Determining the Reliability of Using Cosmogenic Nuclides to Date Arctic Beach Sediment

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This project is a one year pilot study of determine the reliability of using cosmogenic 10Be, 26Al, and other nuclides to date beach sediment. The test site, Prescott Island, on eastern Prince of Wales Island, NWT, has sufficient datable rocks of different lithologies and clast sizes; persisted above 60o N geomagnetic latitude since emergence (the latitude at which the production rate for the nuclides has been approximately constant); and already has a well-defined emergence curve based on 46 driftwood radiocarbon dates. Forty large samples representing beaches from 11 kyr to the modern shoreline were collected during a two week field season in 1995. Support is requested for the physical and chemical preparation and analyses of these samples for 10Be, 26Al, 36Cl, and 14C. Raised shorelines, especially those with wide backshore zones that span a long time range and have numerous beach ridges, are used to construct relative sea level curves for coastal regions that have undergone tectonic uplift or isostatic compensation for deglacial unloading. Shoreline emergence curves provide information on the style and rate of uplift, are useful for detecting local and regional tectonic anomalies, can help delimit the boundaries of crustal blocks and determine if they have tilted during the emergence period, provide constraints on paleo-ice mass and thickness, have been used to help infer deep mantle rheology parameters, and are useful in verifying existing isostatic adjustment models. Tight and reliable chronologies are required on suites of beaches to establish accurate emergence curves. Where datable organic material is not available for radiocarbon dating, cosmogenic nuclide dating may provide a reliable alternative. It is uncertain how reliable the cosmogenic nuclide methods can date individual beach ridges or define a relative sea level curve. Total uncertainties for cosmogenic nuclide ages on single beach levels must be at least 6 or 7% to be useful. Although this has been achiev ed on moraines, there are a number of different environmental factors which must be considered when dating beaches. A test of the utility of terrestrial cosmogenic nuclides to date beaches has never been formally conducted.