PALYNOLOGICAL EVIDENCE FOR THE PRESENCE OF COASTAL TUNDRA WITH STANDS OF BOREAL TREES ON THE SOUTHERN MARGIN OF CENTRAL BERINGIA DURING GLACIAL STAGES 2 - 16

Palynological assemblages from Integrated Ocean Drilling Program (IODP) Expedition 323 site U1343, on the edge of the Bering Sea shelf, contain small but persistent amounts of boreal forest taxa throughout marine isotope stages (MIS) 8, 10, 12, 14, and 16. This study expands upon previous results from MIS 1-6, indicating that the southern coast of central Beringia was a glacial refugium for boreal trees and shrubs, including alder (Alnus), birch (Betula), and spruce (Picea).

Fifteen assemblages from glacial stages 8-16 and six from interglacial stage 15 range in age from 258 kya to 615 kya. All are dominated by grass (Poaceae) and sedge (Cyperaceae) pollen, with low percentages of Alnus (2.3 – 15.3%), Betula (0.6 - 3.7%) and Picea (2.0 – 9.7%). Interglacial pollen spectra from stage 15 differ from glacial assemblages in that they contain slightly higher percentages of Poaceae and lower percentages of tree and shrub taxa such as Alnus and Betula. Five modern surface samples from the Bering Sea, forming a transect between the southwestern shelf edge and Norton Sound, were selected and processed as potential modern analogs. The resulting spectra were added to a database of modern pollen rain from 220 lakes and bogs throughout Alaska. Canonical correspondence analysis clusters glacial and interglacial assemblages together within a quadrant defined by high percentages of Poaceae pollen. Closest modern analogs from terrestrial and marine spectra are from sites proximal to the Yukon/Kuskokwim Delta, where the vegetation consists of herbaceous tundra with isolated stands of trees.

It is conceivable that the similarity between glacial and interglacial assemblages from IODP site U1343 is the result of reworking by fluvial systems on the emergent shelf. However, minimal damage to pollen from boreal taxa in assemblages from glacial stages 2-16 argues against long distance transport by rivers to the shelf edge. Analogs from the modern coastal tundra suggest an alternative: Vegetation migrated with the coastline during glacial/interglacial cycles, and maritime conditions on the southern margin of central Beringia supported a belt of heraceous tundra with stands of boreal trees and shrubs throughout MIS 2-16.