A combined modern training set from three salt marshes and tidal flats of Mainland, Shetland Islands, as a tool for local relative sea-level reconstruction

High-resolution reconstructions of the relative sea-level (RSL) evolution are important for managing coastal-protection challenges and for hazard assessment. For the determination of palaeo-tsunami run-up heights in the Shetland Islands, United Kingdom, within the NORSEAT Project (Storegga and beyond – North Sea tsunami deposits offshore Shetland Islands), RSL reconstructions far beyond existing data are needed. Existing RSL data is limited to two time frames (ca. 7900–5990 cal BP and around 3500 cal BP) and include a large vertical error (approximately ±8 m around the time of the Storegga tsunami). More detailed Holocene RSL reconstructions shall be enabled by a combined modern training set of foraminifers and ostracods from three different voes of Shetlands largest island, Mainland. The training set serves as a basis for a RSL transfer function, which relates the elevation of surface samples to the modern microfaunal associations. This transfer function will be a valuable tool for high-resolution RSL reconstructions from the Holocene stratigraphic record around the Shetland Islands as shown by previous studies in Northern Germany.

Investigations of 44 surface samples, which were collected from three salt marshes and adjacent tidal flats (south Dales Voe, Dury Voe and north Dales Voe), are in progress. First results show highly diverse foraminifer and ostracod associations in the salt marsh and very low occurrence of microfauna in the very coarse parts of the tidal flat. Small areas of very muddy tidal flats suggest higher abundances than the latter. Aside from the investigation of the microfaunal distribution, analyses of environmental parameters like the grain-size distribution and the carbonate and organic matter content are still in progress. Multivariate statistics will determine the main influencing factor of the microfauna distribution between these environmental proxies and the elevation relative to mean sea level.

If the training set is feasible for a RSL transfer function, in a next step, it will be applied to Holocene deposits from offshore cores around Shetland that were conducted within the NORSEAT Project. The resulting new RSL reconstructions will enable more accurate determination of run ups of the currently identified palaeo-tsunamis (Storegga and two younger events).