Late Holocene Beach Evolution: Sediment Starvation under a Falling Sea Level

On the paraglacial coast of Northern Ireland the late Holocene sea level history involves a rise from a lowstand at 13ka to cross the present level by around 7kaBP reaching a few metres above present by 5ka BP and a Subsequent fall to present sea level. `Raised' or `stranded' beaches associated with the late Holocene highstand are distributed widely around the Northern Ireland Coast but have received little geomorphological or sedimentological study. In this paper we report on investigations at Millin Bay on the Ards Peninsula where a raised beach is exposed in a 3m-high scarp at the rear of the modern beach. Sedimentological analysis, coupled with GPR investigations provide insights into the development and preservation of the raised beach. A compressed basal peat on which the beach rests, represents a former back-barrier lagoon developed oil poorly drained underlying glacial clay. The raised beach itself comprises sand and gravel with occasional shell concentrations, similar to modern beaches in the locality and is topped by a relict aeolian dune deposit. However, the late Holocene beach is over 3m thick and over 50m wide whereas the contemporary beach comprises a thin (typically< 30cm) veneer overlying bedrock. This points to an abundance of sediment in the late Holocene compared to the modern sediment-starved condition. We attribute the reduction in sediment supply to stranding of sediment Sources (eroding glacial bluffs) as sea level fell. The implications for environmental change during future sea level rise are discussed.