Glacial Isostatic Adjustment Models Research Articles

A global semi-empirical glacial isostatic adjustment (GIA) model based on Gravity Recovery and Climate Experiment (GRACE) data

Abstract. The effect of glacial isostatic adjustment (GIA) on the shape and gravity of the Earth is usually described by numerical models that solve for both glacial evolution and Earth's rheology, being mainly constrained by the geological evidence of local ice extent and globally distributed sea level data, as well as by geodetic observations of Earth's rotation. In recent years, GPS and GRACE observations have often been used to improve those models, especially in the context of regional studies. However, consistency issues between different regional models limit their ability to answer questions from global-scale geodesy. Examples are the closure of the sea level budget, the explanation of observed changes in Earth's rotation, and the determination of the origin of the Earth's reference frame. Here, we present a global empirical model of present-day GIA, solely based on GRACE data and on geoid fingerprints of mass redistribution. We will show how the use of observations from a single space-borne platform, together with GIA fingerprints based on different viscosity profiles, allows us to tackle the questions from global-scale geodesy mentioned above. We find that, in the GRACE era (2003–2016), freshwater exchange between land and oceans has caused global mean sea level to rise by 1.2±0.2 mm yr−1, the geocentre to move by 0.4±0.1 mm yr−1, and the Earth's dynamic oblateness (J2) to increase by 6.0±0.4×10-11 yr−1.

ITRF2014, Earth Figure Changes, and Geocenter Velocity: Implications for GIA and Recent Ice Melting

AbstractUsing a selection of Global Navigation Satellite System vertical velocities from the latest solution of the International Terrestrial Reference Frame (ITRF) ITRF2014, we calculate the degree‐1 and degree‐2 spherical harmonics coefficients (SHC) of the solid Earth figure changes at different dates, with realistic errors that take into account the inhomogeneity of the network. We find that the SHC are globally close to zero except the zonal coefficients, which show values notably larger than those derived from different glacial isostatic adjustment (GIA) models and which have tended to increase during the time span of observations. We show that these differences are most probably due to global recent ice melting (RIM). Assuming elastic RIM deformation, we then investigate the Earth's geocenter velocity and the geoid oblateness time evolution (J2‐rate) derived from our SHC estimations. The obtained geocenter velocity reaches 0.9 ± 0.5 mm/year in 2013 with a z‐component of 0.8 ± 0.4 mm/year, which is slightly larger than previous estimations. We compare our J2‐rate estimations with observations. Our estimations show a similar acceleration in J2 after 2000. However, our estimates are notably larger than the observations. This indicates either that the J2‐rate due to GIA processes is lower than expected (as proposed by Nakada et al., 2015, 2016) or that the deformation induced by RIM is not purely elastic, or both. Finally, we show that viscous relaxation or phase transitions in the mantle transition zone may only partly explain this discrepancy. This raises the question of the accuracy of current mass estimations of RIM and GIA models.

Present‐Day Vertical Land Motion of Australia From GPS Observations and Geophysical Models

AbstractThe secular rate of Australia's vertical surface deformation due to past ice‐ocean loading changes is not consistent with present vertical velocities observed by a previously sparse network of Global Positioning System (GPS) sites. Current understanding of the Earth's rheology suggests that the expected vertical motion of the crust should be close to zero given that Australia is located in the far field of past ice sheet loading. Recent GPS measurements suggest that the vertical motion of the Australian continent at permanent sites is between 0 and −2 mm/year. Here we investigate if vertical deformation due to previous ice sheet loading can be recovered in the time series of Australian GPS sites through enlarging the number of sites compared to previous studies from ~20 to more than 100 and through the application of improved data filtering. We apply forward geophysical models of elastic surface displacement induced by atmospheric, hydrologic, nontidal ocean, and ice loading and use independent component analysis as a spatiotemporal filter that includes multivariate regression to consider temporally correlated noise in GPS. Using this approach, the common mode error is identified, and subsequent multivariate regression leads to an average reduction in trend uncertainty of ~35%. The average vertical subsidence of the Australian continent is substantially different to vertical motion predicted by glacial isostatic adjustment and surface mass transport models.

Evaluation of the glacial isostatic adjustment (GIA) models for Antarctica based on GPS vertical velocities

Due to the scarcity of data, modeling the glacial isostatic adjustment (GIA) for Antarctica is more difficult than it is for the ancient ice sheet area in North America and Northern Europe. Large uncertainties are observed in existing GIA models for Antarctica. Modern space-based geodetic measurements provide checks and constraints for GIA models. The present-day uplift velocities of global positioning system (GPS) stations at 73 stations in Antarctica and adjacent regions from 1996 to 2014 have been estimated using GAMIT/GLOBK version 10.5 with a colored noise model. To easily analyze the effect of difference sources on the vertical velocities, and for easy comparison with both GIA model predictions and GPS results from Argus et al. (2014) and Thomas et al. (2011), seven sub-regions are divided. They are the northern Antarctic Peninsula, the Filchner-Ronne Ice Shelf, the Amundsen Sea coast, the Ross Ice Shelf, Mount Erebus, inland Southwest Antarctica and the East Antarctic coast, respectively. The results show that the fast uplift in the north Antarctic Peninsula and Pine Island Bay regions may be caused by the elastic response to snow and ice mass loss. The fast subsidence near Mount Erebus may be related to the activity of a magma body. The uplift or subsidence near the East Antarctic coast is very slow while the uplift for the rest regions is mainly caused by GIA. By analyzing the correlation and the associated weighted root mean square (WRMS) between the GIA predictions and the GPS velocities, we found that the ICE-6G_C (VM5a) model and the Geruo13 model show the most consistency with our GPS results, while the W12a and IJ05_R2 series models show poor consistency with our GPS results. Although improved greatly in recent years, the GIA modeling in Antarctica still lags behind the modeling of the North American. Some GPS stations, for example the Bennett Nunatak station (BENN), have observed large discrepancies between GIA predictions and GPS velocities. Because of the large uncertainties in calculating elastic responses due to the significant variations of ice and snow loads, the GPS velocities still cannot be used as a precise constraint on GIA models.

A baseline Antarctic GIA correction for space gravimetry

Within the past decade, newly collected GPS data and geochronological constraints have resulted in refinement of glacial isostatic adjustment (GIA) models for Antarctica. These are critical to estimating and understanding ice mass changes at present-day. A correction needs to be made when using space gravity for ice mass balance assessments due to the fact that any vertical movements of the solid Earth masquerade as changes in ice mass, and these must be carefully removed. The main upshot of the new Antarctic GIA models is a downward revision of negative ice mass trends deduced from the Gravity Recovery and Climate Experiment (GRACE), resulting from a reduced GIA correction. This revision places GRACE inferred trend in mass balance within the one sigma uncertainty of mass balance deduced by altimetry. Because uncertainties in Holocene ice history and the low viscosity rheology beneath the West Antarctic Ice Sheet (WAIS) continue to vex further improvement in predictions of present-day GIA gravity rate, more emphasis has been given to regional-scale GIA models. Here we use a Bayesian method to explore the gravimetric GIA trend over Antarctica, both with and without the impact of a late Pleistocene Antarctic ice loads, along with the contribution of oceanic loads. We call this model without loads associated with Antarctica a baseline for regional GIA models to build upon. We consider variations of the radial mantle viscosity profile and the volume of continental-scale ice sheets during the last glacial cycle. The modeled baseline GIA is mainly controlled by the lower mantle viscosity and continental levering caused by ocean loading. We find that the predicted baseline GIA correction weakly depends on the ice history. This correction averages to +28.4 [16.5–41.9, 95% confidence] Gt/yr, of which +23.6±5.3 are contributed by East Antarctica. In contrast, with late-Pleistocene Antarctic-proximal ice included, the total modeled mass trend due to GIA is +73.7 [30.1–114.7] Gt/yr. A baseline GIA correction of 28.4 Gt/yr is of order 50% of the mean net mass trend measured during the period 1992-2017. The statistical analysis provides tools for properly synthesizing any regional Antarctic GIA model with a self-consistent far-field component. This may prove important for accounting for both global and regional 3-D variations in mantle viscosity.

SELEN<sup>4</sup> (SELEN version 4.0): a Fortran program for solving the gravitationally and topographically self-consistent sea-level equation in glacial isostatic adjustment modeling

Abstract. We present SELEN4 (SealEveL EquatioN solver), an open-source program written in Fortran 90 that simulates the glacial isostatic adjustment (GIA) process in response to the melting of the Late Pleistocene ice sheets. Using a pseudo-spectral approach complemented by a spatial discretization on an icosahedron-based spherical geodesic grid, SELEN4 solves a generalized sea-level equation (SLE) for a spherically symmetric Earth with linear viscoelastic rheology, taking the migration of the shorelines and the rotational feedback on sea level into account. The approach is gravitationally and topographically self-consistent, since it considers the gravitational interactions between the solid Earth, the cryosphere, and the oceans, and it accounts for the evolution of the Earth's topography in response to changes in sea level. The SELEN4 program can be employed to study a broad range of geophysical effects of GIA, including past relative sea-level variations induced by the melting of the Late Pleistocene ice sheets, the time evolution of paleogeography and of the ocean function since the Last Glacial Maximum, the history of the Earth's rotational variations, present-day geodetic signals observed by Global Navigation Satellite Systems, and gravity field variations detected by satellite gravity missions like GRACE (the Gravity Recovery and Climate Experiment). The “GIA fingerprints” constitute a standard output of SELEN4. Along with the source code, we provide a supplementary document with a full account of the theory, some numerical results obtained from a standard run, and a user guide. Originally, the SELEN program was conceived by Giorgio Spada (GS) in 2005 as a tool for students eager to learn about GIA, and it has been the first SLE solver made available to the community.

Uplift of Trail Ridge, Florida, by Karst Dissolution, Glacial Isostatic Adjustment, and Dynamic Topography

AbstractThe present elevation of Trail Ridge, a purported Pleistocene paleoshoreline (2.21 ± 0.38 Ma) up to ~70 m high, is attributed to isostatic uplift from karst‐induced mass loss of the underlying Florida Platform. However, glacial isostatic adjustment, dynamic topography (DT), and elastic flexure also contribute to the observed topography. We investigated the combined contribution of these geophysical processes to the elevation of Trail Ridge by adapting a numerical model of karst‐induced Airy isostatic uplift that is driven by sea level change since ~3 Ma with a gravitationally self‐consistent glacial isostatic adjustment model. We treat DT as a free parameter and explore a spectrum of scenarios, from dynamic subsidence (−25 m/Myr) to dynamic uplift (+25 m/Myr), with values consistent with mantle convection simulations for the region. We demonstrate a trade‐off between the age of Trail Ridge, karstification efficiency, and DT rate that necessitates robust estimates of two of these variables when solving for the third. For example, adopting spring‐efflux‐derived estimates of subsurface carbonate dissolution (1 m/38,000 yr) and a 10 m/Myr dynamic topographic uplift for the northern Florida Platform predicts an age of ~2.1 Ma for a model 70 m elevation marker, consistent with the bounds of the electron spin resonance optical dating geochronology. In contrast, by assuming a more rapid carbonate dissolution efficiency (1 m/11,000 yr) and dynamic subsidence of −15 m/Myr predicts a similar age. A refined karst‐induced isostatic uplift model age for Trail Ridge requires both improved rates of regional karst dissolution, dynamic topographic change, and elastic flexure.

Towards assessing the influence of sediment loading on Last Interglacial sea level

SUMMARY Locally, the elevation of last interglacial (LIG; ∼122 ka) sea level markers is modulated by processes of vertical displacement, such as tectonic uplift or glacial isostatic adjustment, and these processes must be accounted for in deriving estimates of global ice volumes from geological sea level records. The impact of sediment loading on LIG sea level markers is generally not accounted for in these corrections, as it is assumed that the impact is negligible except in extremely high depositional settings, such as the world's largest river deltas. Here we perform a generalized test to assess the extent to which sediment loading may impact global variability in the present-day elevation of LIG sea level markers. We numerically simulate river sediment deposition using a diffusive model that incorporates a migrating shoreline to construct a global history of sedimentation over the last glacial cycle. We then calculate sea level changes due to this sediment loading using a gravitationally self-consistent model of glacial isostatic adjustment, and compare these predictions to a global compilation of LIG sea level data. We perform a statistical analysis, which accounts for spatial autocorrelation, across a global compilation of 1287 LIG sea level markers. Though limited by uncertainties in the LIG sea level database and the precise history of river deposition, this analysis suggests there is not a statistically significant global signal of sediment loading in LIG sea level markers. Nevertheless, at sites where LIG sea level markers have been measured, local sea level predicted using our simulated sediment loading history is perturbed up to 16 m. More generally, these predictions establish the relative sensitivity of different regions to sediment loading. Finally, we consider the implications of our results for estimates of tectonic uplift rates derived from LIG marine terraces; we predict that sediment loading causes 5–10 m of subsidence over the last glacial cycle at specific locations along active margin regions such as California and Barbados, where deriving long-term tectonic uplift rates from LIG shorelines is a common practice.

Late Holocene ice-mass changes recorded in a relative sea-level record from Joinville Island, Antarctica

AbstractRecent ice-mass loss driven by warming along the Antarctic Peninsula has resulted in rapid changes in uplift rates across the region. Are such events only a function of recent warming? If not, does the Earth response to such events last long enough to be preserved in Holocene records of relative sea level (RSL), and thus have a bearing on global-scale glacial isostatic adjustment (GIA) models (e.g. ICE-6G)? Answering such questions in Antarctica is hindered by the scarcity of RSL reconstructions within the region. Here, a new RSL reconstruction for Antarctica is presented based on beach ridges from Joinville Island on the Antarctic Peninsula. We find that RSL has fallen 4.9 ± 0.58 m over the past 3100 yr, and that the island experienced a significant increase in the rate of RSL fall from 1540 ± 125 cal. (calibrated) yr B.P. to 1320 ± 125 cal. yr B.P. This increase in the rate of RSL fall is likely due to the viscoelastic response of the solid Earth to terrestrial ice-mass loss from the Antarctic Peninsula, similar to the Earth response experienced after ice-mass loss following acceleration of glaciers behind the collapsed Larsen B ice shelf in 2002 C.E. Additionally, slower rates of beach-ridge progradation from 695 ± 190 cal. yr B.P. to 235 ± 175 cal. yr B.P. potentially reflect erosion of beach ridges from a RSL rise induced by a local glacial advance. The rapid response of the Earth to minor ice-mass changes recorded in the RSL record further supports recent assertions of a more responsive Earth to glacial unloading and at time scales relevant for GIA of Holocene and Pleistocene sea levels. Thus, current continental and global GIA models may not accurately capture the ice-mass changes of the Antarctic ice sheets at decadal and centennial time scales.

Late Quaternary sea-level history of Saipan, Commonwealth of the Northern Mariana Islands, USA: A test of tectonic uplift and glacial isostatic adjustment models

Abstract In 1979, S. Uyeda and H. Kanamori proposed a tectonic model with two end members of a subduction-boundary continuum: the “Chilean” type (shallow dip of the subducting plate, great thrust events, compression, and uplift of the overriding plate) and a “Mariana” type (steep dip of the subducting plate, no great thrust events, tension, and no uplift). This concept has been used to explain variable rates of Quaternary uplift around the Pacific Rim, yet no uplift rates have been determined for the Mariana Islands themselves, one of the end members in this model. We studied the late Quaternary Tanapag Limestone, which rims much of the eastern and southern coasts of Saipan, Northern Mariana Islands, with elevations of ∼13 m to ∼30 m. Samples from 12 well-preserved corals (Acropora, Porites, and Goniastrea) yielded U-series ages ranging from ca. 134 ka to ca. 126 ka. These ages correlate the emergent reef of the Tanapag Limestone with the last interglacial period, when sea level was several meters above present. Ages and measured reef elevations from the Tanapag Limestone, along with paleo–sea-level data, yield relatively low late Quaternary uplift rates of 0.002–0.19 m/k.y., consistent with the Uyeda-Kanamori model. A review of data from other localities near subduction zones around the Pacific Basin, however, indicates that many coastlines do not fit the model. Uplift rates along the Chilean coast are predicted to be relatively high, but field studies indicate they are low. On some coastlines, relatively high uplift rates are better explained by subduction of seamounts or submarine ridges rather than subduction zone geometry. Despite the low long-term uplift rate on Saipan, the island also hosts an emergent, low-elevation (+3.9–4.0 m) reef with corals in growth position below a notch (+4.2 m). The corals are dated to 3.9–3.1 ka. The occurrence of this young, emergent reef is likely not due to tectonic uplift; instead, it is interpreted to be the result of glacial isostatic adjustment processes after the end of the last glacial period. Our findings are consistent with similar observations on tectonically stable or slowly uplifting islands elsewhere in the equatorial Pacific Ocean and agree with numerical models of a higher-than-present Holocene sea level in this region due to glacial isostatic adjustment processes.

Challenges in relative sea-level change assessment highlighted through a case study: The central coast of Atlantic Patagonia

There are areas in the world where our understanding of past sea-level changes is less robust than in others. This is partly due to the difficulty past investigators had adopting i) high-resolution elevation measurement techniques (largely available only in the last decade) and ii) standardized methodological approaches to reconstruct past relative sea levels starting from field measurements. The central coast of Atlantic Patagonia (Argentina), preserves a unique succession of coastal landforms, which scientific investigations suggest to have been formed discontinuously since at least marine isotope stage 11 up to the Holocene. Patagonian coastal deposits (in particular raised beach ridges, marine terraces and river mouth terraces) and erosional landforms (slope angles, marine notches) were studied in the past with the aim of reconstructing relative sea-level changes and provide geological evidence to constrain geophysically based models for an area affected by significant vertical displacement due to isostatic adjustment and possibly by tectonic uplift. So far, there is no general agreement about the interpretation of Patagonia landforms and deposits in terms of sea-level index points. This is certainly a gap within a planetary scale overview of sea-level change and represents an obstacle for the construction of a sound model of glacial isostatic adjustment for the area. In this paper we critically analyse previous works on sea-level change along the central coast of Atlantic Patagonia and highlight the major sources of uncertainty, including choices about calculating past relative sea levels from the elevation of indicators and the vertical datum used. We also provide a comparison between different vertical datums (tidal, gravimetric and geodetic) at a real site (Puerto Deseado) where Differential Global Positioning System measurements were performed by our team in 2016, and address the problem from a trans-disciplinary point of view. In order to obtain accurate sea-level change estimates in the area, we suggest the use of a large spectrum of sea-level markers with well-defined indicative meanings and the adoption of specific technical choices capable of minimizing instrumental errors.

Modelling of glacially-induced stress changes in Latvia, Lithuania and the Kaliningrad District of Russia

We model the change of Coulomb Failure Stress (δCFS) during the Weichselian glaciation up until today at 12 locations in Latvia, Lithuania and Russia that are characterised by soft-sediment deformation structures (SSDS). If interpreted as seismites, these SSDS may point to glacially-induced fault reactivation. The δCFS suggests a high potential of such reactivation when it reaches the instability zone. We show that δCFS at all 12 locations reached this zone several times in the last 120,000 years. Most notably, all locations exhibit the possibility of reactivation after ca. 15 ka BP until today. Another time of possible activity likely happened after the Saalian glaciation until ca. 96 ka BP. In addition, some models suggest unstable states after 96 ka BP until ca. 28 ka BP at selected locations but with much lower positive δCFS values than during the other two periods. For the Valmiera and Rakuti seismites in Latvia, we can suggest a glacially-induced origin, whereas we cannot exactly match the timing at Rakuti. Given the (preliminary) dating of SSDS at some locations, at Dyburiai and Ryadino our modelling supports the interpretation of glacially-induced fault reactivation, while at Slinkis, Kumečiai and Liciškėnai they likely exclude such a source. Overall, the mutual benefit of geological and modelling investigations is demonstrated. This helps in identifying glacially-induced fault reactivation at the south-eastern edge of the Weichselian glaciation and in improving models of glacial isostatic adjustment.

Countrywide Observations of Plate Spreading and Glacial Isostatic Adjustment in Iceland Inferred by Sentinel‐1 Radar Interferometry, 2015–2018

AbstractEast‐west and vertical ground velocities for 2015–2018 are retrieved over 81% of Iceland from Sentinel‐1 radar interferometry, using satellite images from six different tracks. Only summertime images are considered, to avoid snow cover. Average line‐of‐sight velocity fields for 2015–2018 for each track are estimated using a simple approach: single master interferometry time series together with a linear component estimation for each pixel. The line‐of‐sight velocity fields are combined and their signal is decomposed to extract approximate east (near‐East) and approximate vertical (near‐Up) velocities. Only pixels passing a coherence and outlier criteria are considered, resulting in 81% coverage of Iceland. The 19% of missing coverage is mostly glaciers and farmland. We find a general agreement between the near‐East velocity field and a revised plate spreading model, and the near‐Up velocity field and a glacial isostatic adjustment model. Models and their residuals suggest a difference in rheology between the rift zones in Iceland.

Holocene sea levels in Southeast Asia, Maldives, India and Sri Lanka: The SEAMIS database

We assembled a database of Holocene relative sea-level index points (n = 213) and marine (n = 211) and terrestrial (n = 122) limiting points for the broader South and Southeast Asian region including the Maldives, India and Sri Lanka. The standardized review of published age-elevation information from corals, deltaic, estuarine and mangrove deposits, beachrocks and tidal notches, yielded a new suite of relative sea-level index and limiting points produced according to a standardized protocol. Expected spatial variability in Holocene relative sea-level change due to glacial isostatic adjustment was accounted for, by first subdividing the study area into ten geographic sub-regions from the Central Indian Ocean to the Western Tropical Pacific, and second by comparing sub-regional relative sea-level data to model predictions of glacial isostatic adjustment. Results show that some of the regionally constrained relative sea-level data are characterized by significant inconsistencies that cannot be explained by glacial isostatic adjustment. Such inconsistencies of standardized relative sea-level data become particularly obvious in areas around the Red River Delta in Vietnam, the Gulf of Thailand, the northwest coast of Malaysia and the Spermonde Archipelago in Indonesia. Based on a critical evaluation of the reviewed relative sea-level indicators, we discuss possible sources of local divergence and identify regions where data are currently insufficient to constrain glacial isostatic adjustment predictions. The remaining quality-controlled and consistent relative sea-level data show that glacial isostatic adjustment and syn-/post-formational influences such as tectonic uplift, subsidence and compaction were the dominant local drivers of Holocene relative sea-level change. Collectively, the results of this review suggest that Holocene sea levels in South and Southeast Asia and surrounding regions have been controlled by a variety of global and local drivers and imply that additional index points from the Java Sea in Indonesia would be valuable to better assess the spatial variability, and to calibrate geophysical models of glacial isostatic adjustment.

NKG2016LU: a new land uplift model for Fennoscandia and the Baltic Region

We present the official land uplift model NKG2016LU of the Nordic Commission of Geodesy (NKG) for northern Europe. The model was released in 2016 and covers an area from 49° to 75° latitude and 0° to 50° longitude. It shows a maximum absolute uplift of 10.3 mm/a near the city of Umeå in northern Sweden and a zero-line that follows the shores of Germany and Poland. The model replaces the NKG2005LU model from 2005. Since then, we have collected more data in the core areas of NKG2005LU, specifically in Norway, Sweden, Denmark and Finland, and included observations from the Baltic countries as well. Additionally, we have derived an underlying geophysical glacial isostatic adjustment (GIA) model within NKG as an integrated part of the NKG2016LU project. A major challenge is to estimate a realistic uncertainty grid for the model. We show how the errors in the observations and the underlying GIA model propagate through the calculations to the final uplift model. We find a standard error better than 0.25 mm/a for most of the area covered by precise levelling or uplift rates from Continuously Operating Reference Stations and up to 0.7 mm/a outside this area. As a check, we show that two different methods give approximately the same uncertainty estimates. We also estimate changes in the geoid and derive an alternative uplift model referring to this rising geoid. Using this latter model, the maximum uplift in Umeå reduces from 10.3 to 9.6 mm/a and with a similar reduction ratio elsewhere. When we compare this new NKG2016LU with the former NKG2005LU, we find the largest differences where the GIA model has the strongest influence, i.e. outside the area of geodetic observation. Here, the new model gives from − 3 to 4 mm/a larger values. Within the observation area, similar differences reach − 1.5 mm/a at the northernmost part of Norway and − 1.0 mm/a at the north-western coast of Denmark, but generally within the range of − 0.5 to 0.5 mm/a.

The uppermost mantle seismic velocity structure of West Antarctica from Rayleigh wave tomography: Insights into tectonic structure and geothermal heat flow

We present a shear wave model of the West Antarctic upper mantle to ∼200km depth with enhanced regional resolution from the 2016-2018 UK Antarctic Seismic Network. The model is constructed from the combination of fundamental mode Rayleigh wave phase velocities extracted from ambient noise (periods 8-25s) and earthquake data by two-plane wave analysis (periods 20-143s). We seek to (i) image and interpret structures against the tectonic evolution of West Antarctica, and (ii) extract information from the seismic model that can serve as boundary conditions in ice sheet and glacial isostatic adjustment modelling efforts. The distribution of low velocity anomalies in the uppermost mantle suggests that recent tectonism in the West Antarctic Rift System (WARS) is mainly concentrated beneath the rift margins and largely confined to the uppermost mantle (<180km). On the northern margin of the WARS, a pronounced low velocity anomaly extends eastward from beneath the Marie Byrd Land dome toward Pine Island Bay, underlying Thwaites Glacier, but not Pine Island Glacier. If of plume-related thermal origin, the velocity contrast of ∼5% between this anomaly and the inner WARS translates to a temperature difference of ∼125-200C∘. However, the strike of the anomaly parallels the paleo-Pacific convergent margin of Gondwana, so it may reflect subduction-related melt and volatiles rather than anomalously elevated temperatures, or a combination thereof. Motivated by xenolith analyses, we speculate that high velocity zones imaged south of the Marie Byrd Land dome and in the eastern Ross Sea Embayment might reflect the compositional signature of ancient continental fragments. A pronounced low velocity anomaly underlying the southern Transantarctic Mountains (TAM) is consistent with a published lithospheric foundering hypothesis. Taken together with a magnetotelluric study advocating flexural support of the central TAM by thick, stable lithosphere, this points to along-strike variation in the tectonic history of the TAM. A high velocity anomaly located in the southern Weddell Sea Rift System might reflect depleted mantle lithosphere following the extraction of voluminous melt related to Gondwana fragmentation. Lithospheric thickness estimates extracted from 1D shear wave velocity profiles representative of tectonic domains in West Antarctica indicate an average lithospheric thickness of ∼85km for the WARS, Marie Byrd Land, and Thurston Island block. This increases to ∼96km in the Ellsworth Mountains. A surface heat flow of ∼60mW/m2 and attendant geotherm best explains lithospheric mantle shear wave velocities in the central WARS and in the Thurston Island block adjacent to Pine Island Glacier; a ∼50mW/m2 geotherm best explains the velocities in the Ellsworth Mountains, and a ∼60mW/m2 geotherm best explains a less well-constrained velocity profile on the southern Antarctic Peninsula. We emphasise that these are regional average (many hundreds of km) heat flow estimates constrained by seismic data with limited sensitivity to upper crustal composition.

Holocene reef growth in the tropical southwestern Atlantic: Evidence for sea level and climate instability

The Holocene epoch offers a potential analogue for understanding future sea-level variability as both SST's and Global Mean Sea Levels (GMSL) were at times higher than observed today. However, GMSL can differ significantly from Relative Sea Level (RSL), even at far-field sites remote from margins of former ice sheets. Much of this spatial variability has been shown to be consistent with the predictions of glacial isostatic adjustment (GIA) models. Whilst it is generally accepted that RSL at far-field sites reached its maximum during the mid-Holocene, there have been many interpretations of sea level fall following the highstand from ∼6 ka. Here, we present a RSL history from several tectonically stable, far-field sites in eastern Brazil, derived from 17 microatoll and 45 fossil reef flat ages. Our results show evidence for two periods of RSL instability during the Holocene which differ from GIA predictions, including a hiatus in reef growth ∼3.7–2.5 ka. These results are broadly synchronous with several other locations in the Southern Hemisphere suggesting global rather than regional climatic forcing mechanisms are responsible. Variations in SST and southern hemisphere ice sheet dynamics are proposed as possible controlling mechanisms for the observed RSL oscillations beginning at ∼3.7 ka and 2 ka respectively. We suggest that these global processes combined with increased precipitation (and higher sediment flux) from several regional climatic forces created inhospitable conditions for reef growth, contributing to the observed hiatus and reduced reef flat accretion during the late Holocene (∼2 ka to present).

Testing the mid-Holocene relative sea-level highstand hypothesis in North Wales, UK

Accurate Holocene relative sea-level curves are vital for modelling future sea-level changes, particularly in regions where relative sea-level changes are dominated by isostatically induced vertical land movements. In North Wales, various glacial isostatic adjustment (GIA) models predict a mid-Holocene relative sea-level highstand between 4 and 6 ka, which is unsubstantiated by any geological sea-level data but affects the ability of geophysical models to model accurately past and future sea levels. Here, we use a newly developed foraminifera-based sea-level transfer function to produce a 3300-year-long late-Holocene relative sea-level reconstruction from a salt marsh in the Malltraeth estuary on the south Anglesey coast in North Wales. This is the longest continuous late-Holocene relative sea-level reconstruction in Northwest Europe. We combine this record with two new late-Holocene sea-level index points (SLIPs) obtained from a freshwater marsh at Rhoscolyn, Anglesey, and with previously published regional SLIPs, to produce a relative sea-level record for North Wales that spans from ca. 13,000 BP to the present. This record leaves no room for a mid-Holocene relative sea-level highstand in the region. We conclude that GIA models that include a mid-Holocene sea-level highstand for North Wales need revision before they are used in the modelling of past and future relative sea-level changes around the British Isles.

Benchmarked computation of time-domain viscoelastic Love numbers for adiabatic mantles

SUMMARYThe viscoelastic load Love numbers encapsulate the Earth’s rheology in a remarkably efficient fashion. When multiplied by a sudden increment of spherical harmonic load change, they give the horizontal and vertical surface displacements and gravity change at all later times. Incremental glacial load changes thus need only be harmonically decomposed, multiplied by the Love numbers and summed to predict the Earth’s response to glacial load redistributions. The computation of viscoelastic Love numbers from the elastic, viscous and adiabatic profiles of the Earth is thus the foundation upon which many glacial isostatic adjustment models are based. Usually, viscoelastic Love numbers are computed using the Laplace transform method, employing the correspondence principle to convert the viscoelastic equations of motion into the elastic equations with complex material parameters. This method works well for a fully non-adiabatic Earth, but can accommodate realistic partially adiabatic and fully adiabatic conditions only by changing the Earth’s density profile. An alternative method of Love number computation developed by Cathles (1975) avoids this dilemma by separating the elastic and viscous equations of motion. The separation neglects a small solid-elastic/fluid-elastic transition for compressible deformation, but allows freely defining adiabatic, partially adiabatic or fully non-adiabatic profiles in the mantle without changing the Earth’s density profile. Here, we update and fully describe this method and show that it produces Love numbers closely similar to those computed for fully non-adiabatic earth models computed by the correspondence principle, finite element and other methods. The time-domain method produces Love numbers as good as those produced by other methods and can also realistically accommodate any degree of mantle adiabaticity. All method implementations are available open source.

Glacial Isostatic Adjustment, Intraplate Strain, and Relative Sea Level Changes in the Eastern United States

AbstractDetermining the motion of the North American plate is difficult due to horizontal intraplate deformation, caused mainly by glacial isostatic adjustment (GIA) in the parts of the plate that are not deforming tectonically. Determination of plate motions also depends on whether the geocenter of the geodetic reference frame coincides with the center of plate rotation. Any difference/error in the International Terrestrial Reference Frame geocenter will introduce an apparent deformation of the plate. We address the plate rotation and what correction models can best account for this internal deformation. We use continuous GPS sites located far from active tectonic boundaries to derive the velocity field in the eastern United States with uncertainties based on a colored noise model. We hypothesize that the velocities will be described by a rigid plate model once the deformation due to GIA and any geocenter/plate rotation center errors have been removed. We find the smallest intraplate residuals with both a geocenter correction and the ICE‐6G model applied. We further compare the coastal GPS vertical velocities with mean relative sea level change rate and find a ~1.9 mm/year difference, which matches the long‐term average global sea level change rate from tide gauges. The entire North American plate is deforming when analyzed at the ~1 mm/year level, and there is no instantaneously stable part due to the long‐wavelength GIA deformation. An accurate estimate of plate rotation cannot be determined from a geographic subset of the plate but only by removing the strain component via a GIA model or a simultaneous estimation of the spatially varying strain rate.