INCREASING HUMIDITY DURING THE YOUNGER DRYAS REVEALED BY PALYNOLOGICAL AND SEDIMENTALOGICAL ANALYSIS OF CORES FROM BAKER ISLAND, SOUTHEAST ALASKA

Emerging evidence of increasing humidity during the Younger Dryas (YD) in the North Pacific (1) is supported by analysis of cores from Alaska’s Alexander Archipelago. Consistent with other records from southeast Alaska, palynomorph assemblages reveal a decline in Pinus (pine) pollen indicative of cooler temperatures at the onset of the YD. A decline in monolete spores suggests an initial decrease in humidity. However, greater percentages of fern spores and Picea (spruce) pollen during the second half of the YD indicate increasingly humid conditions.

Two lake sediment cores from Baker Island contain the entire Younger Dryas (YD) interval, which coincides with a change in pollen assemblages and sediment grain size. AMS dates on terrestrial plant macrofossils have ages of 11,590 – 12,627 and 12,712 – 12,916 cal yr. BP, bracketing the YD. Sedimentologically, this interval is represented by ~ 25 cm of silt; a layer of gravel near the top of the interval has higher magnetic susceptibility values than the overlying Holocene gyttja. The YD interval is underlain by an 8 cm-thick black tephra layer.

Palynological analyses reveal that pine dominated the assemblages (60% of total pollen grains) at ~13,500 cal yr. BP, with lesser amounts of monolete spores (ferns, 20%) and Alnus(alder, 15%). Pine pollen decreases to 5% at the onset of the YD interval (~12,900 cal yr. BP) while alders and ferns increase to 75% and 70%, respectively. Shortly afterwards, (~12,700 cal yr. BP), pine pollen briefly increases to 20%, followed by a decrease to minimal frequency (5%) about halfway through the YD (~12,200 cal yr. BP). Percentages of alder pollen and fern spores also decrease to 55% and 35%, respectively, during the first half of the YD. Beginning at ~11,900 cal yr. BP, spruce increase to 40% and fern spores increase to a 65%. A gravel layer at ~12,000 cal yr. BP may also be indicative of wetter conditions that increased sediment input into the lake. These data expand the region experiencing greater effective moisture during the latter half of the YD (1) significantly further south to Baker Island.

1 Kaufman et al., QSR 29 (2010): 1445-1452.