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Shaun James O'Callaghan, Durham University (United Kingdom)
Sarah Bradley, Durham University (United Kingdom)
Richard Hardy, Durham University (United Kingdom)
Glenn Milne, Durham University (United Kingdom)
Ian Shennan, Durham University (United Kingdom)
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Existing models of palaeotides predict changes in tidal ranges since the time of the Last Glacial Maximum (LGM) of more than 50% compared to present. These changes result from: i) fluctuations in sea level; ii) the changing location of the coast, and; iii) sedimentation influx and other local processes. Accurate reconstruction of palaeotides requires correct quantification of these terms. Since most geological indicators of past sea level record a palaeo-tide level rather than paleo-mean sea level accurate reconstructions of palaeotidal amplitudes are an essential factor within the larger context of Quaternary environmental change.
Previous studies vary in i) the number of tidal constituents used; ii) the geological time scales for which reconstructions are made, and; iii) the extent to which a correct quantification of sediment infill exists for inner coastal areas. The number of tidal constituents needed to accurately explain tidal behaviour increases in shallow water areas where bottom friction and localised sedimentary processes dominate. Existing studies include the application of a uniform depth reduction technique, which neglects the spatial and temporal effects of isostatic rebound processes on a continental shelf scale. They apply either one (M2) or six (M2, S2, μ2, M4, M6 and MS4) tidal constituents which limits the accuracy of palaeotidal amplitude predictions especially in shallow water areas such as the southern North Sea and adjacent estuaries. Secondly, limited availability of sea level index points restricts some previous studies to the Holocene. Finally, previous studies, limited to only a few estuaries, underline the importance of sediment based processes by demonstrating that isostatic rebound processes alone are insufficient to explain differences in tidal ranges within estuaries as these changes are also a function of sediment infill.
This paper applies an existing tidal model of the present day to produce numerical reconstructions of palaeotidal elevations in 1,000 year intervals from the LGM to present using twenty six tidal constituents. Subtracting independently derived relative sea level reconstructions from present day bathymetry for each 1,000 year interval provides the model palaeobathymetry. Using this bathymetry as the primary boundary condition produces palaeotidal amplitude reconstructions from the LGM to present for a range of scenarios of vertical Earth movements due to the loading of ice and water through time. Our results provide new reconstructions of palaeobathymetry and coastline position over the past 20,000 years and confirm bathymetry as an important process driver for palaeotidal amplitudes at the continental shelf scale. At present we are coupling the existing palaeotidal model with a river basin model to predict sediment flux at the estuarine scale and asses its importance in paleotidal amplitudes at inner coastal, shallow water areas.
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