Analysis of more than 1200 radiocarbon dated samples that constrain relative sea-levels in Great Britain over the past 16 000 yr provides estimates of current land-level changes (negative of relative sea-level change). Maximum relative land uplift occurs in central and western Scotland, ca. 1.6 mm yr-1, and maximum subsidence is in southwest England, ca. 1.2 mm yr-1. Sediment consolidation, arising from autocompaction as the sediment accumulates and from land drainage, increases the subsidence in areas with thick sequences of Holocene sediments, with an average effect equivalent to at least an extra ca. 0.2 mm yr-1 land subsidence, but more in parts of southeast England, 0.5-1.1 mm yr-1. Modelled changes in tidal range during the mid- to late Holocene in eastern England suggest that the calculated rate of land subsidence is overestimated unless such changes are quantified. The effect is most significant for large coastal lowlands, the Fenland and Humber (ca. 0.5 and 0.6 mm yr-1), that were tidal embayments during the mid- to late Holocene.

Holocene land- and sea-level changes in Great Britain

Estuaries are sites of port, industrial, urban and recreational development and also important to many forms of animal life. They often form sinks for sediment and thus for contaminants associated with the sediment which arise from anthropogenic activities in their hinterland and along their shores. These contaminants can adversely affect estuarine ecosystems and are the subject of international agreements on environmental protection. For both of these reasons, it is important to increase our knowledge of the distribution, concentration, controlling influences on, and impacts of, estuarine contamination. Evidence from around the world shows that although estuaries are sinks for contaminants from the terrestrial environment, there is significant transport of marine material up-estuary as bed load sediment whilst fine-grained terrestrial material may be transported seawards in suspension. Major movement of contaminants from estuaries onto the continental shelf probably occurs only during floods and storms and, in general, the impact on shelf seas is relatively minor and confined to the coastal zone. The chief exception to this rule is in the Far East where sediment from major Chinese rivers may be dispersed up to 300 km across the shelf.

Estuaries as Repositories of Historical Contamination and their Impact on Shelf Seas

Summary

Observations of postglacial relative sea level (RSL) history at sites beyond the margins of the main accumulations of Wu¨rm–Wisconsin–Devensian ice may be invoked to test the accuracy of global models of the glacial isostatic adjustment process. The existing database of RSL time-series for the British Isles forms a continuous network of sites that sample the coasts of Scotland, England and Wales fairly uniformly, and which may be especially important in this regard. Not only are these 14C data for the Holocene epoch of very high quality in terms of our knowledge of the coastal environment and relationship to palaeo-sea level of each age measurement, but they are especially numerous and relate to an island that was itself partly glaciated in the north, essentially ice-free in the south, and located well outboard of the massive ice-sheet that covered Fennoscandia at the Last Glacial Maximum (LGM).



Taken together, the time-series from 55 distinct locations allow us to perform a fairly stringent test of the recently formulated ICE-4G (VM2) global model of the isostatic adjustment process. Even without modification, this model has been shown to fit most of the time-series in the UK data set rather well. In order to satisfy very recently published a priori constraints on the maximum thickness of the Scottish ice-sheet, however, we find that the lithospheric thickness, the feature of the radial viscoelastic structure to which the rebound data from Scotland are most sensitive, must be somewhat reduced. Whereas the assumed thickness of the lithosphere in ICE-4G (VM2) was near to 120 km, best fits to the Scottish data require a lithospheric thickness near to 90 km. This reduced value for lithospheric thickness agrees exceedingly well with the latest estimates of this Earth property based on the inversion of oceanic geoid and topography data, as the asymptotic thickness characteristic of the oldest ocean floor. This would appear to be the best possible a priori estimate for the thickness of the cratonic lithospheres upon which the largest ice-sheets of the late Pleistocene are known to have rested. We also demonstrate explicitly that inferences of the shallow viscoelastic structure beneath the British Isles are strongly influenced by the details of the history of deglaciation at remote locations, at Antarctica in particular.

/pdf/on-the-postglacial-isostatic-adjustment-of-the-british-isles-gphilufr6w.pdf

On the postglacial isostatic adjustment of the British Isles and the shallow viscoelastic structure of the Earth

Abstract Analysis of cores collected from Late Devensian (Weichselian) and Holocene sediments on the floor of the North Sea provides evidence of the transgression of freshwater environments during relative sea-level rise. Although many cores show truncated sequences, examples from the Dogger Bank, Well Bank and 5 km offshore of north Norfolk reveal transitional sequences and reliable indicators of past shoreline positions. Together with radiocarbon-dated sea-level index points collected from the Holocene sediments of the estuaries and coastal lowlands of eastern England these data enable the development and testing of models of the palaeogeographies of coastlines in the western North Sea and models of tidal range changes through the Holocene epoch. Geophysical models that incorporate ice-sheet reconstructions, earth rheology, eustasy, and glacio- and hydroisostasy provide predictions of sea-level relative to the present for the last 10 ka at 1-ka intervals. These predictions, added to a model of present-day bathymetry, produce palaeogeographic reconstructions for each time period. The palaeogeographic maps reveal the transgression of the North Sea continental shelf. Key stages include a western embayment off northeast England as early as 10 ka bp; the evolution of a large tidal embayment between eastern England and the Dogger Bank before 9 ka bp with connection to the English Channel prior to 8 ka bp; and Dogger Bank as an island at high tide by 7.5 ka bp and totally submerged by 6 ka bp. Analysis of core data shows that coastal and saltmarsh environments could adapt to rapid rates of sea-level rise and coastline retreat. After 6 ka bp the major changes in palaeogeography occurred inland of the present coast of eastern England. The palaeogeographic models provide the coastline positions and bathymetries for modelling tidal ranges at each 1-ka interval. A nested hierarchy of models, from the scale of the northeast Atlantic to the east coast of England, uses 26 tidal harmonics to reconstruct tidal regimes. Predictions consistently show tidal ranges smaller than present in the early Holocene, with only minor changes since 6 ka bp. Recalibration of previously available sea-level index points using the model results rather than present tidal-range parameters increases the difference between observations and predictions of relative sea-levels from the glacio-hydro-isostatic models and reinforces the need to search for better ice-sheet reconstructions.

Modelling western North Sea palaeogeographies and tidal changes during the Holocene

The Pliocene–Pleistocene peripheral marine basins of the Mediterranean Sea in southern Italy, from Basilicata and western Calabria to northern and eastern Sicily, represent tectonically formed coastal embayments and narrow straits. Here, units of cross-stratified, mixed silici–bioclastic sand, 25 to 80 m thick, record strong tidal currents. The Central Mediterranean Sea has had a microtidal range of ca 35 cm, and the local amplification of the tidal wave is attributed to tides enhanced in some of the bays and to the out-of-phase reversal of the tidal prism in narrow straits linking the Tyrrhenian and Ionian basins. The siliciclastic sediment was generated by local bedrock erosion, whereas the bioclastic sediment was derived from the contemporaneous, foramol-type cool-water carbonate factories. The cross-strata sets represent small to medium-sized (10 to 60 cm thick) two-dimensional dunes with mainly unidirectional foreset dip directions. These tidalites differ from the classical tidal rhythmites deposited in mud-bearing siliciclastic environments. Firstly, the foreset strata lack mud drapes and, instead, show segregation of siliciclastic and bioclastic sand into alternating strata. Secondly, the thickness variation of the successive silici–bioclastic strata couplets, measured over accretion intervals of 2 to 3 m and analysed statistically, reveal only the shortest-term, diurnal and semi-diurnal tidal cycles. Thirdly, the record of diurnal and semi-diurnal tidal cycles is included within the pattern of neap-spring cycles. Differences between these sediments and classical tidal rhythmites are attributed to the specific palaeogeographic setting of a microtidal sea, with the tidal currents locally enhanced in peripheral basins. It is suggested that this particular facies of mud-free, silici–bioclastic arenite rhythmites in the stratigraphic record might indicate a specific type of depositional sub-tidal environment of straits and embayments and the shortest-term tidal cycles.

The record of tidal cycles in mixed silici–bioclastic deposits: examples from small Plio–Pleistocene peripheral basins of the microtidal Central Mediterranean Sea

Abstract The influences of sea-level, climate, human activity and coastal morphology on post-glacial sediment flux and deposition in the Tees Estuary were considered in a multidisciplinary investigation of the Late Pleistocene and Holocene sedimentary record. The following tripartite division was identified using a combination of lithostratigraphic and geochemical data: a Late Glacial laminated clay providing evidence of a former proglacial lake and a proxy record of climate change; an early-mid-Holocene intercalated sequence of tidal silts and clays and peats; and a late Holocene succession characterized by increasing evidence of human activity and metal contamination. Sea-level change has been identified as the main control on sedimentation via decelerating sea-level rise and changing tidal dynamics between c. 8 and 3 ka bp. Climate controlled the sequence of rhythmite thickness in the Late Glacial clays, whilst increased wetness after c. 3 ka bp may have encouraged terrestrial sediment influx. Enhanced sediment supply to the coastal zone can also be attributed to increasing human activity in the catchment from the Bronze Age onward, first as a consequence of clearance, and subsequently as a result of mining and industrial expansion. Fine-grained sediment flux has almost exclusively been from the Tees catchment to the coast, extending offshore during rebound-induced collapse and erosion of the Late Glacial lake basin. The only notable onshore sediment flux has been the deposition of marine sands in the outer estuary between c. 6.5 and 3.5 ka bp.

Sediment provenance and flux in the Tees estuary: the record from the Late Devensian to the present

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Sediment provenance and flux in the Tees estuary: the record from the Late Devensian to the present