INVESTIGATING CLIFF RETREAT RATES AT DÚN AONGHASA AND DÚN DUCHATHAIR, ARAN ISLANDS, IRELAND, USING SUBLITTORAL GEOMORPHOLOGY AND ECOLOGY

The Aran Islands, off Ireland's west coast, are characterized by steep Atlantic-facing cliffs. Semicircular partial walls of two Bronze Age stone forts – Dún Aonghasa and Dún Duchathair – are truncated at vertical cliff edges, and a long-standing question is to what extent the present geometry of these forts reflects their original relation to the cliffs. Addressing whether they were built intentionally at precipices, or whether 3000 years of erosion brought the cliffs to them, requires insight into erosion and retreat rates. Therefore we are looking beneath the cliffs for relevant geomorphic and biologic data that may hold clues for cliff retreat rates on the centennial to millennial scale, which in turn may inform archaeological interpretations and preservation strategies for the forts.

Underwater remotely-operated vehicle imagery collected in 2021 reveals a nearshore zone characterized by abundant large boulders (up to ~3m diameter), grading to more scattered boulders in deeper water, interspersed with well-sorted gravel. Superimposed on these are depth-controlled biological community structures, with dense sublittoral Laminaria-dominated kelp forests giving way to faunal turf further down (primarily composed of red and green fleshy macroalgae, dead men’s fingers soft coral Alcyonium digitatum, and elephant’s hide sponge Pachymatisma johnstonia).

Boulders are rounded, suggesting a history of overturning and rolling, although mature faunal development on some boulders indicates decadal-term stability. In general, larger boulders tend to have denser cover and larger individual fauna, suggesting less frequent disruption. However, even the most heavily-turfed boulders have lower sides and bases free of organisms, indicating regular scouring by current-entrained sand and gravel in this high-energy environment. The extent of rounding, successional stage, and overall fairly low-density boulder cover suggests infrequent rock falls with substantial reworking of material between fall events.