A valley system investigation of Holocene coastal change and Irish Sea palaeogeography in Cardigan Bay, Wales, UK

Of the three substantial areas of postglacial aerial exposure on the Irish Sea shelf (Liverpool Bay, Cardigan Bay and the Bristol Channel), Cardigan Bay is the largest and the one about which we know the least. This article provides the first empirical sea level data for Cardigan Bay since the 1980s. It examines the proposition that the valleys of north Cardigan Bay were subject to a long period of marine influence that is not modelled in current palaeogeographic reconstructions. Detailed lithostratigraphic investigations traced the extent of Early to Mid-Holocene marine influence in the Dysynni valley. Multiple microfossil proxies were used to calculate sea level data points. Two sea level index points from a back barrier marsh indicated relative sea level rise from -5.55 to -3.13 mOD between 7790−7630 and 6399−6296 cal. a BP. Freshwater/terrestrial limiting points from 7 km inland bracketed a period of marine influence between 9889−9545 and 4783−4416 cal. a BP. Pollen recorded a transition from saltmarsh, fen and reed swamp environments to alder carr and then birch woodland. The results were combined with existing lithostratigraphic and biostratigraphic data to show that the overdeepened glacial valleys of north Cardigan Bay were subject to marine influence from before 12 000 cal. a BP, with different palaeogeographic histories dependent on their depths and rates of postglacial sedimentation.