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

doi:10.21203/rs.3.rs-2750922/v1

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

https://doi.org/10.21203/rs.3.rs-2750922/v1

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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.

Geology

Geomorphology

Geochemistry

extreme-wave event

coastal hazard

North Sea

tsunami deposits

coastal lake

late Holocene

Tsunami deposits are an integral component used in the assessment of the long-term hazard of tsunami events, in particular in coastal regions with a short and fragmented historical record (Switzer and Jones, 2008; Weiss and Bourgeois, 2012; Engel et al., 2016). Even though a range of indicative sedimentary, geochemical and micropalaeontological signatures have been established (Dawson and Shi, 2000; Switzer and Jones, 2008; Engel et al., 2016; Chagué, 2020; Spiske, 2020), the identification of tsunami deposits in the coastal sedimentary record poses challenges at the site scale. Tsunami deposits may vary considerably in structure and thickness depending on the hydrodynamics of the tsunami, the availability and nature of source sediments, the coastal bathymetry and topography, the depositional setting (i.e., onshore, lacustrine, offshore) and potential post-depositional changes.

Despite an apparent scarcity of tsunamis in the shallow North Sea basin, the Shetland Islands have become an important field laboratory in which to study tsunami deposits since the early 1990s (Smith, 1993; Bondevik et al., 2003, 2005; Dawson et al., 2006, 2020a; Costa et al., 2015; Cascalho et al., 2016; Buck and Bristow, 2020), with summaries of the findings presented in Smith et al. (2004, 2019), Long (2015), Costa et al. (2021) and Bondevik (2022). The original studies on Shetland have played a substantial role in the advancement of tsunami sedimentology and wider tsunami geoscience. Most studies since have investigated the abundant sedimentary evidence of the tsunami triggered by the Storegga submarine slides at the Norwegian shelf margin dated to 8175–8120 cal. a BP (Bondevik et al., 2012). The deposits are accessible and usually preserved in thick coastal sections of blanket peat along many of the Shetland fjords, locally called ‘voes’. The deposits mostly consist of normally graded sand up to 40 cm thick, with large rip-up clasts, small boulders and pieces of wood embedded in the sand (Bondevik et al., 2003), and increased concentration of heavy minerals at the base (Cascalho et al., 2016). The deposits thin and fine inland, with sharp, in some cases erosional lower boundaries. The run-up of the Storegga tsunami is estimated to be up to 20–25 m above present sea level (Bondevik et al., 2005; Dawson et al., 2020a). Deposits of the Storegga tsunami were also found in coastal lakes, where they create an erosional boundary, are normally graded with coarse sand and fine gravel at the bottom as well as lake mud clasts and organic macro-remains in the matrix. There is further evidence of a c. 5500 cal. a BP tsunami deposit found at Garth Loch at the east coast of Mainland (Bondevik et al., 2003, 2005) and possibly in southern Norway (Nielsen, 2020). The youngest tsunami deposit identified so far on the Shetland Islands was dated to 1500 cal. a BP and occurs as a thin inland fining sand layer at Dury Voe (Bondevik et al., 2005) and Basta Voe (Dawson et al., 2006). Despite this solid evidence, Long (2015) considers the event “uncertain” and takes possible alternative depositional processes (e.g., aeolian) into account. The trigger mechanism, however, is indeed unknown (Bondevik et al., 2005; Dawson et al., 2006).

Here, we present a new sedimentary record of a late Holocene tsunami from Loch Flugarth, northern Mainland. We demonstrate the tsunami origin based on a range of sedimentary criteria typically observed in tsunami deposits and discuss the deposit’s relation to the youngest inferred tsunami on Shetland. Our objective is to add to the tsunami database on Shetland, improve the chronology and stimulate investigations of the unknown post-Storegga trigger mechanisms to inform coastal hazard assessment at a regional level in the North Sea basin.

The Shetland Islands (1.3°E, 60.4°N) comprise a group of more than hundred islands, located at the northern margin of the North Sea, about 165 km northeast of the Scottish mainland (Fig. 1A) (Mykura et al., 1976; Bennett et al., 1992). The geology of the Shetland Islands is dominated by metamorphic and metasedimentary rocks, which were formed during the Caledonian Orogeny. The archipelago is tectonically divided by the major north-south running fault system of the Walls boundary Fault (Mykura et al., 1976; Gillen, 2003), which represents a continuation of the Great Glen Fault (GGF) (Fig. 1A). The GGF extends in southwestern direction all the way through the Scottish mainland (Pringle, 1970; Gillen, 2003).

The study site of Loch Flugarth is located in the North Roe area (northwestern part of Mainland; Fig. 1B) and is embedded into the Sand Voe Group, comprising thick banded hornblendic and granulithic gneisses with thick lenses of amphibolite and metagabbro near the top of the Sand Voe Group (Mykura et al., 1976). The Loch is crossed by the Flugarth Fault running from SW to NE (Pringle, 1970).

The freshwater lake of Loch Flugarth covers an area of 0.16 km². It is very shallow with a maximum water depth of c. 2.4 m (Murray and Pullar, 1903) and no thermal stratification (Murray and Pullar, 1908; Bennett et al., 1992). Such polymictic lakes with with a non-varved record, a very simple and shallow bathymetry and limited fetch are ideal to preserve event layers as disturbing internal processes such as hyperpycnal flows, erosion from the flanks or interflows are unlikely (Schillereff et al., 2014).

The lake level is c. 2 m higher than mean sea level inside the very shallow and usually calm Sand Voe bay, from which Loch Flugarth is separated by a c. 70 m wide sandy barrier (Chapelhow, 1965). The barrier, which is crossed by an artificial cut, forms a flat beach with mostly sands and a small gravel component (Flinn, 1974). Stabilised dunes and till backing the beach show signs of recent erosion, whereas only the largest storm surges manage to overtop the barrier. The barrier seems to have formed inside Sand Voe bay first as a spit fed by the longshore drift, eventually forming a closed barrier. The area behind the barrier was then cut off and turned into a freshwater lake gradually filling with sediment (Flinn, 1974).

Changing relative sea levels control the onshore accommodation space and sensitivity of coastal lakes to receive sediments during overwash by tsunamis or storm surges and waves (Liu, 2004; Szczuciński, 2020). Thus far, Holocene sea-level index points are scarce in Shetland. Based on very few dated basal peats and a limiting shell from Ronas Voe to the southwest of the study site, the relative sea level (RSL) is assumed to have risen at a high rate until c. 7000–6000 BP and subsequently slowed down (Hoppe et al., 1965; Bondevik et al., 2005; Smith et al., 2019). For the last 1500 years, RSL rise is assumed to have been minimal (Bondevik et al., 2005; Dawson et al., 2006) with tide gauge measurements from Lerwick showing no trend in RSL since ca. 1950 CE (Wahl et al., 2013). Therefore, a stable geomorphic environment can be expected for overwash across the coastal barrier into Loch Flugarth during extreme-wave events of the last c. 1500 years. Under such conditions, small coastal lakes are excellent traps for tsunami deposits with good preservation potential (Kelsey et al., 2005; Kempf et al., 2017; Dawson et al., 2020b).

3.1 Field methods

In the central northern part of Loch Flugarth at a water depth of c. 2.0 m, three short sediment cores (FLUG 2–4) with a length of up to 91.7 cm were taken at a distance of < 30 m to each other using a zodiac and a UWITEC gravity corer (Figs. 1D, S1, S2). The cores were taken directly from the lake bottom (Fig. 1D) and kept in transparent PVC liners (60 mm Ø) (Figure S2b). Core FLUG 1 was taken using a Russian chamber corer that was opened at a depth of 200 cm below the the subaquatic sediment surface to generate an undisturbed stratigraphic sample at a depth of 200–250 cm. Additionally, three surface samples of modern sedimentary environments were taken from the lake margin at the foot of the barrier (FLUG-M1) and from the uppermost intertidal zone inside Sand Voe (FLUG-M2). In this study, we focus on the basal part of cores FLUG 2 and 3. The upper part of these cores has been analysed for storm overwash and the reconstruction of regional storminess by Hess et al. (2023b).

3.2. Laboratory Methods

After splitting and describing the cores at the laboratory of the Geological Survey of Belgium at Brussels, a Geotek multi-sensor core logger was used at the Renard Centre of Marine Geology, Ghent University, Belgium. After obtaining high-resolution photographs (Geoscan IV line-scan camera), bulk density (γ-ray attenuation densometer) and magnetic susceptibility (Bartington point sensor MS2E) were measured at a resolution of 2 mm.

X-ray computer tomography (CT) scans were generated at Ghent University Hospital by a Siemens Somatom Definition Flash Medical X-ray CT scanner. The voxel sizes of the image stacks are 0.15 mm × 0.15 mm × 0.3 mm. The stacked core images were analysed qualitatively using the Fiji software package (Schindelin et al., 2012).

For grain-size analysis of the bottom sand unit in FLUG 3, a Fritsch Analysette 22 NeXT Nano laser diffractometer with a measurable range of 0.01–3800 µm was used at the Laboratory for Geomorphology and Geoecology, Heidelberg University, Germany. The thickness of each sample was 2 mm which is 1/5 of what is recommended for tsunami deposits (Spiske, 2020) – resulting in 88 samples for the basal core section of 18 cm. Samples were dried at 105°C and carefully pestled. Organic matter was dissolved by adding H₂O₂ (30%). Aggregates were dispersed using 0.1 mol Na₄P₂O₇ (44.6 g l^–1) and samples were put in an overhead shaker for 12 h. GRADISTAT v9.1 (Blott and Pye, 2001) was used to calculate univariate statistical measures following Folk and Ward (1957).

The Elementar soliTOC cube of the Laboratory for Geomorphology and Geoecology, Heidelberg University, was used to measure total organic carbon (TOC) and total inorganic carbon (TIC). The soliTOC uses a temperature ramp procedure with dry combustion and CO₂ detection at each temperature plateau. The detection limit is < 15 µg (Elementar, 2020).

Elemental concentrations were measured with an Avaatech (GEN-4) X-ray fluorescence core scanner at the Institute of Earth Sciences, Heidelberg University. The core scanner uses an OXFORD ‘Neptune 5200’ series 100 W X-ray source with a Rh anode. The slit size was set to 5.0 mm down-core and 10.0 mm cross-core. Two different energy levels were applied for measurement: at 10 kV with a current of 500 µA without a filter and a dwell time of 10 s, and at 30 kV beam with a 1500 µA current, a Pd-thick filter and dwell time of 10 s. The data were processed using the bAxil software (Brightspec, 2015). To remove potential elemental bias due to topographic variations of the core surface, e.g., water content or porosity, the elements were normalised by dividing the intensity counts of each element by the XRF total during each run excluding coherent and incoherent scattering (Kern et al., 2019).

A principal component analysis (PCA) was first carried out on 13 centered log-ratio transformed elements (Al, Br, Ca, Fe, K, Mn, Mo, P, Rb, S, Si, Sr, Zr) and subsequently on indicative element ratios. The first five principal components (PC) from the second PCA were used for the hierarchical clustering of principal components (HCPC) to identify the most significant variables and to identify potential differences between types of sand layers in the cores. The PCA and the HCPC were performed in R studio (Kassambara, 2017) by applying the R packages FactoMineR (v2.4) for computing and visualised using factoextra (v1.0.7). Significance tests were applied to analyse whether the differences between the clusters are statistically significant. A Kruskal-Wallis test (R package vegan v2.5-7) and a Dunn’s post-hoc test (R package dunn.test v1.3.5) identified characteristic variables controlling the clusters. The output resulted in Bonferroni-corrected p-values with α > 0.05.

A total of 9 samples from selected core depths representing the different facies were processed for palynological analysis following standard techniques including: freeze-drying, weighing (range of sample dry weights: 0.1–0.7 g), spiking with Lycopodium spores (Batch Nr. 050220211), treatment with HCl, NaOH and HF, heavy liquid separation, sieving (10 µm) and mounting of the residues on glass slides using glycerin jelly. All samples were screened to identify the most common palynomorphs.

A combination of ¹⁴C and ¹³⁷Cs dating was used for a Bayesian age-depth model for the entire stratigraphy of FLUG 2 and 3. Samples for ¹⁴C dating (five macro-plant fragments, one bulk peat) were measured by accelerator mass spectrometry (AMS) at the Poznán Radiocarbon Laboratory, Poland (Goslar et al., 2004). Calibration of the ¹⁴C data was carried out using the IntCal20 dataset (Reimer et al., 2020) (Table S2). For ¹³⁷Cs dating, the upper 15 cm of core FLUG 2 were sampled at a resolution of 1 cm. After freeze-drying and homogenisation, ¹³⁷Cs was measured by γ-spectrometry on a high-purity Ge detector (Canberra BEGe5030) at Ravensburg-Weingarten University, Germany. The ¹³⁷Cs activity was measured for 24 h in each sample. The detection probability was calculated by LabSocs software (Canberra) by also considering self-absorption in the sample and in the sample cup. The depths of the three prominent ¹³⁷Cs peaks of 1986, 1963 and 1959 were determined using the Aquascope model (Smith et al., 2005) (Table S1). Each of the peaks decreases with the sum of three exponential functions for wash-off, increasing fixation and equilibrium processes, respectively. The sum of the nine exponential functions was convolved with a Gaussian curve to account for bioturbation. For Bayesian age-depth modelling rBacon (Blaauw and Christen, 2011) was applied. Changing sedimentation rates or event layers were considered in the model according to Blaauw and Christen (2011) (Table S3). For the discussion, late Holocene ¹⁴C dates of tsunami stratigraphies in the Shetland Islands presented in Bondevik et al. (2005) were recalibrated using Calib software (v8.2) (Stuiver and Reimer, 1993) and the IntCal20 dataset (Reimer et al., 2020) (Table S4).

The bulk density, CT scans, XRF data, TOC data and the age model were first published in Hess et al. (2023b). The grain-size data, magnetic susceptibility and qualitative pollen screening are original data of the present paper.

4.1. Facies model of Loch Flugarth

Facies of the cores were classified based on visual interpretation, PCA and HCPC (Table 1). Based on visual inspection and core description, four different facies types were identified in cores FLUG 2 and 3, briefly described in Table 1. Both cores which show very good correlation (Fig. 2).

Core FLUG 3 can be subdivided into seven different units. The six upper units I–VI consist of varying distribution of the three different organic-rich facies types, interspersed with 40 thin, sub-cm-scale layers of Facies 4 (Fig. 2). In contrast, Unit VII shows a thick section of Facies 4 at the base (Unit VIIb; 78.5–91.7 cm b.s.) overlain by a section of Facies 2 and Facies 1 with a thin layer of Facies 4 in between (Unit VIIa, 73–78.5 cm b.s.). More details on the lithostratigraphy of the entire core can be found in Hess et al. (2023b).

Table 1

Facies classification of cores FLUG 2 and 3 based on visual interpretation and multivariate statistical analyses (PCA, HCPC), and the correlation of both schemes.
Visual interpretation		PCA and HCPC
Facies	Description	Cluster	Description
Facies 1	Dark-brown muddy peat, with diatom fragments	Cluster 4	Muddy peat
Facies 2	Organic-rich, light-brown mud	Cluster 2	Organic-rich mud with sand
Facies 3	Organic-rich, brown mud with a minor sand component	Cluster 2	Organic-rich mud with sand
Facies 4	Pale-yellow sand	Cluster 3	Thin sand layers
Facies 4	Pale-yellow sand	Cluster 1	Thick basal sand (Unit VIIb)

The thick basal Unit VIIb and the remaining 40 thin sand layers above, which were identified based on the CT scans, share a similar sand source. However, the multi-proxy data reveal differences that go beyond the mere thickness. Compared to the thin sand layers above, the sand of Unit VIIb is finer and less sorted, free of carbonate, and the pore space is partially filled by a mud matrix.

The magnetic susceptibility is highest in Unit VIIb, much higher than in the thinner sand layers. Elemental ratios of Sr/Br and Zr/K reach maximum values in Unit VIIb and are up to almost an order of magnitude higher than in the thin sand layers above. In contrast, S/Ti values, which peak in Facies 1, are lowest in Unit VIIb, much lower than in the thin sand layers (Fig. 2).

These differences are corroborated by the PCA and the HCPC. Initially, element counts were used for the PCA. The first two PCs explain 47.2% (PC1) and 14.0% (PC2) of the variance of the whole dataset, respectively (Figure S5). PC1 is driven by the variance of Al, Br, K, Rb, S, Si and Zr, whereas PC2 is mainly driven by Ti. These results were used to establish ratios with conservative elements (e.g., Ti, Fe, Si) as denominator, mostly Ti as it is neither contributed by internal biogenic processes nor is it influenced by redox conditions or diagenetic overprinting (Chagué, 2020). The PCA on ratios shows four separate clusters and a clear distinction of Unit VIIb (Cluster 1) from the remaining sand layers (Cluster 3) and the organic-rich background facies (clusters 2, 4) (Figure S6). While measurements from Unit VIIb show high ratios of Zr/K, Sr/Br or Zr/Rb, measurements of the thin sand layers are related to high ratios of K/Ti, Si/Zr and Si/Ti. The HCPC based on the eigenvalues reveals five dimensions and also four clusters, very similar to those of the PCA. Facies 4 is again clearly separated into clusters 1 and 3 (Fig. 3a, Table 1) with Zr/K and Sr/Br showing the strongest significance for the basal sands of Unit VIIb (Cluster 1). In contrast, the upper thin sand layers (Cluster 3) are mostly driven by Si/Ti and K/Ti (Figs. 4, S6).

An initial screening of pollen content across core FLUG3 also revealed differences between Unit VIIb and the remaining part of the core: the seven samples taken between 3 and 73 cm b.s. are all rich in terrestrial pollen and spores. They are dominated by Ericaceae (heath), Poaceae (grasses) and Cyperaceae (sedges) pollen representing the heath and grasslands around Loch Flugarth. These samples also contain various pollen grains from herbs and fern spores, as well as sporadic arboreal pollen such as Betula (birch), Alnus (alder), Pinus (pine) and Salix (willow). In contrast, the two samples from Unit VIIb (81 and 86 cm b.s.) contain only very few, reworked pollen grains of poor preservation.

4.2. Subdivision of Unit VIIb

The grain-size analysis allows for an internal differentiation of Unit VIIb into four sublayers (Fig. 5). From bottom to top, Sublayer 1 (91.7–88.5 cm b.s.) has a lower section of unimodal, moderately to poorly sorted massive sand, and highest values of Zr/K and bulk density. The upper part is normally graded, shifting to bimodal (sand and minor silt component) and trimodal (sand and minor silt and clay components) grain-size distributions (GSD) (Fig. 6), which are very poorly to extremely poorly sorted. Zr/K values and bulk density decrease in the upper part.

Sublayer 2 (88.5–82.7 cm b.s.) has a lower inversely graded section changing from a mixture of sand and silt to pure sand, with better sorting towards the top, slightly increasing bulk density as well as stable magnetic susceptibility and Zr/K values. The upper part is again normally graded with a distinct change from silty sand to sandy silt at 83.9 cm b.s. and the highest values for magnetic susceptibility of the entire sequence. The uppermost part of Sublayer 2 is less sorted, shows a decrease in Zr/K and an initial increase of TOC from < 3% to around 6%. This might be associated with mm-scale muddy peat clasts embedded in the silty to sandy matrix, the largest of which is > 1 cm in vertical extent (Figs. 5, S8).

There is a sharp contact to Sublayer 3 (82.7–80.6 cm b.s.) with a very thin inversely graded base followed by a normally graded top section with increasing concentration of organic matter and small mud clasts. Zr/K decreases in the entire sublayer, along with bulk density and magnetic susceptibility, while TOC remains at about 6%.

Sublayer 4 (80.6–78.6 cm b.s.) of massive sand has an erosive lower boundary, elevated concentration of organic content (TOC up to 7.5%) and mm-scale muddy peat clasts. It shows a distinct peak in magnetic susceptibility and bulk density associated with a weak increase in Zr/K values, and is better sorted than Sublayer 3. Its upper boundary to Facies 3 is sharp, where TOC rises to up to 20%, with magnetic susceptibility, bulk density and Zr/K values being relatively low (Fig. 5).

4.3. Age of Unit VIIb

Age-depth modelling based on ¹⁴C and ¹³⁷Cs reveals a chronostratigraphy of c. 1450 years. Two outliers (FLUG 3-30-31, FLUG 3 14C-4; Table S2, Figure S3) both creating significant age inversions in the stratigraphy were not considered for further analyses. After removing all instantaneous depositional events (sand layers ≥ 2 mm) (Table S3) by identifying their upper and lower boundaries in the age-depth model and subsequent reintegration at their real depth (cf. Sabatier et al., 2022), the basal sand layer of Unit VIIb reveals an age of 426–787 (median: 569) cal. a CE. The averaged accumulation rate across the entire core is 20 a cm^-1 (Figure S3). Further details on the construction of the age model can be found in Supplementary Note 1.

5.1 Facies interpretation

The three types of organic-rich facies mainly represent lake-internal sedimentation with a contribution of organic matter through surface discharge from the peat-bog catchment. Highest TOC, Br and S in Facies 1 (HCPC Cluster 4), which are best preserved in fine-grained organic matter (Engel et al., 2012; Chagué, 2020; Biguenet et al., 2021), may indicate increased rainfall and surface discharge (McIlvenny et al., 2013). The sand component in Facies 3 (Cluster 2) might relate to phases of higher aeolian sand input from the sand barrier of Sand Voe, a process that has been identified in several coastal lakes and peat bogs in the UK (e.g., Swindles et al., 2018; Kylander et al., 2020).

The thin sand layers of Facies 4 have been interpreted as overwash deposits originating from the dune, beach and uppermost subtidal of Sand Voe during major storm events (Hess et al., 2023b). This is confirmed by the large overlap of Endmember 1 of the endmember modelling analysis of GSD of the entire core by Hess et al. (2023b), representing the thin layers of sandy storm deposits, and the GSD of the modern beach sand (Fig. 6).

Facies 4 is separated into two distinct clusters, i.e., the thin storm deposits and the basal Unit VIIb. The basal sand of Unit VIIb is dominated by high values of Zr/K, Fe/Ti and Sr/Rb. Zr/K most likely reflects the concentration of zircon as one of the highest-density minerals (Davies et al., 2015), which, despite generally low percentages, has been shown to be enriched in tsunami deposits (Costa et al., 2015; Chague, 2020). Zircon grains require high-energy flows to generate sufficient shear stress to become mobilised (Cuven et al., 2013). Furthermore, the very low S/Ti ratio (Fig. 4) may be driven by high Ti concentrations associated with heavy mineral content (cf. Cuven et al., 2013; Chague, 2020). In contrast, the storm deposits are dominated by K and Si, i.e., mainly quartz and feldspar varieties (Chagué, 2020) as in the beach sand, which is confirmed by the HCPC (Fig. 3) as well as the Kruskal-Wallis test and the Dunn’s post-hoc test (Fig. 4).

5.2 Evidence for tsunami deposition in Unit VIIb

The entire Unit VIIb stands out against the background sediments in terms of bulk density and magnetic susceptibility, which is caused by high-density ferromagnetic iron oxides or heavy minerals and is a typical signature of tsunami deposits in lakes (e.g., Wagner et al., 2007; Kempf et al., 2017). Suspension grading, which is observed in sublayers 1–3, is a common sequential pattern in tsunami deposits. It forms when a tsunami loaded with suspended sediment impacts a coastal lake or a coastal plain still inundated from the previous wave. Grains settle as a function of their size, shape and density under Stoke’s law of hydraulic equivalents, with smaller and lighter particles increasing towards the top (Dawson and Shi, 2000; Spiske, 2020). Therefore, the heavy mineral content, in the current case reflected by Zr/K, tends to be higher in the lower parts of sublayers formed by suspension grading (cf. Bahlburg and Weiss, 2007; Moore et al., 2011; Cascalho et al., 2016; Spiske, 2020). The increased heavy mineral content in combination with the finer sand grains and mud component, which can be seen in the GSD (Fig. 6), indicates a larger source area of the dune, the beach, the Sand Voe and marine areas outside of Sand Voe, at water depths out of reach for storm waves. The poorer sorting (including a small clay component in a few trimodal samples from the upper part of the normally graded sublayers; Figs. 5, 6), the larger source area, and the increased concentration of heavy minerals are typical signatures of tsunami deposits when compared to storm deposits (Switzer and Jones, 2008; Engel et al., 2016; Spiske, 2020).

In between sublayers, sharp and undulating erosional boundaries are common (Cuven et al., 2013; Spiske, 2020), such as at the base of Sublayers 3 and 4 (Fig. 5). Tsunamis erode underlying fine-grained and/or organic-rich substrate onshore during both run-up and backwash. The cohesive clasts become entrained in the turbulent flow and embedded into the sandy tsunami deposit as rip-up clasts (Cuven et al., 2013; Spiske, 2020), which have been reported from various coastal lake archives (e.g., Bondevik et al., 1997; Kelsey et al., 2005; Wagner et al., 2007; Kempf et al., 2015, 2017).

Sublayer 2 shows a distinct inversely graded section, which may relate to the process of kinetic sieving where grain-grain collisions in the basal part of the flow cause finer particles to trickle down and settle before the larger particles (Sohn, 1997). Such traction carpets indicate high shear stress from the suspension-laden flow and have been identified in several tsunami deposits (Moore et al., 2011; Falvard and Paris, 2017). However, an upward reduction in bulk density with increasingly coarser particles as observed by Moore et al. (2011) is not reflected by the present data.

A distinct mud cap on top of the entire tsunami sequence, such as observed in an organic-poor, gyttja-type lake (e.g., Kempf et al., 2015, 2017) or clay-rich marsh deposit (e.g., Cuven et al., 2013), cannot unequivocally be inferred from the CT scans (Figure S6). However, the sequence of Unit VIIb is topped by a layer of muddy peat (Facies 1), that would be expected to settle after the disturbance of a tsunami, at the waning stage, similar to a mud cap as the lightest material to settle out of suspension. Likewise, there is increasing concentration of finer particles and organic matter in the upper part of Sublayer 3, also reflecting the waning stage between two waves, a pattern that has been found in other tsunami deposits of shallow coastal lakes as well (e.g., Bondevik, 2022; Bondevik et al., 1997). The low concentration and poor preservation of pollen in Unit VIIb compared to all other facies of the core is a common observation made in tsunami deposits of the region (e.g., Smith et al., 2004) and elsewhere (e.g., Chagué-Goff et al., 2012).

The characteristics of Unit VIIb overlap with typical criteria of tsunami erosion and deposition in the proximal coastal lake environment sensu Kempf et al. (2017). Alternative mechanisms can be excluded, as we can clearly distinguish Unit VIIb from the site-specific pattern of storm deposits, while the sorting is too poor for aeolian input. Furthermore, we can exclude major changes to the framework conditions of the sedimentary archive of Loch Flugarth, as relative sea-level, which is the most crucial factor controlling the local coastal geomorphic setting, is assumed to have not changed notably (Dawson et al., 2020a).

It needs to be mentioned that the investigated gravity cores possibly do not reach the base of the candidate tsunami deposit. Based on the substantial thickness of other regional tsunami deposits in coastal lake settings (e.g., Grauert et al., 2001; Bondevik et al., 2005; Wagner et al., 2007), the tsunami record might continue with depth However, this neither affects the interpretation presented above nor the chronological estimates discussed in the following section. The finding of further organic-rich (presumably related to Facies 1–3) and greyish, more muddy lake sediments at a depth of 250–200 cm (Figure S4) and prior work at the site that indicates two distinctive units possibly corroborating with earlier high energy inundation to the lake basin (Sue Dawson, unpubl. data) lead to the assumption that with more advanced coring techniques, the complete deposit and additional event deposits can be accessed and investigated at Loch Flugarth.

5.3 Timing and trigger of the tsunami

The age range of 426–787 cal. a CE (1524–1163 cal. a BP) for Unit VIIb is considered to represent the time window in which the tsunami occurred. Age overestimation based on dated material with longer residual time before being embedded in their sedimentary contexts is considered negligible, as these cases were manually eliminated before running the age-depth model (Supplementary Note 1, Figure S3, Table S3) (Hess et al., 2023b).

There is one tsunami deposit found at two locations on the Shetland Islands which overlaps with this time range. At Dury Voe, east Mainland, a thin sand layer, located within thick coastal peat was traced in outcrops from the inner fjord up to some 400 m inland, and up to an elevation of 5.6 m above high tide. Two ¹⁴C dates were generated 7–8 cm (2303–1927 cal. a BP) and 1–2 cm (1818–1520 cal. a BP) below the sand layer, respectively. One ¹⁴C dating was generated 1–2 cm above the sand layer (1507–1287 cal. a BP) (Table S4) (Bondevik et al., 2005). These data are maximum ages as the dated objects may be older than their sedimentary contexts. The lower ages pre-date the event by an unknown amount of time as it is unclear how much of the peat stratigraphy was eroded by the tsunami. The location is proximal to the shoreline where shear stress by tsunami flow may still have been high. Therefore, the maximum age generated right on top of the sand layer is more indicative for the timing of the Dury Voe tsunami event and may suggest an age younger than the 1500 cal. a BP estimated by Bondevik et al. (2005).

At the inlet of Basta Voe, northeast coast of Yell, three thin sand layers occur in proximal peat outcrops, the uppermost of which continuously extends for c. 2 km (Dawson et al., 2006; Tappin et al., 2015) and can also be traced in ground-penetrating radar measurements (Buck and Bristow, 2020). The sand layer is bracketed by two strongly overlapping ¹⁴C dates: 0–1 cm below the sand layer, 1546–1345 cal. a BP; 5 cm above the sand layer, 1517–1303 cal. a BP (Table S4) (Bondevik et al., 2005; Dawson et al., 2006). Based on the overlapping, the age range of the upper age range likely covers the event. Chronological data from both sites point to an age of the event around 1400 cal. a BP (or even younger) rather than 1500 cal. a BP, as possible age overestimation due to residual time of the dated material before being embedded in the sedimentary contexts has to be taken into account. This chronological estimate is in accordance with the chronological interpretation of Unit VIIb (Fig. 7). Therefore, we suggest that Unit VIIb represents the Dury Voe tsunami.

A source for the Dury Voe tsunami has not yet been identified. A local coastal or submarine landslide has been discussed (Bondevik et al. 2005; Dawson et al., 2006; Tappin et al., 2015; Ballantyne et al., 2018), but without any physical evidence from seafloor mapping (Long, 2015). Based on their limited size, none of the post-Storegga slides known north of the Shetlands seems capable of triggering a tsunami (Haflidason et al., 2005; Smith et al., 2019).

Recently, Mac Conamhna (2023) reported the possible observation of a tsunami in the earliest written chronicles of the Gaelic world. The chronicle entry from the monastery of Iona in the Outer Hebrides, passed on in three different Medieval annals (Chronicum Scotorum, Annals of Tigernach and Annals of Ulster), mentions “A belch/bursting forth/huge tidal wave/eruption of the sea in the month of October” (Mac Conamhna, 2023). Furthermore, the Annals of Ulster contain the record of “An earthquake in October”. Both events are ascribed to the year 720 CE, indicating a major seismic event triggering a tsunami that impacted the Outer Hebrides and potentially other coastal regions of the northern British Isles. Mac Conamhna (2023) associates this event with seismicity along the GGF running across the Scottish mainland from the southern Hebrides up to the Shetland Islands in a SW–NE direction, and draws a correlation with the deposits of the Dury Voe tsunami on Shetland. However, Fig. 7 shows that maximum ages from all three sites where the Dury Voe tsunami has been identified seem too old, even though there is a small overlap with the age range of the Flugarth deposit from this study. A correlation could only be established under the assumption that the dated material at all three sites were slightly older than their sedimentary contexts.

In addition, there are diverging conclusions on the seismicity of the GGF (Musson, 2007; Piccardi, 2014), which is mostly driven by glacio-isostatic rebound (Davenport et al., 1989; Ringrose, 1989). The two strongest historical earthquakes on Scottish territory with proper documentation, both of which could be associated with the GGF, have estimated magnitudes of 5 (1901 CE) and 5.1 (1816 CE), respectively (Piccardi, 2014), with no reported tsunamis. Onshore palaeoseismological data indicate maximum possible magnitudes of 6.0–7.5 following deglaciation (13,000–6000 BP) and of 6.5 afterwards (Davenport et al., 1989). Based on these data, it is highly speculative whether a submarine segment of the GGF had the ability to generate a major earthquake and tsunami that impacted coastal sections spanning from the Outer Hebrides up to the Shetland Islands and generate sedimentary evidence at several sites with northern and eastern exposure across the Shetland archipelago. Thus, the trigger mechanisms of the Dury Voe tsunami, and the seismic and coastal flooding event recorded near Iona remain elusive, and it is unlikely that they represent the same event based on the current chronological evidence and a lack of sedimentary evidence along the entire northern coast of Scotland.

Sediment cores from Loch Flugarth on northern Mainland, Shetland Islands, show a clear separation between autochthonous organic-rich mud with varying composition and allochthonous sand. The allochthonous sand occurs in thin sub-cm layers throughout the core and results from storm overwash (Hess et al., 2023b). These thin layers differ from the thick basal sand unit of all cores (Unit VIIb) in terms of grain-size distribution, sediment structure and chemical composition. High values of Zr/K and magnetic susceptibility as well as low S/Ti values point to enriched heavy minerals, poorer sorting with partial bi- and trimodality, as well as suspension grading in sublayers, partially with traction carpets and rip-up clasts, indicating deposition by a tsunami. The age-depth model and correlation with tsunami deposits of similar age in the Shetland Islands suggests an age around 1400 cal. a BP (c. 550–650 cal. a CE). Correlation with a historical, allegedly seismic tsunami observed on the Outer Hebrides in 720 CE (Mac Conamhna, 2023) seems questionable based on the chronological evidence.

While this study provides additional information on the geography of the late Holocene Dury Voe tsunami, it prompts the need for (i) further studies on the spatial distribution of its impact through a systematic and intensified palaeotsunami research programme in the North Sea region, and (ii) investigations on possible regional source mechanisms that may represent hazards along the coasts of the North Sea at present and in the future. From a more local perspective, this study corroborates the previous observation of two distinctive units in deeper test cores that may relate to high-energy inundation (Sue Dawson, unpubl. data) and the assumption that the site represents an archive of event deposits beyond the last 1500 years. Loch Flugarth preserves both tsunami and storm deposits that can be unequivocally distinguished based on sedimentological and geochemical criteria. Therefore, we advocate for exploration of the deeper sediment record of Loch Flugarth to improve our chronological estimate of the Dury Voe tsunami and possibly other regional tsunamis such as the Garth tsunami (c. 5500 cal. a BP) (Bondevik et al., 2005) and to extend the palaeotempestological record of the northern North Sea (Hess et al., 2023b).

Acknowledgements

This study was funded by Belgian Science Policy Belspo (BR/175/PI/GEN-EX). The possibility for Geotek core logging at Renard Center for Marine Geology, Ghent University, and the support by Evelien Boes are acknowledged. The permission to access Loch Flugarth was kindly granted by local landowners. We thank Oliver A. Kern for support with grain-size data presentation.

Data availability

Data reported here are stored at https://doi.org/10.11588/data/QJEZHT and freely accessible under a Creative Commons Attribution 4.0 International License (CC BY 4.0) (Hess et al., 2023a). Further supplementary data and information are available as Supplementary Material in the online version of the paper. Correspondence and requests for materials should be addressed to M.E.

Code availability

This study does not use custom code or mathematical algorithm that is deemed central to the conclusions.

Author contributions

M.E.: Conceptualization (lead), Fieldwork (equal), Project administration (equal), Methodology (equal), Investigation (supporting), Writing–original draft (lead), Writing–review & editing (equal); K.H.: Conceptualization (supporting), Methodology (equal), Investigation (lead), Visualization (lead), Writing–original draft (supporting), Writing–review & editing (equal); S.D.: Conceptualization (supporting), Fieldwork (equal), Writing–review & editing (equal); T.P.: Fieldwork (equal), Methodology (equal), Writing–review & editing (equal); A.K.: Methodology (equal), Investigation (supporting), Writing–review & editing (equal); P.V.: Methodology (equal), Investigation (supporting), Writing–review & editing (equal); E.K.: Methodology (equal), Investigation (supporting), Writing–review & editing (equal); P.K.: Fieldwork (equal), Methodology (equal), Writing–review & editing (equal); I.S.: Funding acquisition (equal), Project administration (equal), Writing–review & editing (equal); V.M.A.H.: Fieldwork (equal), Funding acquisition (equal), Project administration (equal), Writing–review & editing (equal).

Competing interests

None of the authors has any competing interests.

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

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Abstract

Figures

1. Introduction

2. Physical Setting

3. Methods

3.1 Field methods

3.2. Laboratory Methods

4. Results

4.1. Facies model of Loch Flugarth

4.2. Subdivision of Unit VIIb

4.3. Age of Unit VIIb

5. Discussion

5.1 Facies interpretation

5.2 Evidence for tsunami deposition in Unit VIIb

5.3 Timing and trigger of the tsunami

6. Conclusions

Declarations

References

Supplementary Files

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