A CONTINUOUS RECORD OF GLACIAL-INTERGLACIAL CYCLES SPANNING MORE THAN 700 KYR FROM LAKE JUNÍN, PERÚ

Lake Junín (11.0°S, 76.2°W), a shallow (z_max~12 m), intermontane, high-elevation (4080 masl) lake in the inner-tropics of the Southern Hemisphere, spans ~300 km². It is dammed by coalescing alluvial fans that are >250 ka that emanate from glacial valleys. Lake Junín has not been overrun by glacial ice in several hundred thousand years and is ideally located to receive glacigenic sediment. The Junín basin is underlain by carbonate rocks that have provided a source of Ca and HCO₃ ions; precipitation of CaCO₃ in the western margin of the lake during the present interglacial period has occurred at ~1mm yr^-1. An airgun seismic survey revealed a strong reflector at ~105 meters depth, which marks the base of the lacustrine section. Drilling focused on three sites. Site 1, located near the depocenter and most distal to glacial sources, yielded a composite sediment thickness of ~95m; Site 2, proximal to glacial outwash fans, yielded a composite thickness of 28 m; Site 3, located at an intermediate distance yielded a sediment thickness of 55m. The stratigraphy of Site 1 is marked by ~8 glacial/interglacial cycles; the latter are characterized by low bulk density and magnetic susceptibility (MS)and high CaCO₃. These units are intercalated with glacigenic sediment that has high density and MS, and low CaCO₃. The age model for Site 1 is based on AMS radiocarbon dates on terrestrial macrofossils and U/Th ages on authigenic CaCO₃. Strong and protracted interglacial periods appear to be associated with intervals of reduced variability of solar insolation in the Southern Hemisphere tropics. During these intervals there is strong covariation (r²>0.9) between the d¹³C and d¹⁸O of authigenic calcium carbonate, and d¹⁸O values are relatively enriched (-12 to -2‰); examples include interglacial periods correlative with marine isotope stages (MIS) 1, 13, and 15. The rate of glacial flour flux was greatest during MIS 2, despite the observation that the extent of ice was smaller during this interval than during prior glacial cycles. This reflects the progressive progradation of glacial outwash fans towards the lake’s depocenter over multiple glacial cycles. Sediment correlative with the LGM is marked by cycles of grain size variability that may be associated with short term variations in glacier mass balance, ablation rates, and/or subglacial plumbing.