2500 years of late Holocene relative sea-level change at Gress, Isle of Lewis, northwest Scotland

The late Holocene relative sea-level (RSL) history of Scotland is spatially and temporally variable, as it lies close to the boundaries of the former British-Irish Ice Sheet (BIIS) and within the maximum sea-level fingerprint of Antarctic melt. It is therefore an interesting location to understand the interplay of drivers of RSL and the consequences on rates of change, over centennial to millennial timescales. However, there are few late Holocene RSL records from the region, especially islands offshore of mainland Scotland. Along mid-latitude coastlines, salt-marsh deposits provide ideal archives of late Holocene sea level. In this study, we combine stratigraphy, sedimentology (grain size analysis and loss-on-ignition) and diatom biostratigraphy to reconstruct late Holocene sea level, at a newly studied salt marsh at Gress, on the eastern coastline of the Isle of Lewis in the Outer Hebrides. Rather than the typical quantitative transfer function approach, we instead utilise a qualitative visual assessment method to reconstruct RSL due to poor performance by the UK modern diatom transfer function at this location. By combining ¹⁴C dates and Bayesian modelling, we derive a chronological model for the core to assess the timing of any RSL change. We consequently present a new, near-continuous RSL record at Gress which shows a stable to slowly falling RSL trend over the last ~2500 years. At ~AD 580, the disappearance of Sphagnum moss, a typical freshwater species, accompanies the appearance of brackish diatoms species, highlighting a potential increase in the proximity of marine conditions which may indicate regionally rising RSL from this time.