Pleistocene stratigraphy, coastal bluffs of northeastern Bristol Bay
Coastal bluffs rimming northeastern Bristol Bay expose a remarkable sequence of Pleistocene glacial, marine, eolian, and volcanic deposits. Research between 1996 and 2000 applied an array of analytical approaches and geochronological methods to reconstruct the major Pleistocene paleoenvironmental changes represented by these deposits, and to integrate these local glacial and sea-level records into a regional and global framework.
Glaciers advanced onto the continental shelf of northeastern Bristol Bay at least three and as many as six times prior to the late Wisconsin. The oldest glaciations are represented by glacial-marine sediment in coastal exposures on Hagemeister Island. The extent of amino acid (isoleucine) epimerization in fossil molluscs indicates that at least one, and possibly four, older middle Pleistocene glacial intervals are represented, with age estimates spanning ~500 to 280 ka and averaging ~400 ± 100 ka. The youngest glacial-marine drift on Hagemeister Island may correlate with the eruption of the Togiak tuya. A 40Ar/39Ar age on basalt that overlies pillow lava indicates that the volcano erupted through glacial ice at least 300 m thick 263 ± 22 ka. The youngest drift in the Togiak Bay region overlies the Old Crow tephra (OCt) in several coastal exposures. These new OCt localities are the most proximal sites yet discovered and yielded relatively coarse-grained glass for new fission track (FT) analysis. A new isothermal plateau FT age of 131 ± 12 ka provides precise age on the OCt.
One remarkable 18-m-high coastal bluff of Togiak Bay exposes marine, lacustrine, fluvial, glacial, volcanic, and organic deposits that record the 50,000-year transition from the peak of the last interglaciation to the early Wisconsin. The basal unit of the exposure is dominated by stratified sand and silt up to 4.3 m above sea level and contains marine diatoms and pollen with a relatively high proportion of spruce and shrubs. An infrared stimulated luminescence (IRSL) age of 144 ± 13 ka from near the base of the exposure overlaps at 2sd with the peak of oxygen-isotope stage (OIS) 5e. The marine sand and silt are overlain by peaty silt with diatoms that record the transition from marine to lacustrine conditions. Spruce and shrubs are nearly absent from the pollen assemblage during this interval, which, on the basis of the tundra pollen assemblage and an IRSL age of 113 ± 10 ka from 0.6 m below the lacustrine mud, probably corresponds with OIS 5d. The organic-rich mud is overlain by stratified sand and organic-rich silt that apparently records shallowing of the lake, during which time spruce and shrubs reappear in the pollen assemblage (OIS 5c ?); a subsequent deepening of the lake (OIS 5b ?); followed by floodplain aggradation (OIS 5a ?). Sometime after, the eruption of the 70 ka lava, an outlet of the Ahklun Mountains ice cap advanced over the site and deposited ~7 m of bouldery drift.
The sedimentary sequence at Togiak Bay contains at least four tephra beds. Major- and trace-element chemistry provide the basis for correlating two of the tephras with tephra at nearby sites. A third tephra (the “Togiak tephra”) closely resembles the widespread OCt in morphology, but is distinct in trace-element geochemistry. The tephras, luminescence ages, and correlations with marine isotope stages, have provided the geochronological control to place the paleoecological changes recorded at Togiak Bay into a regional and global context. The evidence indicates that temperatures lowered before eustatic sea level fell. The surficial drift of the Togiak Bay area also overlies a 70 ± 10 ka basaltic lava flow dated by thermoluminescence (TL) analysis of underlying baked sediment. The drift delimits flat piedmont lobes that spread out onto the continental shelf and terminated >100 km from their source areas during the early Wisconsin.
References
Kaufman, D.S., Manley, W.F., Forman, S.L., Layer, P., 2001a, Pre-late-Wisconsin glacial history, coastal Ahklun Mountains, southwestern Alaska—New amino acid, thermoluminescence, and 40Ar/39Ar results: Quaternary Science Reviews 20, 337-352.
Kaufman, D.S., Manley, W.F., Forman, S.L., Hu, F.S., Preece, S.J., Westgate, J.A., and Wolfe, A.P., 2001b, Paleoenvironment of the last interglacial-to-glacial transition, Togiak Bay, southwestern Alaska: Quaternary Research 55, 190-202.
