GLACIAL LACUSTRINE RECORDS OF GLACIATION IN THE TROPICAL ANDES

Lake sediment cores taken from glacial lakes are often interpreted as records of glacial activity. Alpine glaciers are especially sensitive to changes in precipitation and temperature, and thus the history of glaciation can be used in the reconstruction of past climates. Owing to steep east-to-west moisture gradients across the Andes, glaciers in the eastern cordillera are more sensitive to changes in temperature whereas those in the drier, western, cordillera are more sensitive to changes in precipitation. The objective of this study is to compare continuous records of glaciation from Andean glacial lakes in order to delineate records of past changes in temperature and precipitation in the tropical Andes. Furthermore, we compare our glacial records with the ∂¹⁸O record of precipitation recorded in ice cores and carbonate lakes in the region. Multiple lacustrine sediment cores were taken from two sites in the central Peruvian Andes in June 2011: one set from the eastern cordillera of the Andes and one set from the western cordillera. These cores were measured for two proxy indicators of glacial erosion: magnetic susceptibility (MS) and organic carbon content (TOC); cores were dated by radiocarbon. Laguna Yanacocha, located in the eastern cordillera of the Peruvian Andes (10.558°S, 75.927°W; 4358 m asl) yielded an ~2.6 meter-long sediment core that spans more than 16,000 cal yr. The sediment record reveals a two-step last deglaciation; the initial decline in glacigenic sediment input began about 14,800 cal yr BP and lasted until about 10,600 cal yr BP at which time glacial erosion increased and remained high until about 9000 cal yr BP. For the past 9000 cal yr, glacigenic sediment input into Laguna Yanacocha has been at a minimum. An ~6.7 meter-long core from a bog at the upper end of Laguna Shiurococha, located in the western cordillera (11.905°S, 75.960°W; 4585 m asl), records more than 4000 years of sedimentation. This core reveals a sharp decline in glacigenic sedimentation around 4300 cal yr BP, with very little glacial input thereafter. The absence in either of these cores of a sediment signal of the Little Ice Age, which is well documented in other lake cores and in moraines from the region, may be due to the opening of upvalley lake basins that served to trap sediment during the latest Holocene.