Ongoing Glacial Isostatic Adjustment Research Articles

Influence of GIA Uncertainty on Climate Model Evaluation With GRACE/GRACE‐FO Satellite Gravimetry Data

AbstractGlobal coupled climate models are in continuous need for evaluation against independent observations to reveal systematic model deficits and uncertainties. Changes in terrestrial water storage (TWS) as measured by satellite gravimetry missions GRACE and GRACE‐FO provide valuable information on wetting and drying trends over the continents. Challenges arising from a comparison of observed and modelled water storage trends are related to gravity observations including non‐water related variations such as, for example, glacial isostatic adjustment (GIA). Therefore, correcting secular changes in the Earth's gravity field caused by ongoing GIA is important for the monitoring of long‐term changes in terrestrial water from GRACE in particular in former ice‐covered regions. By utilizing a new ensemble of 56 individual realizations of GIA signals based on perturbations of mantle viscosities and ice history, we find that many of those alternative GIA corrections change the direction of GRACE‐derived water storage trends, for example, from gaining mass into drying conditions, in particular in Eastern Canada. The change in the sign of the TWS trends subsequently impacts the conclusions drawn from using GRACE as observational basis for the evaluation of climate models as it influences the dis‐/agreement between observed and modelled wetting/drying trends. A modified GIA correction, a combined GRACE/GRACE‐FO data record extending over two decades, and a new generation of climate model experiments leads to substantially larger continental areas where wetting/drying trends currently observed by satellite missions coincide with long‐term predictions obtained from climate model experiments.

Glacial isostatic adjustment and post-seismic deformation in Antarctica

Abstract This chapter reviews glacial isostatic adjustment (GIA) and post-seismic deformation in Antarctica. It discusses numerical models and their inputs, and observations and inferences that have been made from them. Both processes are controlled by mantle viscosity but their forcings are different. Ongoing GIA induced by the loss of ice since the last glacial maximum (LGM) could have amounted to 5–15 m of global sea-level rise. However, mantle viscosity is so low in parts of West Antarctica ( c. 10 18 Pa s) that changes in ice thickness over the last centuries and decades have controlled the current uplift rates there. The uplift due to GIA has promoted ice-sheet stability since the LGM, and in West Antarctica GIA is a significant negative feedback on the current decline of the ice sheet. Post-seismic deformation following the 1998 earthquake near the Balleny Islands south of New Zealand has been detected in global navigation satellite system (GNSS) data and compared to model outputs. The best-fitting viscosity for this area is c. 10 19 Pa s, similar to GIA-based estimates for the Antarctic Peninsula. Future work should focus on unifying descriptions of viscosity across geodynamic models, and integrating information from seismic, gravity, experimental and geological data.

The near-surface structure in the area of the Børglum fault, Sorgenfrei-Tornquist Zone, northern Denmark: Implications for fault kinematics, timing of fault activity and fault control on tunnel valley formation

Previous studies showed evidence that the Sorgenfrei-Tornquist Zone (STZ) in northern Denmark is seismically active and fuelled the demand for further analyses of the fault structure and its seismic hazard potential. We focus on the area of the Børglum fault, which represents the northern boundary fault of the STZ and extends at least 250 km from the western Baltic Sea across Jutland into the southeastern North Sea. The Børglum fault has previously been interpreted to represent a glacially-triggered fault. Two shear-wave reflection seismic sections were acquired to image the near-surface structure in the vicinity of the Børglum fault. Detailed insights into the acquisition, processing and interpretation of the seismic sections will be provided. The seismic sections indicate that the Børglum fault in the study area is not an isolated fault, but most likely represents a complex fault system with a strike-slip component. The fault interpretation is based on systematic vertical reflector offsets, the observation of steeply dipping thin transparent zones that separate the offset reflectors, abrupt lateral changes in the reflector pattern and in some cases, the presence of a fault shadow (a diffuse zone on a seismic section below a fault). This interpretation is mainly based on the identification of positive flower structures on the seismic sections, and supported by the presence of elongated sedimentary mini-basins and topographic depressions in the study area, which are partly interpreted to represent strike-slip basins. A WNW-ESE striking tunnel-valley system is imaged on the seismic sections. The spatial relationship between the faults and the tunnel valley suggests a fault-control on tunnel-valley system formation in this area. The seismic sections indicate that faults in the study area were probably active at the end of the Saalian glaciation (MIS 6). In addition, soft-sediment deformation structures and shear-deformation bands developed in Late Pleniglacial to Lateglacial sediments point to repeated phases of fault activity. The reflector pattern on one of the seismic sections indicates that a major fault nearly reaches the Earth's surface. A step in the topography above the fault probably represents a fault-scarp. Because the ongoing glacial-isostatic adjustment will lead to a change in the stress magnitude in this area, future movements along the faults are possible.

Upper mantle density and surface gravity change in Fennoscandia, determined from GRACE monthly data

Precise gravity measurements have been repeatedly observed in Fennoscandia since the 1960-ties, first by relative gravimeters, then by absolute gravimeters, for studying the temporal change of gravity related with the on-going glacial isostatic adjustment (GIA). The results of such studies are vital for understanding GIA processes and tuning GIA and Earth structure models.This study uses monthly repeated data from the twin satellite mission GRACE for 164 months between 2003 and 2016 for estimating the temporal change of surface gravity, its ratio (f) to the land uplift rate as well as the upper mantle density related with the viscous mass flow in the mantle. The maximum negative change of gravity is estimated to −1.8 μGal/yr, and f is estimated to −0.172 ± 0.018 μGal/mm.The solutions for f from the three independent techniques (relative, absolute and satellite gravity observations) were found to agree statistically without significant biases, and they were merged in a joint solution to −0.166 ± 0.004 μGal/mm, corresponding to a mean upper mantle mass flow density of 3402.5 ± 95 kg/m3, which decreases towards the uplift center.

Boulder barricades in an emerging hypertidal environment: the case of Ungava Bay, northern Quebec, Canada

ABSTRACTBoulder barricades are poorly documented Holocene coastal landforms of Arctic and Subarctic regions. To understand the terrain factors and coastal processes explaining the occurrences of boulder barricades, we mapped and described these periglacial landforms along the emerging hypertidal coast of Ungava Bay, in Nunavik, northern Quebec, Canada. This study examines the spatial distribution as well as the different types and configurations of boulder barricades, and identifies and discusses the formation processes of these ridges, which are affected by the ongoing glacial isostatic adjustment (GIA) process. We distinguish four types of ice-formed boulder ridges: low-tide-, intertidal-, high-tide- and raised boulder barricades. Our results highlight the importance of specific terrain factors (coastal configurations, intertidal and backshore slopes, widths of intertidal zones, susceptibility to coastal erosion) and related coastal processes (ice-pushing and ice-rafting intensified by strong tidal currents), forming these different types of barricades. Due to the GIA process, this coastal region is characterized by a large number of evolutionary sequences (i.e. flights) of boulder barricades, from the intertidal zone to the emerged landscape. These sea-ice-related landforms have significant effects on the coastal geomorphology of the Ungava Bay region by successively enclosing and isolating many ponds and lakes.

Structural Inheritance Control on Intraplate Present‐Day Deformation: GPS Strain Rate Variations in the Saint Lawrence Valley, Eastern Canada

AbstractStructural inheritance is one of the key factors commonly proposed to control the localization of strain and seismicity in continental intraplate regions, primarily on the basis of a first‐order spatial correlation between seismicity and inherited tectonic structures. In this paper, we present new GPS (Global Positioning System) velocity and strain rate analyses that provide strong constraints on the magnitude and style of present‐day strain localization associated with the inherited tectonic structures of the Saint Lawrence Valley, eastern Canada. We analyze 143 continuous and campaign GPS stations to calculate velocity and strain rate patterns, with specific emphases on the combination of continuous and campaign velocity uncertainties, and on the definition of robustness categories for the strain rate estimations. Within the structural inheritance area, strain rates are on average 2–11 times higher than surrounding regions and display strong lateral variations of the style of deformation. These GPS velocity and strain rate fields primarily reflect ongoing glacial isostatic adjustment (GIA). Their comparison with GIA model predictions allows us to quantify the impact of the structural inheritance and the associated lithosphere rheology weakening. Outside of the major tectonic inheritance area, GPS and GIA model strain rates agree to first order, both in style and magnitude. In contrast, the Saint Lawrence Valley displays strong strain amplification with GPS strain rates 6–28 times higher than model‐predicted GIA strain rates. Our results provide the first quantitative constraints on the impact of lithospheric‐scale structural inheritance on strain localization in intraplate domains.

The impact of water loading on postglacial decay times in Hudson Bay

Ongoing glacial isostatic adjustment (GIA) due to surface loading (ice and water) variations during the last glacial cycle has been contributing to sea-level changes globally throughout the Holocene, especially in regions like Canada that were heavily glaciated during the Last Glacial Maximum (LGM). The spatial and temporal distribution of GIA, as manifested in relative sea-level (RSL) change, are sensitive to the ice history and the rheological structure of the solid Earth, both of which are uncertain. It has been shown that RSL curves near the center of previously glaciated regions with no ongoing surface loading follow an exponential-like form, with the postglacial decay times associated with that form having a weak sensitivity to the details of the ice loading history. Postglacial decay time estimates thus provide a powerful datum for constraining the Earth's viscous structure and improving GIA predictions. We explore spatial patterns of postglacial decay time predictions in Hudson Bay by decomposing numerically modeled RSL changes into contributions from water and ice loading effects, and computing their relative impact on the decay times. We demonstrate that ice loading can contribute a strong geographic trend on the decay time estimates if the time window used to compute decay times includes periods that are temporally close to (i.e. contemporaneous with, or soon after) periods of active loading. This variability can be avoided by choosing a suitable starting point for the decay time window. However, more surprisingly, we show that across any adopted time window, water loading effects associated with inundation into, and postglacial flux out of, Hudson Bay and James Bay will impart significant geographic variability onto decay time estimates. We emphasize this issue by considering both maps of predicted decay times across the region and site-specific estimates, and we conclude that variability in observed decay times (whether based on existing or future data sets) may reflect this water loading signal.

Inferring regional vertical crustal velocities from averaged relative sea level trends: A proof of concept

AbstractThis study demonstrates that relative sea level trends calculated from long-term tide gauge records can be used to estimate relative vertical crustal velocities in a region with high accuracy. A comparison of the weighted averages of the relative sea level trends estimated at six tide gauge stations in two clusters along the Eastern coast of United States, in Florida and in Maryland, reveals a statistically significant regional vertical crustal motion of Maryland with respect to Florida with a subsidence rate of −1.15±0.15 mm/yr identified predominantly due to the ongoing glacial isostatic adjustment process. The estimate is a consilience value to validate vertical crustal velocities calculated from GPS time series as well as towards constraining predictive GIA models in these regions.

Present-Day 3D Velocity Field of Eastern North America Based on Continuous GPS Observations

The Saint Lawrence River valley in eastern Canada was studied using observations of continuously operating GPS (CGPS) stations. The area is one of the most seismically active regions in eastern North America characterized by many earthquakes, which is also subject to an ongoing glacial isostatic adjustment. We present the current three-dimensional velocity field of eastern North America obtained from more than 14 years (9 years on average) of data at 112 CGPS stations. Bernese GNSS and GITSA software were used for CGPS data processing and position time series analysis, respectively. The results show the counterclockwise rotation of the North American plate in the No-Net-Rotation model with the average of 16.8 ± 0.7 mm/year constrained to ITRF 2008. We also present an ongoing uplift model for the study region based on the present-day CGPS observations. The model shows uplift all over eastern Canada with the maximum rate of 13.7 ± 1.2 mm/year and subsidence to the south mainly over northern USA with a typical rate of −1 to −2 mm/year and the minimum value of −2.7 ± 1.4 mm/year. We compared our model with the rate of radial displacements from the ICE-5G model. Both models agree within 0.02 mm/year at the best stations; however, our model shows a systematic spatial tilt compared to ICE-5G. The misfits between two models amount to the maximum relative subsidence of −6.1 ± 1.1 mm/year to the east and maximum relative uplift of 5.9 ± 2.7 mm/year to the west. The intraplate horizontal velocities are radially outward from the centers of maximum uplift and are inward to the centers of maximum subsidence with the typical velocity of 1–1.6 ± 0.4 mm/year that is in agreement with the ICE-5G model to the first order.

Observed secular gravity trend at Onsala station with the FG5 gravimeter from Hannover

Abstract Annual absolute gravity measurements with a FG5 instrument were performed in Onsala Space Observatory by the Institute of Geodesy of the Leibniz Universität Hannover from 2003 to 2011 and have been continued with the upgraded meter FG5X in 2014. Lantmäteriet, Gävle, with their FG5 absolute gravimeter have visited Onsala since 2007. Because small systematic errors may be inherent in each absolute gravimeter, their measuring level and a resulting bias (offset) between the instruments must be controlled over time by means of inter-comparison. From 2007 to 2014, 8 direct comparisons took place well distributed over the time span. A complete re-processing of the absolute gravity observations with the Hannover instrument has been conducted to improve the reduction of unwanted gravity effects. A new tidal model is based on continuous time series recorded with the GWR superconducting gravimeter at Onsala since 2009. The loading effect of the Kattegat is described with a varying sea bottom pressure (water and air mass load) and has been validated with the continuous gravity measurements. For the land uplift,which is a result of the still ongoing glacial isostatic adjustment in Fennoscandia, a secular gravity trend of −0.22 μGal/yr was obtained with a standard deviation of 0.17 μGal/yr. That indicates a slight uplift but is still not significantly different from zero.

Closing the sea level budget at the Last Glacial Maximum

Establishing the volume of excess ice contained in the global ice sheets during the Last Glacial Maximum (LGM; ∼26,000–19,000 y ago) remains a longstanding problem in Ice Age climate dynamics. Expressed as the equivalent lowering of global mean sea level (GMSL), estimates of this value have varied from 105 (1) to 163 m (2), with many estimates suggesting ∼120 m (3). This wide range introduces substantial uncertainties in LGM global boundary conditions (ice sheet height and extent and continental shelf exposure) that strongly influence climate through their impacts on atmospheric and ocean circulation and global temperature. More recently, this uncertainty has influenced our understanding of present sea level change, whereby the ongoing glacial isostatic adjustment (GIA) of the solid Earth to the redistribution of mass since the LGM must be accounted for in satellite measurements of sea level and of current mass changes from shrinkage of glaciers and ice sheets. In PNAS, Lambeck et al. (4) present a comprehensive analysis of nearly 1,000 paleo-sea level markers to reconstruct GMSL change over the last 35,000 y, with their estimate for the LGM GMSL of 134–140 m being significantly larger than most published estimates but similar to a recent reassessment of the Barbados sea level record (130 m) that had been the basis for many estimates of ∼120 m (5). If it stands, this reconstruction will reduce uncertainties in our understanding of Ice Age climate and modern sea level change. At the same time, however, it raises a significant challenge: is it possible to explain this total sea level change from our best estimates of the contributions from individual LGM glaciers and ice sheets?

A Test of Recent Inferences of Net Polar Ice Mass Balance based on Long-Wavelength Gravity

Abstract A comprehensive analysis of satellite datasets has estimated that the ice sheets of Greenland, West Antarctica, the Antarctic Peninsula, and East Antarctica experienced a net mass loss of −100 ± 92 Gt yr−1 over the period 1992–2000 and −298 ± 58 Gt yr−1 over the period 2000–11, representing an increase of −198 ± 109 Gt yr−1 between the two epochs. The authors demonstrate that the time rate of change of the degree-four zonal harmonic of Earth's gravitational potential provides an independent check on these mass balances that is less sensitive to uncertainties that have contaminated previous analyses of the degree-2 zonal harmonic [e.g., due to ongoing glacial isostatic adjustment (GIA), solid Earth body tides, and core–mantle coupling]. For the period 2000–11, the signal implied by the ice sheet mass flux cited above is (3.8 ± 0.6) × 10−11 yr−1, whereas the change in the harmonic across the two epochs is (2.3 ± 1.1) × 10−11 yr−1. In comparison, using satellite laser ranging (SLR) data, the authors estimate a GIA-corrected value of (3.8 ± 0.6) × 10−11 yr−1 for the epoch 2000–11 and a change across the two epochs of (5.3 ± 1.6) × 10−11 yr−1. The authors conclude that the former supports recent estimates of melting over the last decade, whereas the latter suggests either that estimated melt rates for the earlier epoch were too high or that the uncertainty associated with the SLR-based inference of during the earlier epoch is underestimated.

On the Accuracy of Glacial Isostatic Adjustment Models for Geodetic Observations to Estimate Arctic Ocean Sea-Level Change

Arctic Ocean sea-level change is an important indicator of climate change. Contemporary geodetic observations, including data from tide gages, satellite altimetry and Gravity Recovery and Climate Experiment (GRACE), are sensitive to the effect of the ongoing glacial isostatic adjustment (GIA) process. To fully exploit these geodetic observations to study climate related sea-level change, this GIA effect has to be removed. However, significant uncertainty exists with regard to the GIA model, and using different GIA models could lead to different results. In this study we use an ensemble of 14 contemporary GIA models to investigate their differences when they are applied to the above-mentioned geodetic observations to estimate sea-level change in the Arctic Ocean. We find that over the Arctic Ocean a large range of differences exists in GIA models when they are used to remove GIA effect from tide gage and GRACE observations, but with a relatively smaller range for satellite altimetry observations. In addition, we compare the derived sea-level trend from observations after applying different GIA models in the study regions, sea-level trend estimated from long-term tide gage data shows good agreement with altimetry result over the same data span. However the mass component of sea-level change obtained from GRACE data does not agree well with the result derived from steric-corrected altimeter observation due primarily to the large uncertainty of GIA models, errors in the Arctic Ocean altimetry or steric measurements, inadequate data span, or all of the above. We conclude that GIA correction is critical for studying sea-level change over the Arctic Ocean and further improvement in GIA modelling is needed to reduce the current discrepancies among models.

The effect of melting land-based ice masses on sea-level around the Australian coastline

Changes in relative sea-level (RSL) are generally caused by variations in sea surface heights from steric effects (thermal expansion and salinity changes) and the mechanical response of the Earth to past and current redistributions of ice and water between land and oceans. This paper focuses on the latter, where we present scenario calculations of the spatial variability in present-day RSL change around the Australian coastline resulting from melting land-based ice masses. Three scenarios are investigated: (1) the ongoing effect of glacial isostatic adjustment (GIA) arising from ice- and water-load redistribution during the last glacial–interglacial transition; (2) the effect of present-day changes in the Greenland and West and East Antarctic ice sheets (GIS, WAIS and EAIS, respectively) and two regions of major mountain glaciation, Alaska and Patagonia; and (3) a hypothetical complete melting of the GIS, WAIS and EAIS occurring over 5000 years. The first scenario shows falling RSL around Australia of the order of 0.4 to 1.2 times the average value around the coast (equivalent to a RSL fall of between 0.2 and 0.6 mm/a). For the second scenario, the spatial variability is strongly dependent upon the location of each ice mass relative to Australia. For Greenland and Patagonia, the resulting changes to the Earth's rotation strongly affect the spatial variability, while the direct gravitational effect is more important when considering the Antarctic ice sheets. The variability associated with the first two scenarios becomes clearer when examining RSL change estimates for the locations of tide-gauge stations around the Australian coast, especially for the ongoing GIA (a south-to-north increase in the simulated rate of RSL change), the WAIS (east-to-west increase) and the EAIS (south-to-north increase), with the melting of the EAIS potentially having the greatest influence on the variability of the melting land-based ice contribution to RSL change around Australia. The spatial variability associated with the third scenario is strongly influenced over century-length time-scales by the resulting changes in the Earth's rotation and the direct gravitational attraction of the ice masses, while after several thousand years the uplift of the continent by mantle material displaced towards it by increased ocean loading becomes more prominent. It must, however, be kept in mind that the spatial variability associated with these scenarios is generally a small proportion of the total RSL change, and that the steric (especially thermosteric) contribution is not included in these results.

Geological constraints on glacio-isostatic adjustment models of relative sea-level change during deglaciation of Prince Gustav Channel, Antarctic Peninsula

The recent disintegration of Antarctic Peninsula ice shelves, and the associated accelerated discharge and retreat of continental glaciers, has highlighted the necessity of quantifying the current rate of Antarctic ice mass loss and the regional contributions to future sea-level rise. Observations of present day ice mass change need to be corrected for ongoing glacial isostatic adjustment, a process which must be constrained by geological data. However, there are relatively little geological data on the geometry, volume and melt history of the Antarctic Peninsula Ice Sheet (APIS) after Termination 1, and during the Holocene so the glacial isostatic correction remains poorly constrained. To address this we provide field constraints on the timing and rate of APIS deglaciation, and changes in relative sea-level (RSL) for the north-eastern Antarctic Peninsula based on geomorphological evidence of former marine limits, and radiocarbon-dated marine-freshwater transitions from a series of isolation basins at different altitudes on Beak Island. Relative sea-level fell from a maximum of c. 15 m above present at c. 8000 cal yr BP, at a rate of 3.91 mm yr −1 declining to c. 2.11 mm yr −1 between c. 6900–2900 cal yr BP, 1.63 mm yr −1 between c. 2900–1800 cal yr BP, and finally to 0.29 mm yr −1 during the last c. 1800 years. The new Beak Island RSL curve improves the spatial coverage of RSL data in the Antarctic. It is in broad agreement with some glacio-isostatic adjustment models applied to this location, and with work undertaken elsewhere on the Antarctic Peninsula. These geological and RSL constraints from Beak Island imply significant thinning of the north-eastern APIS by the early Holocene. Further, they provide key data for the glacial isostatic correction required by satellite-derived gravity measurements of contemporary ice mass loss, which can be used to better assess the future contribution of the APIS to rising sea-levels.

Relationship between glacial isostatic adjustment and gravity perturbations observed by GRACE

[1] The Gravity Recovery and Climate Experiment space gravity mission provides one of the principal means of estimating present-day mass loss occurring in polar regions. Extraction of the mass loss signal from the observed gravity changes is complicated by the need to first remove the signal of ongoing glacial isostatic adjustment (GIA) since the Last Glacial Maximum. This can be problematic in regions such as Antarctica where the GIA models are poorly constrained by observation and their accuracy is not well known. We present a new methodology that permits the GIA component to be represented mathematically by a simple, linear expression of the ratio of viscoelastic Love numbers that is valid for a broad range of Earth and ice-load models. The expression is shown to reproduce rigorous computations of surface uplift rates to within 0.3 mm/yr, thus providing a means of inverting simultaneously for present-day mass loss and ongoing GIA with all the accuracy of a fully detailed forward model.

Using Holocene relative sea-level data to inform future sea-level predictions: An example from southwest England

Holocene relative sea-level data contain information on vertical land movements along coasts and, hence, can provide vital input for predictions of future sea-level change. At Thurlestone, in southwest England, late Holocene coastal sediments were cored and sampled in coastal back-barrier marshes. The presence of a basal sedimentary unit containing salt-marsh microfossils made it possible to obtain precise estimates of late Holocene relative sea-level change from the sediments. This is important because previous studies have suggested that the southwest of England is experiencing the fastest rates of land subsidence in the British Isles. Ten new late Holocene basal sea-level index points fill an important gap in the palaeosea-level data set for southwest England. Another 15 early and middle Holocene sea-level index points are available from previous work. The data show that relative sea level rose by about 10 m between 9000 and 7000 cal. yr BP and a further 8 m in the last 7000 yr. In the last 2000 yr, relative sea level rose on average by 0.9 mm/yr. The coast is currently subsiding by 1.1 mm/yr due to ongoing glacial isostatic adjustment (GIA). The Bradley et al. (2009) GIA model, which is used in the United Kingdom to determine land-motion rates for input into future sea-level predictions, underestimates the rate of coastal subsidence by about 0.16 mm/yr, but performs better than other models. Our data validate the land-motion rates currently used in regional sea-level projections.

Time-variable ice loss in Asian high mountains from satellite gravimetry

Substantial amount of glacial ice is considered to be melting in the Asian high mountains. Gravimetry by GRACE satellite during 2003–2009 suggests the average ice loss rate in this region of 47 ± 12 Gigaton (Gt) yr − 1 , equivalent to ∼ 0.13 ± 0.04 mm yr − 1 sea level rise. This is twice as fast as the average rate over ∼ 40 years before the studied period, and agrees with the global tendency of accelerating glacial loss. Such ice loss rate varies both in time and space; mass loss in Himalaya is slightly decelerating while those in northwestern glaciers show clear acceleration. Uncertainty still remains in the groundwater decline in northern India, and proportion of almost isostatic (e.g. tectonic uplift) and non-isostatic (e.g. glacial isostatic adjustment) portions in the current uplift rate of the Tibetan Plateau. If gravity increase associated with ongoing glacial isostatic adjustment partially canceled the negative gravity trend, the corrected ice loss rate could reach 61 Gt yr − 1 .

Spatial variability of late Holocene and 20th century sea-level rise along the Atlantic coast of the United States

Accurate estimates of global sea-level rise in the pre-satellite era provide a context for 21 st century sea-level predictions, but the use of tide-gauge records is complicated by the contributions from changes in land level due to glacial isostatic adjustment (GIA). We have constructed a rigorous quality-controlled database of late Holocene sea-level indices from the U.S. Atlantic coast, exhibiting subsidence rates of <0.8 mm a –1 in Maine, increasing to rates of 1.7 mm a –1 in Delaware, and a return to rates <0.9 mm a –1 in the Carolinas. This pattern can be attributed to ongoing GIA due to the demise of the Laurentide Ice Sheet. Our data allow us to defi ne the geometry of the associated collapsing proglacial forebulge with a level of resolution unmatched by any other currently available method. The corresponding rates of relative sea-level rise serve as background rates on which future sea-level rise must be superimposed. We further employ the geological data to remove the GIA component from tide-gauge records to estimate a mean 20 th century sea-level rise rate for the U.S. Atlantic coast of 1.8 ± 0.2 mm a –1 , similar to the global average. However, we fia distinct spatial trend in the rate of 20 th century sea-level rise, increasing from Maine to South Carolina. This is the fi rst evidence of this phenomenon from observational data alone. We suggest this may be related to the melting of the Greenland ice sheet and/or ocean steric effects.

Glacial Isostatic Adjustment over Antarctica from combined ICESat and GRACE satellite data

The glacial history of Antarctica during the most recent Milankovitch cycles is poorly constrained relative to the Northern Hemisphere. As a consequence, the contribution of mass changes in the Antarctic ice sheet to global sea-level change and the prediction of its future evolution remain uncertain. The process of Glacial Isostatic Adjustment (GIA) represents the ongoing response of the solid Earth to the Late-Pleistocene deglaciation and, therefore, provides information about Antarctic glacial history. Moreover, insufficient knowledge of GIA hampers the determination of present-day changes in the Antarctic mass balance through satellite gravity measurements. Previous studies have laid the theoretical foundation for distinguishing between signals of ongoing GIA and contemporary ice mass change through the combination of satellite gravimetry and satellite altimetry. This distinction is made possible by the fact the GIA-induced changes (involving relatively dense rock) will produce a different combination of topography and gravity change than those produced by variations in ice or firn thickness (due to the lower density of these materials); however, no conclusive results have been produced to date. Here we show that, by combining laser altimetry and gravity data from the ICESat and GRACE satellite missions over the period March 2003–March 2008, the GIA contribution can indeed be isolated. The inferred GIA signal over the Antarctic continent, which represents the first result derived from direct observations by satellite techniques, strongly supports Late-Pleistocene ice models derived from glacio-geologic studies. The GIA impact on GRACE-derived estimates of mass balance is found to be 100 ± 67 Gt/yr.