EARLY HOLOCENE (10.7 CAL KA) GLACIER RE-ADVANCE INFERRED FROM LACUSTRINE SEDIMENT, EMERALD LAKE, KENAI PENINSULA, ALASKA

Downcore changes in physical and biological characteristics of lacustrine sediments from Emerald Lake (EL) were used to reconstruct the Holocene glacier history of Grewingk Glacier (GG), which drains the Grewingk-Yalik Ice Complex on Kenai Peninsula, Alaska. Emerald Lake (59°03’N, 151°37’W) is a threshold lake, receiving meltwater and clastic sediment when GG overtops the topographic divide that separates it from EL. Glacier meltwater discharge is represented in sediment cores from EL by distinct light-gray, stony mud, with high density and low organic-matter content. Sub-bottom acoustical profiles were used to locate two core sites: one with low sedimentation rate and one with high rate to maximize both the length and resolution of the sedimentary sequence recovered in the 4-m-long cores. The age model for the composite sequence is based on 13 ¹⁴C ages and a ²¹⁰Pb profile. A sharp transition from the basal inorganic mud to organic-rich mud ~11.4 cal ka marks the initial retreat of the GG below the divide of EL. The overlaying organic-rich mud is interrupted by stony mud that records a brief re-advance as ice overtopped the divide again ~10.7 cal ka, followed by the final glacial-interglacial transition ~9.8 cal ka. The glacier did not spill meltwater into the lake again until the Little Ice Age, from around AD 1350-1770. The retreat of GG below the divide at 11.4 cal ka took place as temperature and productivity increased across southern Alaska following the Younger Dryas; the subsequent readvance above the divide at 10.7 cal ka corresponds with cooling beginning ~11.0 cal ka in south-central Alaska (Kaufman et al. 2010, QSR 29:1145). Since monitoring began in 1966, the mass balance of Wolverine Glacier, a USGS benchmark glacier 150 km northeast of EL, corresponds with winter precipitation, which is related to the Aleutian Low. Assuming that the controls on mass balance were similar, the newly documented advance at 10.7 cal ka was likely caused by enhanced winter precipitation.