PALYNOLOGICAL AND SEDIMENTALOGICAL EVIDENCE OF THE YOUNGER DRYAS IN SOUTHEAST ALASKA

Lake sediment cores obtained from Baker Island in Southeast Alaska contain the entire Younger Dryas (YD) interval, which coincides with a change in vegetation cover. Macrofossils recovered at 380 cm and 450 cm depth in the core have ages of 11,590 – 12,627 and 12,712 – 12,916 cal yr BP, respectively, bracketing the YD. Sedimentologically, this interval is represented by ~ 25 cm of silt with intermitted layers of gravel, resulting in higher magnetic susceptibility values than the overlying Holocene gyttja. The YD sediment is underlain by an 8 cm black tephra similar to that recovered from Leech Lake, on Heceta Island, where its age is estimated to be approximately 14,500 cal yr BP (J. Addison, pers. comm., 2015). Beneath the tephra is a pale blue clay unit interpreted as glacial flour.

Palynological analysis reveals that pine (Pinus) is the dominant tree taxon (~60% of total pollen grains) at ~14,500 cal yr BP, but pine pollen decreases by ~55% near the base of the YD interval. Pines appear to expand briefly (~5 % increase) near the end of the YD but do not regain pre-YD abundance. Alder (Alnus) increases by 33% near the base of the YD interval and maintains this abundance throughout the YD interval. Spruce (Picea) increases by ~5% near the base of the YD, followed by an additional 10% increase after the YD. Ferns increase by 45% near the base of the YD and decrease by 30% after the YD. These data suggest that the YD climate may have been cooler and more humid. Additional dates and further palynological analyses will constrain the duration of the YD in Southeast Alaska and clarify the impact of this climate perturbation on the regional vegetation.