Glacio-isostatic Adjustment Research Articles

Early–middle Holocene relative sea-level oscillation events recorded in a submerged speleothem from the Yucatán Peninsula, Mexico

Geomorphological and biological archives of relative sea-level change in the western North Atlantic-Caribbean region following the Last Glacial Maximum have traditionally supported the hypothesis of a punctuated rise towards the present sea level. Such records, however, are often at insufficient resolution to discern centennial-scale changes. In caves where the water table is closely controlled by sea level, active periods of speleothem growth constraining maximum sea level, used in combination with marine overgrowths constraining minimum sea level, are a promising alternative archive recording sea-level variability at higher resolution. Here, we present a U-Th-dated early–middle Holocene speleothem record from a submerged cave on the tectonically stable Yucatán Peninsula, Mexico. Our record shows that polychaetes (Annelida, Polychaeta) colonised a sub-aerially deposited stalagmite during four individual submergence events. Submergence events occurred at approximately 8.9, 8.6, 8.4 and 6.0 ka, which we attribute to previously unrecognised minor sea-level oscillation events (OE1–OE4) above and below −6.12 ± 0.1 m relative to present sea level (r.s.l.). Combining these results with mangrove-derived relative sea-level constraints from another submerged cave on the Yucatán Peninsula, we are able to suggest that OE1 and OE2 did not reach as high as −5.26 m r.s.l., but that OE3 and OE4 exceeded −5.22 m r.s.l. We conclude that subsidence of the North American ice-load bulge was the main cause of relative sea-level rise. Superimposed on the glacio-isostatic adjustment were periods of widespread northern hemisphere cooling and ice margin re-advance, resulting in a relative sea-level fall on four occasions during the early–middle Holocene.

Terminal Antarctic melting inferred from a far-field coastal site

The contribution of the Antarctic ice sheet (AIS) to deglacial sea-level rise is poorly constrained. This shortfall gives rise to concerns because incorrect AIS estimates impact significantly on our ability to predict sea-level change in the course of global warming. Given the scarcity of geological data and the complexity of the Antarctic response to glacio-isostatic adjustment processes, there is a need for accurate data to constrain the timing of the ice-sheet retreat. Here, we provide such data on the Holocene Antarctic melting through an isolated geographic site on the northern hemisphere, which we show is sensitive to the Antarctic signal. Using both our site, and other mid-latitudinal relative sea-level sites, our model provides a consensus estimate that the AIS released water corresponding to 5 m equivalent sea level at 8 ka and ceased melting at 6 ka. This is different to most AIS models, which release an equal amount of water at 11 ka or after 6 ka. Our findings change model assumptions about the terminal AIS melting and show that future Holocene sea-level research should focus on broad shelves and large coast embayments in the mid latitudes.

Accelerated West Antarctic ice mass loss continues to outpace East Antarctic gains

While multiple data sources have confirmed that Antarctica is losing ice at an accelerating rate, different measurement techniques estimate the details of its geographically highly variable mass balance with different levels of accuracy, spatio-temporal resolution, and coverage. Some scope remains for methodological improvements using a single data type. In this study we report our progress in increasing the accuracy and spatial resolution of time-variable gravimetry from the Gravity Recovery and Climate Experiment (GRACE). We determine the geographic pattern of ice mass change in Antarctica between January 2003 and June 2014, accounting for glacio-isostatic adjustment (GIA) using the IJ05_R2 model. Expressing the unknown signal in a sparse Slepian basis constructed by optimization to prevent leakage out of the regions of interest, we use robust signal processing and statistical estimation methods. Applying those to the latest time series of monthly GRACE solutions we map Antarctica's mass loss in space and time as well as can be recovered from satellite gravity alone. Ignoring GIA model uncertainty, over the period 2003–2014, West Antarctica has been losing ice mass at a rate of −121±8 Gt/yr and has experienced large acceleration of ice mass losses along the Amundsen Sea coast of −18±5 Gt/yr2, doubling the mass loss rate in the past six years. The Antarctic Peninsula shows slightly accelerating ice mass loss, with larger accelerated losses in the southern half of the Peninsula. Ice mass gains due to snowfall in Dronning Maud Land have continued to add about half the amount of West Antarctica's loss back onto the continent over the last decade. We estimate the overall mass losses from Antarctica since January 2003 at −92±10 Gt/yr.

A data-driven approach for assessing ice-sheet mass balance in space and time

AbstractCombinations of various numerical models and datasets with diverse observation characteristics have been used to assess the mass evolution of ice sheets. As a consequence, a wide range of estimates have been produced using markedly different methodologies, data, approximation methods and model assumptions. Current attempts to reconcile these estimates using simple combination methods are unsatisfactory, as common sources of errors across different methodologies may not be accurately quantified (e.g. systematic biases in models). Here we provide a general approach which deals with this issue by considering all data sources simultaneously, and, crucially, by reducing the dependence on numerical models. The methodology is based on exploiting the different space–time characteristics of the relevant ice-sheet processes, and using statistical smoothing methods to establish the causes of the observed change. In omitting direct dependence on numerical models, the methodology provides a novel means for assessing glacio-isostatic adjustment and climate models alike, using remote-sensing datasets. This is particularly advantageous in Antarctica, where in situ measurements are difficult to obtain. We illustrate the methodology by using it to infer Antarctica’s mass trend from 2003 to 2009 and produce surface mass-balance anomaly estimates to validate the RACMO2.1 regional climate model.

Late Holocene sea-level change in Arctic Norway

Relative sea-level data from the pre-industrial era are required for validating geophysical models of glacio-isostatic adjustment as well as for testing models used to make sea-level predictions based on future climate change scenarios. We present the first late Holocene (past ∼3300 years) relative sea-level reconstruction for northwestern Norway based on investigations in South Hinnøya in the Vesterålen – Lofoton archipelago. Sea-level changes are reconstructed from analyses of salt-marsh and estuarine sediments and the micro-organisms (foraminifera and testate amoebae) preserved within. The ‘indicative meaning’ of the microfauna is established from their modern distributions. Records are dated by radiocarbon, 201Pb, 137Cs and chemostratigraphical analyses. Our results show a continuous relative sea-level decline of 0.7–0.9 mm yr−1 for South Hinnøya during the late Holocene. The reconstruction extends the relative sea-level trend recorded by local tide gauge data which is only available for the past ∼25 years. Our reconstruction demonstrates that existing models of shoreline elevations and GIA overpredict sea-level positions during the late Holocene. We suggest that models might be adjusted in order to reconcile modelled and reconstructed sea-level changes and ultimately improve understanding of GIA in Fennoscandia.

Response of salt structures to ice-sheet loading: implications for ice-marginal and subglacial processes

During the past decades the effect of glacioisostatic adjustment has received much attention. However, the response of salt structures to ice-sheet loading and unloading is poorly understood. Our study aims to test conceptual models of the interaction between ice-sheet loading and salt structures by finite-element modelling. The results are discussed with regard to their implications for ice-marginal and subglacial processes. Our models consist of 2D plane-strain cross-sections, which represent simplified geological cross-sections from the Central European Basin System. The model layers represent (i) sedimentary rocks of elastoplastic rheology, (ii) a viscoelastic diapir and layer of salt and (iii) an elastoplastic basement. On top of the model, a temporarily variable pressure simulates the advance and retreat of an ice sheet. The durations of the individual loading phases were defined to resemble the durations of the Pleistocene ice advances in northern central Europe. The geometry and rheology of the model layers and the magnitude, spatial distribution and timing of ice-sheet loading were systematically varied to detect the controlling factors. All simulations indicate that salt structures respond to ice-sheet loading. An ice advance towards the diapir causes salt flow from the source layer below the ice sheet towards the diapir, resulting in an uplift of up to +4 m. The diapir continues to rise as long as the load is applied to the source layer but not to the crest of the diapir. When the diapir is transgressed by the ice sheet the diapir is pushed down (up to −36 m) as long as load is applied to the crest of the diapir. During and after ice unloading large parts of the displacement are compensated by a reversal of the salt flow. Plastic deformation of the overburden is restricted to the area immediately above the salt diapir. The displacements after unloading range between −3.1 and +2.7 m. Larger displacements are observed in models with deep-rooted diapirs, thicker ice sheets, longer duration of the loading phase, thicker salt source layers and lower viscosity of the salt. The rise or fall of diapirs triggered or amplified by ice-sheet loading are likely to affect glacigenic deformation, erosion and deposition above the diapir and within the rim synclines. Ice-load induced uplift in front of an ice sheet will provide favourable conditions for the formation of push moraines, for example by creating a topographic obstacle and inclining potential detachments. Subglacial subsidence of salt structures will enhance erosion by providing a preferential drainage pathway and fracturing of the overburden of the salt structure and thereby contribute to the incision of tunnel valleys. However, the resulting displacements are probably too low to have a marked effect on the advance or retreat pattern of the ice sheets.

Late Holocene sea- and land-level change on the U.S. southeastern Atlantic coast

Abstract Late Holocene relative sea-level (RSL) reconstructions can be used to estimate rates of land-level (subsidence or uplift) change and therefore to modify global sea-level projections for regional conditions. These reconstructions also provide the long-term benchmark against which modern trends are compared and an opportunity to understand the response of sea level to past climate variability. To address a spatial absence of late Holocene data in Florida and Georgia, we reconstructed ~ 1.3 m of RSL rise in northeastern Florida (USA) during the past ~ 2600 years using plant remains and foraminifera in a dated core of high salt-marsh sediment. The reconstruction was fused with tide-gauge data from nearby Fernandina Beach, which measured 1.91 ± 0.26 mm/year of RSL rise since 1900 CE. The average rate of RSL rise prior to 1800 CE was 0.41 ± 0.08 mm/year. Assuming negligible change in global mean sea level from meltwater input/removal and thermal expansion/contraction, this sea-level history approximates net land-level (subsidence and geoid) change, principally from glacio-isostatic adjustment. Historic rates of rise commenced at 1850–1890 CE and it is virtually certain (P = 0.99) that the average rate of 20th century RSL rise in northeastern Florida was faster than during any of the preceding 26 centuries. The linearity of RSL rise in Florida is in contrast to the variability reconstructed at sites further north on the U.S. Atlantic coast and may suggest a role for ocean dynamic effects in explaining these more variable RSL reconstructions. Comparison of the difference between reconstructed rates of late Holocene RSL rise and historic trends measured by tide gauges indicates that 20th century sea-level trends along the U.S. Atlantic coast were not dominated by the characteristic spatial fingerprint of melting of the Greenland Ice Sheet.

Recent rates of sedimentation on irregularly flooded Boreal Baltic coastal wetlands: Responses to recent changes in sea level

Boreal Baltic coastal wetlands differ markedly from temperate salt marshes by their generally low maximum elevation (between 0 and 1m above m.s.l.), low seaward gradients and the irregular nature of flooding that is characteristic of the NE Baltic Sea coastal region. As a result of these factors these wetlands have been considered to be threatened by future sea level rise. This study presents results for two Boreal Baltic coastal wetland sites in Estonia using the 210Pb and 137Cs radiometric dating methods to investigate the sedimentary development of these coastal systems. Recent coastal evolution has been largely driven by continuing glacio-isostatic adjustment (GIA), with maximum rates of 2.8mmyr−1 around the NW Estonian coast and the inherited geomorphological setting of generally flat-lying coastal topography, resulting in coastal emergence. Broad agreement exists between calculated rates of sedimentation identified within the core sequences. Average rates of sedimentation using the 210Pbexcess CF:CS (or ‘simple’) model range between 0.2 and 1.3mmyr−1. These rates are corroborated using 137Cs, which also suggests an increase in sedimentation rates during recent decades approaching maximum values for current land uplift. Additionally, the 210Pbexcess CRS model reveals periods of sedimentation greatly in excess of these values in response to coastal flooding from known storm activity. This study indicates that changes in sea level caused by variations in atmospheric pressure and storm surges can contribute a significant sedimentary component, which coupled with GIA processes has driven coastal wetland development/emergence and the historical progradation of these wetland systems. The recent acceleration in the rate of global sea-level rise may subtly alter this relationship. However current rates of GIA and sedimentation will continue to maintain the progradation of Boreal Baltic coastal wetlands in the coming decades.

Retreat history of the East Antarctic Ice Sheet since the Last Glacial Maximum

The East Antarctic Ice Sheet (EAIS) is the largest continental ice mass on Earth, and documenting its evolution since the Last Glacial Maximum (LGM) is important for understanding its present-day and future behaviour. As part of a community effort, we review geological evidence from East Antarctica that constrains the ice sheet history throughout this period (∼30,000 years ago to present). This includes terrestrial cosmogenic nuclide dates from previously glaciated regions, 14C chronologies from glacial and post-glacial deposits onshore and on the continental shelf, and ice sheet thickness changes inferred from ice cores and continental-scale ice sheet models. We also include new 14C dates from the George V Land – Terre Adélie Coast shelf. We show that the EAIS advanced to the continental shelf margin in some parts of East Antarctica, and that the ice sheet characteristically thickened by 300–400 m near the present-day coastline at these sites. This advance was associated with the formation of low-gradient ice streams that grounded at depths of >1 km below sea level on the inner continental shelf. The Lambert/Amery system thickened by a greater amount (800 m) near its present-day grounding zone, but did not advance beyond the inner continental shelf. At other sites in coastal East Antarctica (e.g. Bunger Hills, Larsemann Hills), very little change in the ice sheet margin occurred at the LGM, perhaps because ice streams accommodated any excess ice build up, leaving adjacent, ice-free areas relatively unaffected. Evidence from nunataks indicates that the amount of ice sheet thickening diminished inland at the LGM, an observation supported by ice cores, which suggest that interior ice sheet domes were ∼100 m lower than present at this time. Ice sheet recession may have started ∼18,000 years ago in the Lambert/Amery glacial system, and by ∼14,000 years ago in Mac.Robertson Land. These early pulses of deglaciation may have been responses to abrupt sea-level rise events such as Meltwater Pulse 1a, destabilising the margins of the ice sheet. It is unlikely, however, that East Antarctica contributed more than ∼1 m of eustatic sea-level equivalent to post-glacial meltwater pulses. The majority of ice sheet recession occurred after Meltwater Pulse 1a, between ∼12,000 and ∼6000 years ago, during a period when the adjacent ocean warmed significantly. Large tracts of East Antarctica remain poorly studied, and further work is required to develop a robust understanding of the LGM ice sheet expansion, and its subsequent contraction. Further work will also allow the contribution of the EAIS to post-glacial sea-level rise, and present-day estimates of glacio-isostatic adjustment to be refined.

Late Wisconsinan glaciation and postglacial relative sea-level change on western Banks Island, Canadian Arctic Archipelago

The study revises the maximum extent of the northwest Laurentide Ice Sheet (LIS) in the western Canadian Arctic Archipelago (CAA) during the last glaciation and documents subsequent ice sheet retreat and glacioisostatic adjustments across western Banks Island. New geomorphological mapping and maximum-limiting radiocarbon ages indicate that the northwest LIS inundated western Banks Island after ~3114CkaBP and reached a terminal ice margin west of the present coastline. The onset of deglaciation and the age of the marine limit (22–40m asl) are unresolved. Ice sheet retreat across western Banks Island was characterized by the withdrawal of a thin, cold-based ice margin that reached the central interior of the island by ~14calkaBP. The elevation of the marine limit is greater than previously recognized and consistent with greater glacioisostatic crustal unloading by a more expansive LIS. These results complement emerging bathymetric observations from the Arctic Ocean, which indicate glacial erosion during the Last Glacial Maximum (LGM) to depths of up to 450m.

Geological constraints on deglaciation and glacio-isostatic adjustment models of relative sea-level change on the northern Antarctic Peninsula

Geological constraints on deglaciation and glacio-isostatic adjustment models of relative sea-level change on the northern Antarctic Peninsula

Melting ice sheets 400,000 yr ago raised sea level by 13 m: Past analogue for future trends

Marine Isotope Stage (MIS) 11 was possibly the longest (∼423,000–362,000 yr ago) and warmest interglacial of the entire Quaternary Period. This resulted from a special arrangement of Earth's orbital parameters-similar to the present interglacial (MIS 1)-suggesting that MIS 11 provides an analogue for future natural climate forcing and sea levels. Although precise documentation of MIS 11 sea level history is, therefore, crucial – especially considering additional impetus from anthropogenic warming – the maximum MIS 11 sea level remains highly contentious. Estimates from onshore indicators have ranged from below present sea level to about +20m, resulting from uncertainties in reliability of sea level indicators, estimates of their subsequent displacement by vertical crustal motion and in dating. We used a clear and unambiguous sea level indicator (interface between subtidal and intertidal deposits) in well exposed, regressive raised shoreline deposits along South Africa's southern coast. The coast is stable in terms of tectonic setting and falls within a zone of low predicted glacio-isostatic adjustment (GIA) and low sensitivity for plausible model parameters. The sea level indicators were directly dated using thermally transferred optically stimulated luminescence (TT-OSL), which gave a mean age of ∼390ka, corresponding with MIS 11. We determined a precise maximum elevation of +14m for the MIS 11 sea level indicator, which corrected for minor crustal uplift and GIA yielded a eustatic sea level of +13m ±2m. The complete MIS 11 transgressive/regressive shoreline succession is preserved and the wave-cut platform extends 950m inland from the present shoreline, indicating a prolonged highstand. We have also documented sea level fluctuations during MIS 11, including an earlier (lower) highstand close to present sea level, which may explain the lower elevation range of some previous estimates. At one locality, MIS 11 deposits form a sea cliff against which MIS 5e sediments (at an elevation of +6.2m) abut. The +13m MIS 11 sea level tallies with total melting of the Greenland and West Antarctica ice sheets, regarded as most susceptible to melting. Our data predict notably higher sea levels in the future, regardless of anthropogenic influence on climate.

Post-glacial sea-level changes around the Australian margin: a review

It has been known since Rhodes Fairbridge’s first attempt to establish a global pattern of Holocene sea-level change by combining evidence from Western Australia and from sites in the northern hemisphere that the details of sea-level history since the Last Glacial Maximum vary considerably across the globe. The Australian region is relatively stable tectonically and is situated in the ‘far-field’ of former ice sheets. It therefore preserves important records of post-glacial sea levels that are less complicated by neotectonics or glacio-isostatic adjustments. Accordingly, the relative sea-level record of this region is dominantly one of glacio-eustatic (ice equivalent) sea-level changes. The broader Australasian region has provided critical information on the nature of post-glacial sea level, including the termination of the Last Glacial Maximum when sea level was approximately 125m lower than present around 21,000–19,000 years BP, and insights into meltwater pulse 1A between 14,600 and 14,300cal.yrBP. Although most parts of the Australian continent reveals a high degree of tectonic stability, research conducted since the 1970s has shown that the timing and elevation of a Holocene highstand varies systematically around its margin. This is attributed primarily to variations in the timing of the response of the ocean basins and shallow continental shelves to the increased ocean volumes following ice-melt, including a process known as ocean siphoning (i.e. glacio-hydro-isostatic adjustment processes).Several seminal studies in the early 1980s produced important data sets from the Australasian region that have provided a solid foundation for more recent palaeo-sea-level research. This review revisits these key studies emphasising their continuing influence on Quaternary research and incorporates relatively recent investigations to interpret the nature of post-glacial sea-level change around Australia. These include a synthesis of research from the Northern Territory, Queensland, New South Wales, South Australia and Western Australia. A focus of these more recent studies has been the re-examination of: (1) the accuracy and reliability of different proxy sea-level indicators; (2) the rate and nature of post-glacial sea-level rise; (3) the evidence for timing, elevation, and duration of mid-Holocene highstands; and, (4) the notion of mid- to late Holocene sea-level oscillations, and their basis.Based on this synthesis of previous research, it is clear that estimates of past sea-surface elevation are a function of eustatic factors as well as morphodynamics of individual sites, the wide variety of proxy sea-level indicators used, their wide geographical range, and their indicative meaning. Some progress has been made in understanding the variability of the accuracy of proxy indicators in relation to their contemporary sea level, the inter-comparison of the variety of dating techniques used and the nuances of calibration of radiocarbon ages to sidereal years. These issues need to be thoroughly understood before proxy sea-level indicators can be incorporated into credible reconstructions of relative sea-level change at individual locations. Many of the issues, which challenged sea-level researchers in the latter part of the twentieth century, remain contentious today. Divergent opinions remain about: (1) exactly when sea level attained present levels following the most recent post-glacial marine transgression (PMT); (2) the elevation that sea-level reached during the Holocene sea-level highstand; (3) whether sea-level fell smoothly from a metre or more above its present level following the PMT; (4) whether sea level remained at these highstand levels for a considerable period before falling to its present position; or (5) whether it underwent a series of moderate oscillations during the Holocene highstand.

Where might we find evidence of a Last Interglacial West Antarctic Ice Sheet collapse in Antarctic ice core records?

Abundant indirect evidence suggests that the West Antarctic Ice Sheet (WAIS) reduced in size during the Last Interglacial (LIG) compared to the Holocene. This study explores this possibility by comparing, for the first time, ice core stable isotope records for the LIG with output from a glacio-isostatic adjustment (GIA) model. The results show that ice core records from East Antarctica are remarkably insensitive to vertical movement of the solid land motion driven by a simulated hypothetical collapse of the WAIS. However, new and so far unexplored sites are identified which are sensitive to the isostatic signal associated with WAIS collapse and so ice core proxy data from these sites would be effective in testing this hypothesis further.

Use of local tidal records to identify relative sea level change: accuracy and error for decision makers

The Intergovernmental Panel on Climate Change (IPCC) considers eustatic sea level rise to be a major impact driven by climate change. Relative sea level change, whether positive or negative, will affect industries, communities and ecology along the world’s coastlines and estuaries. Estimates of global eustatic sea level rise between 1961 and 2003 are 1.8 ± 0.5 mm a−1, reflecting results from validated global tide gauge records. Over the last two decades, several studies have used automatic tide gauge records with at least 80 years of data to generate global prediction models. The IPCC recognises that global change is not uniform, therefore local policy for flood management and coastal protection should rely on local change models that incorporate glacio-isostatic adjustment (GIA) and apply accurate data correction techniques. Some of the longest tidal records are held within the Northern Hemisphere, e.g. Cascais, Amsterdam, Aberdeen, Sheerness and Newlyn. The UK provides several important case studies highlighting changes in relative sea level between the north and the south, primarily due to variations in GIA rates of land uplift and subsidence. Tide gauge records are held by a variety of governmental, non-governmental and private organisations. However, each source may typically compile data in different ways, relying on diverse equipment and recording techniques, often with variations in frequency, length, quality and corrections applied. Even within a single organisation there may be differences in dataset quality. This paper examines some of the key sources of error when working with historical tidal datasets in local geographic areas and aims to identify the limitations of locally derived data thereby assisting in the determination of relative sea level trends that are of widespread value to infrastructure and policy makers.

Was pre–twentieth century sea level stable?

Sea level rise (SLR) ranks high on the list of climate change issues because the expected acceleration from the current rate (about 3.1 millimeters per year) poses threats to coastal regions. Tide gauge, salt marsh, and archaeological records, and modeling of glacioisostatic adjustment (GIA) have led to the widely accepted idea that late Holocene (the past ∼2000 years) sea level was stable prior to acceleration beginning around 1850–1900 C.E. For instance, according to the Intergovernmental Panel on Climate Change Fourth Assessment Report, before the last century, sea level had “stabilized” over the past 2000 years, rising at a mean rate of 0–0.2 millimeter per year [Bindoff et al., 2007]. Others maintain that sea level was “nearly stable” over the past few thousand years [Nicholls and Cazenave, 2010], pre–twentieth century rates were “close to zero” [Church et al., 2008], or “stable from at least BC 100 until AD 950” and “stable, or slightly falling” from 1350 until the nineteenth century [Kemp et al., 2011].

Relative sea-level response to Little Ice Age ice mass change in south central Alaska: Reconciling model predictions and geological evidence

Integration of geological data and glacio-isostatic adjustment (GIA) modelling shows that it is possible to decouple complex mechanisms of relative sea-level (RSL) change in a tectonically active glacial environment. We model a simplest solution in which RSL changes in upper Cook Inlet, Alaska, are a combination of the interplay of tectonic and isostatic processes driven by the unique rheology of this tectonically active location. We calculate interseismic uplift during latter part of the penultimate earthquake cycle to vary from 0.3 to 0.7mm/yr. Diatom based reconstructions of RSL from tidal marsh sediment sequences coupled with detailed age models, from AD 1400 to the AD 1964 great earthquake, show deviations from a purely tectonically driven model of regional RSL. Glacial isostatic modelling, constrained by GPS data, predicts up to 70cm sea-level change due to mountain glacier mass balance changes during the Little Ice Age. Misfits between the GIA model predictions and RSL reconstructions in the 19th and 20th century highlight that the tidal marshes of upper Cook Inlet potentially record a hemispheric-wide acceleration in sea level and that other more complex Earth process combinations may contribute to regional RSL change.

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

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

Impact Assessment of Current and Future Sea-Level Change on Coastal Archaeological Resources—Illustrated Examples From Northern Newfoundland

ABSTRACT Coastal archaeological heritage is potentially vulnerable to increased erosion resulting from predicted future sea-level rise and increased storminess. As all sites cannot be protected, it is essential that heritage managers know which sites and landscapes are most at risk so they can prioritize resources and decision-making most effectively. One method of doing so is through desk-based modeling of coastal vulnerability. This article outlines the advantages and limitations of such an approach and demonstrates the application of a desk-based model to case studies from Newfoundland's coast. The rate of future sea-level rise around Newfoundland is complicated by its glacio-isostatic recovery since the last ice age. The first step therefore in this assessment is to combine output from glacio-isostatic adjustment models with appropriate rates of global eustatic sea-level rise. Next, these data are integrated with existing information on coastal characteristics (topography, surficial geology, erosion rates) to assess coastal sensitivity to sea-level rise. Finally, overlay of known archaeological sites identifies those locations at greatest risk from destructive coastal changes. The results demonstrate the effectiveness of such models for regional-scale analyses but caution against the use of low resolution data to generate site-specific predictions.

Ecosystem Resilience and Threshold Response in the Galápagos Coastal Zone

BackgroundThe Intergovernmental Panel on Climate Change (IPCC) provides a conservative estimate on rates of sea-level rise of 3.8 mm yr−1 at the end of the 21st century, which may have a detrimental effect on ecologically important mangrove ecosystems. Understanding factors influencing the long-term resilience of these communities is critical but poorly understood. We investigate ecological resilience in a coastal mangrove community from the Galápagos Islands over the last 2700 years using three research questions: What are the ‘fast and slow’ processes operating in the coastal zone? Is there evidence for a threshold response? How can the past inform us about the resilience of the modern system?Methodology/Principal FindingsPalaeoecological methods (AMS radiocarbon dating, stable carbon isotopes (δ13C)) were used to reconstruct sedimentation rates and ecological change over the past 2,700 years at Diablas lagoon, Isabela, Galápagos. Bulk geochemical analysis was also used to determine local environmental changes, and salinity was reconstructed using a diatom transfer function. Changes in relative sea level (RSL) were estimated using a glacio-isostatic adjustment model. Non-linear behaviour was observed in the Diablas mangrove ecosystem as it responded to increased salinities following exposure to tidal inundations. A negative feedback was observed which enabled the mangrove canopy to accrete vertically, but disturbances may have opened up the canopy and contributed to an erosion of resilience over time. A combination of drier climatic conditions and a slight fall in RSL then resulted in a threshold response, from a mangrove community to a microbial mat.Conclusions/SignificancePalaeoecological records can provide important information on the nature of non-linear behaviour by identifying thresholds within ecological systems, and in outlining responses to ‘fast’ and ‘slow’ environmental change between alternative stable states. This study highlights the need to incorporate a long-term ecological perspective when designing strategies for maximizing coastal resilience.