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

波安置块和巨石领域的气旋和海啸洪水在全新世的时间尺度令人印象深刻的证据。不幸的是，它们用于沿海灾害评估受到阻碍的海啸和风暴搬运碎屑的区分困难，并通过适当的和准确的测年方法，这是需要产生强大的年表粗碎屑记录的缺乏。通常应用的AMS-14 C，U/Th或ESR测年的珊瑚礁岩石和海洋生物附着在碎屑取决于一个-主要是假设-生物死亡和巨石位移之间的巧合，推断事件年表可能会受到海洋14 C水库的影响和改造生物。这里提出的研究解决了创新方法的必要性，通过测试最近开发的光释光（OSL）表面曝光测年技术，以确定沿海巨石的运输。通过测量暴露的岩石表面的发光信号的深度依赖性重置，并将其与已知年龄样品的信号深度剖面进行比较，OSL表面暴露测年能够模拟巨石运输的直接沉积年龄。因此，它有望克服现有测年技术的局限性，并破译复杂的运输历史的碎屑经历了多个阶段的暴露和埋葬。在考古学背景下，OSL表面暴露于岩石表面的成功应用表明，候选的粗碎屑记录应具有以下特征：（i）暴露时间短于预期测年范围（约10-100 ka）的翻转巨石;（ii）具有适用于OSL测年的石英或长石显著百分比的巨石岩性;（iii）暴露巨石表面的有限侵蚀;以及（iv）相同岩性的已知年龄岩石表面，以校准特定于地点的模型参数（即岩石表面处的局部光通量）。我们在卡波德特拉法尔加（加的斯，西班牙）的研究基地提供了所有这些先决条件。风暴和/或海啸波沉积了长达7米的碎屑，这些碎屑由潮间和潮上台地上的更新世海滩岩石组成，并假设许多较小的碎屑存在多个位错。因此，我们的目标是确定具有单个和多个位错的巨石的年代。最近暴露的岩石表面，古建筑以及采石场提供已知暴露年龄的表面。应评估使用人工暴露岩石样品（为期1年和2年）作为模型校准参考样品的情况。在最好的情况下，拟定的调查不仅应测试沿海漂石进场的一般适用性，还应提供有关特拉法尔加极端波浪事件的频率-强度关系的定量信息。因此，它将允许验证或伪造与1755年里斯本海啸的相关性，这是假设的最大的碎屑。

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.