Lateglacial and early Holocene climate oscillations in the Matanuska Valley, south-central Alaska

Here we present multi-proxy data from two cores taken from Hundred Mile Lake in the Matanuska Valley of south-central Alaska to investigate the climate, vegetation and deglaciation history of the last 14,000 years. The chronology of the cores was controlled by five AMS dates. Sediment lithology changes from clay at ∼14–13 ka (1 ka=1000 cal BP), through marl at 13–8 ka, to gyttja at 8–0 ka. The transition from clay to marl probably represents increased productivity of the lake in a stabilizing watershed, induced by initial Allerød climate warming after ice retreat, as calcite precipitation of marl was facilitated by Chara photosynthesis under high water temperature in summer. The δ18O record obtained from bivalve Pisidium mollusc shells shows several large shifts between 13 and 8 ka. A negative excursion of ∼2‰ in δ18O at 12.4–11.5 ka is suggestive of a regional expression of the Younger Dryas (GS-1) cooling event. A 4.5‰ negative shift from −10.5‰ at 13–11 ka to −15‰ at 10.5–8 ka occurred during the peak carbonate interval around 10.5 ka. This surprisingly large negative shift in δ18O values during the early Holocene thermal maximum has not been documented elsewhere in the region. This shift suggests a major change in atmospheric circulation patterns, possibly through more frequent easterly flow of warm and dry air masses that are also depleted in 18O. Pollen results from marl sediments indicate vegetation change from a herb tundra, through shrub birch-dominated tundra, to an alder forest, which follows closely with other regional pollen sequences in south-central Alaska. The results from this study suggest that the climatic shift during the early Holocene was of greater magnitude than the Younger Dryas event, implying the importance of regional feedback processes in high latitudes in controlling regional climate response to large-scale forcing.