Submerged bedrock shore platforms, Orkney Islands, UK: A new record of significant, though chronologically uncertain sea-level change and coastal erosion

Abstract

Reconstructions of sea level change in NW Europe are primarily based on records of relative sea level (RSL) recovered from terrestrial environments, above modern sea level. This deficit in marine-based records results from the highly limited number of sea level indicators observed in modern submarine settings, as well as the often-limited absolute chronology information available. This sampling bias introduces significant uncertainty in former RSL predictions, negatively impacting efforts to accurately model ice-sheet histories and isostatic response. Here we present new seabed mapping data (i.e. high-resolution multibeam bathymetry) from northern Scotland to address this data gap. Encircling the Orkney Islands we identify an exceptional sequence of submerged terraces ranging from -5 to -95 m below modern sea level, carved in bedrock. We interpret these bedrock terraces as relict shore platforms, based on their spatial distribution and a range of geomorphological characteristics. Shore platform development was linked to contemporaneous landward coastline erosion and cliff formation, and each landform pair (i.e. terrace = shore platform and accompanying seacliff / escarpment) likely represents a single sea-level stillstand event of considerable duration (possibly millennia). These wide and well preserved shore platforms attest to formation during multiple, separate periods of RSL stillstand, and we estimate that 5–7 RSL stillstands are recorded offshore Orkney. We discuss their potential age – spanning more than the last glacial cycle (i.e. Middle - Late Pleistocene) – and explore the wider implications for Quaternary coastal erosion and sea-level change in the region. This study shows how marine geological data and geomorphological analysis can be used to identify palaeo-sea-level indicators within a glacio-isostatically complex region. Despite a current lack of absolute chronological constraint, we believe these observations may provide crucial information towards understanding sea level change within the NW European region.