Luminescence dating of offshore tsunami deposits from the Shetland Islands, United Kingdom

Abstract. With onshore geological evidence of three tsunami events, the Shetland Islands (UK) yield the most-detailed Holocene tsunami record in the North Sea region. Lately, tsunami deposits were also identified in shallow waters offshore the Shetland Islands. These offshore tsunami deposits may carry the potential to further improve our understanding of both local and basin-wide, long-term tsunami hazard. Making use of the offshore record, however, requires establishing robust event chronologies. By using cores collected from two shallow-marine embayments located in the eastern Shetlands (Dury Voe and Colgrave Sound), we present the first chronology for offshore tsunami evidence from this region based on luminescence dating by resolving critical methodological challenges. The offshore cores show four distinct layers that contrast with the background shelf sediments. We find micro X-ray fluorescence mapping to be a rapid and effective method for assessing the geometries and internal architectures of these candidate tsunami deposits, which helps to guide sampling for luminescence dating. Complex radiation fields and time-dependency of environmental dose rates are tackled by implementing representative water contents, site-specific internal potassium contents, modeled dose rates considering the effect of radioactive disequilibria, and stratigraphic heterogeneity on the scale of gamma radiation. We demonstrate how robust luminescence-based chronologies can be achieved through measuring silt- and sand-sized K-feldspars with a pIRIR₁₃₀ protocol in combination with Bayesian age-depth modeling under these conditions. Our luminescence chronologies indicate sedimentary evidence of the Storegga tsunami at offshore Dury Voe and Colgrave Sound, and additionally of the late-Holocene tsunami at offshore Dury Voe. Overall, these results demonstrate both the great potential of luminescence dating for offshore tsunami deposits and the benefits of the associated chronologies for regional tsunami hazard assessment.