Characterization of glacial sediments from a 700,000-year-old Lake Junín drill core

Lake Junín (11.0°S, 76.2°W) is an intermontane lake at an elevation of 4085 masl in Junín, Peru. The lake spans ~300 km2 and has a water depth of ~12m. It is dammed at its northern and southern ends by glacial alluvial fans that have been dated >250 ka, indicating that the lake is at least this old. Lake Junín has never been overridden by ice in the past 1 million years making it one of the few lakes in the tropical Andes that predates the last maximum extent of glaciation and has a continuous record of waxing and waning of nearby Cordilleran glaciers. In July and August of 2015, piston cores were obtained from three sites in Lake Junín. These cores were overlapped to form a continuous record spanning the past ~700 ka. Siliciclastic flux, magnetic susceptibility (MS), mean grain size, and Ti/Ca of sediments from Lake Junín provide proxy records of glacial erosion and glaciation in adjacent valleys. Seven periods of glaciation are determined by relatively high magnetic susceptibility, low CaCO3, high clastic flux and Ti+Si+Al /Ca ratio. Mean grain size during all glacial cycles, irrespective of glacial extent is ~1- 2 microns. The dominance of fine glacial sediments and absence of coarse sediments in the lake core indicate that there is no ice rafted debris (IRD) in Lake Junín. This supports the hypothesis that while the glaciers reached the lake edges, it never calved into the lake. The oldest glacial cycle has relatively coarser sediments (>3 microns) but this distinctive change in grain size is hypothesized to indicate a fluvial depositional environment indicating the birth of Lake Junín to be around ~600,000 years. Despite smaller ice extent in the LLGM, siliciclastic flux between MS2-4 is higher than in other glacial periods which indicates that the glacier at the time was warm based with a higher activity ratio. Furthermore, comparison between skewness and mean grain sizes of glacial sediments in Lake Junín show that the two are inversely related. Coarser sediments have negative skewness with a finer tail while finer sediments have a positive 3 skewness. This variation in mean grain size distributions reflects a mixed signal of different mean grain sizes either from a single dominant valley with different rock types or from different valleys with prominently different rock types. In general, mean grain sizes are very fine (1-2 microns) so the sediments would have followed Stokes Law and travelled into the lake either as an interflow or overflow.

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