Paper No. 10
Presentation Time: 9:00 AM-6:00 PM
EVIDENCE FOR A MESIC GLACIAL REFUGIUM IN CENTRAL BERINGIA: A 150 KYR PALYNOLOGICAL RECORD FROM IODP EXPEDITION 323 SEDIMENT
WESTBROOK, Rachel E.1, FOWELL, Sarah J.1, BIGELOW, Nancy H.2 and VANLANINGHAM, Sam3, (1)Dept. of Geosciences, University of Alaska Fairbanks, P.O. Box 755780, Fairbanks, AK 99775-5780, (2)Alaska Quaternary Center, University of Alaska Fairbanks, P.O. Box 755940, Fairbanks, AK 99775-5940, (3)Institute of Marine Science, University of Alaska Fairbanks, P.O. Box 757220, Fairbanks, AK 99775-7220, rewestbrook@alaska.edu
Palynological assemblages from IODP Expedition 323 (Bering Sea Expedition) site U1343, on the edge of the Bering Sea Shelf, permit reconstruction of the terrestrial vegetation of south-central Beringia. It has been hypothesized that the lowlands of central Beringia acted as a glacial refugium for boreal vegetation, which expanded into eastern and western Beringia as glaciers retreated. This hypothesis has been difficult to test, because sampling has been restricted to eastern and western Beringia and islands in the Bering Sea. Samples from site U1343 provide a record of central Beringian vegetation over the past 150 kyr at a resolution of ~10 kyr. Although it is possible that some of the pollen deposited during glacial
stages is derived from interglacial sediment reworked by rivers flowing across the emergent shelf, we postulate that such sources contribute only 1-5% of the total sediment found at the Bering slope site. Thus we consider assemblages from site U1343 to be a robust proxy for the glacial vegetation of the central Beringian coast. Pollen and spore assemblages are dominated by grass (Poaceae ≥ 15%) and sedge (Cyperaceae ≥ 11%), but small percentages of spruce (Picea ≤ 5%), birch (Betula ≤ 15%) and alder (Alnus ≤ 17%) are consistently present throughout glacial/interglacial cycles, suggesting that trees and shrubs remained in central Beringia during glacial maxima. The presence of Sphagnum spores (4-11%) in all samples indicates the persistence of locally or regionally mesic conditions during marine oxygen isotope stages (MIS) 1-6. These results support the existence of a mesic refugium during glacial stages. However, Sphagnum percentages are consistently higher during MIS 6 than MIS 2. Application of a paludification index (angiosperm pollen / Sphagnum spores) as a proxy for saturated organic matter reveals a degree of site paludification during MIS 6 comparable to that of interglacial stages (MIS 1 and 5). Minimum site paludification during MIS 2 coincides with the lowest shrub/herb ratios in our record, suggesting that conditions were drier and woody plants were more sparse during the last glacial maximum. Differences in the relative abundance of shrubs and trees and the degree of paludification between MIS 2 and MIS 6 may be due to differences in temperature and/or the extent of regional sea-ice cover.
