Sue Dawson’s research while affiliated with University of Dundee and other places

Tsunami deposits around the North Sea basin are needed to assess the long-term hazard of tsunamis. Here, we present sedimentary evidence of the youngest tsunami on the Shetland Islands from Loch Flugarth, a coastal lake on northern Mainland. Three gravity cores show organic-rich background sedimentation with many sub-centimetre-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 14 C, 137 Cs and Bayesian age-depth modelling, was found in all cores. High-resolution grain-size analysis identified four normally graded or massive sublayers with inversely graded traction carpets at the base of two sublayers. A thin organic-rich 'mud' drape and a 'mud' cap cover the two uppermost 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 storm layers above. Multivariate statistical analysis of X-ray fluorescence core scanning data also distinguishes both sand units: Zr, Fe and Ti dominate the thick basal sand, while the thin storm layers are high in K and Si. Enriched Zr and Ti in the basal sand layer, in combination with increased magnetic susceptibility, may be related to higher heavy mineral content reflecting an additional marine sediment source below the storm-wave base that is activated by a tsunami. Based on reinterpretation of chronological data from two different published sites and the chronostratigraphy of the present study, the tsunami seems to date to c. 1400 cal. a BP. Although the source of the tsunami remains unclear, the lack of evidence for this event outside of the Shetland Islands suggests that it had a local source and was smaller than the older Storegga tsunami (8.15 cal. ka BP), which affected most of the North Sea basin.

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 ning and landward thinning sand sheets which are vertically con ned 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, re ecting 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 14 C, 137 Cs and Bayesian age-depth modelling, was found in all three cores. High-resolution grain-size analysis identi ed 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 uorescence 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 re ecting 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.

Severe storms, their extreme waves and surges pose the greatest natural hazard to the coasts of northwestern Europe, commonly resulting in infrastructural damages and high financial losses. Proxy records of past storminess are important for assessing future risks that may arise from storm surges and assessing whether storm activity has increased in recent decades. High-resolution records of North Atlantic storminess are generally limited to instrumental weather data or historical documentation of the past 50 to 200 years. Since the most destructive and severe storms passing over Europe originate in the North Atlantic, the Shetland Islands serve as a window to cyclogenesis in this region. In our research, we extracted lacustrine sediments of the coastal freshwater lake Loch of Flugarth on Mainland, Shetland Islands, that is separated from the ocean by a low sand and gravel barrier. A series of distinct sand layers intercalated in the otherwise fine-grained, organic-rich lake deposits and examined using particle-size analysis, microfossils, TOC and XRF, may represent storm overwash or aeolian transport mechanisms, both either pointing towards individual storm events or shorter phases of high storm activity. Based on radiocarbon data of some selected layers, the investigated sediment sequence covers ca. 1500 years and a Bayesian age-depth model is being established. In combination with a hydrodynamic wave model based on Delft3D-Flow, we simulate a critical threshold value at which waves may reach the lake to determine the sensitivity of the sedimentary archive. With the inclusion of historical documentation and observations, our multi-methodological approach aims at reaching a better understanding of the recurrence pattern of extreme storm events on the Shetland Islands over the last 1500 years. This implies further insights into the parameters driving extra-tropical storms in the wider region as well as the role and variability of the North Atlantic Oscillation across the targeted time frame.

This paper reviews the evidence for onshore sedimentary imprints of tsunami inundation known from the geological record of the Atlantic basin. The central aim of the paper is to offer a broad overview on the main deposits and key localities that have been documented along the Atlantic coastlines, and which attest to the local or regional impact of tsunamis during historical, pre-historical and recent geological times. Considerable detail is devoted to summarising key diagnostic criteria used to identify the deposits as tsunami-derived, and to set each deposit in its own unique geomorphological setting and context, always referring to the latest scientific knowledge of the events that generated them. The paper also discusses the relationships between the different tsunamigenic sources that concurred to the formation of the deposits, as well as critical information on magnitude and frequency, as inferred from the sedimentary responses preserved in the sediment archives. Documented case studies range from the well-studied landslide-triggered and earthquake-triggered Storegga and 1755 Lisbon tsunamis, respectively, to collapse- and eruption-triggered tsunamis in the Atlantic archipelagos, to other less well-known events in the south Atlantic, Caribbean and Arctic. Despite its less frequent tsunami recurrence, the Atlantic coastal stratigraphy presents some world-class case studies and outstanding outcrops, from which critical knowledge can be gained with respect to some of the most enigmatic aspects of tsunami science.