PRE-LATE-GLACIAL TO RECENT GLACIER EXTENTS AND CONSTRAINTS ON ELA DEPRESSION IN THE JAHUACOCHA VALLEY, CORDILLERA HUAYHUASH, PERÚ

Glacier equilibrium line altitude (ELA) estimates in the tropical Andes vary widely, due in part to uncertainty of the age of glacial features used to determine the ELAs and in part due to real differences in ELAs throughout the Andean cordillera. For example, there is a large ELA difference or a steep gradient between the Cordillera Blanca (8-9º S) and the Junin plain (11º S). The little-studied Cordillera Huayhuash (~10°15’ S) lies between these regions, and provides an opportunity to determine, with good age control, the last glacial maximum (LGM) extent and paleo-ELAs from pre-late-Glacial to recent times. We present results from a pilot study to map glacial features, determine the glacial chronology using cosmogenic ages of moraine ridges, and reconstruct paleo-ELAs in the Jahuacocha valley, headwaters of the Rio Achin, on the western flank of the Cordillera Huayhuash. Four major moraine sets, comprising a minimum of ten ridges, were mapped in the Jahuacocha valley, a major valley draining the cordillera. Moraines were mapped using hand-held global positioning system units, aerial photographs, Advanced Spaceborne Thermal Emission and Reflection Radiometer (ASTER) satellite data, and the recently-released 3 arc second (~90 m) shuttle radar topography mission (SRTM) digital elevation model for South America. Glacial lake cores may contain material to help constrain the moraine ages further, and deformed lake sediments within one of the (probable) late-Glacial moraines indicates a period of advance after a significant still-stand. In addition, rock samples were collected from moraine ridge tops to determine cosmogenic ages. Cosmogenic ages will help to determine which of the ridges correspond to the LGM, and will help to put the older moraines in a chronologic context. From the mapped moraine positions and valley shapes we reconstruct probable glacier area and ELA for each stage. By combining the reconstructions from the Jahuacocha valley with the LGM extent and ELAs across the Cordillera Huayhuash we will get an improved understanding of the spatial gradients in ELA and ELA depression at a critical tropical location.