Holocene glacier fluctuations and tephra fall inferred from lacustrine sediment, Emerald Lake, Alaska

Downcore changes in physical and biological characteristics of lacustrine sediments from Emerald Lake were used to reconstruct the Holocene glacier history of Grewingk Glacier, which drains the Grewingk-Yalik Ice Complex on Kenai Peninsula, Alaska. Emerald Lake is a threshold lake, receiving meltwater and clastic sediment when Grewingk Glacier overtops the topographic divide that separates it from Emerald Lake. Glacier meltwater discharge is represented in sediment cores from Emerald Lake 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 a low sedimentation rate (Core 2; 18 m depth) and one with a high rate (Core 3; 50 m depth) to maximize both the length and resolution of the sedimentary sequence recovered in the ~3-m-long cores. Bulk density, sedimentation rate, stratigraphy, organic-matter content, and chlorophyll were used to record environmental changes since ~12 cal ka, with 14C and 210Pb for geochronology. Ages were assigned to tephra beds in Cores 2 and 3: 18 and 9 beds respectively. A diffuse transition from the basal inorganic mud to organic-rich mud ~11.4 cal ka marks the initial retreat of the Grewingk Glacier below the divide of Emerald Lake. 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 1350-1900, consistent with documented LIA advances on the Kenai Peninsula. The retreat is estimated from lichen ages on a bouldery moraine on the topographic divide and is consistent with the previously estimated age of the Grewingk Glacier terminal moraine (AD 1858). The retreat of Grewingk Glacier below the divide at 11.4 cal ka took place as temperature and productivity increased across southern Alaska; the subsequent readvance above the divide at 10.7 cal ka corresponds with cooling beginning ~11.0 cal ka in south-central Alaska. Decreased precipitation in southern Alaska from 5.5 to 4.0 cal ka lowered the level of Emerald Lake and sedimentation rate decreased. The initial LIA advance over the divide (AD 1350) and peak meltwater input into Emerald Lake (AD 1660) coincide with documented solar minima, suggesting solar variability influences Grewingk Glacier fluctuations.