The late Holocene tsunami in the Shetland Islands (UK) identified in Loch Flugarth, north Mainland

Abstract

Abstract To assess the long-term hazard of tsunamis, particularly in regions with a short and fragmented historical record, sedimentary deposits of tsunamis are an essential tool. In the North Sea region, evidence of tsunamis is scarce. The Shetland Islands are an exception, as they contain abundant deposits of the Storegga tsunami (c. 8150 cal. a BP), and additionally more fragmented evidence of younger tsunami events c. 5500 and c. 1500 cal. a BP. Sediments of the youngest tsunami (the “Dury Voe” event) have only been found at two sites so far, marked by thin landward fining and landward thinning sand sheets which are vertically confined by peat. Here, we present sedimentary evidence for the youngest Shetland tsunami from the small coastal lake of Loch Flugarth, northern Mainland. Three gravity cores of up to 91.7 cm length were taken behind the barrier separating the lake from a shallow marine embayment. The cores show organic-rich background deposition with many sub-cm-scale sand layers, reflecting recurring storm overwash and a sediment source limited to the active beach and uppermost subtidal zone. A basal 13 cm-thick sand layer, dated to 426–787 cal. a CE based on 14C, 137Cs and Bayesian age-depth modelling, was found in all three cores. High-resolution grain-size analysis identified four normally graded sublayers with inversely graded traction carpets in the lower part of two sublayers. An organic-rich ‘mud’ drape and ‘mud’ cap cover the upper two sublayers, which also contain small rip-up clasts. Grain-size distributions show a difference between the basal sand layer and the coarser and better sorted thin storm layers. Principal component analysis of X-ray fluorescence core scanning data also distinguishes both sand units: Zr, Fe and Ti dominate the basal sand, while the thin storm layers are high in K and Si. The enrichment of the basal sand layer in Zr and Ti, in combination with increased magnetic susceptibility, may be related to higher heavy mineral content in the basal sand reflecting the additional marine sediment source of a tsunami deposit below the storm-wave base. Based on reinterpretation of chronological data from the two published sites and the chronostratigraphy of the present study, the Dury Voe tsunami seems to be slightly younger, i.e., closer to 1400 cal. a BP. Although the source of the tsunami remains unclear, the lack of evidence for this event outside of the Shetlands suggests that it was smaller than the older Storegga tsunami, which affected most of the North Sea basin.