range of results for the same palaeoglacier. When a range of methods is used to reconstruct ELAs across a region, resulting variations may be related to the method rather than the past climate. Palaeoclimatic interpretation of ELAs that were reconstructed by different methods may prompt spurious inferences if the ELA range is the result of methodological differences rather than climatic variation. We address the relationship and degree of variation between methods by comparing terminus‐to‐headwall‐altitude ratio (THAR), accumulation–area ratio (AAR), and accumulation‐area balance ratio (AABR) methods for palaeoglaciers in four valleys in the tropical Andes. Valleys in the eastern cordillera of the Peruvian Andes bordering the Junin Plain (11° S, 76° W, ca. 4100 m a.s.l.) are presently ice‐free but were glaciated repeatedly during the Pleistocene. We use a combination of 90‐m shuttle radar topography mission (SRTM) data, 15‐m Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) data, and 1:25 000 topographic maps to reconstruct ELAs. Within each of three groups of moraines, map‐based THAR and AABR estimates of ELA tend to be highest, followed by DEM‐derived THAR ELAs, with AAR‐reconstructed ELAs somewhat lower in this region. ELA estimates for the local Last Glacial Maximum (LLGM) range from ca. 4250 to 4570 m a.s.l., with ΔELAs of ca. −220 to −550 m (depending on valley and method used). Within individual valleys, ΔELAs for the same palaeoglaciers calculated by different methods vary by ± 100 m. ELAs of the LLGM glaciers and those of the largest glaciers to occupy the Junin valleys (> 65 ka) are not markedly different from each other, regardless of the method used in their calculation, which is largely a reflection of the valley hypsometry in the region. Our analysis suggests relatively low decreases in temperature during glacial periods in the region (ca. 2.5± 1°C). Copyright © 2005 John Wiley & Sons, Ltd.