Denise Brushett

Late Pleistocene ice contact glaciomarine deltas indicate the deglacial sea level to which the deltas were graded at successive ice positions and record the inland limit of marine submergence. Precise age determinations for topset foreset contacts provide a valuable chronology of the timing and rate of deglaciation as well as the duration of marine submergence and its relationship to postglacial uplift. The present study aims to test the reliability of using terrestrial in situ produced cosmogenic nuclide (TCN) exposure dating on raised glaciomarine deltas where a radiocarbon based chronology exists for comparison. The Auburn Plains glaciomarine delta in southern Maine was exposure dated using cosmogenic 10Be measured in five sand samples collected in a vertical profile (94 to 265 cm depth) beneath the sediment mixing zone. The samples contained 1.46 x 104 atom g 1 of inherited 10Be produced in the quartz prior to delta deposition. After minor (< 1%) adjustments for snow and vegetation cover, and change in the integrated production rate due to isostatic uplift (1.6%), the apparent exposure age (ignoring erosion) of the Auburn Plains delta is 11.0 " 0.9 ka (2s). A sensitivity analysis was performed to evaluate the influence on the apparent exposure age of uncertainties in bulk density and surface erosion. While the apparent exposure age is a minimum age due to a potentially high sensitivity to erosion which would reduce the exposure age, the precision of the measurement is comparable to radiocarbon dating. When compared to published maximum limiting radiocarbon ages for the Auburn Plains delta (mean = 14.7 ka), an average constant erosion rate of 0.035 mm/yr-1 (or 52 cm of sediment removed) would be required to explain the disparity.

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Pages: 90
Supervisor:John Gosse