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Kurt Lambeck - One of the best experts on this subject based on the ideXlab platform.

  • The Scandinavian Ice Sheet: from MIS 4 to the end of the Last Glacial Maximum
    Boreas, 2010
    Co-Authors: Kurt Lambeck, Anthony W Purcell, J. Zhao, Nils-olof Svensson
    Abstract:

    Glacial Rebound modelling, to establish constraints on past ice sheets from the observational evidence of palaeo-shoreline elevations, is well established for the post- Last Glacial Maximum (post-LGM) period, for which the observational evidence is relatively abundant and well distributed spatially and in time. This is particularly the case for Scandinavia. For the earlier part of the Glacial cycle this evidence becomes increasingly sparse and uncertain such that, with the exception of the Eemian period, there are very few, if any, direct sea-level indicators that constrain any part of the Scandinavian Ice Sheet evolution before the LGM. Instead, we assume that ice-sheet basal conditions during Marine Isotope Stage 3 (MIS 3) are the same as those for the LGM, focus on establishing these conditions from the Rebound analysis for the LGM and LateGlacial period, and then extrapolate to the earlier period using observationally constrained locations of the ice margins. The Glacial Rebound modelling and inversion follow previously established formulations, with the exception that the effects of water loading from proGlacial lakes that form within the Baltic Basin and elsewhere have been included. The data set for the inversion of the sea- and lake-level data has been extended to include marine-limit data in order to extend the observational record further back in time. The result is a sequence of time slices for the Scandinavian Ice Sheet from the time of MIS 4 to the LateGlacial that are characterized by frozen basal conditions until late in the LGM interval when rapid thinning occurred in the eastern and southern sectors of the ice sheet. The primary function of these models is as an interpolator between the fragmentary observational constraints and to produce quantitative models for the glaciation history with predictive capabilities, for example regarding the evolution of the Baltic Basin.

  • Glacial Rebound and crustal stress in finland
    2003
    Co-Authors: Kurt Lambeck, Anthony W Purcell
    Abstract:

    TIIVISTELMA TABLE OF CONTENTS 1 1 SUMMARY 3 2 REVIEW OF THE Glacial Rebound MODELS FOR SCANDINAVIA .. 9 3 IMPROVEMENTS IN MODELLING METHODS AND IN EXTERNAL MODEL PARAMETERS 11 4 OBSERVATIONAL DATA 13 4.1 Geological data 13 4.2 Geodetic data 14 5 AN IMPROVED Rebound MODEL FOR FENNO-SCANDINAVIA ........ 19 5.1 Earth rheology 19 5.2 Model-parameter estimation procedure 19 5.3 Results: variants of the reference model 20 5.4 Results: time-dependent scaling of the ice load 27 5.5 Results: spatially variable scaling of the ice load 33 5.6 Results: the revised ice sheet 39 5.7 Baltic Ice-Lake data: a check of model parameters 41 6 RESULTS FROM GEOLOGICAL DATA INVERSIONS 47 7 PRESENT-DAY TIDE GAUGE RECORDS: A COMPARISON OF OBSERVATIONS AND PREDICTIONS 55 7.1 Summary 58 8 CRUSTAL STRESS CONSIDERATIONS 61 8.

  • dependence of horizontal stress magnitude on load dimension in Glacial Rebound models
    Geophysical Journal International, 2002
    Co-Authors: Paul Johnston, Patrick Wu, Kurt Lambeck
    Abstract:

    Summary It has been proposed that the deglaciation of the Northern Hemisphere triggered large earthquakes within intraplate environments and in this paper we examine this hypothesis by evaluating quantitatively the stress state in the lithosphere produced by time-dependent surface loads. A series of models demonstrate the dependence of horizontal incremental stress in an elastic plate overlying an inviscid or viscoelastic mantle on the lateral extent of a load applied at the surface. The horizontal stress is largest when the dominant wavelength (that is twice the diameter) of the load is close to eight times the elastic thickness of the plate when the mantle is inviscid and, for the particular viscosity model employed in this paper, close to 12 times the elastic thickness for a viscoelastic mantle. At wavelengths close to the critical wavelength, the horizontal incremental stress may be up to six times as large as the vertical incremental stress. For appropriate earth-model parameters amplification of horizontal stress is close to maximum for ice loads with a radius of 280 km, comparable to the dimensions of the former ice sheet over Great Britain. This amplification may be sufficiently large that loading by small ice sheets can lead to failure on marginally stable faults, in contrast to the behaviour for large ice sheets. The models also predict greater fault instability for Fennoscandia than for the larger Laurentide ice sheet, consistent with the observation of large postGlacial faults in northern Sweden. The model is used to predict the stability of faults and style of faulting due to Rebound stresses in the absence of a background tectonic stress field since the last Glacial maximum (∼ 18 000 years ago) in Northern Europe. Within the formerly glaciated region thrust faulting is predicted to occur at the end of deglaciation and normal faulting is predicted to occur in peripheral regions for the entire period since the last Glacial maximum.

  • estimates of present day Glacial Rebound in the lambert glacier region antarctica
    Geophysical Research Letters, 1999
    Co-Authors: Dan Zwartz, Kurt Lambeck, Paul Johnston, Paul Tregoning, John O Stone
    Abstract:

    Changes in the ice load since the time of the Last Glacial Maximum (LGM) contribute to the present- day vertical motion of the Antarctic continent. The obser- vation of these motions will reveal information on the ice load history. Predictions of uplift rates along a transect across the Lambert Glacier region, East Antarctica, from the coast to the southernmost rock outcrops of the Prince Charles Mountains have been computed for three deglacia- tion scenarios. The relative vertical velocities between sites on the transect are -7 to +7 mm/yr and are large enough to be detected from continuous GPS observations taken at permanent sites over several years. When available, such in- formation will discriminate between the currently available models for deglaciation of East Antarctica.

  • tests of Glacial Rebound models for fennoscandinavia based on instrumented sea and lake level records
    Geophysical Journal International, 1998
    Co-Authors: Kurt Lambeck, Catherine Smither, Martin Ekman
    Abstract:

    SUMMARY Evidence for changing sea levels in northwestern Europe related to Glacial Rebound is found in both the geological record of the past millennia and in the instrumental records of the past two centuries. The latter records are of two types: records of sealevel change, primarily from the Baltic and the Gulfs of Finland and Bothnia, and records of the tilting of some of the larger lakes in both Finland and Sweden. The sealevel records are particularly important because of their long duration and high quality, their large number and good spatial distribution, and the spatially coherent background noise. The two instrumental data types are complementary and provide constraints on the upper-mantle rheology and on the distribution of ice during the late Glacial stage. Comparisons of the observed rates of change of the water levels with models for Glacial Rebound yield earth models with a lithospheric thickness of 80‐100 km and an uppermantle viscosity of (4‐5)◊1020 Pa s, eVective parameters that are consistent with those obtained from the analysis of the geological evidence for the same region. The mareograph results support ice-sheet models in which the Late Weichselian ice thickness over the eastern and southern parts of Fennoscandia is relatively thinner than that for the western region, also consistent with the interpretation of the geological evidence for sea-level change. In addition, the instrumental records provide constraints on the eustatic sea-level change for about the past 100 years. A satisfactory separation of the earth rheology parameters from this rate of change can be achieved by estimating the latter only from those records for which the predicted isostatic eVects are small. A check on these results is possible by using the lake-level records to establish constraints on the earth-model parameters and the sea-level records to constrain also the eustatic change. All approaches lead to an average eustatic sea-level rise for the past century of about 1.1±0.2 mm yr’1.

John Wahr - One of the best experts on this subject based on the ideXlab platform.

  • modelling post Glacial Rebound with lateral viscosity variations
    Geophysical Journal International, 2005
    Co-Authors: Archie Paulson, Shijie Zhong, John Wahr
    Abstract:

    SUMMARY Observations of isostatic adjustment of the Earth's surface due to Glacial loading provide important constraints on mantle viscosity structure. We solve the forward problem of Glacial isostatic adjustment in two complementary ways: a spectral method for strictly 1-D (spherically symmetric) earth models, and a finite element method that can accommodate 3-D viscosity structure. We discuss how each method may be augmented in three ways: to accommodate motion of the centre of mass, to implement a gravitationally self-consistent ocean load via the sea level equation and to include the influence of polar wander. With all these effects implemented, the two methods are benchmarked against each other. We also study the influence of lateral viscosity variations upon measurements of post-Glacial Rebound (PGR) in two ways: first by observing the effect of viscosity perturbations in an idealized model and second by developing a realistic 3-D viscosity model and comparing it with results of related 1-D (spherically symmetric) models. The 3-D viscosity structure is derived starting from seismic tomography models. We conclude from both approaches that PGR observations are sensitive to both the local viscosity structure and to the viscosity structure beneath the loaded region, even if it is removed from where the observations are made. In particular, PGR measurements made at Hudson Bay tend to reflect the local viscosity structure beneath Hudson Bay; PGR measurements made along the east coast of North America, being sensitive to both the local (east coast) viscosity structure as well as the loaded (continental) viscosity structure, are not reproducible with a 1-D viscosity model.

  • Greenland mass balance from GRACE
    Geophysical Research Letters, 2005
    Co-Authors: Isabella Velicogna, John Wahr
    Abstract:

    [1] We use 22 monthly GRACE (Gravity Recovery and Climate Experiment) gravity fields to estimate the linear trend in Greenland ice mass during 2002–2004. We recover a decrease in total ice mass of 82 ± 28 km3 of ice per year, consistent with estimates from other techniques. Our uncertainty estimate is dominated by the effects of GRACE measurement errors and errors in our post Glacial Rebound (PG) correction. The main advantages of GRACE are that it is sensitive to the entire ice sheet, and that it provides mass estimates with only minimal use of supporting physical assumptions or ancillary data.

  • what might grace contribute to studies of post Glacial Rebound
    Space Science Reviews, 2003
    Co-Authors: John Wahr, I Velicogna
    Abstract:

    The NASA/DLR satellite gravity mission GRACE, launched in March, 2002, will map the Earth's gravity field at scales of a few hundred km and greater, every 30 days for five years. These data can be used to solve for time-variations in the gravity field with unprecedented accuracy and resolution. One of the many scientific problems that can be addressed with these time-variable gravity estimates, is post Glacial Rebound (PGR): the viscous adjustment of the solid Earth in response to the deglaciation of the Earth's surface following the last ice age. In this paper we examine the expected sensitivity of the GRACE measurements to the PGR signal, and explore the accuracy with which the PGR signal can be separated from other secular gravity signals. We do this by constructing synthetic GRACE data that include contributions from a PGR model as well as from a number of other geophysical processes, and then looking to see how well the PGR model can be recovered from those synthetic data. We conclude that the availability of GRACE data should result in improved estimates of the Earth's viscosity profile.

  • phase transitions and heat conduction in post Glacial Rebound
    Geophysical Journal International, 2002
    Co-Authors: M E Tamisiea, John Wahr
    Abstract:

    SUMMARY We have developed a method for including phase boundary conditions into post-Glacial Rebound models that allows for conduction of latent heat away from the boundary. This method returns the chemical boundary results if latent heat conducts away from the phase boundary too slowly to allow the transition to proceed, as is commonly argued. This is not necessarily the case, however. For example, the secular change of the geoid and the vertical uplift rates for phase boundaries with latent heat conduction can differ from the chemical boundary results by up to 10 and 15 per cent, respectively. When modelling the phase transition, we consider two scenarios: the latent heat is released either at a narrow boundary that separates the two phases or over a thick mixed region of the two phases. In the case where the phase transition occurs over a thick enough region (5‐10 km), the final results are close to the results obtained by considering a phase boundary that ignores the release of latent heat completely. This thick boundary formulation also suggest that the phase boundaries could respond nearly instantaneously, changing both the elastic load and body Love numbers. However, we have not considered kinetics, the energetics of the mechanisms of the phase transitions, in this formulation. This work suggests a greater knowledge of the kinetics near equilibrium phase transitions is required. A naive calculation indicates that the kinetics will not be a significant factor for post-Glacial Rebound but will be a limiting factor for earth tides.

  • an analysis of anisotropic mantle viscosity and its possible effects on post Glacial Rebound
    Physics of the Earth and Planetary Interiors, 1997
    Co-Authors: John Wahr
    Abstract:

    Abstract This paper presents a preliminary analysis of the possibility of using post-Glacial Rebound observations to place constraints on the anisotropy of upper-mantle viscosity. We extend existing analyses by considering a visco-elastic, spherical, self-gravitating Earth. We show that three independent viscosity parameters are required to describe a Maxwell solid with transversely isotropic viscosity and isotropic elasticity. We develop a theoretical formalism for including the effects of transversely isotropic visco-elasticity in post-Glacial Rebound models. Using this formalism to describe anisotropy in the upper mantle, we model the free air gravity anomaly, relative sea-level changes, and present-day horizontal motion caused by the Rebound, to assess their dependence on the upper-mantle viscosity parameters. We find that as long as those parameters are no larger than the lower-mantle (isotropic) viscosity, the results for the gravity and sea-level changes are notably sensitive to only one of the three viscosity parameters. This suggests, at least for the case where the lower-mantle viscosity is significantly larger than the upper-mantle viscosity, that post-Glacial Rebound studies of gravity and sea-level will be hard pressed to usefully constrain viscous anisotropy. We do find that the horizontal motion is relatively sensitive to transverse isotropy and so might be used, in principle, to constrain the anisotropy. In practice, though, it is likely that the strong dependence of horizontal motion on details of the ice model, coupled with the uncertainties in existing ice models, would make such attempts impractical.

Paul Johnston - One of the best experts on this subject based on the ideXlab platform.

  • dependence of horizontal stress magnitude on load dimension in Glacial Rebound models
    Geophysical Journal International, 2002
    Co-Authors: Paul Johnston, Patrick Wu, Kurt Lambeck
    Abstract:

    Summary It has been proposed that the deglaciation of the Northern Hemisphere triggered large earthquakes within intraplate environments and in this paper we examine this hypothesis by evaluating quantitatively the stress state in the lithosphere produced by time-dependent surface loads. A series of models demonstrate the dependence of horizontal incremental stress in an elastic plate overlying an inviscid or viscoelastic mantle on the lateral extent of a load applied at the surface. The horizontal stress is largest when the dominant wavelength (that is twice the diameter) of the load is close to eight times the elastic thickness of the plate when the mantle is inviscid and, for the particular viscosity model employed in this paper, close to 12 times the elastic thickness for a viscoelastic mantle. At wavelengths close to the critical wavelength, the horizontal incremental stress may be up to six times as large as the vertical incremental stress. For appropriate earth-model parameters amplification of horizontal stress is close to maximum for ice loads with a radius of 280 km, comparable to the dimensions of the former ice sheet over Great Britain. This amplification may be sufficiently large that loading by small ice sheets can lead to failure on marginally stable faults, in contrast to the behaviour for large ice sheets. The models also predict greater fault instability for Fennoscandia than for the larger Laurentide ice sheet, consistent with the observation of large postGlacial faults in northern Sweden. The model is used to predict the stability of faults and style of faulting due to Rebound stresses in the absence of a background tectonic stress field since the last Glacial maximum (∼ 18 000 years ago) in Northern Europe. Within the formerly glaciated region thrust faulting is predicted to occur at the end of deglaciation and normal faulting is predicted to occur in peripheral regions for the entire period since the last Glacial maximum.

  • estimates of present day Glacial Rebound in the lambert glacier region antarctica
    Geophysical Research Letters, 1999
    Co-Authors: Dan Zwartz, Kurt Lambeck, Paul Johnston, Paul Tregoning, John O Stone
    Abstract:

    Changes in the ice load since the time of the Last Glacial Maximum (LGM) contribute to the present- day vertical motion of the Antarctic continent. The obser- vation of these motions will reveal information on the ice load history. Predictions of uplift rates along a transect across the Lambert Glacier region, East Antarctica, from the coast to the southernmost rock outcrops of the Prince Charles Mountains have been computed for three deglacia- tion scenarios. The relative vertical velocities between sites on the transect are -7 to +7 mm/yr and are large enough to be detected from continuous GPS observations taken at permanent sites over several years. When available, such in- formation will discriminate between the currently available models for deglaciation of East Antarctica.

  • sea level change Glacial Rebound and mantle viscosity fornorthern europe
    Geophysical Journal International, 1998
    Co-Authors: Kurt Lambeck, Catherine Smither, Paul Johnston
    Abstract:

    Northwestern Europe remains a key region for testing models of Glacial isostasy because of the good geological record of crustal response to the Glacial unloading since the time of the Last Glacial Maximum. Models for this Rebound and associated sea-level change require a detailed knowledge of the ice-sheet geometry, including the ice thickness through time. Existing ice-sheet reconstructions are strongly model-dependent, and inversions of sea-level data for the mantle response may be a function of the model assumptions. Thus inverse solutions for the sea-level data are sought that include both ice- and earth-model parameters as unknowns. Sea-level data from Fennoscandia, the North Sea, the British Isles and the Atlantic and English Channel coasts have been evaluated and incorporated into the solutions. The starting ice sheet for Fennoscandia is based on a reconstruction of a model by Denton & Hughes (1981) that is characterized by quasi-parabolic cross-sections and symmetry about the load centre. Both global (northwestern Europe as a whole) and regional (subsets of the data) solutions have been made for earth-model parameters and ice-height scaling parameters. The key results are as follows. (1) The response of the upper mantle to the changing ice and water loads is spatially relatively homogenous across Scandinavia, the North Sea and the British Isles. (2) This response can be adequately modelled by an effective elastic lithosphere of thickness 65–85 km and by an effective upper-mantle viscosity (from the base of the lithosphere to the 670 km depth seismic discontinuity) of about 3–4×1020 Pa s. The effective lower-mantle viscosity is at least an order of magnitude greater. (3) The ice thickness over Scandinavia at the time of maximum glaciation was only about 2000 m, much less than the 3400 m assumed in the Denton & Hughes model. (4) The ice profiles are asymmetric about the centre of the ice sheet with those over the western part being consistent with quasi-parabolic functions whereas the ice heights over the eastern and southern regions increase much more slowly with distance inwards from the ice margin.

  • Sea‐level change, Glacial Rebound and mantle viscosity fornorthern Europe
    Geophysical Journal International, 1998
    Co-Authors: Kurt Lambeck, Catherine Smither, Paul Johnston
    Abstract:

    Northwestern Europe remains a key region for testing models of Glacial isostasy because of the good geological record of crustal response to the Glacial unloading since the time of the Last Glacial Maximum. Models for this Rebound and associated sea-level change require a detailed knowledge of the ice-sheet geometry, including the ice thickness through time. Existing ice-sheet reconstructions are strongly model-dependent, and inversions of sea-level data for the mantle response may be a function of the model assumptions. Thus inverse solutions for the sea-level data are sought that include both ice- and earth-model parameters as unknowns. Sea-level data from Fennoscandia, the North Sea, the British Isles and the Atlantic and English Channel coasts have been evaluated and incorporated into the solutions. The starting ice sheet for Fennoscandia is based on a reconstruction of a model by Denton & Hughes (1981) that is characterized by quasi-parabolic cross-sections and symmetry about the load centre. Both global (northwestern Europe as a whole) and regional (subsets of the data) solutions have been made for earth-model parameters and ice-height scaling parameters. The key results are as follows. (1) The response of the upper mantle to the changing ice and water loads is spatially relatively homogenous across Scandinavia, the North Sea and the British Isles. (2) This response can be adequately modelled by an effective elastic lithosphere of thickness 65–85 km and by an effective upper-mantle viscosity (from the base of the lithosphere to the 670 km depth seismic discontinuity) of about 3–4×1020 Pa s. The effective lower-mantle viscosity is at least an order of magnitude greater. (3) The ice thickness over Scandinavia at the time of maximum glaciation was only about 2000 m, much less than the 3400 m assumed in the Denton & Hughes model. (4) The ice profiles are asymmetric about the centre of the ice sheet with those over the western part being consistent with quasi-parabolic functions whereas the ice heights over the eastern and southern regions increase much more slowly with distance inwards from the ice margin.

  • Glacial Rebound of the british isles iii constraints on mantle viscosity
    Geophysical Journal International, 1996
    Co-Authors: Kurt Lambeck, Catherine Smither, Paul Johnston, Masao Nakada
    Abstract:

    SUMMARY Observations of sea-level change since the time of the last Glacial maximum provide important constraints on the response of the Earth to changes in surface loading on time-scales of 103-104 years. This response is conveniently described by an effective elastic lithospheric thickness and effective viscosities for one or more mantle layers. Considerable trade-off between the parameters describing these layers can occur, and different combinations can give rise to comparable predictions of sea-level change. In particular, the trade-off between lithospheric thickness and upper-mantle viscosity can be important, and for any reasonable value for the lithospheric thickness a corresponding mantle viscosity structure can be found that gives a plausible comparison of sealevel predictions with observations. In particular, thin-lithosphere models will lead to low estimates for the upper-mantle viscosity, while thick-lithosphere models lead to high viscosity values. However, either solution may represent only a local minimum in the model parameter space, and may not correspond to the optimum solution. It becomes important, therefore, that in the inversion of observational data, a comprehensive search is conducted throughout the entire model-parameter space, to ensure that the solution identified does indeed correspond to the optimum solution. The sea-level data for the British Isles lend themselves well to such an inversion because of the relatively high quality of the data, the good geographic distribution of the data relative to the former ice sheet, and reasonable observational constraints on the dimensions of the former ice sheet and on its retreat. Furthermore, because of the contribution to the sea-level signal from the distant ice sheets, as well as from the melt-water load, the observational data base for the region also has some resolving power for the viscosity of the deeper mantle. The parameter space explored is defined by up to five mantle layers, the lithosphere of effective elastic thickness D,, and a series of upper-mantle layers, i = 2-4, extending down to depths of 200, 400 and 670 km, respectively, each of viscosity qi, and a lower-mantle layer of viscosity qlm extending down to the coremantle boundary. The range of parameters explored is 30 < D, I 120 km, 3 x lOI9 Iqi

Thomas S James - One of the best experts on this subject based on the ideXlab platform.

  • a comparison of eastern north american seismic strain rates to Glacial Rebound strain rates
    Geophysical Research Letters, 1994
    Co-Authors: Thomas S James, Allison L Bent
    Abstract:

    Glacial Rebound strain-rates computed using a simple Laurentide Glacial loading model are of the order of 10−9 per year within the region of glaciation and extending several hundred kilometers beyond. The horizontal strain-rates receive approximately equal contributions from horizontal and vertical velocities, a consequence of the spherical geometry adopted for the Earth model. In the eastern United States and southeastern Canada the computed strain-rates are 1–3 orders of magnitude greater than an estimate of the average seismic strain-rate [Anderson, 1986] and ∼1 order of magnitude greater than predicted erosional strain-rates. The predicted Glacial Rebound strain-rates are not, in general, oriented in such a way as to augment the observed state of deviatoric stress, possibly explaining why the seismic strain-rates are much smaller than the Glacial Rebound strain-rates. An exception to this may be seismically active regions in the St. Lawrence valley.

  • a comparison of vlbi data with the ice 3g Glacial Rebound model
    Geophysical Research Letters, 1993
    Co-Authors: Thomas S James, A Lambert
    Abstract:

    Crustal motion predicted by the ICE-3G Glacial Rebound model exhibits a pattern of tangential (horizontal) divergence away from the centres of uplift, which in North America and Europe are located around Hudson Bay and the Gulf of Bothnia. Tangential velocities reach peak magnitudes of 1–2 mm/yr, and must be included when predicting Very-Long-Baseline-Interferometry (VLBI) baseline-length change rates due to post-Glacial Rebound. Out of 18 observed VLBI baselines examined 3 are situated such that their predicted length rates are around their 2σ uncertainties or greater. It is encouraging that 2 of these baselines exhibit predicted length rates within 2σ of the observed rates.

  • A comparison of VLBI data with the Ice‐3G Glacial Rebound Model
    Geophysical Research Letters, 1993
    Co-Authors: Thomas S James, A Lambert
    Abstract:

    Crustal motion predicted by the ICE-3G Glacial Rebound model exhibits a pattern of tangential (horizontal) divergence away from the centres of uplift, which in North America and Europe are located around Hudson Bay and the Gulf of Bothnia. Tangential velocities reach peak magnitudes of 1–2 mm/yr, and must be included when predicting Very-Long-Baseline-Interferometry (VLBI) baseline-length change rates due to post-Glacial Rebound. Out of 18 observed VLBI baselines examined 3 are situated such that their predicted length rates are around their 2σ uncertainties or greater. It is encouraging that 2 of these baselines exhibit predicted length rates within 2σ of the observed rates.

  • the hudson bay free air gravity anomaly and Glacial Rebound
    Geophysical Research Letters, 1992
    Co-Authors: Thomas S James
    Abstract:

    Current models of the Laurentide deglaciation predict only 15% to 30% of the observed Hudson Bay Free-air gravity anomaly low. Unless these models are able to be modified, the source of the remainder of this Free-air gravity anomaly must be sought elsewhere, such as in mantle convection.

  • The Hudson Bay free‐air gravity anomaly and Glacial Rebound
    Geophysical Research Letters, 1992
    Co-Authors: Thomas S James
    Abstract:

    Current models of the Laurentide deglaciation predict only 15% to 30% of the observed Hudson Bay Free-air gravity anomaly low. Unless these models are able to be modified, the source of the remainder of this Free-air gravity anomaly must be sought elsewhere, such as in mantle convection.

Masao Nakada - One of the best experts on this subject based on the ideXlab platform.

  • Effects of water load on geophysical signals due to Glacial Rebound and implications for mantle viscosity
    Earth Planets and Space, 2001
    Co-Authors: Jun’ichi Okuno, Masao Nakada
    Abstract:

    We investigate the effects of the ocean function on predictions of the sea-level changes and other geophysical signals due to Glacial Rebound. To precisely predict these signals, a realistic ocean function including the effects of the palaeotopography, the distribution of ice sheet and meltwater influx is required. The adoption of a precise ocean function is very important in simulating the observables in Hudson Bay for an earth model with a low lower mantle viscosity of ∼10^21 Pa s. In this case, the contribution from water loads can be comparable to that from ice loads. In the Fennoscandian region, however, the predictions are less sensitive to the details of the ocean function, because the width of the Gulf of Bothnia is very small compared with that of Hudson Bay. With an assumption that the ice model is represented by ARC3+ANT4b, we have examined the viscosity structure using relative sea-levels, gravity anomaly and solid surface gravity changes in North America and northern Europe. This study suggests a lower mantle viscosity of greater than 10^22 Pa s and a upper mantle viscosity of (4 ∼ 10) × 10^20 Pa s.

  • Glacial Rebound of the british isles iii constraints on mantle viscosity
    Geophysical Journal International, 1996
    Co-Authors: Kurt Lambeck, Catherine Smither, Paul Johnston, Masao Nakada
    Abstract:

    SUMMARY Observations of sea-level change since the time of the last Glacial maximum provide important constraints on the response of the Earth to changes in surface loading on time-scales of 103-104 years. This response is conveniently described by an effective elastic lithospheric thickness and effective viscosities for one or more mantle layers. Considerable trade-off between the parameters describing these layers can occur, and different combinations can give rise to comparable predictions of sea-level change. In particular, the trade-off between lithospheric thickness and upper-mantle viscosity can be important, and for any reasonable value for the lithospheric thickness a corresponding mantle viscosity structure can be found that gives a plausible comparison of sealevel predictions with observations. In particular, thin-lithosphere models will lead to low estimates for the upper-mantle viscosity, while thick-lithosphere models lead to high viscosity values. However, either solution may represent only a local minimum in the model parameter space, and may not correspond to the optimum solution. It becomes important, therefore, that in the inversion of observational data, a comprehensive search is conducted throughout the entire model-parameter space, to ensure that the solution identified does indeed correspond to the optimum solution. The sea-level data for the British Isles lend themselves well to such an inversion because of the relatively high quality of the data, the good geographic distribution of the data relative to the former ice sheet, and reasonable observational constraints on the dimensions of the former ice sheet and on its retreat. Furthermore, because of the contribution to the sea-level signal from the distant ice sheets, as well as from the melt-water load, the observational data base for the region also has some resolving power for the viscosity of the deeper mantle. The parameter space explored is defined by up to five mantle layers, the lithosphere of effective elastic thickness D,, and a series of upper-mantle layers, i = 2-4, extending down to depths of 200, 400 and 670 km, respectively, each of viscosity qi, and a lower-mantle layer of viscosity qlm extending down to the coremantle boundary. The range of parameters explored is 30 < D, I 120 km, 3 x lOI9 Iqi

  • Glacial Rebound of the British Isles—III. Constraints on mantle viscosity
    Geophysical Journal International, 1996
    Co-Authors: Kurt Lambeck, Catherine Smither, Paul Johnston, Masao Nakada
    Abstract:

    SUMMARY Observations of sea-level change since the time of the last Glacial maximum provide important constraints on the response of the Earth to changes in surface loading on time-scales of 103-104 years. This response is conveniently described by an effective elastic lithospheric thickness and effective viscosities for one or more mantle layers. Considerable trade-off between the parameters describing these layers can occur, and different combinations can give rise to comparable predictions of sea-level change. In particular, the trade-off between lithospheric thickness and upper-mantle viscosity can be important, and for any reasonable value for the lithospheric thickness a corresponding mantle viscosity structure can be found that gives a plausible comparison of sealevel predictions with observations. In particular, thin-lithosphere models will lead to low estimates for the upper-mantle viscosity, while thick-lithosphere models lead to high viscosity values. However, either solution may represent only a local minimum in the model parameter space, and may not correspond to the optimum solution. It becomes important, therefore, that in the inversion of observational data, a comprehensive search is conducted throughout the entire model-parameter space, to ensure that the solution identified does indeed correspond to the optimum solution. The sea-level data for the British Isles lend themselves well to such an inversion because of the relatively high quality of the data, the good geographic distribution of the data relative to the former ice sheet, and reasonable observational constraints on the dimensions of the former ice sheet and on its retreat. Furthermore, because of the contribution to the sea-level signal from the distant ice sheets, as well as from the melt-water load, the observational data base for the region also has some resolving power for the viscosity of the deeper mantle. The parameter space explored is defined by up to five mantle layers, the lithosphere of effective elastic thickness D,, and a series of upper-mantle layers, i = 2-4, extending down to depths of 200, 400 and 670 km, respectively, each of viscosity qi, and a lower-mantle layer of viscosity qlm extending down to the coremantle boundary. The range of parameters explored is 30 < D, I 120 km, 3 x lOI9 Iqi

  • holocene Glacial Rebound and sea level change in nw europe
    Geophysical Journal International, 1990
    Co-Authors: Kurt Lambeck, Paul Johnston, Masao Nakada
    Abstract:

    SUMMARY Observations of Late Pleistocene and Holocene sea-level change relative to the crust exhibit very considerable variations across NW Europe in consequence of the response of the Earth’s crust to the deglaciation of Fennoscandia and of the water added to the oceans from the melting of all Late Pleistocene ice sheets. Inversion of sea-level observations from a site near the centre of the Fennoscandian ice sheet and from three sites located beyond the margin of the ice sheet at the time of maximum glaciation yield a range of plausible models for the Earth’s response and for the ice models. Further constraints on this range of models is placed by a comparison of observed sea-levels with predicted values at other sites near the former ice sheet margins. The resulting mantle parameters are: upper mantle viscosity (3-5) X 10’’ Pa s; lower mantle viscosity (2-7) X loz1 Pa s; lithospheric thickness 100150 km. These values represent effective parameters that describe the response of the Earth to surface loading of short to intermediate wavelengths on a time-scale of 104yr. The lower mantle viscosity is poorly constrained but the marked increase from upper to lower mantle is a characteristic of all plausible solutions. The inversion places a constraint on the total volume of ice in the Fennoscandian ice sheet such that the equivalent sea-level rise from this contribution is about 13-14 m. A less well-determined constraint of about 10 m equivalent sea-level rise is suggested for the Barents-Kara ice sheet. The inversion also indicates that a small amount of melt-water, from ice sheets far away from Europe, continued to be added into the oceans during Late Holocene time so as to raise the equivalent sea-level by about 3 m during the past 6000 yr, consistent with similar inversions of data from sites in the Australian and Pacific regions.