LARGE ICE-SHOVED RAMPARTS REVEAL LATE HOLOCENE EXTREME WINDS AND HIGH LAKE LEVELS IN SOUTHERN ALASKA

In the central Kenai Peninsula, we examined large berms on 16 lakes, which were apparently formed during spring break-ups by the bulldozing action of ice pans driven by strong NE winds. These “ice-shoved ramparts” (ISR’s), 1-11 feet high, are typically found on the SW ends of lakes. On closed-basin lakes (with no outlets) the ISR’s are situated well above modern lake levels (mean 19.3 feet, range 4.6-32 feet, n=12). This evidence of high lake levels indicates periods of extremely wet climate. No such berms are known to have formed within the historical period. We have excavated and dated soil profiles in 14 ramparts (37 AMS radiocarbon dates). Stratigraphic evidence typically shows two or more ice thrust events within a single rampart, dating several thousand years apart. ISR dates fall within the last 5200 calibrated years BP, especially 500-2400 years BP during the Neoglaciation. These late Holocene dates are significant because the west-central Kenai Peninsula has been free of glacial ice for the last 19,000 years. We have located but not examined ISR’s on the west side of Cook Inlet near Lake Clark, NW of Tok at Moosehead Lake and Lake George, and south of Fairbanks at Birch Lake. We suggest that ISR’s represent extreme climatic anomalies that involve a major reorganization of late Holocene North Pacific weather patterns, directing extreme precipitation and high springtime winds into southern Alaska. This hitherto unknown climate history could be important for calibrating General Circulation Models (GCM’s) forecasting climate change in the North Pacific basin.