Northeastern Section - 53rd Annual Meeting - 2018

Paper No. 34-25
Presentation Time: 8:00 AM-12:00 PM


LAMA SHERPA, Tshering, KATZ, Sarah A. and RODBELL, Donald, Geology Department, Union College, Schenectady, NY 12308

Lake Junín (11.0°S, 76.2°W, 4085 masl) is an intermontane, high-elevation lake in the inner-tropics of the Southern Hemisphere that spans ~300 km2. With a maximum water depth of ~12m, Lake Junin is dammed at its northern and southern ends by alluvial fans that emanate from glacial valleys in both cordillera. These fans can be traced to moraines that are >250 ka, indicating that the lake is at least this old. During the maximum extent of late Cenozoic glaciation, glaciers reached the lake edge but at no time over the last 1 million years, or more, has Lake Junín been overridden by ice. Lake Junín is thus one of the few lakes in the tropical Andes that predates the maximum extent of glaciation and is in a geomorphic position to record the waxing and waning of alpine glaciers in nearby cordillera. In July and August of 2015, piston cores were obtained from three sites in Lake Junin. Multiple overlapping cores from the deepest water site (Site 1) extend to ~100 m below lake floor (mblf), and those from two shallow water, paleoglacier-proximal sites (Sites 2 and 3) extend ~23 and 51 mblf, respectively. Site 1 records ~7 glacial and interglacial cycles spanning the past ~700 ka, whereas shallow water locations appear to be dominated by sediment deposited during the last glacial-interglacial cycle. The record of siliciclastic flux, magnetic susceptibility (MS), mean grain size, and Ti/Ca of sediment provide proxy records of glacial erosion and the extent of ice in adjacent valleys. Comparison of these proxies with a local speleothem record of regional hydrologic balance reveals that during marine isotope stages 2 and 3 glacial erosion was strongly coupled to regional precipitation. However, beginning ~22 ka, proxies of glacial erosion began to decline in spite of sustained moist conditions. This indicates that the tropical Andes began to warm several millennia prior to the onset of the decline in global ice volume and increase in sea level. Mean grain size of glacial sediment in Lake Junin is between 1 and 2µm and varies by a factor of two, largely in concert with proxies of clastic flux and glaciation. The rapid response of glacial erosion to regional hydroclimate suggests that paleoglaciers were warm-based and had high activity ratios.