CONTROLS ON THE COMPOSITION AND DISTRIBUTION OF LATE PLEISTOCENE AND HOLOCENE SEDIMENT IN LAKE JUNIN, PERU

Lake Junín is a large (~300 km²), high-elevation (~4085 m) lake occupying an intermontane basin in the tropical South American Andes (11° S, 76.1° W) that is highly sensitive to changes in temperature and precipitation. It is one of few lakes in the tropical Andes that preserves a continuous sedimentary record spanning the last glacial cycle and is ideally situated to record long-term variations in the South American Summer Monsoon. Lake Junín was drilled in 2015 to recover overlapping sediment cores from three locations, penetrating to a maximum depth of ~100 m below lake floor. These cores will provide the longest independently-dated record of water balance, glacier variability, and paleomagnetic secular variation for the region. Prior to drilling, we collected short cores (2 - 9 m length) using a modified Livingstone corer at 9 locations across a NE-SW transect to characterize the controls on sediment composition and distribution since deglaciation. These cores each extend into glacigenic sediments, indicating recovery of the complete postglacial (late Pleistocene and Holocene) sequence. All cores exhibit a similar two-component stratigraphy: a lower section of glacial sediments characterized by gray glacial flour and minerogenic material sourced from glacial outwash fans to the east and an upper section of non-glacial sediments composed primarily of beige marl (authigenic carbonate) with intermittent brown mud. Data from two overlapping cores near the center of the lake basin confirm this stratigraphy, with low % CaCO₃ (avg. 16%) and high magnetic susceptibility (MS) (avg. 4.8 SI´10^-5) in the lower section and high % CaCO₃ (avg. 48%) and low MS (avg. -1.3 SI´10^-5) in the upper section. A radiocarbon date of terrestrial grass provides a minimum limiting median calibrated age of 18,950 cal yr BP for this transition, which is earlier than other records show for regional deglaciation. Preliminary MS data from 7 of the 9 short cores show that the carbonate/glacial contact deepens from NE to SW, and the thickness of the upper section displays a significant positive correlation with distance from the NE shore. Quantification of late Pleistocene and Holocene sediment dynamics in Lake Junín will facilitate more accurate interpretation of their stratigraphic significance in the longer drill core record.