Glacial Isostatic Adjustment Models Research Articles

Holocene relative sea-level movements along the North Norfolk Coast, UK

Relative changes in Holocene sea level are reconstructed from foraminiferal assemblages in seven cores recovered from the intertidal zone of North Norfolk (Eastern England). A total of 33 radiocarbon and infra-red stimulated luminescence dates provide a detailed chronological framework for these changes. A transfer function approach, developed from modern UK foraminiferal datasets, establishes a series of 21 new sea-level index points for this coastline which are combined with 18 existing index points and a further number of limiting values. Errors associated with changing tidal ranges through the Holocene are reduced by adopting recently developed tidal models for this period. This new approach allows the use of foraminiferal assemblages to be extended from the upper saltmarsh environment (traditionally used for sea-level studies) to lower elevations in the tidal frame and include clastic detrital sediments. The results confirm the previously observed trend of continuously rising sea-levels in this area during the last 10 000 years which is consistent with a Glacial Isostatic Adjustment model. Relative sea-level (RSL) rises from an observed minimum of approximately − 13.50 m OD at about 7500 cal. BP to approximately − 4.0 m at about 5300 cal. BP, which equates to a mean rise of c. 4.3 mm yr − 1 . Subsequently, RSL rose gradually to present levels, although there is evidence that a minor reverse in this trend occurred between 4000 and 2000 cal. BP when sea-level fell by about 1 m to c. − 3.5 m. The rate of land subsidence for North Norfolk is 0.54 ± 0.03 mm yr − 1 (inverse of Late-Holocene sea-level rise).

Ice Sheet and Solid Earth Influences on Far-Field Sea-Level Histories

Previous predictions of sea-level change subsequent to the last glacial maximum show significant, systematic discrepancies between observations at Tahiti, Huon Peninsula, and Sunda Shelf during Lateglacial time (approximately 14,000 to 9000 calibrated years before the present). We demonstrate that a model of glacial isostatic adjustment characterized by both a high-viscosity lower mantle (4 x 10(22) Pa s) and a large contribution from the Antarctic ice sheet to meltwater pulse IA (approximately 15-meters eustatic equivalent) resolves these discrepancies. This result supports arguments that an early and rapid Antarctic deglaciation contributed to a sequence of climatic events that ended the most recent glacial period of the current ice age.

Sensitivity of glacial isostatic adjustment models with shallow low-viscosity earth layers to the ice-load history in relation to the performance of GOCE and GRACE

Abstract The GOCE satellite mission, which is planned by ESA for launch in August 2006, is designed to map the static global gravity field with centimeter accuracy in geoid height at 100 km or better resolution. Such a global high resolution gravity field might be able to constrain properties of shallow low-viscosity zones (LVZs) using glacial isostatic adjustment (GIA) models. In (L.L.A. Vermeersen, The potential of GOCE in constaining the structure of the crust and lithosphere from post-gracial rebound, Space Sci. Rev. 108 (2003) 105–113.) and (W. van der Wal, H.H.A. Schotman, L.L.A. Vermeersen, Geoid heights due to a crustal low viscosity zone in glacial isostatic adjustment modeling; a sensitivity analysis for GOCE, Geophys. Res. Lett. 31 (2004) 10.1029/2003GL019139.) it is shown that a crustal low-viscosity zone (CLVZ) introduces variations in geoid height up to several decimeters with spatial scales down to hundred kilometers underneath and just outside formerly glaciated areas. In (W. van der Wal, H.H.A. Schotman, L.L.A. Vermeersen, Geoid heights due to a crustal low viscosity zone in glacial isostatic adjustment modeling; a sensitivity analysis for GOCE, Geophys. Res. Lett. 31 (2004) 10.1029/2003GL019139.) it is shown that the response is sensitive to both changes in the properties of the CLVZ and the Late Pleistocene ice-load history. In this study we quantify the sensitivity to ice-load history, and investigate the effect of an asthenospheric low-viscosity zone (ALVZ) just below the lithosphere. We show, using spherical harmonic degree amplitudes, that GOCE is predicted to be sensitive to differences in the load history up to degree 130 for a CLVZ and degree 70 for an ALVZ. The sensitivity of GRACE, using the realized performance over a 111-day period (GGM01S, (B.D. Tapley, S. Bettadpur, M. Watkins, C. Reigber, The gravity recovery and climate experiment: Mission overview and early results, Geophys. Res Lett. 31 (2004) 10.1029/2004GL019920.)) is limited to lower degrees. This means that GOCE, and to a lesser degree GRACE, could provide information on the ice-load history in the presence of an LVZ, provided that our knowledge of the earth mode is sufficient. To extract information about the LVZ from the degree amplitudes, we show that it is possible to largely remove the influence of the ice-load history and thus compute spectral signatures for different properties of the LVZ. We focus on the continental shelf-areas of western Europe to show the sensitivity of estimates of present-day sea level change to the inclusion of an LVZ.

Geodetic constraints on glacial isostatic adjustment in Europe

Direct measurements of surface deformation due to Glacial Isostatic Adjustment (GIA) in Europe have been so far mostly limited to the present‐day uplift area. Here, we use permanent GPS networks results to investigate the significance of GIA effects in Europe south of Fennoscandia. We show that uplift in Fennoscandia is surrounded by subsidence reaching as far south as the Alps, with a maximum vertical rate of 1.5 mm/yr between 50.5–53°N. Horizontal velocity gradients show shortening between Fennoscandia and north‐central Europe with strain rates of ∼10−9 yr−1 and principal compressional strain axis pointing to the Gulf of Bothnia in a radial pattern. We find a very good quantitative agreement with the 3D surface displacement predicted by Milne et al. (2001), although the increase of misfit in far‐field of Fennoscandia suggests that geodetic data outside of the uplift area may bring additional constraints to the rheological parameters used in GIA models.

Upper mantle viscosity from continuous GPS baselines in Fennoscandia

Abstract The permanently recording BIFROST (Baseline Inferences for Fennoscandian Rebound Observations, Sea Level and Tectonics) GPS network was set into operation during the early to mid-1990s to monitor the three-dimensional crustal deformation field in Fennoscandia. We have employed 2500 days of BIFROST GPS data to estimate rates of baseline component change (length, transverse and up). Baselines are less sensitive than single-site position estimates to perturbations in satellite orbits and reference frame realizations. Hence, the baseline time series represent a particularly robust data set for model parameter estimation. We use this data set to outline an evaluation scheme for GPS observations of intraplate horizontal movements. Our results show that the data can be used to infer parameters in a model that simulates the glacial isostatic adjustment (GIA) of the region. This process is widely believed to dominate the present-day deformation field, and analysis of the length and transverse components support this view. The data prefer an elastic lithosphere of thickness 120 km and an upper mantle viscosity in the range ( 3 – 10 ) × 1 0 20 Pa s (to 95% confidence). These results are consistent with those obtained in a previous analysis that considered single-site position estimates. This suggests that the problems associated with perturbations in satellite orbits and reference frame realizations have not significantly impacted the analysis of single-site positions from the BIFROST time series. Subtracting the best-fitting GIA model predictions from the observed baseline rates leaves a horizontal residual field of magnitude less than 1 mm year−1.

A 16 000-year record of near-field relative sea-level changes, northwest Scotland, United Kingdom

Isolation basins, raised tidal marshes, coastal wetlands and dune systems around Arisaig in northwest Scotland produce a ∼16 000-year record of relative sea-level (RSL) change from the time of local deglaciation following the Last Glacial Maximum (LGM) to the present. New sea-level index points from Mointeach Mhor North define the culmination of the mid-Holocene RSL highstand to ∼7600–7400 cal yr BP and 6.74±0.2 m above present. Numerous sea-level index points indicate that the RSL highstand persisted more than 1000 years before the onset of any significant RSL fall. This points to a gradual cessation of melting of the Laurentide and Antarctic ice sheets, rather than an abrupt termination. Isolation basin evidence, from a marine limit ∼34 to 38 m above present down to sites close to current sea level, constrain the acceleration of eustatic sea-level rise during meltwater pulse 1a to ∼30 mm yr −1 or ∼11 km 3 yr −1 meltwater discharge. Comparison of RSL observations with glacial isostatic adjustment (GIA) model predictions show differences in the pattern of residuals dependent upon the GIA model used. Changes to the far-field ice model components can partly explain the patterns of residuals.

Evidence for enhanced coastal sea level rise during the 1990s

Sea level rise over the last 55 years is estimated to have been 1.7 ± 0.2 mm yr−1, based upon 177 tide gauges divided into 13 regions with near global coverage and using a Glacial Isostatic Adjustment (GIA) model to correct for land movements. We present evidence from altimeter data that the rate of sea level rise around the global coastline was significantly in excess of the global average over the period 1993–2002. We also show that the globally‐averaged rate of coastal sea level rise for the decade centered on 1955 was significantly larger than any other decade during the past 55 years. In some models of sea level rise, enhanced coastal rise is a pre‐cursor of global average rise. It remains to be seen whether the models are correct and whether global‐average rates in the future reflect the high rates of coastal rise observed during the 1990s.

The signatures of tectonics and glacial isostatic adjustment revealed by the strain rate in Europe

SUMMARY Tectonics and glacial isostatic adjustment (GIA) are simultaneously taken into account in order to quantitatively define their role in crustal deformation in Europe. A spherical finiteelement model, based on the thin viscous shell approach and suitable for predicting tectonic deformation, and a spherical stratified viscoelastic Earth model, based on the normal mode approach to quantifying the effects of GIA, are used to predict intraplate deformation in Europe. Model predictions are compared with the geodetic strain rate obtained from ITRF2000 velocity solutions. Our results confirm that both geophysical processes influence intraplate deformation in Europe, with tectonics playing the leading role south of Potsdam and GIA being the only mechanism north of Onsala. Both geophysical processes affect the deformation at intermediate latitudes, where the contributions to the deformation coming from tectonics and GIA are of the same magnitude and the combined tectonic plus GIA model succeeds in reproducing the eigendirections of the local predominantly SSW‐NNE directed compression. The stiffening in the East European Platform is crucial for shielding the northeastern regions from the compressional effects of Africa‐Eurasia convergence and to allow SE‐NW directed extension in Fennoscandia driven by GIA.

Geoid heights due to a crustal low viscosity zone in glacial isostatic adjustment modeling: A sensitivity analysis for GOCE

Seismic observations and the occurrence of post‐seismic deformation in several continental regions indicate the presence of one or more layers with relative low (1018–1019 Pas) viscosity in the Earth's crust. This study simulates the effect of thickness, viscosity and depth of such a low viscosity zone (LVZ) on geoid heights that are the result of glacial isostatic adjustment to Pleistocene ice loads. It is found that an LVZ introduces variations in geoid heights up to 1 meter with spactial scales larger than a few hundred kilometers underneath and just outside former glaciated areas. The response to changes in the properties of the LVZ is shown to be wavelength‐dependent. The LVZ induced geoid signal is within the accuracy of the upcoming GOCE satellite mission, which means that in principle it should be possible to detect the presence of a supposed LVZ with GOCE data.

Continuous GPS measurements of postglacial adjustment in Fennoscandia: 2. Modeling results

Data collected under the auspices of the BIFROST GPS project yield a geographically dense suite of estimates of present‐day, three‐dimensional (3‐D) crustal deformation rates in Fennoscandia [Johansson et al., 2002]. A preliminary forward analysis of these estimates [Milne et al., 2001] has indicated that models of ongoing glacial isostatic adjustment (GIA) in response to the final deglaciation event of the current ice age are able to provide an excellent fit to the observed 3‐D velocity field. In this study we revisit our previous GIA analysis by considering a more extensive suite of forward calculations and by performing the first formal joint inversion of the BIFROST rate estimates. To establish insight into the physics of the GIA response in the region, we begin by decomposing a forward prediction into the three contributions associated with the ice, ocean, and rotational forcings. From this analysis we demonstrate that recent advances in postglacial sea level theory, in particular the inclusion of rotational effects and improvements in the treatment of the ocean load in the vicinity of an evolving continental margin, involve peak signals that are larger than the observational uncertainties in the BIFROST network. The forward analysis is completed by presenting predictions for a pair of Fennoscandian ice histories and an extensive suite of viscoelastic Earth models. The former indicates that the BIFROST data set provides a powerful discriminant of such histories. The latter yields bounds on the (assumed constant) upper and lower mantle viscosity (νUM, νLM); specifically, we derive a 95% confidence interval of 5 × 1020 ≤ νUM ≤ 1021 Pa s and 5 × 1021 ≤ νLM ≤ 5 × 1022 Pa s, with some preference for (elastic) lithospheric thickness in excess of 100 km. The main goal of the (Bayesian) inverse analysis is to estimate the radial resolving power of the BIFROST GPS data as a function of depth in the mantle. Assuming a reasonably accurate ice history, we demonstrate that this resolving power varies from ∼200 km near the base of the upper mantle to ∼700 km in the top portion of the lower mantle. We conclude that the BIFROST data are able to resolve structure on radial length scale significantly smaller than a single upper mantle layer. However, these data provide little constraint on viscosity in the bottom half of the mantle. Finally, elements of both the forward and inverse analyses indicate that radial and horizontal velocity estimates provide distinct constraints on mantle viscosity.

Combined effects of tectonics and glacial isostatic adjustment on intraplate deformation in central and northern Europe: Applications to geodetic baseline analyses

We use a suite of spherical, thin sheet, finite element model calculations to investigate the pattern of horizontal tectonic deformation within Europe. The calculations incorporate the effects of Africa‐Eurasia convergence, Atlantic Ridge push forces, and changes in the lithospheric strength of the East European and Mediterranean subdomains. These predictions are compared to the deformation computed for the same region using a spherically symmetric, self‐gravitating, viscoelastic Earth model of glacial isostatic adjustment. The radial viscosity profile and ice history input into the GIA model are taken from a model that “best fits” three‐dimensional crustal velocities estimated from the BIFROST Fennoscandian GPS network. The comparison of the tectonic and GIA signals includes predictions of both crustal velocity maps and baseline length changes associated with sites within the permanent ITRF2000 and BIFROST GPS networks. Our baseline analysis includes reference sites in northern and central Europe that are representative of sites at the center, edge, and periphery of the GIA‐induced deformation. Baseline length change predictions associated with all three reference sites are significantly impacted by both tectonic and GIA effects, albeit with distinct geometric sensitivities. In this regard, several of our tectonic models yield baseline rates from Vaas, Onsala, and Potsdam to sites below 55°N which are consistent with observed trends. We find that a best fit to the ITRF2000 data set is obtained by simultaneously considering the effects of GIA plus tectonics, where the latter is modeled with a relatively weak Mediterranean subdomain. In this case, the tectonic model contributes to the observed shortening between Onsala/Potsdam and sites to the south, without corrupting the extension observed for baselines extending from these reference sites and sites to the north; this extension is well reconciled by the GIA process alone.

Investigation of glacial isostatic adjustment in the northeast U.S. using GPS measurements

We have been observing three‐dimensional site velocities at over 60 permanent GPS stations in the northeastern U.S for the purpose of inferring mantle viscosity and thus improving current models of glacial isostatic adjustment (GIA). We estimate the upper and lower mantle viscosity by comparing radial site velocities from a subset of continuous GPS measurements with numerical GIA predictions. Our inferences are consistent with previous estimates obtained using different methods and data sets.

A Summary of Glaciation

CLIMATE SCIENCE The EPILOG (Environmental Processes of the Ice Age: Land, Oceans, Glaciers) program originated in 1998, under the aegis of the IMAGES (International Marine Global Change Study) program of IGBP/PAGES (International Geosphere-Biosphere Programme/Past Global Changes). It was charged with developing a comprehensive description of Earth during the last glacial maximum (LGM) 21,000 years ago. A primary goal was to explore the transitions into and out of a full glacial state by updating the pictures of sea surface temperature, glacial ice distribution, and albedo provided in 1981 by the landmark study CLIMAP. The EPILOG 2000 workshop summarized the current knowledge about continental ice sheets and sea level immediately before, during, and after the LGM. A collection of papers drawn from the workshop addresses four main issues: (i) the extent, volume, and evolution of glaciers and ice sheets; (ii) the magnitude of global sea level change caused by the growth and decay of ice sheets; (iii) the isostatic response to the global redistribution of ice and water; and (iv) the relation between ice sheets and global climate change. These papers discuss geological reconstructions of ice sheet extent, records of sea level change, ice sheet modeling, geophysical models of glacial isostatic adjustment, and geochemical proxies of ice volume. From this body of work, several important conclusions emerge. First, sea level at the LGM was 118 to 135 meters below the modern value, bracketing the lower bound for sea level drop of 127 meters and much less than the maximum value of 163 meters estimated by CLIMAP. Second, although new estimates of the margins of ice sheets agree in most cases with the boundaries drawn by CLIMAP, the amount and distribution of ice were substantially less than the CLIMAP estimates. Finally, some or all of the ice sheets must have been thinner than the minimum value given by CLIMAP. — HJS Quat. Sci. Rev. 21 , 1 (2002).

Global geoid and sea level changes due to present‐day ice mass fluctuations

We predict gravitationally self‐consistent global geoid and relative sea level (RSL) perturbations due to present‐day melting of ice complexes, including the Antarctic and Greenland ice sheets and a suite of mountain glaciers and ice sheets. Classic analyses of sea level change indicate that these perturbations will depart significantly from eustatic (i.e., geographically uniform) trends [e.g., Woodward, 1888; Farrell and Clark, 1976], although this result has not always been appreciated in modern analyses. Mass flux of individual ice reservoirs will produce unique geometries of sea level change, and this distinctiveness admits the possibility of using global geoid, sea surface, and RSL signatures of recent climate change to infer the ongoing mass balance of each reservoir rather than simply the net mass flux. As an example, we show that perturbations to the geoid arising from noneustatic water loads associated with each ice reservoir are sufficiently large (at low degrees) to be theoretically measurable within 5 years by the GRACE satellite mission. We complete the study by reanalyzing tide gauge data at 23 sites selected by Douglas [1997] in a recent analysis of global RSL rise. Traditionally, estimates of global sea level rise are generated by taking the mean of a set of secular tide gauge trends that have been corrected for the influence of ongoing glacial isostatic adjustment (GIA) related to the late Pleistocene glacial cycles. The common assumption in such studies is that the geographic scatter in the residual, GIA‐corrected trends is due to errors in the GIA model or unmodeled processes (e.g., tectonics). We consider a large suite of GIA model predictions and apply a least squares approach to the GIA‐corrected tide gauge trends to estimate the weighting of various present‐day sea level signatures. We find that the fit to the residual RSL trends is significantly improved and that the procedure is able to resolve a long‐standing observation of anomalously low sea level rates in Europe. This preliminary analysis, which is relatively insensitive to changes in the assumed geometry of the present‐day mass balance, assumes that ocean thermal expansion is globally uniform. However, the procedure can be easily extended to incorporate a realistic steric contribution once the geometry of the process is sufficiently well constrained.

Ice sheets and sea level of the Last Glacial Maximum

This paper outlines the general issues regarding ice sheets and sea level of the Last Glacial Maximum (LGM), which formed the basis of an EPILOG Project workshop. Papers in this special issue of Quaternary Science Reviews provide a comprehensive assessment of these issues from the perspective of geological reconstructions of ice sheet extent, records of sea level change, ice sheet modelling, geophysical models of glacial isostatic adjustment, and geochemical proxies of ice volume. This new assessment has substantially narrowed the uncertainties in the total changes in ice sheets and sea level and their proxies, suggesting a net decrease in eustatic sea level at the LGM ranging from 120 to 135 m.

Glacial isostatic adjustment in Fennoscandia for a laterally heterogeneous earth

Glaciation and deglaciation in Fennoscandia during the last glacial cycles has significantly perturbed the Earth’s equilibrium figure. Changes in the Earth’s solid and geoidal surfaces due to external and internal mass redistributions are recorded in sequences of ancient coastlines, now either submerged or uplifted, and are still visible in observations of present-day motions of the surface and glacially induced anomalies in the Earth’s gravitational field. These observations become increasingly sophisticated with the availability of GPS measurements and new satellite gravity missions. Observational evidence of the mass changes is widely used to constrain the radial viscosity structure of the Earth’s mantle. However, lateral changes in earth model properties are usually not taken into account, as most global models of glacial isostatic adjustment assume radial symmetry for the earth model. This simplifying assumption contrasts with seismological evidence of significant lateral variations in the Earth’s crust and upper mantle throughout the Fennoscandian region. We compare predictions of glacial isostatic adjustment based on an ice model over the Fennoscandian region for the last glacial cycle for both radially symmetric and fully 3-D earth models. Our results clearly reveal the importance of lateral variations in lithospheric thickness and asthenospheric viscosity for glacially induced model predictions. Relative sea-level predictions can differ by up to 10–20m, uplift rate predictions by 1–3mmyr−1 and free-air gravity anomaly predictions by 2–4mGal when a realistic 3-D earth structure as proposed by seismic modelling is taken into account.

Modelling of stresses in the Fennoscandian lithosphere induced by Pleistocene glaciations

The seismicity observed in Fennoscandia is usually explained by tectonic stresses. The question of the contribution of stresses induced by glacial-isostatic adjustment to the seismicity has not finally been settled yet. In order to quantify the isostatic stress field generated by the Fennoscandian glacial load, we consider layered Maxwell-viscoelastic half-spaces loaded by axisymmetric discs. In particular, we calculate the maximum shear-stress distribution induced by the load model and the associated stress regimes in the lithosphere. Emphasis is placed on a discussion of the sensitivity of the calculated isostatic stresses to variations in load cross-section, lithosphere thickness and asthenosphere viscosity. We also examine the effects produced by the viscous relaxation of the lithosphere, whose viscosity is parameterized according to the stratification expected for temperature-activated diffusion creep. Our results show that the isostatic shear stresses typically reach ∼2 MPa near the surface at the present time but that their distribution may be modified strongly by the details of the earth and load models investigated. The viscous relaxation of the lithosphere becomes particularly important, if more than one glaciation is considered. Since the last ice age in Fennoscandia consisted of several glaciations, the model of a perfectly elastic lithosphere conventionally employed in models of glacial-isostatic adjustment therefore may not be justified when calculating the isostatic stresses.

MANTLE VISCOSITY, GLACIAL ISOSTATIC ADJUSTMENT AND THE EUSTATIC LEVEL OF THE SEA

Tide gauge recordings of the secular variation of relative sea level are known to be strongly influenced by the ongoing global process of glacial isostatic adjustment. The east coast of the North American continent is heavily instrumented with tide gauge installations, many of which have been carefully maintained for over 50 years. Since this region traverses the collapsing forebulge of the Laurentide ice sheet and since the process of collapse is extremely well constrained on the basis of radio-carbon dated relative sea level histories from a dense set of locations, the region is globally unique in enabling an accurate decontamination of the tide gauge data using the 14C records themselves. It is shown herein that the decontaminated data define a residual signal which varies only slightly along the coast and which consists of an average rate of sea level rise near 2 mm yr-1. The relative sea level histories from sites along this coast also provide an excellent basis for testing theoretical models of the global glacial isostatic adjustment process that must be employed to decontaminate the tide gauge records at sites for which 14C records are unavailable. It is demonstrated that a mantle viscosity profile determined by the formal inversion of 14C controlled relative sea level histories from sites within the margins of the northern hemisphere ice sheets that existed at last glacial maximum enables a gravitationally and topographically self-consistent global model of glacial isostatic adjustment to accurately reconcile east coast rsl data. No viscosity structure has previously been derived that was successful in this regard. The global model based on this structure is therefore expected to provide an excellent basis for the removal of glacial isostatic adjustment effects from tide gauge recordings. The viscosity structure itself is also extremely close to models of the radial variation that have previously been shown to fit the requirements of non-hydrostatic geoid anomalies. This has important geodynamic implications concerning mantle rheology as it would appear to establish that transient and therefore non-linear creep mechanisms are not involved, since ‘short’’ timescale and ‘‘long’’ timescale viscosities are the same.

The changing role of the lithosphere in models of glacial isostasy: a historical review

During the last 125 years, the role of the lithosphere in models of glacial-isostatic adjustment experienced several changes. Following the postulation of glacial isostasy by Jamieson in 1865, the lithosphere was generally regarded as comparable in importance for the adjustment process to the fluid substratum. This changed with the initiation of quantitative modelling by Van Bemmelen and Berlage and by Haskell in 1935, whereupon effects due to the lithosphere were commonly neglected in interpretations of postglacial uplift for 30 years. After the development of a layered viscous earth model with an elastic surface layer by McConnell in 1965, the lithosphere was eventually reintroduced into models of glacial isostasy. Subsequent studies largely confirmed the original ideas regarding the importance of the lithosphere for the adjustment process, although the effects are pronounced only for short-wavelength deformations. Using this response characteristic of the lithosphere, estimates of its thickness have recently become available for several tectonic provinces.

Observations of postglacial uplift at Churchill, Manitoba

Churchill, Manitoba, is located near the centre of postglacial uplift caused by the Earth's recovery from the melting of the Laurentide Ice Sheet. The value of present-day uplift at Churchill has important implications in the study of postglacial uplift in that it can aid in constraining the thickness of the ice sheet and the rheology of the Earth. The tide-gauge record at Churchill since 1940 is examined, along with nearby Holocene relative sea-level data, geodetic measurements, and recent absolute gravimetry measurements, and a present-day rate of uplift of 8–9 mm/a is estimated. Glacial isostatic adjustment models yield similar estimates for the rate of uplift at Churchill. The effects of the tide-gauge record of the diversion of the Churchill River during the mid-1970's are discussed.