Deglaciation Model Research Articles

Construction of an evolutionary deglaciation model for the Irish midlands based on the integration of morphostratigraphic and geophysical data analyses

AbstractAlternative, established models for the deglaciation of the midlands of Ireland are tested against an interpretation of a suite of deglacial sediments covering an area of 600 km2 in the east central midland area. Interpretation of the sediments is based on geomorphological mapping, lithostratigraphic characterization of exposures and geotechnical data supported by electrical resistivity tomography (ERT) and ground penetrating radar (GPR). GPR depicted small‐scale sedimentological and deformational structures within low‐conductivity soft sediments, such as cross‐bedding, planar bedding, channel‐like features and faulting planes, and revealed the internal architecture of eskers, glaciodeltas, subaqueous fans and raised bogs. ERT data permitted the detection of depth to bedrock and the lithological characterization of unconsolidated sediments. The ten sites presented were surveyed by traditional mapping methods and/or geophysical techniques. This allowed the construction of a local model of the deglaciation of the area which recognized five main stages. An ice sheet covering most of Ireland withdrew as a single body as far as the midlands. At this stage, two main directions of ice retreat are identified from the spatial distribution of meltwater/overflow channels, esker and morainic ridges, and ice‐marginal glaciolacustrine deposits. A pattern of deglacial sedimentation into an expanding ice‐marginal glacial lake is depicted. The glacial lake was dammed to the west by two ice dome fronts, one decaying to the north‐west and another to the south‐west, and by the Shannon Basin watershed to the east. Glacial lake outlets identified along the watershed and the altitude of the topset/foreset interface zone depicted in glaciodeltaic deposits allowed the identification of three lake water levels. The highest level is at 87–89 m Ordnance Datum (OD), the second lake level at 84 m OD and the third at 78 m OD. Copyright © 2012 John Wiley & Sons, Ltd.

A new glacial isostatic adjustment model for Antarctica: calibrated and tested using observations of relative sea-level change and present-day uplift rates

We present a glacial isostatic adjustment (GIA) model for Antarctica. This is driven by a new deglaciation history that has been developed using a numerical ice-sheet model, and is constrained to fit observations of past ice extent. We test the sensitivity of the GIA model to uncertainties in the deglaciation history, and seek earth model parameters that minimize the misfit of model predictions to relative sea-level observations from Antarctica. We find that the relative sea-level predictions are fairly insensitive to changes in lithospheric thickness and lower mantle viscosity, but show high sensitivity to changes in upper mantle viscosity and constrain this value (95 per cent confidence) to lie in the range 0.8–2.0 × 1021 Pa s. Significant misfits at several sites may be due to errors in the deglaciation history, or unmodelled effects of lateral variations in Earth structure. When we compare our GIA model predictions with elastic corrected GPS uplift rates we find that the predicted rates are biased high (weighted mean bias = 1.8mm yr–1) and there is a weighted root-mean-square (WRMS) error of 2.9mm yr–1. In particular, our model systematically over-predicts uplift rates in the Antarctica Peninsula, and we attempt to address this by adjusting the Late Holocene loading history in this region, within the bounds of uncertainty of the deglaciation model. Using this adjusted model the weighted mean bias improves from 1.8 to 1.2mm yr–1, and the WRMS error is reduced to 2.3mm yr–1, compared with 4.9mm yr–1 for ICE-5G v1.2 and 5.0mm yr–1 for IJ05. Finally, we place spatially variable error bars on our GIA uplift rate predictions, taking into account uncertainties in both the deglaciation history and modelled Earth viscosity structure. This work provides a new GIA correction for the GRACE data in Antarctica, thus permitting more accurate constraints to be placed on current ice-mass change.

MODES OF SEDIMENTATION AND GLACIOLOGICAL ASPECTS OF THE PERMO-CARBONIFEROUS DWYKA GROUP IN THE ELANDSVLEI - ELANDSDRIF AREA, SOUTH AFRICA

Research Article| June 01, 2012 MODES OF SEDIMENTATION AND GLACIOLOGICAL ASPECTS OF THE PERMO-CARBONIFEROUS DWYKA GROUP IN THE ELANDSVLEI – ELANDSDRIF AREA, SOUTH AFRICA H.J. BLIGNAULT; H.J. BLIGNAULT Consulting Geologist, 3 Hof Avenue, Stellenbosch, South Africa, e-mail: hennieb@iafrica.com Search for other works by this author on: GSW Google Scholar J.N. THERON J.N. THERON Department of Earth Sciences, University of Stellenbosch, Private Bag X1, Matieland 7602, South Africa, e-mail: jntfynbo@iafrica.com Search for other works by this author on: GSW Google Scholar South African Journal of Geology (2012) 115 (2): 211–224. https://doi.org/10.2113/gssajg.115.2.211 Article history first online: 09 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation H.J. BLIGNAULT, J.N. THERON; MODES OF SEDIMENTATION AND GLACIOLOGICAL ASPECTS OF THE PERMO-CARBONIFEROUS DWYKA GROUP IN THE ELANDSVLEI – ELANDSDRIF AREA, SOUTH AFRICA. South African Journal of Geology 2012;; 115 (2): 211–224. doi: https://doi.org/10.2113/gssajg.115.2.211 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySouth African Journal of Geology Search Advanced Search Abstract A field investigation resulted in a detailed stratigraphic profile of the full sequence of the Permo-Carboniferous, glaciomarine Dwyka Group in the Elandsvlei-Elandsdrif area. Six cycles of a basal tillite unit capped by a terminal zone of stratified sediments, are documented. Of note is the recurrence in the tillite units of the association diamictite-boulder beds-eskers. This facies association is interpreted as a subglacial deformation tillite, with the boulder beds as products of the deforming-bed process. The sediments of the terminal zones indicate a sub-ice shelf environment. Nine advance/retreat fluctuations are interpreted from the facies associations. The immature eskers and lack of sorted sediments point to a low level of melt water supply, pointing to a polar ice sheet. Westward ice-flow is supported by macrofabric data, basal and boulder pavements. An analysis of the boulder lithologies shows that the source area stayed the same throughout the glacial period. It is proposed that part of the onset area due east is an extrusive-intrusive complex with a chert-carbonate-banded iron formation component. This investigation supports a series of glacial advance/retreat cycles rather than the previous model of deglaciation. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Lateglacial surface exposure dating in the Monadhliath Mountains, Central Highlands, Scotland

We constrain the deglaciation history of the Monadhliath Mountains since the Last Glacial Maximum using glacial geomorphology and surface exposure dating. Collectively these data clarify the glacial history of the Monadhliath Mountains at the close of the Devensian cold period. 10Be exposure ages indicate deglaciation of the Last Devensian ice sheet at 15.1 ± 1.4 ka at ca 640 m OD. The exposure ages are consistent with basal radiocarbon ages from nearby Loch Etteridge, 5 km to the south-east (15.3 ± 0.2 kcal BP11ka is used for ages in this study, kcal BP for calibrated ages and 14C kyrs BP for uncalibrated ages 1.). Boulders from moraines in three Monadhliath corries yielded exposure ages between 11.8 ka and 9.8 ka (470–600 m OD), suggesting that a Lateglacial advance occurred during the Younger Dryas stadial. The limited extent of these Younger Dryas glaciers in the Monadhliath Mountains results from the combined effect of a drier climate experienced in the eastern part of the Central Highlands ice cap and topographical factors. Our reconstruction largely supports the deglaciation model of Golledge et al. (2008) and confirms that a SW to NE precipitation gradient dominated Scotland during the Younger Dryas.

Greenland uplift and regional sea level changes from ICESat observations and GIA modelling

We study the implications of a recently published mass balance of the Greenland ice sheet (GrIS), derived from repeated surface elevation measurements from NASA’s ice cloud and land elevation satellite (ICESat) for the time period between 2003 and 2008. To characterize the effects of this new, high-resolution GrIS mass balance, we study the time-variations of various geophysical quantities in response to the current mass loss. They include vertical uplift and subsidence, geoid height variations, global patterns of sea level change (or fingerprints), and regional sea level variations along the coasts of Greenland. Long-wavelength uplifts and gravity variations in response to current or past ice thickness variations are obtained solving the sea level equation, which accounts for both the elastic and the viscoelastic components of deformation. To capture the short-wavelength components of vertical uplift in response to current ice mass loss, which is not resolved by satellite gravity observations, we have specifically developed a high-resolution regional elastic rebound (ER) model. The elastic component of vertical uplift is combined with estimates of the viscoelastic displacement fields associated with the process of glacial-isostatic adjustment (GIA), according to a set of published ice chronologies and associated mantle rheological profiles. We compare the sensitivity of global positioning system (GPS) observations along the coasts of Greenland to the ongoing ER and GIA. In notable contrast with past reports, we show that vertical velocities obtained by GPS data from five stations with sufficiently long records and from one tide gauge at the GrIS margins can be reconciled with model predictions based on the ICE-5G deglaciation model and the ER associated with the new ICESat-derived mass balance.

Sea level change along the Italian coast during the Holocene and projections for the future

Published and new sea level data are used to provide projections of sea level change in Italy for the year 2100 by adding new isostatic and tectonic component to the IPCC and Rahmstorf projections. Comparison of the observations from more than 130 sites (with different geomorphological and archaeological sea level markers) with the predicted sea level curves provides estimates of the vertical tectonic contribution to the relative sea level change. The results are based on the most recent ANU model for the ice sheets of both hemispheres, including an alpine deglaciation model. On the basis of the eustatic, tectonic and isostatic components to the sea level change, projections are provided for marine inundation scenarios for the Italian coastal plains for the year 2100, that today are at elevations close to current sea level.

Clearing observed PGR in GRACE data aimed at global viscosity inversion: Weighted Mass Trends technique

[1] The time-variable gravity field shows the effects of large present-day mass variations associated with hydrological phenomena and melting ice-sheets, resulting in a shaded Post Glacial Rebound (PGR) signal. A meaningful viscosity inversion based on a global scale comparison of GRACE data and PGR predictions is thus hard to obtain. We derive a weighted surface mass distribution in water equivalent, starting from an initial guess, which portrays the secular effects of present-day phenomena. The gravity field it generates is then carefully removed from GRACE data, resulting in a gravity pattern where the effects of PGR are clearer and ready to be compared with the predictions. On the basis of viscoelastic stratified Earth models and different Pleistocene deglaciation models, we show that the quality of a global preliminary viscosity inversion greatly improves.

Termination of the Last Glacial Maximum sea-level lowstand: The Sunda-Shelf data revisited

The sea-level history around the last glaciation is in the focus of recent, controversial debates. A profound understanding of sea-level changes during this time interval is, however, essential since sea level is a central parameter in the climate system as well as a major force on continental margin sedimentation. Here, we present a seismic record together with augmented data from the Sunda Shelf [Hanebuth, T.J.J., Stattegger, K., Saito, Y., 2002. The architecture of the central Sunda Shelf (SE Asia) recorded by shallow-seismic surveying. Geo-Marine Letters 22, 86–94.] and compare our results in a careful evaluation with the sparse existing data sets of global validity, i.e. the Bonaparte Gulf record [Yokoyama, Y., Lambeck, K., DeDeckker, P., Johnston, P., Fifield, L.K., 2000. Timing of the Last Glacial Maximum from observed sea-level minima. Nature 406, 713–716.; Yokoyama, Y., De Deckker, P., Lambeck, K., Johnston, P., Fifield, L.K., 2001. Sea-level at the Last Glacial Maximum: evidence from nortwestern Australia to constrain ice volumes for oxygen isotope stage 2. Paleogeography Paleoclimatology Paleoecology 165, 281–297.], the Barbados coral record [Fairbanks, R.G., 1989. A 17,000-year glacio-eustatic sea level record: influence of glacial melting dates on the Younger Dryas event and deep ocean circulation. Nature 342, 637–642.; Peltier, W.R., Fairbanks, R.G., 2006. Global glacial ice volume and Last Glacial Maximum duration from an extended Barbados sea level record. Quaternary Science Reviews 25 (23–24), 3322–3337.] and the latest numerical model of continental deglaciation [Peltier, W.R., Fairbanks, R.G., 2006. Global glacial ice volume and Last Glacial Maximum duration from an extended Barbados sea level record. Quaternary Science Reviews 25 (23–24), 3322–3337.]. Sea level seems to have been lower shortly prior to the conventional Last Glacial Maximum (LGM; 21–19 cal kyr BP). The time interval around this glacial lowstand is not covered by ages from the Sunda Shelf, but documented by an ancient barrier — tidal-flat system. These palaeo-coastal relict forms indicate such an early lowstand some 5 m deeper than sea level was during LGM times. The LGM sea level on the Sunda shelf is recalculated to − 123 ± 2 m modern water depth. This depth fits nicely with the lowstand data derived from Barbados and the Bonaparte Gulf. The recently assumed 19-kyr sea-level rise is supported by the Sunda and Bonaparte data sets combined, although it might have started already as early as at 19.6 cal kyr BP lasting for some 800 kyr with an amplitude of at least 10 m. This early pulse-like rise might have played a crucial role in the physical preservation of the high-glacial to early deglacial deposits on the Sunda Shelf. The modelled sea-level history is, thus, supported with respect to an initial high-glacial lowstand prior to the LGM, which might be in apparent contrast to observations from Bonaparte. Nevertheless, field data suggest a glacial sea-level evolution about 10 m deeper than the model. Also, the gradual rising trend from 26 to 16 cal kyr BP, as deduced from the model, can definitively not be approved by any field data. However, our knowledge is still unsatisfactory and an expansion of field data from suited areas is urgently needed.

Using postglacial sea level, crustal velocities and gravity-rate-of-change to constrain the influence of thermal effects on mantle lateral heterogeneities

Lateral heterogeneities in the mantle can be caused by thermal, chemical and non-isotropic pre-stress effects. Here, we investigate the possibility of using observations of the glacial isostatic adjustment (GIA) process to constrain the thermal contribution to lateral variations in mantle viscosity. In particular, global historic relative sea level, GPS in Laurentide and Fennoscandia, altimetry together with tide-gauge data in the Great Lakes area, and GRACE data in Laurentide are used. The lateral viscosity perturbations are inferred from the seismic tomography model S20A by inserting the scaling factor β to determine the contribution of thermal effects versus compositional heterogeneity and non-isotropic pre-stress effects on lateral heterogeneity in mantle viscosity. When β = 1, lateral velocity variations are caused by thermal effects alone. With β < 1, the contribution of thermal effect decreases, so that for β = 0, there is no lateral viscosity variation and the Earth is laterally homogeneous. These lateral viscosity variations are superposed on four different reference models which differ significantly in the lower mantle viscosity. The Coupled Laplace Finite Element method is used to predict the GIA response on a spherical, self-gravitating, compressible, viscoelastic Earth with self-gravitating oceans, induced by the ICE-4G deglaciation model. Results show that the effect of β on uplift rates and gravity rate-of-change is not simple and involves the trade-off between the contribution of lateral viscosity variations in the transition zone and in the lower mantle. Models with small viscosity contrast in the lower mantle cannot explain the observed uplift rates in Laurentide and Fennoscandia. However, the RF3S20 model with a reference viscosity profile simplified from Peltier's VM2 with the value of β around 0.2–0.4 is found to explain most of the global RSL data, the uplift rates in Laurentide and Fennoscandia and the BIFROST horizontal velocity data. In addition, the changes in GIA signals caused by changes in the value of β are large enough to be detected by the data, although uncertainty in other parameters in the GIA models still exists. This may encourage us to further utilize GIA observations to constrain the thermal effect on mantle lateral heterogeneity as geodetic and satellite gravity measurements are improved.

Geodetic measurements of postglacial adjustments in Greenland

We analyze data from seven continuous Global Positioning System (GPS) receivers and one tide gauge, all located along the edge of the Greenland ice sheet, to determine vertical uplift rates. We compare our results with predictions based on the ICE‐5G deglaciation model of Peltier (2004). Results from the GPS receiver at Kellyville (−1.2 ± 1.1 mm/a) and from the tide gauge at Nuuk (−2.2 ± 1.3 mm/a), indicate that ICE‐5G overestimates the subsidence rates at those locations by 2.1 and 1.1 mm/a, respectively. Kellyville and Nuuk are located along the southwestern margin of the Greenland ice sheet, and the observed negative uplift rates are consistent with independent evidence that the ice margin along the southwestern edge readvanced during the last ∼8 ka to its current position. The ICE‐5G glaciation‐deglaciation history includes a readvance between the latitudes of 62°N and 72°N. The GPS measurements suggest the ICE‐5G readvance may be too large or mistimed. Our GPS results at Qaqortoq, located at the southern tip of Greenland, suggest a secular subsidence rate of −0.3 ± 1.1 mm/a, while ICE‐5G predicts an uplift rate of 1.0 mm/a. ICE‐5G assumes no ice sheet readvance in south Greenland, including no readvance of the Qassimiut lobe. The difference of 1.3 ± 1.1 mm/a can tentatively be explained as due to a ∼33 km readvance of the Qassimiut lobe during the last ∼3 ka. For the other GPS sites, the observed/predicted uplift rates are 3.6 ± 1.1/−0.1 mm/a at Thule, 0.0 ± 1.1/2.0 mm/a at Scoresbysund, and −0.4 ± 1.1/−1.7 mm/a at Kulusuk. For Thule, Kulusuk, and Scoresbysund the differences between the observed and predicted rates are on the order of 1.3–3.7 mm/a, though with opposite signs, and indicate that ICE‐5G does not exactly reproduce the correct rebound signal at those locations.

Annually laminated Late Pleistocene sediments from Llangorse Lake, South Wales, UK: a chronology for the pattern of ice wastage

This paper presents the results of microfacies analysis of proglacial lake sediments from the Llangorse basin in South Wales. It is the first detailed micromorphological investigation of mineral-laminated lake sediments in the UK. Four microfacies of the mineral-laminated sequence are identified and subsequently compared to modern and palaeo-datasets. The laminated sediments are interpreted as glaciolacustrine varves, thereby validating previous interpretations. The total number of varves in the core indicates that Glacial Lake Llangorse existed for a minimum of 574 years. This sequence cannot be ‘fixed’ to a specific age other than the latter part of the Dimlington Stadial. The varve thickness decreases through the core, suggesting that the ice margin, which initially impounded the lake, became progressively more distal to the locus of sedimentation. These data, when related to Lewis' model of deglaciation in the area, imply that the ice margin retreated at an average rate of 5.2 m a −1 .

Isolating the PGR signal in the GRACE data: impact on mass balance estimates in Antarctica and Greenland

SUMMARY Redistribution of mass over the Earth and within the mantle changes the gravity field whose variations are monitored at high spatial resolution by the presently flying GRACE space gravity mission from NASA or, at longer wavelengths, by the Satellite Laser Ranging (SLR) constellation. In principle, GRACE data allow one to study the time evolution of various Earth phenomena through their gravitational effects. The correct identification of the gravitational spatial and temporal fingerprints of the individual hydrologic, atmospheric, oceanographic and solid Earth phenomena is thus extremely important, but also not trivial. In particular, it has been widely recognized that the gravitational estimates of present-day ice mass loss in Greenland and Antarctica, and the related effect on sea level changes, depend on an accurate determination of the Postglacial Rebound (PGR) after Pleistocene deglaciation, which in turn depends on the assumed solid Earth parameters and deglaciation model. Here we investigate the effect of the uncertainty of the solid Earth parameters (viscosity, litospheric thickness) and of different deglaciation processes on PGR in Greenland and Antarctica. We find that realistic constraints to the trend in ice mass loss derived from GRACE data determine a range of variation substantially wider than commonly stated, ranging from an important ice loss of −209 Gt yr −1 to an accumulation of +88 Gt yr −1 in Antarctica, and Greenland ablation at a rate between −122 and −50 Gt yr −1 . However, if we adopt the set of most probable Earth parameters, we infer a substantial mass loss in both regions, −171 ± 39 and −101 ± 22 Gt yr −1 for Antarctica and Greenland, respectively.

Preliminary investigations of outwash environments in the Tyne Valley: implications for Late Devensian (Dimlington Stadial) deglaciation

Whilst recent detailed sedimentological investigations have been carried out on deglacial valley fill sediments of Late Pleistocene age in many UK river valleys, there is a paucity of high-resolution work on the deglacial history of the Tyne Valley. Combined with a poorly resolved chronology, this represents an appreciable gap in our understanding of Devensian palaeoenvironments in northeastern Britain. Confusion still exists in terms of understanding the mode of deglaciation and the resulting depositional landforms that characterize the present-day morphology of the valley. This paper reconstructs part of the deglacial history of the lower Tyne Valley using information gathered through a combination of geomorphological mapping and description and interpretation of sedimentary sequences. The evidence indicates the sediments are glaciofluvial in origin, deposited within a supra-glacial, rather than sub-glacial environment. Further work currently underway, including the development of a rigorous geochronology based on optically stimulated luminescence (OSL) will help to place these deposits within a secure, regional model of deglaciation, which can be compared with other models developed for adjacent areas of northern Britain.

Surging ice and break-down of an ice dome – a deglaciation model for the Gulf of Bothnia

Based on studies of striae in the Kvarken area in the northern part of the Baltic basin, and re-interpretation of stratigraphic sequences earlier interpreted as evidence of a readvance of the ice margin in the Gulf of Bothnia a model for the deglaciation of the Gulf is proposed. The model implies quick break-up of the ice due to areal thinning of an ice lobe in the Gulf and change from cold-based to warm-based ice. Wide calving bays were formed close to and into the centre of the ice in the northern part of the Gulf. The change of balance caused sudden collapses in the centre followed by surges upon the warming base of soft ductile sediments. The effect was a westward shift of the ice centre from the Gulf area towards the mountain range. The ice sheet changed from a thick one with a subaquatic margin into a thin, terrestrial one, thus becoming more sensitive to the rising temperature at the end of the Weichselian glaciation.

Effects of lateral variations in lithospheric thickness and mantle viscosity on glacially induced relative sea levels and long wavelength gravity field in a spherical, self-gravitating Maxwell Earth

The effects of lateral variations in lithospheric thickness and mantle viscosity on glacially induced relative sea level (RSL) changes and the secular rate of change of the Earth's long wavelength gravity field in a spherical, self-gravitating, incompressible visco-elastic earth are investigated using the Coupled-Laplace-Finite-Element method. The ICE-4G deglaciation model is used with gravitationally self-consistent sea levels in realistic oceans to describe the load. Lateral variations in mantle viscosity and lithospheric thickness are inferred from seismic tomography model S20A. The full 3-D earth model, which includes all the lateral heterogeneities in the lithosphere and mantle, gives a better fit to the global RSL data than the related laterally homogeneous model. However, the situation is less clear for the observed secular drift of the low degree geopotential coefficients J˙ l because of the uncertain contribution of recent melting. But, the full 3-D model can fit the J˙ 2 observation if recent melting contributes about 1.0 mm/a of equivalent sea level rise. It predicts that the GIA induced secular gravity rate of change to be detected by the GRACE mission in the southern part of Hudson Bay is about 1.2 to 1.6 μgal/a. Moreover, the contributions of lateral heterogeneities from individual layers in the mantle or in the lithosphere are studied. The contribution from the transition zone (420–670 km) is generally opposite to that from its neighboring layers and thus can mask their effects. As a consequence, the effects from the deep lower mantle become dominant for RSL and secular rate of change of gravity over Laurentide. For the secular rates of change for degrees 2–4 geopotential coefficients, the contribution is mostly from lateral heterogeneities in the deeper mantle. The effects of background viscosity profiles are also investigated and are found to be significant for all these observables.

Global glacial ice volume and Last Glacial Maximum duration from an extended Barbados sea level record

Fundamental characteristics of the climate system during the most recent precessional cycle of the Earth's orbit around the Sun consist of the final expansion of land ice to its maximum extent, the subsequent episode of deglaciation, and the variations of global sea level that accompanied these events. In order to address the important issue of the variation of continental ice volume and related changes in global sea level through the late glacial period, we employ an extended set of observations of the pre-glacial and postglacial history of sea-level rise at the island of Barbados, together with a refined model of continental deglaciation and an accurate methodology for the prediction of postglacial sea-level change. Although our results provide unambiguous evidence that the post LGM rise of eustatic sea-level was very close to the widely supported estimate of 120 m, the data also provide evidence that LGM must have occurred 26,000 years ago, approximately 5000 yr earlier than the usually assumed age.

Role of background viscosity in the investigation of postglacial rebound induced crustal motion in a laterally heterogeneous mantle

The effect of laterally homogeneous background viscosity profile on postglacial rebound induced surface motion on a laterally heterogeneous earth is investigated. The lateral viscosity perturbations are inferred from seismic tomography model S20A. Three different background viscosity models are investigated—they differ in lithospheric thickness and the viscosity jump across the 670 km discontinuity. The coupled Laplace-finite-element method is used to calculate present-day crustal motion on a spherical, self-gravitating, incompressible, viscoelastic earth with self-gravitating oceans, induced by the ICE-4G deglaciation model. Results show that background viscosity significantly affects the magnitude of the uplift rate, and both direction and magnitude of the tangential velocity. Thus a good handle on the background viscosity is very important in the study and interpretation of glacial isostatic data.

Effects of lateral variations in lithospheric thickness and mantle viscosity on glacially induced surface motion on a spherical, self-gravitating Maxwell Earth

The effects of lateral variations in lithospheric thickness and mantle viscosity on surface motions on a spherical, self-gravitating, incompressible viscoelastic earth are investigated using the Coupled Laplace–Finite Element Method. The ICE-4G deglaciation model is used together with gravitationally self-consistent sea levels in realistic oceans to describe the load. Lateral variations in mantle viscosities and lithospheric thickness are inferred from seismic tomography model S20A using well known scaling relationships. The laterally homogeneous background viscosity has viscosity increases across 670 km and 1330 km depths and is a simplification of Peltier's VM2 model. The effect of interaction between ice complexes in North America, Greenland, Eurasia and Antarctica are investigated and found to be significant for tangential motion. The predicted motion in Laurentide and Antarctica are also compared with results obtained with flat-earth models and found that there is general agreement between the two types of modeling although spherical earth models are more realistic because they take into account the interaction between ice complexes and there is no problem with edge effects. The effects of lateral heterogeneity on uplift rate are found to be large enough to be resolved by current GPS technology. It confirms that the effect of the reverse viscosity contrast in the transition zone under the Laurentian and Fennoscandian craton may mask the effect of lateral heterogeneity in the neighboring layers on uplift rate. For tangential motion, lateral heterogeneity is found to be able to overprint the divergent motion from the centers of rebound predicted in laterally homogeneous models. This finding is new for Laurentide and is due to the contribution from the deep part of the lower mantle – although the contribution from the upper mantle is also important on tangential motion, its effects are compromised by the effects from the other layers.

Effects of lateral variations in lithospheric thickness and mantle viscosity on glacially induced surface motion in Laurentia

The effects of lateral variations in lithospheric thickness and mantle viscosity on surface motions in Laurentia are investigated using the Finite Element method. Both a simple axisymmetric ice model and a more realistic ice deglaciation model (ICE4G) are used to investigate the effects of lateral heterogeneity. Beside the laterally homogeneous reference model, six laterally heterogeneous models are used. The lateral heterogeneity in the lithosphere, upper mantle, transition zone and lower mantle are inferred from seismic tomographic model S20A. In general, the magnitude of the effect of lateral heterogeneity is found to depend on the deglaciation history and the relative position between the ice center and the lateral heterogeneity. It is shown that lateral heterogeneity does affect horizontal motion in Laurentia, however because the effects of lateral heterogeneity from different depths give different orientations to the incremental velocity vector, the total effect is not large enough to overwhelm the divergent motion — but just weaken it in southern Laurentia. For land uplift rate, the effect of lateral viscosity variation in the transition zone between 425 and 700 km depth is opposite to that of the upper or lower mantle — thus the net effect of lateral heterogeneity on uplift rate is not very large — but enough to change the shape of the uplift pattern.

Quasi-continuous global positioning system measurements of glacial isostatic deformation in the Northern Transantarctic Mountains

Global positioning system (GPS) measurements have been collected quasi-continuously between November 1996 and January 2001 at two autonomous GPS stations in the Northern Transantarctic Mountains. High-quality data from the two sites at Mt. Coates in the Dry Valleys and Mt. Cocks in the Royal Society Range have resolved significant horizontal motions. The vertical rate at Mt. Coates indicates an uplift of 4.5±2.3 mm/year, most likely due to glacial isostatic motion. Uplift at Mt. Coates deviates significantly from uplift predictions based on deglaciation models ICE-3G and ICE-4G, but is consistent with a model (D91-1.5) in which deglaciation persists up to the late Holocene.