Holocene sea-level change in North Wales : the evolution of the Menai Strait
This study adopts a multidisciplinary approach in order to elucidate aspects of relative sea-level change within North Wales during the Holocene, with particular reference to the evolution of the contemporary tidal channel forming the Menai Strait. Micropalaeontological, sedimentological and geophysical evidence has been utilized in conjunction with radiocarbon dating techniques in order to interpret stratigraphical relationships between sedimentary facies deposited within the northeastern region of the Menai Strait. Micropalaeontological and sedimentological data allied to radiocarbon data obtained from a series of organic-rich horizons demonstrate that relative mean sea level within the northeastern Menai Strait increased from approximately -27m to -6m OD between 11500 and 8000 calendar years BP. Continual, although subsequently reduced rates of relative mean sea-level rise ensured the eventual breaching of a watershed within the central region of the Menai Strait. This rise consequently resulted in the formation of the contemporary tidal strait and the final separation of Anglesey from the mainland at some point between 5600 and 4800 calendar years BP. Relative mean sea level subsequently rose by approximately 1.5m between 6800 and 4000 calendar years BP. The contemporary bathymetrical profile of the main channel probably originated at some point after 6000 calendar years BP. Excellent correlation between sedimentological and geophysical data demonstrate that sedimentary sequences located beneath the sea-floor within the northeastern Menai Strait comprise a series of alternating layers of terrestrial and marine material which constitute a series of laterally extensive near-horizontal layers extending between -28m and -2m OD. The deposits originated during the early to middle Holocene and reflect a pattern of continual and cyclic environmental change, with conditions fluctuating rapidly between those associated with the existence of low-lying terrestrial marsh and those characterized by the presence of an inter-tidal environment. Although a reasonable degree of correlation can be made with similarly dated sequences found on more regional and global scales, it remains unclear if the transgressive and regressive phases that these sequences represent, are wholly or partially attributable to either local physical regimes and processes or are a consequence of fluctuations in the pattern of Late Devensian to early Holocene eustatic sea-level rise. Comparison of the sea-level data obtained from North Wales with sea-level reconstructions derived from pre-existing geophysical models indicate that the modelled outputs underestimate the altitude of relative sea level in North Wales during the early Holocene by up to 25m and further demonstrate that marine conditions existed within this area of the Irish Sea as early as around 14000 calendar years BP.