Late-Pleistocene Glaciation and Climate Around the Junín Plain, Central Peruvian Highlands

Abstract

Modern glaciers in the cordillera west and east of the plain of Laguna Junín northeast of Lima are confined to small summits and west-facing cirques. Snowline today is at about 4900 m on the west and 4800 m on the east. That these small glaciers were larger in the recent past is indicated by small moraines and by areas nearly bare of vegetation peripheral to the present glacier limits. In the Pleistocene a mountain glacier complex covered most of the cordillera and spread to all but the center of the plain, thus damming Laguna Junín. Two phases of glaciation can be distinguished. The younger is marked by sharp moraines dotted with small depressions. The older has smooth landforms, no undrained depressions, and locally a thin cover of loess. Retreat from the younger moraines may not have commenced until about 12,000 years ago. Maximum late-Pleistocene snowline depression is calculated at about 300 m in the western cordillera and 500 m in the eastern, on the basis of the elevation of small cirque lakes. Snowline depression of 300 m could be caused by a depression in mean annual temperature of only 2°C. Greater snowline depression on the east may reflect the southward shift of the tropical rain zone in the Amazonian lowlands, as a secondary effect of the vast Laurentide ice sheet of North America.