A limpet's eye view of post‐glacial isostasy: fixed biological indicators provide new sea‐level index points for the Mid‐Holocene relative highstand in eastern Northern Ireland

Empirical data for Holocene relative sea level (RSL) changes around the coast of north-eastern Ireland are sparse. Fixed biological indicators (FBIs), in the form of bioerosional marks made by limpets (Patella) and endolithic traces of other intertidal/subtidal organisms, are preserved significantly above present mid-tide level on a limestone cliff at Portmuck, in Co. Antrim, Northern Ireland. The limpet marks indicate a mid-tide level (MTL) for the Mid-Holocene relative highstand (MHRH) at +7.8±0.55 m relative to MTL today. This is >3 m higher than previous empirical data for the region but is consistent with recent glacial isostatic adjustment (GIA) models. Shells of intertidal molluscs (Patella, Littorina and Nucella) recovered from fissures below the limpet marks show a significant spread of ¹⁴C ages, with the oldest a close match for the date of the MHRH but others as much as 2750 years younger. They indicate that RSL fell no more than ~2.5 m in that time from a highstand at ~6.3 cal. ka BP, a pattern comparable to that seen in southern Scotland but 1.5–2 ka later. This chronological offset is consistent with models that predict shoreline diachroneity in moving away from the centre of isostatic uplift in southern Scotland.