THE TIMING AND MAGNITUDE OF LATE PLEISTOCENE AND HOLOCENE LAKE LEVEL CHANGE FROM A PAIRED CARBONATE δ18O - STRATIGRAPHY RECORD OF A CORE TRANSECT IN LAKE JUNIN, PERU

The 2015 NSF-ICDP Junín drilling project recovered multiple long sediment cores (~100 m depth) from Lake Junín, Peru (~300 km²; 4085 m.a.s.l.; 11.0°S, 76.1°W; z_max~12 m). These cores span multiple glacial-interglacial cycles and are anticipated to provide the longest independently-dated record of hydroclimate, glacier variability, and paleomagnetic variation for the tropical Andes. To aid in accurate interpretation of the drill core stratigraphy, we investigated spatial variations in sediment composition by collecting 9 short cores (1-9 m length) across a paleoglacier proximal-distal (NE-SW) transect. Each transect core extends into glacigenic sediments, indicating recovery of the complete postglacial (past ~22.5 ka) sequence. Most cores exhibit a four-component stratigraphy: a lower unit of gray glacial flour and minerogenic material sourced from glacial outwash fans to the NE, a lower-middle unit of organic-rich sediments with abundant grass, an upper-middle unit of transitional organic to carbonate sediments, and an upper unit of non-glacial sediments composed primarily of beige marl. We interpret these facies changes to reflect major changes in depositional environment and lake conditions. However, spatial variations in the age, thickness, and transition character among these units reflect the complex stratigraphy of a lake with such shallow water depth and high sensitivity to regional changes in hydrologic balance. Additionally, at least four erosional unconformities exist in the shallow-water cores to the NE, evidencing substantial changes in lake conditions. These unconformities have been radiocarbon dated to provide “benchmarks” for past lake level. These benchmarks can be used to estimate past lake level relative to modern and maximum water depth. The ages of these unconformities and the associated lake level story can be compared to local and regional hydroclimate records as a check on the proxies used.