Postglacial relative sea‐level observations from Ireland and their role in glacial rebound modelling

The British Isles have been the focus of a number of recent modelling studies owing to the existence of a high‐quality sea‐level dataset for this region and the suitability of these data for constraining shallow earth viscosity structure, local to regional ice sheet histories and the magnitude/timing of global meltwater signals. Until recently, the paucity of both glaciological and relative sea‐level (RSL) data from Ireland has meant that the majority of these glacial isostatic adjustment (GIA) modelling studies of the British Isles region have tended to concentrate on reconstructing ice cover over Britain. However, the recent development of a sea‐level database for Ireland along with emergence of new glaciological data on the spatial extent, thickness and deglacial chronology of the Irish Ice Sheet means it is now possible to revisit this region of the British Isles. Here, we employ these new data to constrain the evolution of the Irish Ice Sheet. We find that in order to reconcile differences between model predictions and RSL evidence, a thick, spatially extensive ice sheet of ～600–700 m over much of north and central Ireland is required at the LGM with very rapid deglaciation after 21 k cal. yr BP. Copyright © 2007 John Wiley & Sons, Ltd.     

An improved glacial isostatic adjustment model for the British Isles

The glacial isostatic adjustment (GIA) of the British Isles is of interest due to the constraints that can be provided on key model parameters such as the global meltwater signal, local ice sheet history and viscoelastic earth structure. A number of recent studies have modelled relative sea‐level (RSL) data from this region to constrain model parameters. As indicated in these studies, the sensitivity of these data to both local and global parameters results in a highly non‐unique problem. This study aims to address this inherent non‐uniqueness by combining a previously published British–Irish ice model that is based on the most recent geomorphological data with a new global ice sheet model that provides an accurate prediction of eustatic sea‐level change. In addition, constraints from Global Positioning System (GPS) measurements of present‐day vertical land motion are considered alongside the entirety of RSL data from both Great Britain and Ireland. A model solution is found that provides a high‐quality fit to both the RSL data and the GPS data. Within the range of earth viscosity values considered, the optimal data model fits were achieved with a relatively thin lithosphere (71 km), upper mantle viscosities in the range 4–6 × 10²⁰ Pa s and lower mantle viscosities ≥ 3 × 10²² Pa s. Copyright © 2011 John Wiley & Sons, Ltd.     

Late Holocene vertical land motion and relative sea‐level changes: lessons from the British Isles

Vertical land motion caused by continuing glacial isostatic adjustment is one of several important components of sea‐level change and is not limited just to previously glaciated regions. A national‐scale analysis for the British Isles shows an ellipse of present‐day relative uplift (relative sea‐level fall), ～1.2 mm a^?1, broadly centred on the deglaciated mountains of Scotland. The pattern of three foci of relative subsidence, ～1 mm a^?1, results from the additional interactions of the deglacial meltwater load on the Atlantic basin and the continental shelf, and the signal due to far‐field ice sheets. At a local scale, sediment compaction can more than double the rate of relative land subsidence. Relative land‐level change (the negative of relative sea‐level change) is not the same as vertical land motion. There is a spatial pattern in the difference between relative land‐level change and vertical land motion, with differences at present of approximately ?0.1 to ?0.3 mm a^?1 around the British Isles and +2.5 to ?1.5 mm a^?1 globally. For the wider scientific and user community, whether or not the differences are considered significant will depend upon the location, time frame and spatial scale of the study that uses such information. Copyright © 2011 John Wiley & Sons, Ltd.     

Evaluation of a numerical model of the British–Irish ice sheet using relative sea‐level data: implications for the interpretation of trimline observations

The glacial isostatic adjustment (GIA) of the British Isles is complex due to the interplay between local and non‐local signals. A number of recent studies have modelled the GIA response of the British Isles using relative sea‐level data. This study extends these previous analyses by using output from a numerical glaciological model as input to a GIA model. This is a necessary step towards more realistic GIA models, and although there have been similar studies for the major late Pleistocene ice sheets, this is the first study to do so for the British Isles. We test three reconstructions, classed as ‘minimal’, ‘median’ and ‘maximal’ in terms of their volume at maximum extent, and find it is possible to obtain good data–model fits. The minimal reconstruction is clearly preferred by the sea‐level data. The ice reconstructions tested were not constrained by geomorphological information of past ice extent (lateral and vertical). As a consequence, the reconstructions extend further than much of this information suggests, particularly in terms of ice thickness. It is notable, however, that the reconstructions produce good fits to many sea‐level data from central, mountainous regions (e.g. Scottish highlands), which lends support to the suggestion that trimlines, often used as an constraint on the palaeo ice surface location, are in fact features formed at the transition from warm‐ to cold‐based ice and so mark a minimum constraint on the ice surface altitude. Based on data–model misfits, suggestions are made for improving the ice model reconstructions. However, in many locations, the cause of the misfit could be due to non‐local, predominantly Fennoscandian ice and so interpretation is not straightforward. As a result, we suggest that future analyses of this type consider models and observations for both Fennoscandia and the British Isles. Copyright © 2012 John Wiley & Sons, Ltd.     

Late Devensian and Holocene relative sea‐level change in North Wales,UK

Deglacial sea‐level index points defining relative sea‐level (RSL) change are critical for testing glacial isostatic adjustment (GIA) model output. Only a few observations are available from North Wales and until recently these provided a poor fit to GIA model output for the British‐Irish Ice Sheet. We present results of an integrated offshore geophysical (seismic reflection), coring (drilling rig), sedimentological, micropalaeontological (foraminifera), biostratigraphical (palynology) and geochronological (AMS ¹⁴C) investigation into a sequence of multiple peat/organic sediment horizons interbedded within a thick estuarine–marine sequence of minerogenic clay‐silts to silty sands from the NE Menai Strait, North Wales. Ten new sea‐level index points and nine new limiting dates from the Devensian Late‐glacial and early Holocene are integrated with twelve pre‐existing Holocene sea‐level index points and one limiting point from North Wales to generate a regional RSL record. This record is similar to the most recent GIA predictions for North Wales RSL change, supporting either greater ice load and later deglaciation than in the GIA predictions generated before 2004, or a modified eustatic function. There is no evidence for a mid‐Holocene highstand. Tidally corrected RSL data indicate initial breaching of the Menai Strait between 8.8 and 8.4 ka BP to form a tidal causeway, with final submergence between 5.8 and 4.6 ka BP. Final breaching converted the NE Menai Strait from a flood‐dominated estuary into a high energy ebb tidal delta with extensive tidal scouring of pre‐existing Late‐glacial and Holocene sequences. The study confirms the value of utilising offshore drilling/coring technology to recover sea‐level records which relate to intervals when rates of both eustatic and isostatic change were at their greatest, and therefore of most value for constraining GIA models. Copyright © 2011 John Wiley & Sons, Ltd.     

The role of megatides and relative sea level in controlling the deglaciation of the British–Irish and Fennoscandian ice sheets

Key external forcing factors have been proposed to explain the collapse of ice sheets, including atmospheric and ocean temperatures, subglacial topography, relative sea level and tidal amplitudes. For past ice sheets it has not hitherto been possible to separate relative sea level and tidal amplitudes from the other controls to analyse their influence on deglaciation style and rate. Here we isolate the relative sea level and tidal amplitude controls on key ice stream sectors of the last British–Irish and Fennoscandian ice sheets using published glacial isostatic adjustment models, combined with a new and previously published palaeotidal models for the NE Atlantic since the Last Glacial Maximum (22 ka BP). Relative sea level and tidal amplitude data are combined into a sea surface elevation index for each ice stream sector demonstrating that these controls were potentially important drivers of deglaciation in the western British Irish Ice Sheet ice stream sectors. In contrast, the Norwegian Channel Ice Stream was characterized by falling relative sea level and small tidal amplitudes during most of the deglaciation. As these simulations provide a basis for observational field testing we propose a means of identifying the significance of sea level and tidal amplitudes in ice sheet collapse.     

Sea‐level change in the Irish Sea since the Last Glacial Maximum: constraints from isostatic modelling

Models of glacio‐hydroisostatic sea‐level change have been published for the British Isles that are broadly consistent with the observational evidence, as well as with glaciological constraints. It has been argued, however, that the models fail to represent sea‐level change along the Irish Sea margins and in southern Ireland for the post‐deglaciation period. The argument rests on the interpretation of the depositional environment of the elevated ‘Irish Sea Drift’ on both sides of the Irish Sea: whether this is terrestrial or glaciomarine. The isostatic models for the British Isles are consistent with the former interpretation in that sea‐levels on either side of the Irish Sea, south of about the Isle of Man, are not predicted to have risen above present sea‐level at any time since the deglaciation of the Irish Sea. This implies that ice over both the Irish Sea and Ireland was relatively thin (ca. 600–700 m over Ireland). If the glaciomarine interpretation of the elevated Irish Sea Drift is correct, then the maximum ice thickness over central and southern Ireland would have to reach 2000 m, exceeding that over Scotland. Furthermore, for the resulting sea‐level change to be consistent with the Holocene evidence, this thick ice sheet could not have extended to the eastern side of the Irish Sea. Nor could it have been very thick at its northern and western limits. If such an ice model is extreme and incompatible with glaciological observations then the alternative is to accept the interpretation of the Irish Sea Drift as terrestrial in origin. Copyright © 2001 John Wiley & Sons, Ltd.     

http://www.sciencedirect.com/science/article/pii/S1674987112001302

We present relative sea level （RSL） curves in Antarctica derived from glacial isostatic adjustment （GIA）predictions based on the melting scenarios of the Antarctic ice sheet since the Last Glacial Maximum （LGM）given in previous works.Simultaneously,Holocene-age RSL observations obtained at the raised beaches along the coast of Antarctica are shown to be in agreement with the GIA predictions.The differences from previously published ice-loading models regarding the spatial distribution and total mass change of the melted ice are significant.These models were also derived from GIA modelling; the variations can be attributed to the lack of geological and geographical evidence regarding the history of crustal movement due to ice sheet evolution.Next,we summarise the previously published ice load models and demonstrate the RSL curves based on combinations of different ice and earth models.The RSL curves calculated by GIA models indicate that the model dependence of both the ice and earth models is significantly large at several sites where RSL observations were obtained.In particular,GIA predictions based on the thin lithospheric thickness show the spatial distributions that are dependent on the melted ice thickness at each sites.These characteristics result from the short-wavelength deformation of the Earth.However,our predictions strongly suggest that it is possible to find the average ice model despite the use of the different models of lithospheric thickness.By sea level and crustal movement observations,we can deduce the geometry of the post-LGM ice sheets in detail and remove the GIA contribution from the crustal deformation and gravity change observed by space geodetic techniques,such as GPS and GRACE,for the estimation of the Antarctic ice mass change associated with recent global warming.     

A critical review of the glaciomarine model for Irish sea deglaciation: evidence from southern Britain,the Celtic shelf and adjacent continental slope

In support of their ‘glaciomarine’ model for the deglaciation of the Irish Sea basin, Eyles and McCabe cited the occurrence of distal glaciomarine mud drapes onshore in the Isles of Scilly and North Devon, and of arctic beach‐face gravels and sands around the shores of the Celtic Sea. Glacial and sea‐level data from the southern part of the Irish Sea in the terminal zone of the ice stream and the adjacent continental slope are reviewed here to test this aspect of the model. The suggestion that the glacial sequences of both the Isles of Scilly and Fremington in North Devon are glaciomarine mud drapes is rejected. An actively calving tidewater margin only occurred early in the deglacial sequence close to the terminal zone in the south‐central Celtic Sea. Relative sea‐levels were lower, and therefore glacio‐isostatic depression less, than envisaged in the glaciomarine model. Geochronological, sedimentological and biostratigraphical data indicate that the raised beach sequences around the shores of the Celtic Sea and English Channel were deposited at, or during regression soon after, interglacial eustatic highstands. Evidence for ice‐rafting at a time of high relative sea‐levels is restricted to a phase(s) earlier than the Late Devensian. These data indicate that the raised beach sequences have no bearing on the style of Irish Sea deglaciation. Copyright © 2001 John Wiley & Sons, Ltd.     

Modelling the influence of glacial isostasy on Late Weichselian ice‐sheet growth in the Barents Sea

A fully integrated ice‐sheet and glacio‐isostatic numerical model was run in order to investigate the crustal response to ice loading during the Late Weichselian glaciation of the Barents Sea. The model was used to examine the hypothesis that relative reductions in water depth, caused by glacio‐isostatic uplift, may have aided ice growth from Scandinavia and High Arctic island archipelagos into the Barents Sea during the last glacial. Two experiments were designed in which the bedrock response to ice loading was examined: (i) complete and rapid glaciation of the Barents Sea when iceberg calving is curtailed except at the continental margin, and (ii) staged growth of ice in which ice sheets are allowed to ground at different water depths. Model results predict that glacially generated isostatic uplift, caused by an isostatic forebulge from loads on Scandinavia, Svalbard and other island archipelagos, affected the central Barents Sea during the early phase of glaciation. Isostatic uplift, combined with global sea‐level fall, is predicted to have reduced sea level in parts of the central Barents Sea by up to 200 m. This reduction would have been sufficient to raise the sea floor of the Central Bank into a subaerial position. Such sea‐floor emergence is conducive to the initiation of grounded ice growth in the central Barents Sea. The model indicates that, prior to its glaciation, the depth of the Central Deep would have been reduced from around 400 m to 200 m. Such uplift aided the migration of grounded ice from the central Barents Sea and Scandinavia into the Central Deep. We conclude that ice loading over Scandinavia and Arctic island archipelagos during the first stages of the Late Weichselian may have caused uplift within the central Barents Sea and aided the growth of ice across the entire Barents Shelf. Copyright © 2000 John Wiley & Sons, Ltd.     

Late Quaternary sea‐level change and evolution of Belfast Lough,Northern Ireland: new offshore evidence and implications for sea‐level reconstruction

The interplay of eustatic and isostatic factors causes complex relative sea‐level (RSL) histories, particularly in paraglacial settings. In this context the past record of RSL is important in understanding ice‐sheet history, earth rheology and resulting glacio‐isostatic adjustment. Field data to develop sea‐level reconstructions are often limited to shallow depths and uncertainty exists as to the veracity of modelled sea‐level curves. We use seismic stratigraphy, 39 vibrocores and 26 radiocarbon dates to investigate the deglacial history of Belfast Lough, Northern Ireland, and reconstruct past RSL. A typical sequence of till, glacimarine and Holocene sediments is preserved. Two sea‐level lowstands (both max. ?40 m) are recorded at c. 13.5 and 11.5k cal a bp . Each is followed by a rapid transgression and subsequent periods of RSL stability. The first transgression coincides temporally with a late stage of Meltwater Pulse 1a and the RSL stability occurred between c. 13.0 and c. 12.2k cal a bp (Younger Dryas). The second still/slowstand occurred between c. 10.3 and c. 11.5k cal a bp . Our data provide constraints on the direction and timing of RSL change during deglaciation. Application of the Depth of Closure concept adds an error term to sea‐level reconstructions based on seismic stratigraphic reconstructions.     

AMS 14C dating of deglacial events in the Irish Sea Basin and other sectors of the British–Irish ice sheet

Sedimentary sequences deposited by the decaying marine margin of the British–Irish Ice Sheet (BIIS) record isostatic depression and successive ice sheet retreat towards centres of ice dispersion. Radiocarbon dating by accelerator mass spectrometry (AMS) of in situ marine microfaunas that are commonly associated with these sequences constrain the timing of glacial and sea level fluctuations during the last deglaciation, enabling us to evaluate the dynamics of the BIIS and its response to North Atlantic climate change. Here we use our radiocarbon-dated stratigraphy to define six major glacial and sea level events since the Last Glacial Maximum. (1) Initial deglaciation may have occurred ⩾18.3 kyr ¹⁴C BP along the northwestern Irish coast, in agreement with a deglacial age of ∼22 ³⁶Cl kyr BP for southwestern Ireland. Ice retreated to inland centres and areas of transverse moraine began to form across the north Irish lowlands. (2) Channels cut into glaciomarine deglacial sediments along the western Irish Sea coast are graded to below present sea level, identifying a fall of relative sea level (RSL) in response to isostatic emergence of the coast. (3) Marine mud that rapidly infilled these channels records an abrupt rise in global sea level of 10–15 m ∼16.7 ¹⁴C kyr BP that flooded the Irish Sea coast and may have triggered deglaciation of a marine-based margin in Donegal Bay. (4) Intertidal boulder pavements in Dundalk Bay indicate that RSL ∼15.0 ¹⁴C kyr BP was similar to present. (5) A major readvance of all sectors of the BIIS occurred between 14 and 15 kyr ¹⁴C BP which overprinted subglacial transverse moraines and delivered a substantial sediment flux to tidewater ice sheet margins. This event, the Killard Point Stadial, indicates that the BIIS participated in Heinrich event 1. (6) Subsequent deposition of marine muds on drumlins 12.7 ¹⁴C kyr BP indicates isostatic depression and attendant high RSL resulting from the Killard Point readvance. These events identify a dynamic BIIS during the last deglaciation, as well as significant changes in RSL that reflect a combination of isostatic loading and eustatic changes in global sea level.     

A new Holocene relative sea‐level curve for Terra Nova Bay,Victoria Land,Antarctica

More than 100 radiocarbon dates of penguin guano and remains, shells and seal skin afford ages for raised beaches adjacent to Terra Nova Bay, Antarctica. These dates permit construction of a new relative sea‐level curve that bears on the timing of deglaciation. Recession of the Ross Sea ice‐sheet grounding line from Terra Nova Bay occurred no earlier than 7200 ¹⁴C yr (8000 cal. yr) BP. Retreat along the Victoria Land coast may have been rapid, possibly contributing to eustatic sea‐level rise centred at ca. 7600 cal. yr BP. The presence of a significant amount of ice remaining in the Ross Sea Embayment in Holocene time lessens the chance that Antarctica contributed significantly to meltwater pulse 1A several thousand years earlier. Copyright © 2004 John Wiley & Sons, Ltd.     

On the reconstruction of pleistocene ice sheets: A review

Pleistocene ice sheets can be reconstructed through three separate approaches: (1) Evidence based on glacial geological studies, such as erratic trains, till composition, crossing striations and exposures of multiple tills/nonglacial sediments. (2) Reconstructions based on glaciological theory and observations. These can be either two- or three-dimensional models; they can be constrained by ‘known’ ice margins at specific times; or they can be ‘open-ended’ with the history of growth and retreat controlled by parameters resting entirely within the model. (3) Glacial isostatic rebound after deglaciation provides a measure of the distribution of mass (ice) across a region. A ‘best fit’ ice sheet model can be developed that closely approximates a series of relative sea level curves within an area of a former ice sheet; in addition, the model should also provide a reasonable sea level fit to relative sea level curves at sites well removed from glaciation.This paper reviews some of the results of a variety of ice sheet reconstructions and concentrates on the various attempts to reconstruct the ice sheets of the last (Wisconsin, Weischelian, Würm, Devensian) glaciation. Evidence from glacial geology suggests flow patterns at variance with simple, single-domed ice sheets over North America and Europe. In addition, reconstruction of ice sheets from glacial isostatic sea level data suggests that the ice sheets were significantly thinner than estimates based on 18 ka equilibrium ice sheets (cf. Denton and Hughes, 1981). The review indicates it is important to differentiate between ice divides, which control the directions of glacial flow, and areas of maximum ice thickness, which control the glacial isostatic rebound of the crust upon deglaciation. Recent studies from the Laurentide Ice Sheet region indicate that the center of mass was not over Hudson Bay; that a major ice divide lay east of Hudson Bay so that flow across the Hudson Bay and James Bay lowlands was from the northeast; that Hudson Bay was probably open to marine invasions two or three times during the Wisconsin Glaciation; and that the Laurentide Ice Sheet was thinner than an equilibrium reconstruction would suggest.     

Late quaternary sea‐level changes,crustal movements and coastal evolution in Northumberland,UK

Northumberland lies in the transition between Holocene emergence and submergence and is thus a critical zone for testing models of isostatic rebound. We have collected data from this area to reconstruct relative sea‐level changes and lateral coastline movements for the last 14000 y. These are deposits from tidal marsh, back‐barrier wetland and terrestrial environments producing 47 sea‐level index points from 12 sites. There is no unequivocal evidence for Late Devensian sea levels above present and the reliable sea‐level index points are restricted between −6 m and +2.5 m relative to present and 9.0–2.5 kyr cal. BP. Analysis of these quantifies differential responses to glacio‐ and hydroisostatic rebound, with the northern sites recording a mid‐Holocene sea‐level maximum ca. 2.5 m above present, whereas the southern sites show a maximum ca. 0.5 m above present. These observations show a reasonable fit with the predictions from quantitative models of glacio and hydroisostatic rebound, but there is currently no unique solution of Earth and ice model parameters that will explain all the sea‐level observations. Copyright © 2000 John Wiley & Sons, Ltd.     

Relative sea‐level observations in western Scotland since the Last Glacial Maximum for testing models of glacial isostatic land movements and ice‐sheet reconstructions

Observations of relative sea‐level change and local deglaciation in western Scotland provide critical constraints for modelling glacio‐isostatic rebound in northern Britain over the last 18 000 years. The longest records come from Skye, Arisaig and Knapdale with a shorter, Holocene, record from Kintail. Biostratigraphic (diatom, pollen, dinoflagellate, foraminifera and thecamoebian), lithological and radiocarbon analyses provide age and elevation parameters for each sea‐level index point. All four sites reveal relative sea‐level change that is highly non‐monotonic in time as the local vertical component of glacio‐isostatic rebound and eustasy (or global meltwater influx) dominate at different periods. Copyright © 2006 John Wiley & Sons, Ltd.     

Glacial rebound and sea-level change in the British Isles

Observations of sea levels around the coastline of the British Isles for the past 10,000–15,000 years exhibit a major regional variation and provide an important data base for testing models of glacial rebound as well as models of the Late Devensian ice sheet. A high-resolution rebound model has been developed which is consistent with both the spatial and temporal patterns of sea-level change and which demonstrates that the observations are the result of (i) the glacio-isostatic crustal rebound in response to the unloading of the ice sheet over Britain and, to a lesser degree, of the ice sheet over Fennoscandia, and (ii) the rise in sea-level from the melting Late Pleistocene ice sheets, including the response of the crust to the water loading (the hydro-isostatic effect). The agreement between model and observations is such that there is no need to invoke vertical crustal movements for Great Britain and Ireland of other than glacio-hydro-isostatic origin. The rebound contributions are important throughout the region and nowhere is it sufficiently small for the sea-level change to approximate the eustatic sea-level rise. The observational data distribution around the periphery as well as from sites near the centre of the former ice sheet is sufficient to permit constraints to be established on both earth model parameters specifying the mantle viscosity and lithospheric thickness and the extent and volume of the ice sheet at the time of the last glaciation. Preliminary solutions are presented which indicate an upper mantle viscosity of (3–5)10²⁰ Pas, a lithospheric thickness of about 100 km or less, and an ice model that was not confluent with the Scandinavian ice sheet during the last glaciation and whose maximum thickness over Scotland is unlikely to have exceeded about 1500 m.     

Late Holocene Sea Level Reconstructions Based on Observations of Roman Fish Tanks,Tyrrhenian Coast of Italy

We present estimates for late Holocene relative sea level change along the Tyrrhenian coast of Italy based on morphological characteristics of eight submerged Roman fish tanks (piscinae) constructed between the 1st century B.C. and the 2nd century A.D. Underwater geomorphological features and archaeological remains related to past sea level have been measured and corrected using recorded tidal values. We conclude that local sea level during the Roman period did not exceed 58 ± 5 cm below the present sea level. These results broadly agree with previous observations in the region but contrast with recent analysis that suggests a significantly larger sea level rise during the last 2000 years. Using a glacial isostatic adjustment model, we explain how regional sea level change departs from the eustatic component. Our calculation of relative sea level during the Roman period provides a reference for isolating the long‐wavelength contribution to sea level change from secular sea level rise. Precise determination of sea level rise in the study area improves our understanding of secular, instrumentally observed, variations across the Mediterranean. © 2012 Wiley Periodicals, Inc.