INITIAL RESULTS FROM DEEP DRILLING LAKE JUNIN, PERU

Lake Junín (11.0°S, 76.2°W) is an intermontane, high-elevation (4080 m asl) 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 coalescing 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. Sediment cores obtained between 1980 and 1996 reveal that sediment deposited during the last glacial cycle (~30-16 ka) is dominated by glacial flour whereas sediment deposited during the last 16 ka consists predominantly of authigenic calcite (marl) and ostracod carapaces. 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. Core catchers at the bottom of each 3 meter section of all cores were analyzed for total organic carbon (TOC) and total inorganic carbon [as Ca(Mg)CO₃; TIC] by coulometry. Total carbon (TC) was analyzed by combusting ~10 mg samples at 1000°C and quantifying the resultant CO₂ by coulometry whereas TIC was analyzed by reacting ~10 mg samples in 6N H₃PO₄ and quantifying the resultant CO₂ by coulometry; TOC was determined from TOC=TC-TIC. Over the last glacial postglacial cycle (last ~30 ka), mean CaCO₃ and TOC concentrations in deep-water cores are much higher (~33 and 7.4%, respectively) than those in shallow water settings (~9.5 and 4%). Similarly, mean magnetic susceptibility (MS) is lower in the deep water cores (6.9 SI) than in the most paleoglacier-proximal shallow water site (Site 2, 9.4 SI). The deep water cores apparently record ~6 stadial and interstadial events whereas shallow water locations appear to be dominated by sediment deposited during the last glacial cycle.