Quantifying rates and identifying drivers of rocky cliff retreat along Ireland’s west coast

More than half of the Irish Coast can be considered rocky, featuring cliffs and shore platforms in various environments and comprising different lithologies. In the face of climate and sea-level change, cliff retreat and its drivers had become an important research subject. For investigating coastal changes on millennial scales, modelling maximum likelihoods of possible exposure histories of shore platforms based on beryllium-10 (¹⁰Be) concentrations have become regularly used in the past decade. We applied ¹⁰Be analysis at four sites along Ireland’s west coast to study the interaction between local environment, changes in relative sea level (RSL) and climate, and cliff retreat. Our preliminary results indicate decreasing erosion rates during the Holocene, with till cliffs and hard rock cliffs retreating a few centimetres and millimetres annually, respectively. Depending on factors such as lithology, local RSL change, and exposure to the North Atlantic, initial retreat rates at each site were as much as one order of magnitude higher. While Sandstone cliffs show initial erosion rates of 12–14 mm*year^-1, glacial cliffs initially retreated with several centimetres per year, reaching rates to 10 cm*year^-1, even when located in inner bay settings. Some sites, especially those featuring cliffs of glacial sediments, show indications for pre-Holocene shore platforms. Cliffs attached to pre-Holocene shore platforms experienced an initial increasing retreat rate, accompanied by high rates of Holocene RSL rising, followed by a decreasing retreat rate. Our findings indicate that slower rates of RSL change lead to decreasing cliff retreat rates. Some previous studies on rocky coast erosion around the world have found similar relationships between changes of RSL and cliff retreat rates, while others could not detect any connections. These differences show a potential influence of variations in regional local coast and environmental settings.

The much lower recent cliff retreat rates raising the question of whether retreat of certain cliff configurations has shifted from marine-driven erosion towards more terrestrially driven erosion, and, if so, whether we might anticipate a shift back towards marine forcing as global sea-level rise accelerates.