Changes in wave climate over the northwest European shelf seas during the last 12,000 years

Because of the depth attenuation of wave orbital velocity, wave-induced bed shear stress is much more sensitive to changes in total water depth than tidal-induced bed shear stress. The ratio between wave- and tidal-induced bed shear stress in many shelf sea regions has varied considerably over the recent geological past because of combined eustatic changes in sea level and isostatic adjustment. In order to capture the high-frequency nature of wind events, a two-dimensional spectral wave model is here applied at high temporal resolution to time slices from 12 ka BP to present using paleobathymetries of the NW European shelf seas. By contrasting paleowave climates and bed shear stress distributions with present-day conditions, the model results demonstrate that, in regions of the shelf seas that remained wet continuously over the last 12,000 years, annual root-mean-square (rms) and peak wave heights increased from 12 ka BP to present. This increase in wave height was accompanied by a large reduction in the annual rms wave- induced bed shear stress, primarily caused by a reduction in the magnitude of wave orbital velocity penetrating to the bed for increasing relative sea level. In regions of the shelf seas which remained wet over the last 12,000 years, the annual mean ratio of wave- to (M-2) tidal-induced bed shear stress decreased from 1 (at 12 ka BP) to its present-day value of 0.5. Therefore compared to present- day conditions, waves had a more important contribution to large-scale sediment transport processes in the Celtic Sea and the northwestern North Sea at 12 ka BP.