Magnitude and frequency of extra-tropical North Atlantic cyclones: A chronology from cliff-top storm deposits

Cliff-top storm deposits (CTSDs) are known to occur on cliffs at elevations of up to 50 m above sea level at exposed sites on the deep-water coasts of the British Isles. Eye-witness accounts and time-series field mapping of CTSDs demonstrate their formation over the historical period and their continued modification during major storms, with CTSDs burying a range of modern human-derived debris. Older deposits show surface lichen growth and weathering effects that relate to the relative age of CTSDs. Over longer time spans, buried sand and peat deposits provide dates on phases of CTSD emplacement and mobilisation and so provide an indication of the frequency and timing of the most severe storms. Two adjacent sites in Shetland provide a combination of 14C dates on peat buried beneath storm-deposited boulders, 14C dates on marine shells buried inside CTSD boulder ridges and optically stimulated luminescence (OSL) ages on sands intercalated with storm-derived boulders. Although the precision is low, this evidence identifies phases of exceptional storm activity spanning approximately 400–550, 700–1050, 1300–1900 and since 1950 AD. By the very nature of cliff-top erosion and deposition these records are fragmentary, yet the timeline of major storms recorded in the Shetland cliffs matches fairly well with the more general record of storminess in the North Atlantic derived from the sea salt record in the GISP2 ice core. As such CTSDs represent an overlooked archive of storm sedimentation with the potential to elucidate local storm chronologies as well as informing debate surrounding the palaeo-tempestology and impact of major cyclones and hurricanes.