Relative Sea-level History Research Articles

Glacial isostatic adjustment: New developments from advanced observing systems and modeling

Abstract The measurement of glacial isostatic adjustment (GIA) is one of the key ways in which geophysicists probe the long-term mantle rheology and Pleistocene ice history. GIA models are also tied to global and regional relative sea-level (RSL) histories, to 20th century tide-gauge (TG) data and to space and terrestrial geodetic measurements. Two new types of observation are related to the high-resolution space–gravity data recovered from the Gravity and Climate Experiment (GRACE) satellite pair and the soon-to-be launched Gravity and Ocean Circulation Experiment (GOCE) with on-board three-component gradiometer. Gravity mapping has the unique capability of isolating those regions that lack isostatic equilibrium. When coupled with other space and terrestrial geodetic measurements, such as those of the Global Positioning System (GPS) networks and with multi-decade terrestrial gravity data, new constraints on GIA are in the offing and should soon illuminate new interpretations of ice-sheet history and mantle response. GIA studies also incorporate space-based altimetry data, which now provide multi-decadal coverage over continents, oceans and lakes. As we are approaching 72 monthly solutions of GRACE gravity coefficients for determining the Earth's secular component of gravity change over the continents, a new issue has surfaced: the problem of relying on interannual hydrological modeling to determine the hydrological contribution to the linear trend in the gravity field. Correctly extracting this contribution is germane to using the GIA-driven component for modeling solid-Earth and paleo-climatic parameters. Seismic and heat-flux-based models of the Earth's interior are emerging with ever higher levels of sophistication regarding material strength (or viscosity). A basic question raised is: how good are traditional Newtonian and non-Newtonian viscosity models that only allow radial variations of Earth parameters? In other words: under what circumstances must this assumption be abandoned for joint interpretations of new and traditional data sets. In this short review we summarize the issues raised in the papers forming this special issue (SI) dedicated to GIA.

Subtropical sea-level history from coral microatolls in the Southern Cook Islands, since 300 AD

Relative sea-level (RSL) history is reconstructed for the Southern Cook Islands since 300 AD using Porites sp. coral microatolls. The upper surface or height of living coral of the modern microatolls on Rarotonga was rigorously determined by 400 laser survey measurements to be constrained by the sea-level − 0.36 m (below Mean Sea Level) with a standard deviation of ± 0.008 m, that is equivalent to midway between Mean Low Water Neaps and Mean Low Water Springs tidal datums. Fossil microatoll history on the fringing reefs is fragmentary due to multi-decadal to centennial windows of microatoll growth, cyclone destruction, and preservation through coastal progradation. The mid-late Holocene highstand ceased prior to 500 BC and RSL fell from + 1.3 ± 0.1 m to 0.45 ± 0.15 m by 1000 AD at a rate of − 0.5 mm/yr. The RSL minimum occurred during the late 1700s to 1800s AD at − 0.2 m below present, before rising during the 1900s. During the past 500 yr RSL fluctuated, including an abrupt fall at ~ 1750 AD following a sustained multi-decadal high anomaly. Similarly, low RSL anomalies and slow coral growth rates during the 950 to 1000 AD period are indicative of Pacific Basin-wide climate variability and are consistent with extreme La Niña-like climate anomalies previously reported around the Pacific basin. The fall in RSL during the first millennium AD crossed-over tidal thresholds for rapid coastal progradation, when ambient Mean High Water Neaps and Mean High Water Springs fell below the low tide level of the mid-late Holocene high stand. This coastal response is inversely analogous to the projected response to sea-level rise during the latter part of the modern century.

Controls on Early Diagenetic Overprinting in Icehouse Carbonates: Insights from Modeling Hydrological Zone Residence Times Using CARB3D+

Abstract Icehouse carbonate diagenesis is complex, with carbonate sequences being exposed to multiple meteoric hydrozones (vadose-zone, freshwater-lens, mixing-zone) on multiple occasions. A forward modeling approach using CARB3D+ has enabled quantitative exploration of the nature of this diagenetic overprinting, by explicit simulation of meteoric hydrozone residence times during evolution of an isolated carbonate platform. A sensitivity analysis was performed, encompassing the fundamental controls on hydrozone residence time; frequency and magnitude of sea-level variation, subsidence rate, and climate, the latter via its effect on freshwater-lens thickness and rate of dissolutional lowering of the land surface. Residence time within all meteoric hydrozones is characterized by a high degree of lateral continuity. Vertical continuity is much more limited, with significant variations on a fourth order sequence scale across the platform. Below sequence boundaries prolonged exposure to the vadose zone results from multiple missed beats (when the platform remains exposed at relative sea-level maxima). Overprinting gives total meteoric residence times significantly in excess of the duration of exposure of the overlying sequence boundary. There is a hierarchy of eustatically driven overprint cycles, with higher-frequency cyclicity modifying the primary signal generated by lower-frequency cycles. Fifth-order cycles thus have only a minor impact on hydrozone residence times, whilst third order (seismic scale) cycles are a critical control, and could be used as a first-order approximation of the amount of diagenetic overprinting. Subsidence has a strong control on residence time, with greater overprinting at lower subsidence rates, but in high-subsidence regimes sediment packages move below the zone of meteoric-water influence sufficiently rapidly that residence time can be minimal. High land-surface dissolution rates, which result from a wetter climate, remove a large proportion of the subaerially exposed sediment and reduce vadose-zone residence time. Higher effective recharge can increase the thickness of freshwater lens and mixing zones which increases residence times significantly. Residence times in the vadose zone can be directly linked to a sequence stratigraphic framework, but for the freshwater lens and the mixing zone this is possible only with a well constrained relative sea-level history.

Post-glacial relative sea level, isostasy, and glacial history in Icy Strait, Southeast Alaska, USA

We use the radiocarbon ages of marine shells and terrestrial vegetation to reconstruct relative sea level (RSL) history in northern Southeast Alaska. RSL fell below its present level around 13,900 cal yr BP, suggesting regional deglaciation was complete by then. RSL stayed at least several meters below modern levels until the mid-Holocene, when it began a fluctuating rise that probably tracked isostatic depression and rebound caused by varying ice loads in nearby Glacier Bay. This fluctuating RSL rise likely reflects the episodic but progressive advance of ice in Glacier Bay that started around 6000 cal yr BP. After that time, RSL low stands probably signaled minor episodes of glacier retreat/thinning that triggered isostatic rebound and land uplift. Progressive, down-fjord advance of the Glacier Bay glacier during the late Holocene is consistent with the main driver of this glacial system being the dynamics of its terminus rather than climate change directly. Only after the glacier reached an exposed position protruding into Icy Strait ca. AD 1750, did its terminus succumb – a century before the climate changes that marked the end of the Little Ice Age – to the catastrophic retreat that triggered the rapid isostatic rebound and RSL fall occurring today in Icy Strait.

Relative sea‐level change and postglacial isostatic adjustment along the coast of south Devon, United Kingdom

AbstractPrevious sea‐level studies suggest that southwest Britain has the fastest subsiding coastline in the United Kingdom, but tide‐gauge data, GPS and gravity measurements and geophysical models show little evidence of anomalous subsidence in this region. In this paper we present 15 new sea‐level index points from four coastal barrier systems in south Devon. Eight are from compaction‐free basal sediments and others were corrected for autocompaction. Our data suggest that relative sea level along the south Devon coastline has risen by 21 ± 4 m during the past 9000 years. Sea‐level rise slowed during the middle and late Holocene and a rise of 8 ± 1 m has occurred since ca. 7000 cal. yr BP. Anomalous ages for many rejected points are attributed to sediment reworking during barrier transgression. The relative sea‐level history during the early and middle Holocene shows a good fit with geophysical model predictions, but the geological and modelled data diverge in the later Holocene. Unlike the geophysical models, sea‐level index points cannot differentiate between late Holocene relative sea‐level histories of south Devon and southwest Cornwall. It is suggested that this discrepancy can be resolved by obtaining additional high‐quality sea‐level index points covering the past 4000 years. Copyright © 2008 John Wiley & Sons, Ltd.

Quaternary glaciation history and glaciology of Jakobshavn Isbræ and the Disko Bugt region, West Greenland: a review

The Disko Bugt region in central West Greenland is characterised by permanent ice streams, of which Jakobshavn Isbræ is by far the most important. The first thorough studies on the glaciology of the region were conducted over 150 years ago by H.J. Rink, who introduced the terms 'ice streams' and 'Inland Ice'. Rink's work inspired new field work, which has continued to the present, and the long series of observations are unique for an Arctic region.
 Cooling during the Cenozoic led to ice-sheet growth in Greenland. A number of interglacial occurrences have been reported from the Disko Bugt region, and during the penultimate glacial stage, the Greenland ice-sheet margin extended to the shelf break. During the last glacial maximum, the ice margin probably extended only to the inner part of the banks on the continental shelf, and large floating glaciers may have been present at this time. During the Younger Dryas cold period, the ice margin may have been located at a marked basalt escarpment west of Disko Bugt.
 Disko Bugt was deglaciated rapidly in the early Holocene, around 10 500–10 000 years before present (10.5–10 ka B.P.), but when the ice margin reached the eastern shore of the bay, recession paused, and major moraine systems were formed. With renewed recession, the present ice-margin position was attained around 8–6 ka B.P., and by c. 5 ka B.P. the ice margin was located east of its present position. The subsequent Neoglacial readvance generally reached a maximum during the Little Ice Age, around AD 1850. This was followed by recession that has continued to the present day.
 The relative sea-level history shows a rapid sea-level fall in the early Holocene, and a slow rise in the late Holocene. This development mainly reflects a direct isostatic response to the ice-margin history.
 Jakobshavn Isbræ is the main outlet from the Greenland ice sheet. It drains c. 6.5% of the present Inland Ice, and produces c. 35–50 km3 of icebergs per year, corresponding to more than 10% of the total output of icebergs from the Inland Ice. The velocity of the central part of the ice stream at the front has been around 7 km/year since records began, but has nearly doubled in recent years. Other calf-ice producing glacier outlets in Disko Bugt produce c. 18 km3 per year. The large calf-ice production of Jakobshavn Isbræ may have been initiated at about 8 ka B.P. when the glacier front receded from the iceberg bank (Isfjeldsbanken) near Ilulissat. Ice streams in inner and outer Egedesminde Dyb may have been active during the early Holocene and during the last glacial maximum.

Sea-level fluctuations imply that the Younger Dryas ice-sheet expansion in western Norway commenced during the Allerød

After the first emergence following deglaciation, relative sea level rose by 10 m in western Norway and culminated late in the Younger Dryas (YD). The relative sea-level history, reconstructed by dating deposits in isolation basins, shows a sea-level low-stand between ∼13 640 and 13 080 cal yr BP, a 10 m sea-level rise between ∼13 080 and 11 790 cal yr BP and a sea-level high-stand between ∼11 790 and 11 550 cal yr BP. Shortly after the YD/Holocene boundary, sea level fell abruptly by ∼37 m. The shorelines formed during the sea-level low-stand in the mid-Allerød and during the sea-level high-stand in the YD have almost parallel tilts with a gradient of ∼1.3 m km −1, indicating that hardly any isostatic movement has taken place during this period of sea-level rise. We conclude that the transgression was caused by the major re-advance of the Scandinavian Ice Sheet that took place in western Norway during the Lateglacial. The extra ice load halted the isostatic uplift and elevated the geoid due to the increased gravitational attraction on the sea. Our results show that the crust responded to the increased load well before the YD (starting ∼12 900 cal yr BP), with a sea-level low-stand at 13 640 cal yr BP and the subsequent YD transgression starting at 13 080 cal yr BP. Thus, we conclude that the so-called YD ice-sheet advance in western Norway started during the Allerød, possibly more than 600 years before the Allerød/YD transition.

Geocentric sea-level trend estimates from GPS analyses at relevant tide gauges world-wide

The problem of correcting the tide gauge records for the vertical land motion upon which the gauges are settled has only been partially solved. At best, the analyses so far have included model corrections for one of the many processes that can affect the land stability, namely the Glacial-Isostatic Adjustment (GIA). An alternative approach is to measure (rather than to model) the rates of vertical land motion at the tide gauges by means of space geodesy. A dedicated GPS processing strategy is implemented to correct the tide gauges records, and thus to obtain a GPS-corrected set of ‘absolute’ or geocentric sea-level trends. The results show a reduced dispersion of the estimated sea-level trends after application of the GPS corrections. They reveal that the reference frame implementation is now achieved within the millimetre accuracy on a weekly basis. Regardless of the application, whether local or global, we have shown that GPS data analysis has reached the maturity to provide useful information to separate land motion from oceanic processes recorded by the tide gauges or to correct these latter. For comparison purposes, we computed the global average of sea-level change according to Douglas [Douglas, B.C., 2001. Sea level change in the era of the recording tide gauge. Int. Geophys. Ser., 75, pp. 37–64.] rules, whose estimate is 1.84 ± 0.35 mm/yr after correction for the GIA effect [Peltier, W.R., 2001. Global glacial isostatic adjustment and modern instrumental records of relative sea level history. Int. Geophys. Ser., 75, pp. 65–95.]. We obtain a value of 1.31 ± 0.30 mm/yr, a value which appears to resolve the ‘sea level enigma’ [Munk, W., 2002. Twentieth century sea level: an enigma. Proc. Natl. Acad. Sci. U.S.A., 99(10), pp. 6550–6555].

Mid-Holocene evolution and paleoenvironments of the shoreface–offshore transition, north-eastern Argentina: New evidence based on benthic microfauna

A sedimentary record spanning 5792–5511 cal yr BP and 3188–2854 cal yr BP was recovered at 36° 45′ 43″ S–56 ° 37′ 13″ W, south-west South Atlantic. The sedimentological features and micropaleontological (benthic foraminifera and ostracoda) content were analyzed in order to reconstruct paleoenvironmental conditions. Considerable environmental fluctuations are indicated by all these proxies. Five different stages were distinguished: Stage 1 (ca. 5800–5000 cal yr BP) consists of muddy sand with abundant microfossils. In this interval, species typical for inner marine shelf environments maintained a high abundance. Stage 2 consists of plastic light greenish grey clays barren of microfossils, and probably represents fluvial input from the de la Plata River to the shelf contemporaneous of a lowering of sea level. Stage 3 is composed of brownish yellow sandy silts, and represents increasing marine conditions in the area as reflected by higher faunal diversity and typical foraminifera of inner shelf environments. Stage 4 is made of homogeneous mud, barren of microfossil, which represents a new pulse of fluvial input to the shelf in consequence of a new fall in sea level. The final part of the core (Stage 5) is a coarsening upward sequence, grading from greeny brown clayey sandy silts to coarse shelly sands and represents the modern sedimentation in the area. This interpretation strengthens the stepped model of late-Holocene sea-level fall between 5511–5792 cal yr BP and 2854–3188 cal yr BP in Buenos Aires coast, and agrees with the relative sea-level history previously proposed by some authors from western South Atlantic coasts.

A Revised Late Holocene Sea-Level Record for Northern Massachusetts, USA

Abstract Accelerator mass spectrometry (AMS) radiocarbon dating of basal high-marsh sediments from Romney Marsh, Revere, Massachusetts, provides a revised reconstruction of the late Holocene relative sea-level history of the region. After correction for changes in tidal amplitude, the sea-level change envelope reconstructed from five AMS radiocarbon dates of basal marsh sediments at Romney Marsh in Revere, Massachusetts, indicates a rise in mean sea level (MSL) of close to 2.6 m in the past 3300 years. The data indicate a possible decrease in the average rate of rise from 0.80 ± 0.25 mm/y between 3300 to 1000 YBP to a rate of 0.52 ± 0.62 mm/y between 1000 YBP and the past 150 to 500 years. An increase in the rate of sea-level rise is evident over the past few hundred years. A slowing of the rate of sea-level rise between 1000 YBP and historic times and the increase in the rate of rise to modern values is also evident in other sea-level records from Maine and Connecticut. The coherence between these sea-le...

Late Holocene sea-level changes and isostasy in western Denmark

Cores and exposed cliff sections in salt marshes around Ho Bugt, a tidal embayment in the northernmost part of the Danish Wadden Sea, were subjected to 14C dating and litho- and biostratigraphical analyses to reconstruct paleoenvironmental changes and to establish a late Holocene relative sea-level history. Four stages in the late Holocene development of Ho Bugt can be identified: (1) groundwater-table rise and growth of basal peat (from at least 2300 BC to AD 0); (2) salt-marsh formation (0 to AD 250); (3) a freshening phase (AD 250 to AD 1600?), culminating in the drying out of the marshes and producing a distinct black horizon followed by an aeolian phase with sand deposition; and (4) renewed salt-marsh deposition (AD 1600? to present). From 16 calibrated AMS radiocarbon ages on fossil plant fragments and 4 calibrated conventional radiocarbon ages on peat, we reconstructed a local relative sea-level history that shows a steady sea-level rise of 4 m since 4000 cal yr BP. Contrary to suggestions made in the literature, the relative sea-level record of Ho Bugt does not contain a late Holocene highstand. Relative sea-level changes at Ho Bugt are controlled by glacio-isostatic subsidence and can be duplicated by a glacial isostatic adjustment model in which no water is added to the world's oceans after ca. 5000 cal yr BP.

Late Quaternary in northwestern Russia ‐ Introduction

Following reconnaissance work in 1994, a group offour Norwegian and Russian geologists in the summerof 1995 started working in river sections alongSevernaya Dvina and Vaga in the Arkhangelsk regionof northwestern Russia. This was the initiation of theDanish Norwegian Russian Swedish ArkhangelskProject, which in the following years evolved from shipand vehicle-borne 6 8 person expeditions along Sever-naya Dvina (1996 and 1997) and the Mezen RiverBasin (1997 and 1998) to ship and helicopter-borneexpeditions along the coasts of the White and Barentsseas, the Chyoshskaya Bay and on the Timan Ridge(2000, 2001, 2002) (Fig. 1). The expeditions thereforegrew in size and complexity over the years, as did thegeology encountered in the field areas as we movedfrom areas influenced only by the Scandinavian IceSheet to areas influenced also by the Barents Sea andthe Kara Sea ice sheets. The present volume sum-marizes the work carried out during these eightsummers. Our work was an integral part of theQUEEN Project (Quaternary Environment of theEurasian North), a European Science Foundationproject comprising fieldwork in northern Russia andSiberia and the adjacent seas. Much has already beenpublished in the series of collective volumes from thisproject (Larsen et al. 1999; Thiede et al. 2001, 2004).For the waxing and waning of ice sheets in Eurasiaduring the last ice age, the Arkhangelsk region holds astrategic position as a triple junction, because it wasoverridden by all three major Eurasian ice sheets: theScandinavian, the Barents Sea and the Kara Sea icesheets. While the first was mainly land-based andflowing out from the Fennoscandian mountains, theother two formed on the coastal shelf and moved ontoland. They therefore represent very different types ofice sheet, and this is reflected in their dynamic responseto external forcing. The unravelling of these glacialevents, and its relation to sea level and climate, hasbeen the main focus of our work.The Quaternary history of this region was firstoutlined by Wollosovich (1900) and Ramsay (1904),then by Lavrova (1937). These works contain manyprimary observations that are still valid. Later, com-prehensive surveys by Devyatova & Loseva (1964) andby Devyatova (1982) functioned scientifically as a fieldguide on Severnaya Dvina and in the Mezen Basin.Also regional Quaternary maps compiled by Lavrov(1977, 1991) provided highly useful overviews andnarrowed our scope to critical field areas.Since 1995, members of our project have documen-ted and logged more than 110 localities along the riversand coasts and surveyed many more (Fig. 1).Ice sheet dynamics from field studies and modelsLarsen et al. (2006a) offer a synthesis of the complexglacial and glaciolacustrine history of northwesternRussia, shown for the period 135 to 15 kyr BP in aseries of palaeo-geographical maps. Regional overviewsare presented in Demidov et al. (2006) for theScandinavian Ice Sheet and in Kjaer et al. (2006) for aspecific area around the Kanin Peninsula and Chyoshs-kaya Bay. The reconstruction of a mega-scale thrust-fault complex illustrates the ice/bed interaction andallows a palaeoenvironmental model to be establishedfor the earliest part of the Weichselian (Larsen et al.2006b). Lambeck et al. (2006) use the field observa-tions from this and other regions, e.g. Taimyr, to modelrelative sea level and changing ice cover over Eurasia inthe period c. 150 to 60 kyr BP.Sea-level changeGrosfjeld et al. (2006) give a detailed history of LateSaalian and Eemian relative sea level along the PyozaRiver, and Jensen et al. (2006) describe a MiddleWeichselian transgression which took place duringmajor regional deglaciation of the whole area at thetransition from MIS 4 to 3. The relative sea-level andisostatic history for the Late Saalian to the earlyHolocene is synthesized in Larsen et al. (2006a) andmodelled by Lambeck et al. (2006). Kjaer et al. (2006)explore the wider implications of rapid sea levelchanges and their impact on shelf-ice stability.The Eemian interglacialMarine sediments from the Eemian are widespread andform an important and readily recognizable stratigra-phical marker. A multi-palaeontological study gives adetailed account of hydrographical change and inflowof Atlantic water (Grosfjeld et al. 2006).

Depositional origin of an adverse delta surface slope at Altermarka, northern Norway

Based on grain-size analysis of sediment samples from a delta at Altermarka, it has previously been suggested that two tills were deposited on the delta surface as a glacier planed off the delta topsets and created its adverse surface slope. In this reinvestigation of the delta, there was no evidence that a glacier overrode the surface: the topset beds remain intact and no appreciable till was found. Sediments resembling the ‘till’ samples were located, but soil processes and other sedimentary processes easily account for the wide grain-size distributions of these sediments. As a result, this study cautions against using grain-size analysis, on its own, to interpret the genesis of sediments. Other processes potentially responsible for the adverse surface slope include differential isostatic rebound, recent differential slip along ancient thrust faults, and failure of the delta surface to reach sea level. Based on the sedimentology of the topset beds and the local relative sea level history (indicating the delta (9495±70 14C yr BP) formed about the time of the local marine limit (9560–9375 14C yr BP) 17 m higher than the delta surface), the feature is determined to have built up below sea level with an adverse surface slope.

A Reanalysis and Reinterpretation of Geodetic and Geological Evidence of Glacial-Isostatic Adjustment in the Churchill Region, Hudson Bay

We review the historical, geological, tide-gauge, GPS and gravimetric evidence advanced in favour of, or against, continuing land uplift around Hudson Bay, Canada. We also reanalyse the tide-gauge and GPS data for Churchill using longer time series than those available to previous investigators. The dependence of the mean rate of relative sea-level change obtained from the tide-gauge record on the length and mid-epoch of the observation interval considered is investigated by means of a newly developed linear-trend analysis diagram. For studying the shorter-period variability of the tide-gauge record, the wavelet transform is used. The mean rate of land uplift obtained from GPS is based on a new analysis using IGS solutions of GFZ. To include the post-glacial land uplift, sea-level indicators from the Churchill region representing the relative sea-level history during the past 8000 years are also used. Finally, the values of the four observables are jointly inverted in terms of mantle viscosity. The optimum values are ~3.2 × 1020 Pa s and ~1.6 × 1022 Pa s for the upper- and lower-mantle viscosities, respectively.

Holocene sea levels and palaeoenvironments, Malay-Thai Peninsula, southeast Asia

Sedimentological and palynological investigations of Great Songkhla Lakes, east coast of the Malay-Thai Peninsula, Southeast Asia, reveal sedimentary sequences rich in palynomorph assemblages dominated by pollen of mangroves and freshwater swamps. Compared with other regions in Southeast Asia the assemblages are of relatively low diversity. Geochronological data indicate that the Great Songkhla Lakes record one of the earliest mangrove environments in Southeast Asia (8420-8190 cal. yr BP), which are subsequently replaced by a freshwater swamp at 7880-7680 cal. yr BP owing to the decline of marine influence. Sea-level observations from Great Songkhla Lakes and other areas of the Malay-Thai Peninsula reveal an upward trend of Holocene relative sea level from a minimum of -22 m at 9700-9250 cal. yr BP to a mid-Holocene high stand of 4850-4450 cal. yr BP, which equates to a rise of c. 5.5 mm/yr. The sea-level fall from the high stand is steady at c. - 1.1 mm/yr. Geophysical modelling shows that hydroisostasy contributes a significant spatial variation to the sea-level signal between some site locations (3-4 m during the mid-Holocene), indicating that it is not correct to construct a single relative sea-level history for the Malay-Thai Peninsula.

Late Quaternary sea-level highstands in the central and eastern Indian Ocean: A review

The relative sea-level history of several atolls in the central and eastern Indian Ocean, including the Cocos (Keeling) Islands, Chagos Archipelago, and the Maldives–Laccadive Archipelagoes, has been debated for over a century but takes on a particular significance in the face of anticipated climate change. For each of these central and eastern Indian Ocean atolls Pleistocene limestone is encountered at depths of 6–20 m below sea level. On the Cocos (Keeling) Islands this has been dated to Last Interglacial age. Conglomerate platform underlies the reef islands on Cocos within which a sequence of fossil microatolls of massive and branching Porites records a gradual fall of sea level relative to the atoll. In the Maldives, the significance of outcrops of ‘reef rock’ has been vigorously debated without resolving sea-level history. Although in situ Heliopora occurs on the reef flat of Addu Atoll, dated at around 2700 radiocarbon yrs BP, other evidence for higher sea level remains poorly constrained. Conglomerates of a similar age have been described from the Chagos Archipelago, but it has not been unequivocally demonstrated that they formed under conditions of relatively higher sea level. In contrast to reefs further west in the Indian Ocean, each of these atolls has living microatolls of massive Porites that have been constrained in their upward growth by sea level. Interpretation of the upper surface of two such specimens from the Cocos (Keeling) Islands indicates broad fluctuations in the sea surface over the past century; similar microatolls are described from the Maldives implying little change in sea level over recent years. Regardless of minor past fluctuations, most reef islands in the Maldives are particularly low-lying and appear vulnerable to inundation, and extracting a more detailed sea-level history remains an important challenge.

Large‐scale fluvial grade as a nonequilibrium state in linked depositional systems: Theory and experiment

A graded river conveys its sediment load without net aggradation or degradation. Grade is thought to represent the equilibrium state of a river system subject to steady allogenic forcing. We address the concept of grade in linked depositional systems through a combination of mathematical modeling and flume experiments. We develop a moving boundary morphodynamic model of fluviodeltaic sedimentation and use it to quantify the conditions necessary for a state of sustained grade in an alluvial‐deltaic system developing on a uniformly sloping shelf. To test our theory, we conducted a series of flume experiments in which we constructed laboratory‐scale fluviodeltaic systems under specific relative sea level histories. On large spatiotemporal scales, our theoretical and experimental results suggest an alternative view of grade as an intrinsically nonequilibrium state that requires a fall in relative sea level. Under conditions of stillstand or rise in relative sea level, the equilibrium (graded) profile does not exist, and the alluvial river will always aggrade. With a steady sediment supply, alluvial grade can be sustained if relative sea level falls with a specific, square‐root‐of‐time dependence. Common sequence‐stratigraphic conceptualizations of alluvial river response to sea level change suggest that rivers aggrade and degrade in response to relative sea level rise and fall, respectively. We show that rivers can remain aggradational if the rate of sea level fall is sufficiently small. The boundary between aggradational and degradational states is the graded state described here: that associated with a relative sea level fall that varies with the square root of time.

A review of possible eustatic, isostatic and tectonic contributions in eight late-Holocene relative sea-level histories from the Mediterranean area

This paper consists of a comparison between eight late-Holocene sea-level histories investigated by the author in different Mediterranean areas and sea-level predictions in the same areas made by global isostatic models. The areas include three regions that are generally considered as tectonically relatively stable: (1) the southern coast of France, (2) Sardinia, and (3) the southern coasts of Tunisia, where late-Holocene evidence may be either of submergence, or of emergence. Also considered are: (4) a subsidence region in the northern Adriatic, and actively uplifting areas: (5) in Calabria, and (6) in Nisyros Island (Greece). Lastly, case studies in areas affected by repeated coseismic vertical displacements include: (7) western Crete and Antikythira Island in the Hellenic Arc, and (8) the Levant coasts of Turkey, Syria and the Lebanon. In each case the question of whether the evidence available may or may not be consistent with gradual isostatic or eustatic displacements has been explored. It appears that in non-tectonic areas the hydro-isostatic effects have been overestimated (Sardinia) or underestimated (southern Tunisia) by the same models. In the northern Adriatic, the closest Mediterranean area to the former Scandinavian ice sheet, glacio-eustatic effects are suspected to be almost absent. Our analysis failed to quantify isostatic effects in areas of rapid tectonic uplift (Calabria and Nisyros), but rejected predictions of significant isostatic subsidence in the Hellenic Arc and on the Levant coast. Finally, several data from tectonic and non-tectonic areas are consistent with nearly stable global eustasy since 6000 BP, thus challenging the assertion of significant additional melting of Antarctica after the complete melting of the former Northern Hemisphere ice caps.

On post-glacial sea level - II. Numerical formulation and comparative results on spherically symmetric models

SUMMARY Theoretical approaches to computing gravitationally self-consistent sea-level changes in consequence of ice growth and ablation are comprised of two parts. The first is a mapping between variations in global sea level and changes in ocean height (required to define the surface load), and the second is a method for computing global sea-level change arising from an arbitrary surface loading. In Mitrovica & Milne (2003) (Paper I) we described a new, generalized mapping between sea-level change and ocean height that takes exact account of the evolution of shorelines associated with both transgression and regression cycles and time-dependent marine-based ice margins. The theory is valid for any earth model. In this paper we extend our previous work in three ways. First, we derive an efficient, iterative numerical algorithm for solving the generalized sea-level equation. Secondly, we consider a special case of the new sea-level theory involving spherically symmetric earth models. Specifically, we combine our iterative numerical formulation with viscoelastic Love number theory to derive an extended pseudo-spectral algorithm for solving the new sea-level equation. This algorithm represents an extension of earlier methods developed for the fixed-shoreline case to precisely incorporate shoreline migration processes. Finally, using this special case, we quantitatively assess errors incurred in previous efforts to extend the traditional (fixed shoreline) sea-level equation of Farrell & Clark (1976) to treat time-dependent shorelines. We find that the approximations adopted by Johnston (1993) and Milne (1998) to treat transgression and regression at shorelines introduce negligible (∼1 per cent) error into predictions of post-glacial relative sea-level histories. In contrast, the errors associated with the Peltier (1994) sea-level equation are an order of magnitude larger, and comparable to the error incurred using the traditional sea-level theory. Furthermore, our numerical tests verify the high accuracy of the Milne (1998) approximation for treating the influence of grounded, marine-based ice.

On the hemispheric origins of meltwater pulse 1a

During the glacial–interglacial transition that began subsequent to the Last Glacial Maximum approximately 21,000 calendar years ago, globally averaged (eustatic) sea-level rose by approximately 120 m as climate warmed to its current (Holocene) state. This rise of relative sea-level (RSL) did not occur smoothly, however, but was characterized by the occurrence of one or more episodes of extremely rapid increase. The most extreme of these events has come to be referred to as meltwater pulse 1a, and was initially identified in the coral based record of RSL history from the island of Barbados in the Caribbean Sea. Although it has usually been assumed that this episode of rapid RSL rise was derivative of a partial collapse of the northern hemisphere ice sheets, it has recently been suggested that this pulse could have originated in a dramatic melt-back of the Antarctic Ice Sheet. In this paper the arguments presented in favour of the southern hemisphere source are revisited in order to assess the plausibility of this alternative scenario. Based upon the analyses presented, it is concluded that the evidence previously provided in support of the southern hemisphere scenario is in fact unable to rule out an entirely northern hemisphere source for the meltwater pulse 1a. Since explicit evidence does exist that both the Laurentide and Fennoscandian ice sheets contributed to this event and that Antarctic ice sheet melting occurred significantly later, the southern hemisphere appears not to have been a prime mover of northern hemisphere events.