OSL surface exposure dating of wave-emplaced boulders - Improving the use of coarse-clast records for coastal hazard assessments

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Fields of wave-emplaced blocks and boulders represent impressive evidence of cyclone and tsunami flooding over Holocene time scales. Unfortunately, their use for coastal hazard assessment is impeded by difficulties in the differentiation of tsunami- and storm-transported clasts, and by the absence of appropriate and accurate dating approaches, which are needed to generate robust chronologies of coarse clast records. The commonly applied AMS-14C, U/Th or ESR dating of coral-reef rocks and marine organisms attached to the clasts depends on a - mostly hypothetical - coincidence between the organisms death and boulder displacement, and inferred event chronologies may be biased by the marine 14C-reservoir effect and reworked organisms. The research proposed here addresses the necessity of innovative approaches to date the transport of coastal boulders by testing the recently developed optically stimulated luminescence (OSL) surface exposure dating technique. By measuring the depth-dependent resetting of the luminescence signal in exposed rock surfaces and comparing it to the signal-depth profiles of known-age samples, OSL surface exposure dating is capable to model direct depositional ages for boulder transport. Thereby, it promises to overcome the limitations of existing dating techniques, and to decipher complex transport histories of clasts that underwent multiple phases of exposure and burial. The successful application of OSL surface exposure to rock surfaces in archaeological contexts suggests that candidate coarse-clast records should be characterized by (i) overturned boulders that are exposed for periods shorter than the expected dating range of ~10-100 ka; (ii) a boulder lithology with significant percentages of quartz or feldspar appropriate for OSL dating; (iii) limited erosion of the exposed boulder surfaces; and (iv) known-age rock surfaces of the same lithology to calibrate site-specific model parameters (i.e. the local light flux at the rock surface). Our study site at Cabo de Trafalgar (Cádiz, Spain) offers all of these preconditions. Storm and/or tsunami waves have deposited up to 7 m-long clasts composed of Pleistocene beach rock on the inter- and supratidal platform, and multiple dislocation is assumed for numerous smaller clasts. We thus aim at dating boulders with single and multiple dislocation. Recently exposed rock surfaces, ancient buildings as well as quarries provide surfaces of known exposure age. The use of artificially exposed rock samples (for periods of 1 and 2 years) as reference samples for model calibration shall be evaluated. In the best case, the proposed investigations shall not only test the general suitability of the approach to coastal boulders but also provide quantitative information about the frequency-magnitude relationship of extreme wave events at Trafalgar. Thereby, it will allow to verify or falsify a correlation with the 1755 Lisbon tsunami, which is assumed for the largest clasts.