A coupled cosmogenic-modelling approach to quantifying Holocene erosion of Ireland’s hard-rock coastal cliffs

Abstract. Hard rock cliffs and shore platform are a key feature of the coastal environments in Ireland. Being highly influenced by marine driven erosion, the relationship between changes in relative sea level (RSL) and cliff retreat rates are of major interest when analysing coastal evolution. At the western coast of Ireland, the rates of RSL rise in the Holocene has varied and some locations experienced marine driven erosion earlier than others. Therefore, for understanding coastal hard-rock erosion and the influences of long-term changes, such as RSL changes, studying multiple at locations is crucial.

In this study, we investigated cliff erosion rates on millennial scales at two locations on the western Irish coast – Muckros Head and Ballard Bay – using cosmogenic beryllium-10 concentrations. Our findings indicate that the shore platforms at both locations were formed entirely during the Holocene, and that each site has experienced a long-term deceleration in cliff retreat, reflecting declining rates of RSL rise. The cliff retreat rate decelerated from 14 to 1 mm yr^-1 at Muckros Head within the past ~9500 yrs, and from 25 to 5 mm yr^-1 at Ballard Bay over the last ~5000 yrs. The best-fit models suggest shore platform widening at Muckros Head and steady-state erosion at Ballard Bay. Whereas the pronounced deceleration in cliff retreat observed at Muckros Head, relative to Ballard Bay, is explained by the elevated early-Holocene RSL at the former, differences in contemporary retreat rates between our sites likely reflect contrasting geomorphological and geological conditions and marine environments.