Long-term shoreline retreat rates on Whidbey Island, Washington, USA

Abstract: Long-term retreat rates of Puget Sound''s unconsolidated sediment shorelines have been difficult to quantify, and little systematic research has been completed to constrain retreat in this area. We put forward a new application of cosmogenic ¹⁰Be exposure dating to assess long-term shoreline retreat on Whidbey Island, WA by dating lag boulders exposed on the shore platform as the shoreline erodes. Production of ¹⁰Be in shoreline boulders is modulated by both tidal submergence and topographic shielding from the retreating bluff. By modeling the combined effect of these variables on ¹⁰Be production, the timing of exposure can be determined and used to calculate long-term (10³–10⁴ yr) bluff retreat rates. In rare cases, retreat rates are underestimated due to inherited ¹⁰Be. Within the study area, average retreat rates ranged between 0 and 8cmyr⁻¹. Our results demonstrate the utility of cosmogenic nuclides for determining long-term shoreline retreat rates in areas with thick sediment cover, where large numbers of samples can be collected, and where the pre-depositional history of the boulders is uncomplicated. [Copyright &y& Elsevier]