PALYNOLOGICAL EVIDENCE FOR COASTAL REFUGIA IN SOUTH-CENTRAL BERINGIA

Palynological assemblages from Integrated Ocean Drilling Program (IODP) Expedition 323 site U1343, in the Bering Sea, record the vegetation of the south-central Beringian coast during Marine Isotope Stages (MIS) 1-6. Samples analyzed at ~10 kyr intervals between ~11 ka and ~150 ka are all dominated by grass (Poaceae) and sedge (Cyperaceae). Boreal taxa, including birch (Betula), alder (Alnus), and relatively sparse grains of spruce (Picea) are present throughout the record, revealing little difference between glacial and interglacial assemblages. Principal component analysis of these assemblages and 220 modern pollen spectra from sites across Alaska indicates that both the glacial vegetation (MIS 2, 4, and 6) and the interglacial vegetation (MIS 3 and 5) of coastal Beringia can be characterized as moist herbaceous/shrub tundra, such as that found in the vicinity of the modern Yukon-Kuskokwim Delta.

During glacial stages, falling sea levels exposed interglacial sediment deposited on the Bering Sea shelf to fluvial erosion and reworking. This is one possible explanation for the homogeneity of the IODP site U1343 palynological assemblages. However, the presence of fragile Populus (cottonwood or aspen) pollen in samples from MIS 8 indicates minimal transport of at least some boreal taxa. This suggests the alternative hypotheses that coastal south-central Beringia was continuously covered by moist herbaceous/shrub tundra during MIS 1-6 and that the emergent shelf served as a refugium for isolated stands of boreal trees during glacial stages. If boreal forest taxa repopulated Alaska from coastal refugia, migration rates of these plants were much slower than estimates based on dispersal from habitats south of the ice sheets have indicated.