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At present, exploring the space of rheological parameters in models of glacial isostatic adjustment (GIA) and relative sea level (RSL) which incorporate laterally variable Earth structure is computationally expensive. A single simulation using the Seakon model (Latychev et al., 2005), using contemporary high-performance computing hardware, requires several wall-days & ≈ 1 core-year for one RSL simulation from late Marine Isotope Stage 3 to present day. However, it is well established that the impact from laterally variable mantle viscosity and lithospheric thickness on RSL and GIA is significant (Whitehouse, 2018). We present initial results from using the Tensorflow (Abadi et al.) framework to construct artificial neural networks that emulate the difference in the rate of change of relative sea level and relative radial displacement between model configurations using spherically symmetric (SS) and laterally variable (LV) Earth structures. Using this emulator we can accurately sample the parameter space (≈ 360 realisations of the background (SS) structure) for a given realization of lateral Earth structure (e.g. viscosity variations derived from shear-wave tomographic models) using ≈ 1/10th the amount of parameter vectors as a training set. Average misfits are O(0.1-1%) of the total RSL signal when using the emulator to adjust SS GIA model output to incorporate the impact from LV. We shall report on two case studies which allow us to examine the influence of lateral Earth structure on inferences of background (i.e. global-mean) viscosity. For these case studies, the emulator, in conjunction with a fast SS GIA/RSL model, is used to determine optimal Earth model parameters (elastic lithosphere thickness, upper and lower mantle viscosities) by calculating the model misfits across the parameter space. The first case study uses the regional RSL database of Vacchi et al. (2018) which spans the Canadian Arctic and East Coast with several hundred sea level index points and limiting points for the early to late Holocene. The second case study uses a global database of several thousand contemporary uplift rates derived from GPS data (Schumacher et al., 2018). For the first case study we find two main features from incorporating LV structures compared to the SS configuration: a decrease in the best scoring misfit and a shift of the misfit distribution in the parameter space to favour a reduced upper mantle viscosity and reduced sensitivity to the lower mantle viscosity.ReferencesAbadi, M., Agarwal, A., Barham, P., et al.: TensorFlow: Large-Scale Machine Learning on Heterogeneous Systems, https://www.tensorflow. org/.Latychev, K., Mitrovica, J. X., Tromp, J., et al.: Glacial isostatic adjustment on 3-D Earth models: a finite-volume formulation, GJI, 161, 421–444, https://doi.org/10.1111/j.1365-246x.2005.02536.x, 2005.Schumacher, M., King, M. A., Rougier, J., et al.: A new global GPS data set for testing and improving modelled GIA uplift rates, GJI, 214, 2164–2176, https://doi.org/10.1093/gji/ggy235, 2018.Vacchi, M., Engelhart, S. E., Nikitina, D., et al.: Postglacial relative sea-level histories along the eastern Canadian coastline, QSR, 201, 124–146, https://doi.org/10.1016/j.quascirev.2018.09.043, 2018.Whitehouse, P. L.: Glacial isostatic adjustment modelling: historical perspectives, recent advances, and future directions, Earth Surface Dynamics, 6, 401–429, https://doi.org/10.5194/esurf-6-401-2018, 2018.

The first records of Greenland Vikings date to 985 CE. Archaeological evidence yields insight into how Vikings lived, yet drivers of their disappearance in the 15th century remain enigmatic. Research suggests a combination of environmental and socioeconomic factors, and the climatic shift from the Medieval Warm Period (~900 to 1250 CE) to the Little Ice Age (~1250 to 1900 CE) may have forced them to abandon Greenland. Glacial geomorphology and paleoclimate research suggest that the Southern Greenland Ice Sheet readvanced during Viking occupation, peaking in the Little Ice Age. Counterintuitively, the readvance caused sea-level rise near the ice margin due to increased gravitational attraction toward the ice sheet and crustal subsidence. We estimate ice growth in Southwestern Greenland using geomorphological indicators and lake core data from previous literature. We calculate the effect of ice growth on regional sea level by applying our ice history to a geophysical model of sea level with a resolution of ~1 km across Southwestern Greenland and compare the results to archaeological evidence. The results indicate that sea level rose up to ~3.3 m outside the glaciation zone during Viking settlement, producing shoreline retreat of hundreds of meters. Sea-level rise was progressive and encompassed the entire Eastern Settlement. Moreover, pervasive flooding would have forced abandonment of many coastal sites. These processes likely contributed to the suite of vulnerabilities that led to Viking abandonment of Greenland. Sea-level change thus represents an integral, missing element of the Viking story.

Future sea-level rise poses an existential threat for many river deltas, yet quantifying the effect of sea-level changes on these coastal landforms remains a challenge. Sea-level changes have been slow compared to other coastal processes during the instrumental record, such that our knowledge comes primarily from models, experiments, and the geologic record. Here we review the current state of science on river delta response to sea-level change, including models and observations from the Holocene until 2300 CE. We report on improvements in the detection and modeling of past and future regional sea-level change, including a better understanding of the underlying processes and sources of uncertainty. We also see significant improvements in morphodynamic delta models. Still, substantial uncertainties remain, notably on present and future subsidence rates in and near deltas. Observations of delta submergence and land loss due to modern sea-level rise also remain elusive, posing major challenges to model validation. ▪ There are large differences in the initiation time and subsequent delta progradation during the Holocene, likely from different sea-level and sediment supply histories. ▪ Modern deltas are larger and will face faster sea-level rise than during their Holocene growth, making them susceptible to forced transgression. ▪ Regional sea-level projections have been much improved in the past decade and now also isolate dominant sources of uncertainty, such as the Antarctic ice sheet. ▪ Vertical land motion in deltas can be the dominant source of relative sea-level change and the dominant source of uncertainty; limited observations complicate projections. ▪ River deltas globally might lose 5% (∼35,000 km2) of their surface area by 2100 and 50% by 2300 due to relative sea-level rise under a high-emission scenario.

SUMMARYThe Pacific Coast of Central North America is a geodynamically complex region which has been subject to various geophysical processes operating on different timescales. Glacial isostatic adjustment (GIA), the ongoing deformational response of the solid Earth to past deglaciation, is an important geodynamic process in this region. In this study, we apply earth models with 3-D structure to determine if the inclusion of lateral structure can explain the poor performance of 1-D models in this region. Three different approaches are used to construct 3-D models of the Earth structure. For the first approach, we adopt an optimal 1-D viscosity structure from previous work and add lateral variations based on four global seismic shear wave velocity anomalies and two global lithosphere thickness models. The results based on these models indicate that the addition of lateral structure significantly impacts modelled RSL changes, but the data-model fits are not improved. The global seismic models are limited in spatial resolution and so two other approaches were considered to produce higher resolution models of 3-D structure: inserting a regional seismic model into two of the global seismic models and, explicitly incorporating regional structure of the Cascadia subduction zone and vicinity, that is the subducting slab, the overlying mantle wedge and the plate boundary interface. The results associated with these higher resolution models do not reveal any clear improvement in satisfying the RSL observations, suggesting that our estimates of lateral structure are inaccurate and/or the data-model misfits are primarily due to limitations in the adopted ice-loading history. The different realizations of 3-D Earth structure gives useful insight to uncertainty associated with this aspect of the GIA model. Our results indicate that improving constraints on the deglacial history of the southwest sector of the Cordilleran ice sheet is an important step towards developing more accurate of GIA models for this region.

Many coastal cities are an early casualty in climate-related coastal flooding because of processes resulting in land subsidence and thus enhanced relative sea-level (RSL) rise. Much of the Atlantic coast of North America has been sinking for thousands of years, at a maximum rate of ~20 cm per century as a consequence of solid Earth deformation in response to deglaciation of the Laurentide ice sheet (between ~18,000 and ~7,000 years ago) [e.g. Love et al., Earth's Future, 4(10), 2016]. Karegar et al. [Geophysical Research Letters, 43(7), 2016] have shown that vertical land motion along the Atlantic coast of the USA is an important control on nuisance flooding. A key finding in this study is that while glacial isostatic adjustment (GIA) is the dominant process driving land subsidence in most areas, there can be large deviations from this signal due to the influence of anthropogenic activity impacting hydrological processes. For example, between Maine (45°N) and New Hampshire (43°N), the GPS data show uplift while geological data show long-term subsidence. The cause of this discrepancy is not clear, but one hypothesis is increasing water mass associated with the James Bay Hydroelectric Project in Quebec [Karegar et al., Scientific Reports, 7, 2017].The primary aim of this study is to better constrain and understand the processes that contribute to contemporary and future vertical land motion in this region to produce improved projections of mean sea-level change and nuisance flooding. The first step towards achieving these aims is to determine a GIA model parameter set that is compatible with observations of past sea-level change for this region. We make use of two regional RSL data compilations: Engelhart and Horton [Quaternary Science Reviews, 54, 2012] for northern USA and Vacchi et al. [Quaternary Science Reviews, 201, 2018] for Eastern Canada, comprising a total of 1013 data points (i.e., sea level index points and limiting data points) over 38 regions distributed throughout our study region. These data are well suited to determine optimal GIA model parameters due to the magnitude of other signals being much smaller, particularly in near-field regions such as Eastern Canada. We consider a suite of 32 ice history models that is comprised mainly of a subset from Tarasov et al. [Earth and Planetary Science Letters, 315–316, 2012] as well as the ICE-6G and ANU models. We have computed RSL for these ice histories using a state-of-the-art sea-level calculator and 440 1-D Earth viscosity models per each ice history model to identify a set of Earth model parameters that is compatible with the observations.

Global patterns of sea-level change – often termed “sea-level fingerprints” – associated with future changes in ice/water mass re-distribution are a key component in generating regional sea-level projections. Calculation of these fingerprints is commonly based on the assumption that the isostatic response of the Earth is dominantly elastic on century timescales. While this assumption is accurate for regions underlain by mantle material with viscosity close to that of global average estimates, recent work focusing on the West Antarctic region has shown that this assumption can lead to significant error where the viscosity is significantly lower than typical global average values. Here, we test this assumption for fingerprints associated with glaciers and ice caps. We compare output from a (1D) elastic Earth model to that of a 3D viscoelastic model that includes low-viscosity mantle in three glaciated regions: Alaska, southwestern Canada, and the southern Andes (Randolph Glacier Inventory (RGI) regions 1, 2, and 17, respectively). This comparison indicates that the error incurred by ignoring the non-elastic response is of the order of 1 mm in most areas (or about 1 % of the barystatic signal) over the 21st century with values reaching the centimetre level in glaciated regions. However, in glaciated regions underlain by low-viscosity mantle, the non-elastic deformation can result in relative sea-level changes with magnitudes of up to several tens of centimetres (or several times the barystatic value). The magnitude and spatial pattern of this non-elastic signal is sensitive to variations in both the projected ice history and regional viscosity structure, indicating the need for loading models with high spatial resolution and improved constraints on regional Earth viscosity structure to accurately simulate sea-level fingerprints in these regions. The anomalously low mantle viscosity in these regions also amplifies the glacial isostatic adjustment signal associated with glacier changes during the 20th century, causing it to be an important (and even dominant) contributor to the modelled relative sea-level changes over the 21st century.

The observed crustal uplift rates in Greenland are caused by the combined response of the solid Earth to both ongoing and past surface mass changes. Existing elastic Earth models and Maxwell linear...

The isostatic response of the Earth to past mass exchange between ice sheets and oceans, so-called glacial isostatic adjustment (GIA), is an important geodynamic process in North America. Modelling GIA observables provides one of the few direct methods for estimating Earth viscosity structure. We investigate here the requirement for anomalously high mantle viscosities to fit the relative sea level (RSL) data along the Atlantic and Gulf coasts of North America found in recent GIA studies. We demonstrate that this requirement is primarily related to the modelled geometry and evolution of the peripheral bulge of the Laurentide ice sheet. We show that a 3D Earth model with a global average viscosity in the upper mantle of 0.3 × 10 21 Pa s and 3 × 10 22 Pa s in the lower mantle, which is consistent with both the Richmond Gulf (Hudson Bay) relaxation time and several recent global analyses, is able to produce a much improved fit to the Gulf and Atlantic coast RSL data relative to the 1D Earth model results when no lateral structure is applied. Therefore, we conclude that realistic implementations of lateral structure can explain the markedly different viscosity inferences obtained using 1D GIA models and RSL data from different regions of North America. A necessary caveat is that there are significant differences among the computed RSL curves corresponding to the three different realisations of lateral structure considered here, demonstrating significant uncertainty associated with this model input.

We present a sensitivity analysis aimed at testing whether observables related to glacial isostatic adjustment can support or refute the occurrence of a low viscosity melt-rich layer (MRL) above the mantle transition zone, as required by the 'transition-zone water-filter' model (Bercovici & Karato 2003). In total, 1600 model runs were performed sampling a range of MRL thicknesses (1, 10 and 20 km) and viscosities (10 15 -10 19 Pa·s), plausible viscosity values in the upper and lower mantle regions and four distinct ice histories. To determine decay time constraints, we consider relative sea level (RSL) data from two sites [Angerman River (AR), Sweden and Richmond Gulf (RG), Canada] and use a new method of observational sea level data correction. Comparing model output of postglacial decay times and J 2 to observational constraints, we find numerous possible solutions, largely as a result of parameter trade-off. The investigated observables are sensitive to the existence of an MRL and reasonable variations in its thickness and viscosity. The magnitude and nature of this sensitivity varies between the two data types as well as the adopted background viscosity structure. Decay time results from either considered location do not strictly support or exclude MRL existence. However, both locations offer MRL viscosity requirements for given thicknesses, with AR being more restrictive. RG constraints allow MRL viscosities as low as 10 16 Pa·s (10 km) and 10 17 Pa·s (20 km). AR results narrow these permitted viscosity ranges to 10 18 Pa·s or greater for both 10 and 20 km MRL thicknesses. In the case of a 1 km thick MRL, AR constraints permit the viscosity to be as low as 10 17 Pa·s, whereas those of RG permit any MRL viscosity. The decay time observations are satisfied by only a small subset of 'background' mantle viscosities (regardless of the MRL properties), none of which support a spherically symmetric solution of Earth viscosity. Finally, comparing model output to the observed J 2 value did not provide any constraints on MRL properties. However, our results show that this observable has a strong preference for viscosity values in the lower mantle that are equal to or greater than 10 22 Pa·s.

Karegar et al. (2016, GRL) showed that independent estimates of vertical land motion from geodetic and geologic techniques are critical for understanding coastal surface motion caused by geological versus human-induced processes along the Atlantic coast of North America. Motivated by these results, we extend our analysis to the British Isles where good quality and spatially dense constraints are available from a continuous GNSS network and a state-of-the-art Holocene sea-level database. Glacial Isostatic Adjustment (GIA) along the Atlantic coast of North America causes the land surface to sink (up to -1.5 mm/yr), exacerbating tidal-induced flooding effects of sea-level rise. The British Isles are also subjected to proglacial forebulge collapse associated with the GIA response to the ancient Fennoscandian and British-Irish Ice Sheets. Here, we present an up-to-date and precise analysis based on continuous GNSS (combined GPS and GlONASS observations) and geologic records of late Holocene sea-level change to examine residuals between rates on these different timescales to determine if there is a significant residual and, if so, the processes responsible for the rate change.

Geothermal heat flux (GHF) is known to be an important control on the basal thermal state of an ice sheet which, in turn, is a key factor in governing how the ice sheet will evolve in response to a given climate forcing. In recent years, several studies have estimated GHF beneath the Greenland ice sheet using different approaches (e.g. Rezvanbehbahani et al., Geophysical Research Letters, 2017; Martos et al., Geophysical Research Letters, 2018; Greve, Polar Data Journal, 2019). Comparing these different estimates indicates poor agreement and thus large uncertainty in our knowledge of this important boundary condition for modelling the ice sheet. The primary aim of this study is to quantify the influence of this uncertainty on modelling the past evolution of the ice sheet with a focus on the most recent deglaciation. We build on past work that considered three GHF models (Rogozhina et al., 2011) by considering over 100 different realizations of this input field. We use the uncertainty estimates from Martos et al. (Geophysical Research Letters, 2018) to generate GHF realisations via a statistical sampling procedure. A sensitivity analysis using these realisations and the Parallel Ice Sheet Model (PISM, Bueler and Brown, Journal of Geophysical Research, 2009) indicates that uncertainty in GHF has a dramatic impact on both the volume and spatial distribution of ice since the last glacial maximum, indicating that more precise constraints on this boundary condition are required to improve our understanding of past ice sheet evolution and, consequently, reduce uncertainty in future projections.

The Pacific Coast of Central North America is a geodynamically complex region subject to various geophysical processes with different patterns of vertical land motion. It includes two distinct tectonic regimes: the Cascadia subduction zone and the strike-slip San Andreas fault system. The vertical land motion in this region reflects not only tectonic activity of these plate boundaries, but also isostatic signals associated with different loading effects such as the (de)glaciation of North American ice sheets and the more contemporary, anthropogenically-related groundwater extraction and mountain glacier mass loss. These processes occur over a broad range of timescales and are observed by a variety of measuring techniques.Here we combine geological measurements of relative sea level (RSL) change with contemporary observations of vertical land motion inferred from geodetic data to decipher and thus better understand the contribution from various individual processes. Our results suggest that contemporary vertical land motion is dominated by Cascadia interseismic deformation and the isostatic response to the retreat of the North American ice sheets but is also influenced by other contemporary processes. We present some model results that illustrate the contributions of the above-mentioned processes to RSL projections along this coastline.

The observed crustal uplift rates in Greenland are caused by the combined response of the solid Earth to both ongoing and past surface mass changes. Existing elastic Earth models and Maxwell linear viscoelastic GIA (glacial isostatic adjustment) models together underpredict the observed uplift rates. These models do not capture the ongoing mantle deformation induced by significant ice melting since the Little Ice Age. Using a simple Earth model within a Bayesian framework, we show that this recent mass loss can explain the data-model misfits but only when a reduced mantle strength is considered. The inferred viscosity for sub-centennial timescale mantle deformation is roughly one order of magnitude smaller than the upper mantle viscosity inferred from GIA analysis of geological sea-level data. Reconciliation of geological sea-level and modern crustal motion data may require that the model effective viscosity be treated with greater sophistication than in the simple Maxwell rheological paradigm.

Highlights • Coral reef responses to slow-rate, low-amplitude, high-frequency sea-level changes. • Continuous reef development over the last 6000 years in optimal conditions. • Mid-late Holocene coral assemblages similar to their modern counterparts. • Facies distribution governed by variations in accommodation space. • Prominent role played by reef deposits in the formation and shaping of modern islands. Geological records of coastal system evolution during past higher and/or rising sea levels provide an important baseline for developing projections regarding the response of modern coastal systems to future sea-level rise. The mid-late Holocene corresponds to the most recent window into natural variability prior to the Anthropocene and involves slow-rate and low-amplitude sea-level changes that were mostly governed by a limited glacio-eustatic contribution, most likely sourced from Antarctica, and ‘glacial isostatic adjustment’ processes. This paper documents in unprecedented detail the response of coral reefs and coastal systems to changing accommodation space in relation to mid-late Holocene sea-level changes in French Polynesia. The sea-level curve that underpins this study has a global significance and documents a single short-lived sea-level highstand between 4.10 and 3.40 kyr BP. The amplitude of the highstand is less than one metre, within the range of the predicted sea level at the end of the current century. The reported relative sea-level changes are characterized by slow rates ranging from a few tens of millimetres per year to up to 2.5 mm/yr and by significant sea-level stability (stillstands) lasting more than a century and up to 250 years, defining a step-like pattern. Sea-level variability probably driven by climatic oscillations on interannual to millennial time scales is evidenced during the entire time window. The detailed reconstruction of reef development over the last 6000 years brings valuable information regarding coral reef dynamics and coastal processes during periods of higher sea level and wave energy regimes. The persistence of stable and optimal depositional environments over the last 6000 years is demonstrated by the constant overall composition and diversity of reef communities and the almost continuous development of coral microatolls. The facies distribution as well as the lateral extension and shift of facies belts have been governed by variations in accommodation space, which are controlled by relative sea-level changes and antecedent topography. The widespread development of mid-late Holocene reef deposits in coastal areas suggests that they have played a prominent role in global processes related to the formation and shaping of modern islands.

At the centers of previously glaciated regions such as Hudson Bay in Canada and the Gulf of Bothnia in Fennoscandia, it has been observed that the sea level history follows an exponential form and that the associated decay time is relatively insensitive to uncertainty in the ice loading history. We revisit the issue of decay time sensitivity by computing relative sea level histories for Richmond Gulf and James Bay in Hudson Bay and Ångerman River in Sweden for a suite of reconstructions of the North American and Fennoscandian Ice Sheets and Earth viscosity profiles. We find that while some Earth viscosity models do indeed show insensitivity in computed decay times to the ice history, this is not true in all cases. Moreover, we find that the location of the study site relative to the geometry of the ice sheet is an important factor in determining ice sensitivity, and based on our set of ice sheet reconstructions, conclude that the location of James Bay is not well-suited to a decay time analysis. We describe novel corrections to the RSL data to remove the effects associated with the spatial distribution of sea level indicators as well as for other signals unrelated to regional ice loading (ocean loading, rotation and global mean sea-level changes) and demonstrate that they can significantly affect the inference of viscosity structure. We performed a forward modelling analysis based on a commonly adopted 2-layer, sub-lithosphere viscosity structure to determine how the solution space of viscosity models changes with the input ice history at the three study sites. While the solution spaces depend on ice history, for both Richmond Gulf and Ångerman River there are regions of parameter space where solutions are common across all or most ice histories, indicating low ice load sensitivity for these mantle viscosity parameters. For example, in Richmond Gulf, upper mantle viscosity values of (0.3–0.5)x1021 Pa s and lower mantle viscosity values of (5–50)x1021 Pa s tend to satisfy the data constraint consistently for most ice histories considered in this study. Similarly, the Ångerman River solution spaces contain a solution with an upper mantle viscosity of 0.3 × 1021 Pa s and lower mantle viscosity values of (5–50)x1021 Pa s common to 9 of the 10 ice histories considered there. However, the dependence of the viscosity solution space on ice history suggests that joint estimation of ice and Earth parameters is the optimal approach.

We infer the GIA signal and its uncertainty along the central Pacific coast of North America using 680 sea-level index points and over 20,000 model runs sampling >700 (1-D) Earth viscosity models and 29 ice sheet reconstructions. Due to the large spatial extent and different tectonic settings of the study area, we divided it into three sub-regions (northern, central and southern) for which model parameters were inferred separately. Also, given that this region is tectonically active, the influence of this process (as well as sediment isostatic adjustment) was accounted for where possible by removing it from the data using published estimates. Our results indicate that it is not possible to produce an acceptable to for all of the RSL data with a single set of model parameters, suggesting significant lateral variability in viscous structure. Specifically, low viscosities (1019–1020 Pas) are inferred in the upper mantle within the northern region (southwestern British Columbia and northwest Washington) compared to those inferred (2–5 × 1020 Pas) for the central and southern regions (extending from southern Washington to southern California). High quality model fits were obtained for all data except those from the northern region where no single parameter set was able to capture both the rapid and large RSL fall during the late glacial and the monotonic rise during the mid-to-late Holocene at all localities. This suggests the need for an Earth model that incorporates departures from a linear Maxwell rheology (as applied here) and/or lateral variations in viscosity structure. Using our optimal model parameter sets, we show that GIA is a significant contributor to both contemporary vertical land motion and relative sea level change in our study region and so should be considered when interpreting observations of these signals and for making future relative sea level projections. Model output of present-day vertical land motion at 483 GPS stations and sea-level change at 56 tide gauge stations is provided (with estimated uncertainty) so that these data can be used to study non-GIA processes more accurately.

Sea level rise presents a hazard for coastal populations, and the Mississippi Delta (MD) is a region particularly at risk due to the high rates of land subsidence. We apply a gravitationally self-consistent model of glacial and sediment isostatic adjustment (SIA) along with a realistic sediment load reconstruction in this region for the first time to determine isostatic contributions to relative sea level (RSL) and land motion. We determine optimal model parameters (Earth rheology and ice history) using a new high-quality compaction-free sea level indicator database. Using the optimal model parameters, we show that SIA can lower predicted RSL in the MD area by several meters over the Holocene and so should be taken into account when modeling these data. We compare modeled contemporary rates of vertical land motion with those inferred using GPS. This comparison indicates that isostatic processes can explain the majority of the observed vertical land motion north of latitude 30.7°N, where subsidence rates average about 1 mm/yr; however, subsidence south of this latitude shows large data-model discrepancies of greater than 3 mm/yr, indicating the importance of nonisostatic processes. This discrepancy extends to contemporary RSL change, where we find that the SIA contribution in the Delta is on the order of 10−1 mm/yr. We provide estimates of the isostatic contributions to 20th and 21st century sea level rates at Gulf Coast Permanent Service for Mean Sea Level tide gauge locations as well as vertical and horizontal land motion at GPS station locations near the MD.

Despite elevated summer insolation forcing during the early Holocene, global ice sheets retained nearly half of their volume from the Last Glacial Maximum, as indicated by deglacial records of global mean sea level (GMSL). Partitioning the GMSL rise among potential sources requires accurate dating of ice-sheet extent to estimate ice-sheet volume. Here, we date the final retreat of the Laurentide Ice Sheet with 10 Be surface exposure ages for the Labrador Dome, the largest of the remnant Laurentide ice domes during the Holocene. We show that the Labrador Dome deposited moraines during North Atlantic cold events at ∼10.3 ka, 9.3 ka and 8.2 ka, suggesting that these regional climate events helped stabilize the retreating Labrador Dome in the early Holocene. After Hudson Bay became seasonally ice free at ∼8.2 ka, the majority of Laurentide ice-sheet melted abruptly within a few centuries. We demonstrate through high-resolution regional climate model simulations that the thermal properties of a seasonally ice-free Hudson Bay would have increased Laurentide ice-sheet ablation and thus contributed to the subsequent rapid Labrador Dome retreat. Finally, our new 10 Be chronology indicates full Laurentide ice-sheet had completely deglaciated by 6.7 ± 0.4 ka, which re quires that Antarctic ice sheets contributed 3.6–6.5 m to GMSL rise since 6.3–7.1 ka.

Extinction models generally predict that coastal and neritic fauna benefit during sea-level rise (transgression), whereas sea-level retreat (regression) diminishes their suitable habitat area and promotes evolutionary bottlenecks. Sea-level change also impacts terrestrial island biogeography, but it remains a challenge to evidence how sea-level rise impacts aquatic island biogeography, especially in the subterranean realm. Karst subterranean estuaries (KSEs) occur globally on carbonate islands and platforms, and they are populated by globally-dispersed, ancient ecosystems (termed anchialine). Anchialine fauna currently exhibit a disjunct biogeography that cannot be completely explained by plate tectonic-imposed vicariance. Here we provide evidence that anchialine ecosystems can experience evolutionary bottlenecks caused by habitat reduction during transgression events. Marine-adapted anchialine fauna benefit from habitat expansion during transgressions, but fresh- and brackish-adapted fauna must emigrate, evolve to accommodate local habitat changes, or are regionally eliminated. Phanerozoic transgressions relative to long-term changes in subsidence and relief of regional lithology must be considered for explaining biogeography, evolution, local extirpation or complete extinction of anchialine fauna. Despite the omission of this entire category of environments and animals in climate change risk assessments, the results indicate that anchialine fauna on low-lying islands and platforms that depend upon meteoric groundwater are vulnerable to habitat changes caused by 21st century sea-level rise.

Here we summarize the motivation and issues surrounding the responses of ice sheets and sea level to past climate warming as part of the PALeo constraints on SEA level rise (PALSEA) working group. Papers in this special issue of Quaternary Science Reviews focus on the timescale of glaciations during the late Pliocene, the magnitude of ice-sheet fluctuations and volume leading up to and during the last glacial maximum, the timing and persistence of ice-sheet impacts on deglacial and future relative sea-level change, and relative sea-level change during peak interglacial climate. A more dynamic cryosphere is noted under both late Pliocene and last glacial cycle climate conditions, while relative sea-level changes during the last deglaciation appear to correspond closely with individual ice-sheet deglaciation. Lastly, relative sea-level change during peak interglacial conditions may have fluctuated by as much as a meter, although the sources of such variability (Greenland, Antarctica or elsewhere) remain elusive.

Observations of enhanced volcanic frequency during the last deglaciation have led to the hypothesis that ice unloading in glaciated volcanic terrains can promote volcanism through decompression melting in the shallow mantle or a reduction in crustal magma storage time. However, a direct link between regional climate change, isostatic adjustment, and the initiation of volcanism remains to be demonstrated due to the difficulty of obtaining high-resolution well-dated records that capture short-term climate and volcanic variability traced to a particular source region. Here we present an exceptionally resolved record of 19 tephra layers paired with foraminiferal oxygen isotopes and alkenone paleotemperatures from marine sediment cores along the Southeast Alaska margin spanning the last deglacial transition. Major element compositions of the tephras indicate a predominant source from the nearby Mt. Edgecumbe Volcanic Field (MEVF). We constrain the timing of this regional eruptive sequence to 14.6–13.1 ka. The sudden increase in volcanic activity from the MEVF coincides with the onset of Bolling–Allerod interstadial warmth, the disappearance of ice-rafted detritus, and rapid vertical land motion associated with modeled regional isostatic rebound in response to glacier retreat. These data support the hypothesis that regional deglaciation can rapidly trigger volcanic activity. Rapid sea surface temperature fluctuations and an increase in local salinity (i.e., δ18Osw) variability are associated with the interval of intense volcanic activity, consistent with a two-way interaction between climate and volcanism in which rapid volcanic response to ice unloading may in turn enhance short-term melting of the glaciers, plausibly via albedo effects on glacier ablation zones.

We determine the contribution of glacial isostatic adjustment (GIA) to future relative sea-level change for the North American coastline between Newfoundland and Texas. We infer GIA model parameters using recently compiled and quality-assessed databases of past sea-level changes, including new databases for the United States Gulf Coast and Atlantic Canada. At 13 cities along this coastline, we estimate the GIA contribution to range from a few centimeters (e.g., 3 [−1 to 9] cm Miami) to a few decimeters (e.g., 18 [12–22] cm, Halifax) for the period 2085–2100 relative to 2006–2015 (1−σ ranges given). We provide estimates of uncertainty in the GIA component using two different methods; the more conservative approach produces total ranges (1−σ confidence) that vary from 3 to 16 cm for the cities considered. Contributions from ocean steric and dynamic changes as well as those from changes in land ice are also estimated to provide context for the GIA projections. When summing the contributions from all three processes at the 13 cities considered along this coastline, using median or best-estimate values, the GIA signal comprises 5–38% of the total depending on the adopted climate forcing and location. The contributions from ocean dynamic/steric changes and ice mass loss are similar in amplitude but with spatial variation that approximately cancels, resulting in GIA dominating the net spatial variability north of 35°N.

The last deglaciation is the most recent interval of large-scale climate change that drove the Greenland ice sheet from continental shelf to within its present extent. Here, we use a database of 645 published 10Be ages from Greenland to document the spatial and temporal patterns of retreat of the Greenland ice sheet during the last deglaciation. Following initial retreat of its marine margins, most land-based deglaciation occurred in Greenland following the end of the Younger Dryas cold period (12.9–11.7 ka). However, deglaciation in east Greenland peaked significantly earlier (13.0–11.5 ka) than that in south Greenland (11.0–10 ka) or west Greenland (10.5–7.0 ka). The terrestrial deglaciation of east and south Greenland coincide with adjacent ocean warming. 14C ages and a recent ice-sheet model reconstruction do not capture this progression of terrestrial deglacial ages from east to west Greenland, showing deglaciation occurring later than observed in 10Be ages. This model-data misfit likely reflects the absence of realistic ice-ocean interactions. We suggest that oceanic changes may have played an important role in driving the spatial-temporal ice-retreat pattern evident in the 10Be data.

This study presents a new model of Holocene ice-volume equivalent sea level (ESL), extending a previously published global ice sheet model (Bassett et al., 2005), which was unconstrained from 10 kyr BP to present. This new model was developed by comparing relative sea level (RSL) predictions from a glacial isostatic adjustment (GIA) model to a suite of Holocene sea level index points from China and Malay-Thailand. Three consistent data-model misfits were found using the Bassett et al. (2005) model: an over-prediction in the height of maximum sea level, the timing of this maximum, and the temporal variation of sea level from the time of the highstand to present. The data-model misfits were examined for a large suite of ESL scenarios and a range of earth model parameters to determine an optimum model of Holocene ESL. This model is characterised by a slowdown in melting at ∼7 kyr BP, associated with the final deglaciation of the Laurentide Ice Sheet, followed by a continued rise in ESL until ∼1 kyr BP of ∼5.8 m associated with melting from the Antarctic Ice Sheet. It was not possible to identify an earth viscosity model that provided good fits for both regions; with the China data preferring viscosity values in the upper mantle of less than 1.5 × 10 20 Pa s and the Malay-Thailand data preferring greater values. We suggest that this inference of a very weak upper mantle for the China data originates from the nearby subduction zone and Hainan Plume. The low viscosity values may also account for the lack of a well-defined highstand at the China sites.

Ireland experienced a spatially complex pattern of relative sea-level (RSL) changes and shoreline development caused by the interplay of isostatic and eustatic (ice equivalent sea level) processes since the Last Glacial Maximum (LGM). Using a combination of high-resolution marine geophysical data, vibrocores, foraminiferal analysis and 10 AMS radiocarbon dates, we reconstruct the Late Quaternary evolution and RSL history of Bantry Bay, a large (40 km long, 5–10 km wide) embayment in SW Ireland. The data indicate two infill phases: one before and one after the LGM, separated by glacial and lowstand sediments. The pre-LGM history is not dated and the depositional history is inferred. A large sediment lobe formed at the outer edge of Bantry Bay as a lowstand ice-proximal glacimarine outwash system as the ice retreated after the LGM, at a sea level ca. 80 m lower than present. Iceberg scour immediately west of this location likely relate to the break-up of the local Kerry–Cork Ice Cap. Long curvilinear ridges, seen both offshore and on top of the sediment lobe, probably formed as shoreface ridges under stronger-than-present tidal currents during a period of RSL stability (pre-14.6 ka cal BP). A subsequent infill phase is characterised by a basin-wide erosional (ravinement) surface and the deposition of inter- and sub-tidal estuarine sediments. Although our data support the general trends, our stratigraphic and radiocarbon data suggest a higher sea level between 11 and 13.5 ka cal BP than predicted by existing glacial isostatic adjustment models.

The thickness of the lithosphere inferred in most glacial isostatic adjustment (GIA) modelling studies tends to be significantly thinner than when found through seismic or thermal modelling studies. In those GIA studies, the lithosphere tends to be modelled as a plate of uniform and very high viscosity. We develop and test Earth models that include depth-dependent viscosity in the lithosphere to consider the implications for inferring lithospheric thickness from observed relative sea-level (RSL) changes. We find that when comparing predictions of RSL between the traditional plate lithosphere models and those with viscous structure, the latter produce RSL predictions that most closely resemble those from traditional models that are 10 s of km thinner. The greatest sensitivity to this change in the Earth model is most evident in regions loaded by relatively small ice sheets such as the British Isles. We also find that the effective elastic thickness of the lithosphere models with viscous structure is time-dependent, with thinning by tens of kilometres over a timescale of ∼10 kyr.

The rate of change of surface gravity, dg/dt, and vertical deformation rate of the solid surface, du/dt, are two observables of glacial isostatic adjustment (GIA). They contribute with different information on the same phenomenon. Their relation contains information of the underlying physics and a trustworthy relation allows to combine these observations to strengthen the overall observational accuracy of the phenomenon. In this paper we investigate the predicted relation between dg/dt and du/dt in previously glaciated areas. We use the normal mode approach for one dimensional earth models and solutions of the sea level equation with time-dependent coastline geometry. Numerical predictions of dg/dt and du/dt are computed for Laurentia, Fennoscandia and the British Isles respectively, using six different earth models. Within each region a linear trend is then fitted using the relation dg/dt = C du/dt + dg_0/dt. The estimated C and dg_0/dt differ more between the regions than between different earth models within each region. For Fennoscandia C ≈ −0.163 μGal/mm and for Laurentia C ≈ −0.152 μGal/mm. Maximum residuals between the linear trend and spatially varying model predictions of dg/dt are 0.04 μGal/yr in Fennoscandia and 0.17 μGal/yr in Laurentia. For the British Isles the results are harder to interpret, mainly since this region is located on the zero uplift isoline of Fennoscandia. In addition, we show temporal variation of the relation since the last glacial maximum till present-day. The temporal and spatial variation of the relation between dg/dt and du/dt can be explained by (i) the elastic respectively viscous proportion of the total signal and (ii) the spectral composition of the regional signal. Additional local effects, such as the Newtonian attraction and elastic deformation from local sea level changes, are examined in a case study for six stations in the Nordic absolute gravity network. The influence of these local effects on the relation between View the dg/dt and du/dt is negligible except for extreme locations close to the sea.

We present a revised and extended high Arctic air temperature reconstruction from a single proxy that spans the past ∼12,000 y (up to 2009 CE). Our reconstruction from the Agassiz ice cap (Ellesmere Island, Canada) indicates an earlier and warmer Holocene thermal maximum with early Holocene temperatures that are 4–5 °C warmer compared with a previous reconstruction, and regularly exceed contemporary values for a period of ∼3,000 y. Our results show that air temperatures in this region are now at their warmest in the past 6,800–7,800 y, and that the recent rate of temperature change is unprecedented over the entire Holocene. The warmer early Holocene inferred from the Agassiz ice core leads to an estimated ∼1 km of ice thinning in northwest Greenland during the early Holocene using the Camp Century ice core. Ice modeling results show that this large thinning is consistent with our air temperature reconstruction. The modeling results also demonstrate the broader significance of the enhanced warming, with a retreat of the northern ice margin behind its present position in the mid Holocene and a ∼25% increase in total Greenland ice sheet mass loss (∼1.4 m sea-level equivalent) during the last deglaciation, both of which have implications for interpreting geodetic measurements of land uplift and gravity changes in northern Greenland.

The processes responsible for land surface subsidence in the Mississippi Delta (MD) have been vigorously debated. Numerous studies have postulated a dominant role for isostatic subsidence associated with sediment loading. Previous computational modeling of present-day vertical land motion has been carried out in order to understand geodetic data. While the magnitudes of these measured rates have been reproduced, the model parameter values required have often been extreme and, in some cases, unrealistic. In contrast, subsidence rates in the MD on the 103 year timescale due to delta loading estimated from relative sea level reconstructions are an order of magnitude lower. In an attempt to resolve this conflict, a sensitivity analysis was carried out using a spherically symmetric viscoelastic solid Earth deformation model with sediment, ice, and ocean load histories. The model results were compared with geologic and geodetic observations that provide a record of vertical land motion over three distinctly different timescales (past 80 kyr, past 7 kyr, and past ~15 years). It was found that glacial isostatic adjustment is likely to be the dominant contributor to vertical motion of the Pleistocene and underlying basement. Present-day basement subsidence rates solely due to sediment loading are found to be less than ~0.5 mm yr−1. The analysis supports previous suggestions in the literature that Earth rheology parameters are time dependent. Specifically, the effective elastic thickness of the lithosphere may be

A contribution to the PAGES' PALSEA2 Working Group. ABSTRACT: This paper presents new Holocene relative sea level (RSL) data collected from isolation basins close to the town of Paamiut in south west Greenland. The data shows a rapid fall from a marine limit of c. 52masl at c. 10.9cal.ka BP to close to present by c. 9.5cal.ka BP at rates of up to c. 32mm/yr falling below present for the majority of the Holocene before rising to present in the last 2000 years. The elevation of the RSL lowstand is not well constrained but was at least below 3m. This pattern of rapid RSL fall during the early Holocene matches the pattern seen at other southern Greenland locations suggesting rapid largely simultaneous ice retreat from the area surrounding the Qassimiut Lobe at the start of the Holocene occurring c. 2000 years after the initial deglaciation of the extreme southern tip of Greenland. The RSL histories from this and other southern Greenland locations are distinct to those recorded further north along the west coast and are in broad agreement with a pattern of vertical land motion and RSL predicted by the Huy2 model (Simpson etal. 2009) which predicts an 80m drop in the contribution of vertical land motion to RSL at 10cal.ka BP between Sisimiut and Paamiut on the west coast. Despite this broad scale spatial agreement between the RSL data and the Huy2 model it fails to satisfactorily predict the Holocene RSL histories at Paamiut and other southern Greenland locations. Sensitivity tests indicate that the data model misfits are most likely due to an over estimate of the forcing during the Holocene Thermal Maximum (or the response to this forcing) in southern Greenland and error in the North American ice sheets component of the background deglaciation model. Our new data suggests that much of the southern part of the ice sheet acted differently to the area further north. However RSL changes at Paamiut are also largely impacted by regional and larger scale processes including a bulls eye of uplift centred on the west the impact of the Holocene Thermal Maximum and the influence of the collapse of the North American ice sheets. © 2013 Elsevier Ltd.

Lake Superior represents an important component of the aquatic ecosystem in North America. Along its south shore, ongoing lake-level rise, accelerated erosion, and wetland loss are major environmental concerns to coastal communities. A better prediction of the future of this shore requires placing the instrumental lake-level records in a geological context. However, our knowledge of the late-Holocene history of the world's largest freshwater body remains fragmentary. Here we present a sedimentary record of late-Holocene relative lake-level changes by dating transgressive basal peats resting directly on a sandy substrate along a bathymetric gradient in Bark Bay Slough, Wisconsin. Our record shows a moderate lake-level rise at 1.4±0.2 mm/yr from about 2200 to 1250 cal yr BP as a result of relatively slow differential uplift of Bark Bay relative to the controlling outlet at Port Huron. The rise accelerated to 2.3±0.2 mm/yr at about 1250 cal yr BP when Sault Ste. Marie emerged as the controlling outlet, thereby separating Superior from Lakes Michigan–Huron and giving rise to the modern hydrographic regime of the upper Great Lakes. The timing of this event in our record is about 1000 yr later than that estimated in most previous studies, but our data complement and confirm the younger age of lake separation inferred using a different methodology. Our results not only provide pertinent information for hydrological regulation, navigation operation, and infrastructural design in the upper Great Lakes, but also provide insight into freshwater wetland succession on flooded coasts.

Relative sea-level (RSL) reconstructions from the circum-Caribbean region were interpreted using a glacial isostatic adjustment (GIA) model with the aims to quantify the contribution of this process to both the temporal and spatial forms of the RSL observations and remove the GIA signal to estimate land ice volume change (eustasy) during the mid-to-late Holocene. To infer an optimal GIA parameter set, the RSL data were used to determine best-fitting Earth viscosity model parameters for two different global ice histories. The RSL data indicate a clear preference (95% confidence) for relatively high viscosity values: > 0.8 × 1021 Pas in the upper mantle and > 3 × 1022 Pas in the lower mantle. The data were not able to discriminate (at 95% confidence) between lithospheric thickness values ranging between 71 and 120 km, although the thickest value considered produced the best fits. RSL predictions based on the best-fitting model parameters indicate a spatial variability across the region of up to ~ 7 m during the early to mid-Holocene which is large enough to introduce significant error when using the entire dataset to produce a single regional RSL curve without correcting for GIA. When corrected for GIA, the most precise data from the region indicate about 3–4 m of land ice melt (eustasy) from ~ 7 cal kyr B.P. to ~ 3–2 cal kyr B.P., although we note that there is considerable scatter in these corrected data. This likely reflects GIA model limitations, such as the assumption of lateral homogeneity in a region that contains several plate boundaries, the influence of tectonic processes (which were not modelled), as well as errors and an underestimate of the uncertainty in the RSL reconstructions.

Coral records from the Barbados have been used to infer that, due to ice expansion, sea level was 120 m lower than today during the Last Glacial Maximum. A 3D simulation of the mantle in this region suggests these estimates were biased by the presence of a subducted slab, and indicates the sea-level difference was closer to 130 m.

Ice core records were recently used to infer elevation changes of the Greenland ice sheet throughout the Holocene. The inferred elevation changes show a significantly greater elevation reduction than those output from numerical models, bringing into question the accuracy of the model-based reconstructions and, to some extent, the estimated elevation histories. A key component of the ice core analysis involved removing the influence of vertical surface motion on the δ 18 O signal measured from the Agassiz and Renland ice caps. We re-visit the original analysis with the intent to determine if the use of more accurate land uplift curves can account for some of the above noted discrepancy. To improve on the original analysis, we apply a geophysical model of glacial isostatic adjustment calibrated to sea-level records from the Queen Elizabeth Islands and Greenland to calculate the influence of land height changes on the δ 18 O signal from the two ice cores. This procedure is complicated by the fact that δ 18 O contained in Agassiz ice is influenced by land height changes distant from the ice cap and so selecting a single location at which to compute the land height signal is not possible. Uncertainty in this selection is further complicated by the possible influence of Innuitian ice during the early Holocene (12–8 ka BP). Our results indicate that a more accurate treatment of the uplift correction leads to elevation histories that are, in general, shifted down relative to the original curves at GRIP, NGRIP, DYE-3 and Camp Century. In addition, compared to the original analysis, the 1-σ uncertainty is considerably larger at GRIP and NGRIP. These changes reduce the data-model discrepancy reported by Vinther et al. (2009) at GRIP, NGRIP, DYE-3 and Camp Century. A more accurate treatment of isostasy and surface loading also acts to improve the data-model fits such that the residuals at all four sites for the period 8 ka BP to present are significantly reduced compared to the original analysis. Prior to 8 ka BP, the possible influence of Innuitian ice on the inferred elevation histories prevents a meaningful comparison.

This study evaluates the influence of plausible changes in East Antarctic Ice sheet (EAIS) thickness and the subsequent glacio-isostatic response as a contributor to the Antarctic warming indicated by ice core records during the Last Interglacial period (LIG). These higher temperatures have been estimated primarily using the difference in the δD peak (on average ~ 15‰) in these LIG records relative to records for the Present Interglacial (PIG). Using a preliminary exploratory modelling study, it is shown that introducing a relatively moderate reduction in the amount of thickening of the EAIS over the LIG period introduces a significant increase (up to 8‰) in the predicted elevation-driven only δD signal at the central Antarctic Ice sheet (AIS) ice core sites compared to the PIG. A sensitivity test in response to a large prescribed retreat of marine-based ice in the Wilkes and Aurora subglacial basins (equivalent to ~ 7 m of global mean sea-level rise) results in a distinct elevation signal that is resolvable within the ice core stable isotope records at three sites (Taylor Dome, TALDICE and EPICA Dome C). These findings have two main implications. First, EAIS elevation's only effects could account for a significant fraction of the LIG warming interpreted from ice core records. This result highlights the need for an improved estimate to be made of the uncertainty and size of this elevation-driven δD signal which contributes to this LIG warming and that these effects need to be deconvolved prior to attempting to extract a climatic-only signal from the stable isotope data. Second, a fingerprint of significant retreat of ice in the Wilkes and Aurora basins should be detectable from ice core δD records proximal to these basins and therefore used to constrain their contribution to elevated LIG sea levels, after accounting for ice sheet–climate interactions not considered in our approach.

The last deglaciation of the Scandinavian Ice Sheet (SIS) from ∼ 21 , 000 to 13,000 yr ago is well-constrained by several hundred 10Be and 14C ages. The subsequent retreat history, however, is established primarily from minimum-limiting 14C ages and incomplete Baltic-Sea varve records, leaving a substantial fraction of final SIS retreat history poorly constrained. Here we develop a high-resolution chronology for the final deglaciation of the SIS based on 79 10Be cosmogenic exposure dates sampled along three transects spanning southern to northern Sweden and Finland. Combining this new chronology with existing 10Be ages on deglaciation since the Last Glacial Maximum shows that rates of SIS margin retreat were strongly influenced by deglacial millennial-scale climate variability and its effect on surface mass balance, with regional modulation of retreat associated with dynamical controls. Ice-volume estimates constrained by our new chronology suggest that the SIS contributed ∼ 8 m sea-level equivalent to global sea-level rise between ∼14.5 ka and 10 ka. Final deglaciation was largely complete by ∼10.5 ka, with highest rates of sea-level rise occurring during the Bolling–Allerod, a 50% decrease during the Younger Dryas, and a rapid increase during the early Holocene. Combining our SIS volume estimates with estimated contributions from other remaining Northern Hemisphere ice sheets suggests that the Antarctic Ice Sheet (AIS) contributed 14.4 ± 5.9 m to global sea-level rise since ∼13 ka. This new constraint supports those studies that indicate that an ice volume of 15 m or more of equivalent sea-level rise was lost from the AIS during the last deglaciation.

Some of the world's largest cities are sinking faster than the oceans are rising. Humans are part of the problem, but we can also be part of the solution through monitoring and modeling.

International audience; Most of the policy debate surrounding the actions needed to mitigate and adapt to anthropogenic climate change has been framed by observations of the past 150 years as well as climate and sea-level projections for the twenty-first century. The focus on this 250-year window, however, obscures some of the most profound problems associated with climate change. Here, we argue that the twentieth and twenty-first centuries, a period during which the overwhelming majority of human-caused carbon emissions are likely to occur, need to be placed into a long-term context that includes the past 20 millennia, when the last Ice Age ended and human civilization developed, and the next ten millennia, over which time the projected impacts of anthropogenic climate change will grow and persist. This long-term perspective illustrates that policy decisions made in the next few years to decades will have profound impacts on global climate, ecosystems and human societies - not just for this century, but for the next ten millennia and beyond.

During the last deglaciation, sea levels rose as ice sheets retreated. This climate transition was punctuated by periods of more intense melting; the largest and most rapid of these—Meltwater Pulse 1A—occurred about 14,500 years ago, with rates of sea-level rise reaching approximately 4 m per century1, 2, 3. Such rates of rise suggest ice-sheet instability, but the meltwater sources are poorly constrained, thus limiting our understanding of the causes and impacts of the event4, 5, 6, 7. In particular, geophysical modelling studies constrained by tropical sea-level records1, 8, 9 suggest an Antarctic contribution of more than seven metres, whereas most reconstructions10 from Antarctica indicate no substantial change in ice-sheet volume around the time of Meltwater Pulse 1A. Here we use a glacial isostatic adjustment model to reinterpret tropical sea-level reconstructions from Barbados2, the Sunda Shelf3 and Tahiti1. According to our results, global mean sea-level rise during Meltwater Pulse 1A was between 8.6 and 14.6 m (95% probability). As for the melt partitioning, we find an allowable contribution from Antarctica of either 4.1 to 10.0 m or 0 to 6.9 m (95% probability), using two recent estimates11, 12 of the contribution from the North American ice sheets. We conclude that with current geologic constraints, the method applied here is unable to support or refute the possibility of a significant Antarctic contribution to Meltwater Pulse 1A.

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

The glacial isostatic adjustment (GIA) of the British Isles is complex due to the interplay between local and non-local signals. A number of recent studies have modelled the GIA response of the British Isles using relative sea-level data. This study extends these previous analyses by using output from a numerical glaciological model as input to a GIA model. This is a necessary step towards more realistic GIA models, and although there have been similar studies for the major late Pleistocene ice sheets, this is the first study to do so for the British Isles. We test three reconstructions, classed as ‘minimal’, ‘median’ and ‘maximal’ in terms of their volume at maximum extent, and find it is possible to obtain good data–model fits. The minimal reconstruction is clearly preferred by the sea-level data. The ice reconstructions tested were not constrained by geomorphological information of past ice extent (lateral and vertical). As a consequence, the reconstructions extend further than much of this information suggests, particularly in terms of ice thickness. It is notable, however, that the reconstructions produce good fits to many sea-level data from central, mountainous regions (e.g. Scottish highlands), which lends support to the suggestion that trimlines, often used as an constraint on the palaeo ice surface location, are in fact features formed at the transition from warm- to cold-based ice and so mark a minimum constraint on the ice surface altitude. Based on data–model misfits, suggestions are made for improving the ice model reconstructions. However, in many locations, the cause of the misfit could be due to non-local, predominantly Fennoscandian ice and so interpretation is not straightforward. As a result, we suggest that future analyses of this type consider models and observations for both Fennoscandia and the British Isles. Copyright © 2012 John Wiley & Sons, Ltd.

The absence of basal peat in the stratigraphic sequences of the southern Bay of Biscay has long precluded the development of Holocene sea-level curves. We have approached this problem by combining the indicative depositional meaning (derived from the micropalaeontological composition and sand content) with radiocarbon ages of 55 borehole samples obtained from three estuarine areas of the southern Bay of Biscay. These new sea-level index points have produced the first complete Holocene sea-level curve from this area. We further reviewed all available sea-level data from SW Europe to provide the regional trend and use these data to calibrate a recently developed isostatic model. Field data and model reconstructions present a good agreement for the region considered. A north-south trend is apparent in the data and this is shown to be dominated by the influence of the deglaciation of Eurasian ice sheets, as suggested by previous studies for this region. However, some data-model discrepancy in the south of the Iberian Peninsula suggests that local factors tend to dominate during the late Holocene. On comparing our results to estimates of recent sea-level rise obtained from tide gauges and high-resolution proxy records, it is clear that this region has experienced a significant acceleration in sea level during the past century or so.

This paper presents new evidence regarding relative sea-level (RSL) changes and vertical land motions at three sites in Greenland since 1300 A.D., a time interval that spans the later part of the Medieval Climate Anomaly (MCA) and the Little Ice Age (LIA). We observe RSL rise at two sites in central west Greenland from c. − 0.80 ± 0.20 m at c. 1300 A.D. to c. − 0.20 m ± 0.25 m at c. 1600 A.D., after which RSL slowed and then stabilised. At a third site in south Greenland, we observe RSL rise from c. − 1.40 ± 0.20 m at c. 1400 A.D. until c. 1750 A.D., after which RSL slowed and was stable during at least the latter part of the 20th century. The c. 1600 A.D. RSL slow-down seen at the two former sites is surprising because it occurs during the LIA when one might expect the ice sheet to be gaining mass and causing RSL to rise. We interpret this RSL slowdown to indicate a period of enhanced regional mass loss from central west Greenland since c. 1600 A.D. and propose two hypotheses for this loss: first, a reduction in precipitation during cold and dry conditions and second, higher air temperatures and increased peripheral surface melt of the ice sheet from this date onwards. The latter hypothesis is compatible with a well-established temperature seesaw between western Greenland and northern Europe and, potentially, a previously identified shift from a positive to generally more negative NAO conditions around 1400 to 1600 A.D. Our study shows how RSL data from Greenland can provide constraints on the timing of ice sheet fluctuations in the last millennium and challenges the notion that during cold periods in northern Europe the ice sheet in west Greenland gained mass.

Vertical land motion caused by continuing glacial isostatic adjustment is one of several important components of sea-level change and is not limited just to previously glaciated regions. A national-scale analysis for the British Isles shows an ellipse of present-day relative uplift (relative sea-level fall), ∼1.2 mm a−1, broadly centred on the deglaciated mountains of Scotland. The pattern of three foci of relative subsidence, ∼1 mm a−1, results from the additional interactions of the deglacial meltwater load on the Atlantic basin and the continental shelf, and the signal due to far-field ice sheets. At a local scale, sediment compaction can more than double the rate of relative land subsidence. Relative land-level change (the negative of relative sea-level change) is not the same as vertical land motion. There is a spatial pattern in the difference between relative land-level change and vertical land motion, with differences at present of approximately −0.1 to −0.3 mm a−1 around the British Isles and +2.5 to −1.5 mm a−1 globally. For the wider scientific and user community, whether or not the differences are considered significant will depend upon the location, time frame and spatial scale of the study that uses such information. Copyright © 2011 John Wiley & Sons, Ltd.