Rate Of Relative Sea-level Rise Research Articles

The Holocene evolution of the barrier and the back-barrier basins of Belgium and the Netherlands as a function of late Weichselian morphology, relative sea-level rise and sediment supply

AbstractFlooding of the southern part of the North Sea occurred between 9000 and 8000 BP, when the rate of relative sea-level rise was on the order of 0.7 cm per year for the Dover Strait Region and 1.6 cm per year for the area north of the Frisian Islands, forcing the shoreline to recede rapidly. When relative sea-level rise decelerated after 7000 BP for the Belgian coast and 6000 BP for the central Netherlands coast, sediment supply by the tidal currents balanced the creation of accommodation space in the estuaries and other back-barrier basins. Consequently, the barrier started to stabilize, and the tidal basins and their inlets silted up. Between 5500 and 4500 BP, the Belgian coastal plain changed into a freshwater marsh with peat accumulation, and the same happened 500–1000 years later in the western provinces of the Netherlands. The E-W running barrier/back-barrier system of the Frisian Islands in the northern Netherlands stayed open until today, however, because of lower sediment supply.The period between 4000 and 2000 BP was relatively quiet due to the strong deceleration of the rate of sea-level rise; peat cushions developed behind the barriers, which were straightened by erosion of the headlands. Major and often catastrophic flooding occurred in the Middle Ages, when the estuaries in the southwestern part of the Netherlands formed.About 226 (± 15%) × 109 m3 sediment, mostly sand, is stored in the barriers and back-barrier basins of the Netherlands, 70% of which was deposited prior to 5000 BP. About 10% of the stored sediment is estimated to be of alluvial origin. Most of the sediment is derived by the erosion of the Pleistocene basement during recession of the barriers, but tide-induced crossshore transport from the North Sea forms an additional source for the barriers and back-barriers of the west-facing coast of the Netherlands.

Suspended Solids Flux Between Salt Marsh and Adjacent Bay: A Long-term Continuous Measurement

The beneficial roles of coastal salt marshes have been well identified as a storm surge protector, a nursery for young fish and a natural filter for pollutants. However, the vectors of nutrients and suspended solids exchanges between the salt marsh and the adjacent bay are not completely known. In this study, suspended solids flux between a salt marsh at Tuckerton, New Jersey, U.S.A. and Great Bay was continuously measured for an extended period of time by an improved monitoring methodology. A field infrared nephelometric turbidimeter was deployed to measure turbidity continuously at the mouth of the tidal creek, and the measured turbidity data were used to derive suspended solids concentrations. Current velocity and water surface elevation were measured concurrently at the same location. During the monitoring period from March to October 1996, suspended solids were found to be imported into the salt marsh from the adjacent bay. The small net import appeared to be inadequate for wetlands areal viability when compared to the relative sea-level rise rates. Results of this study suggested that a minimum of five water sample sets were needed to establish a reasonably good overall TSS-turbidity regression relationship in terms of flux quantification.

Fossil shore platforms and drowned gravel beaches; evidence for high-frequency sea-level fluctuations in the distal Alpine foreland basin

ABSTRACT The Eocene Nummulitic Limestone Formation records progressive backstepping of a shallow marine carbonate ramp at the distal margin of the Early Tertiary Alpine foreland basin during flexurally induced transgression of the European foreland plate. Analysis of paleoshoreline features preserved along the basal unconformity of the basin in southeastern France enables identification of rapid fluctuations in the rate of relative sea-level rise. In the Champsaur district of Les Hautes Alpes, the Priabonian Nummulitic Limestone Formation unconformably overlies pre-Tertiary bedrock across a stepped transgressive ravinement surface. This surface consists of a succession of subplanar bedrock terraces that are separated by high-angle risers. The terraces display an overall staircase geometry, stepping upwards in the direction of shoreline transgression. Progradational gravelly beachface deposits overlie the terraces, and are themselves abruptly overlain either by transgressive shoreface facies or by younger beachface units. The prograded beaches are arranged in an overall landward-stepping stacking pattern. The terraces are interpreted as a flight of fossil wavecut shore platforms that record episodic landward migration of the paleoshoreline onto a paleotopographic high. Mapping of the relict paleoshorelines indicates successive episodes of terrace cutting, beach progradation, drowning, and shoreline backstepping on a late Eocene rocky coastline. The formation of the platforms and preservation of drowned gravel beaches is interpreted as a response to abrupt variations in the rate of relative sea-level rise during transgression of the region by the distal shoreline of the Alpine foreland basin. Periods of slow rise or stillstand enabled platform cutting and subsequent beach progradation, whereas pulses of rapid sea-level rise caused abrupt drowning and landward shift of the shoreline. We speculate that these high-frequency episodic fluctuations in relative sea level are a consequence of glacioeustatic sea-level oscillations of < 0.5 m.y. duration superimposed upon a steady relative sea-level rise resulting from the background flexural subsidence.

Temporal significance of sequence boundaries

This paper analyses the temporal significance of stratigraphic surfaces bounding the marine portions of the depositional sequence, genetic stratigraphic sequence and transgressive–regressive sequence. These bounding surfaces, known as the `correlative conformity' (c.c.), `maximum flooding surface' (MFS) and `conformable transgressive surface' (CTS), respectively, may either be defined on the basis of stratal stacking patterns (which we call `type A surfaces'), or on the basis of water-depth changes and relative sea-level changes (which we call `type B surfaces'). The type A MFS and CTS are time lines in a depositional-dip section, corresponding to the turnaround points from shoreline transgression to regression and vice versa. They separate prograding (coarsening-upward) from retrograding (fining-upward) geometries, with a timing determined by the interplay between the rates of sedimentation and relative sea-level rise in the shoreline area. The timing of type A MFS and CTS is not affected by the offshore variations in sedimentation and subsidence rates, but it is only controlled by the shoreline movements and the associated facies shifts. The type A c.c. separates rapidly prograding and offlapping forced regressive strata from the overlying lower rate prograding and aggrading normal regressive strata. This surface is diachronous, younger basinward, with the rate of offshore sediment transport. The timing of the type A c.c. in the shoreline area corresponds to the end of relative sea-level fall, but it develops under relative sea-level rise conditions offshore. The timing of the type B MFS and CTS depends on the offshore variations in the sedimentation and subsidence rates. These surfaces, defined on the basis of bathymetric changes, become younger and older seaward, respectively, tending to merge together offshore. The type B c.c. marks the end of relative sea-level fall in any point along a depositional-dip section. It is diachronous, older basinward, as its timing depends on the offshore variations in subsidence rates. The diachroneity of type B surfaces reaches a quarter of the period of the highest frequency variable, whichever that is among the eustasy, tectonics or sedimentation controls. Types A and B surfaces merge together in the shoreline area, but they become temporally divergent offshore. Deepening-upward and shallowing-upward facies should not be confused with transgressive and regressive systems tracts. The latter are strictly controlled by the shoreline movements, which determine the direction of facies shifts and the stratal stacking patterns.

Variations in sedimentary environments and accretionary patterns in estuarine marshes undergoing rapid submergence, Chesapeake Bay

Major processes controlling sedimentological characteristics and accretionary rates and patterns were evaluated in two of the most common tidal marsh settings (estuarine tributaries and estuarine embayments) in Chesapeake Bay, one of the largest microtidal estuaries in the world. Textural analyses of 29 vibracores show spatial and temporal trends in grain size and organic matter which reflect differences in depositional settings within the marshes and changes in the controlling processes, as well as broader events in the surrounding watersheds. Spatially, the surficial sediments normally follow expected patterns, with coarsest sediments and lowest organic matter contents found in bay margin or channel margin locations, while the finest, organic-rich sediments occur in interior and submerged upland marshes. However, temporally, the marshes do not always show the expected increasing dominance of organic input over time (indicated by increasing % loss on ignition with decreasing depth), indicative of the marsh building process. Sea-level rise, tidal channel migrations, and anthropogenic effects likely contribute to decreases in organic matter contents in the upper sediment column. Furthermore, accretion rates and stratigraphic characteristics can be affected by these same processes, illustrating the importance of understanding temporal changes in marsh depositional environments over various time scales before sedimentologic and accretionary patterns are evaluated. Examination of the vibracores reveals that the marsh is composed of a complex stratigraphy with at least four major stratigraphic sequences occurring: (1) emerging or developing marsh sequences characteristic of channel margin and interior marshes; (2) submerging or mineral matter enriched marshes, also characteristic of channel and interior marsh areas; (3) high-energy marsh sequences characteristic of bay margin environments; and (4) submerged upland marsh sequences. The sedimentological properties of the stratigraphic sequences are controlled by overwash processes, tidal channel migrations, duration of tidal flooding, various anthropogenic effects, and sea-level rise. Accretion rates and patterns over the last ∼200 years, determined from pollen histories, differ between the marshes along the estuarine tributary (Nanticoke River) and those found in the tidal embayment (Monie Bay). Marshes in the upper-estuarine tributary have a larger riverine sediment input, and trapping of the high sediment loads in the upper estuary often yields marsh accretion rates (maximum of 0.74 cm/yr) which are greater than relative sea-level rise (∼0.4 cm/yr), resulting in stable marshes. By comparison, estuarine processes limit penetration of high sediment loads to the lower reaches of marshes along the tributary and, as a consequence, accretion rates are generally lower than the rate of relative sea-level rise, and marsh loss is prevalent. Overall, accretion rates in the marshes in the embayment are close to or less than the local sea-level rise and do not have as distinctive spatial patterns as the tributary marshes. Although marsh loss has yet to be identified as a significant process here, decreases in organic content of the upper sediment column may signal the early stages of submergence due to sea-level rise.

Sea-level and subsidence data from a Late Holocene back-barrier lagoon (Valle Standiana, Ravenna, Italy)

Late Holocene back-barrier sediments have been studied in a former lagoon reclaimed during the last century. Several shallow boreholes were cored, from which nine organic-rich samples were selected for radiocarbon dating, and others for palynological and palaeontological analysis. This enabled the reconstruction of the evolution of a back-barrier coastal basin during the late Holocene coastline progradation. Since late Roman times, the rate of relative sea-level rise (RSLR) has increased from 1.1 mm/yr to over 2 mm/yr, even after removal of the main human impacts during of the last century. This change is ascribed to sediment compaction related to changes in the hydrological regime, and possibly to an isostatic subsidence wave accompanying the northward shift of the Po Delta during the last 2000 years.

Climate change and the rise and fall of sea level over the millennium

Average rates of relative sea level rise (RSLR) along the coast of the eastern United States were 1–2 mm/yr over the last 2,000 years. Higher rates have occurred in recent centuries, and their onset seems to have preceded the period of modern global warming. A modest rate increase started in the 17th century, well before the modern global warming of this century, but the highest rates have occurred during the last 200 years, with a major acceleration around 1800 A.D. No significant acceleration is apparently associated with the rapidly rising temperatures of the last 100 years.How does one establish possible relationships between rates of sea level rise and climate fluctuations? Tide‐gauge records are too short and noisy to derive a quantitative, empirical relationship between relative sea level rise rates and climate change [Douglas, 1992], and predictions of future rates of sea level rise are largely based on theory. The recent geological record can provide empirical insights, which can be applied toward the assessment of current rates and those predicted for the next century.

The role of relative sea-level rise and changing sediment supply on Holocene gravel barrier development: the example of Porlock, Somerset, UK

The Holocene development of the coastal system at Porlock has been driven by variations in the rate of relative sea-level rise providing a long-term control on longshore sediment supply, which in turn has controlled gravel barrier beach dynamics. Relative sea-level rise rates of between approximately 8.5 mm yr-1 and 2.0 mm yr-1 define a critical range which allowed gravel barrier consolidation and back-barrier alder carr formation between c. 8500 to 6000 cal. yrs BP. A marked deceleration in rise rates around 7500 to 7000 cal. yrs BP caused a reduction in coarse clastic sediment supply, forcing the barrier into the breakdown domain, enhancing its vulnerability to storm events. Back-barrier sedimentation since c. 6000 cal. yrs BP has been exclusively inorganic, and probably included an important catchment-derived component resulting from anthropogenic disturbances.

GREENSAND RESERVOIRS IN SILICICLASTIC SHORELINE SYSTEMS: FACIES MODELS FOR HYDROCARBON EXPLORATION

Greensand reservoirs are particularly significant in siliciclastic shoreline systems. Formation of autochthonous glauconite is restricted almost exclusively to the rising limb of the relative sea level (RSL) curve. Depositional environments that are typically developed in response to a rise in RSL are estuaries, barrier bars and shoreface sands. In this setting, the greensands are diachronous and exploration for hydrocarbon reservoirs should ideally be based on a sequence stratigraphic framework. These deposits are characterised by a number of significant surfaces that differ greatly in their origin, geographic extent and chronostratigraphic significance, and their thickness depends on the rate of RSL rise and sediment supply. The identification of significant surfaces bounding or contained within depositional sequences is critical, especially where drillhole samples are either unreliable or not available for geochemical and palynological analysis. Abrupt physical and chemical changes often occur on or across these surfaces, e.g. porosity, permeability and the presence of Fe-rich authigenic minerals such as glauconite and siderite. Greensands deposited in estuarine, barrier bar and shoreforce environments have excellent oil and gas potentail. Early Cretaceous greensand reservoirs from the North West Shelf are best developed where they overlie ravinement surfaces. Recognition of greensands within estuarine, barrier bar and shoreface environments is dependent on the correct identification of facies and the subsequent interpretation of the bounding discontinuities.

Accretion of a New England (U.S.A.) Salt Marsh in Response to Inlet Migration, Storms, and Sea-level Rise

Sediment accumulation rates were determined at several sites throughout Nauset Marsh (Massachusetts, U.S.A.), a back-barrier lagoonal system, using feldspar marker horizons to evaluate short-term rates (1 to 2 year scales) and radiometric techniques to estimate rates over longer time scales (137Cs,210Pb,14C). The barrier spit fronting theSpartina-dominated study site has a complex geomorphic history of inlet migration and overwash events. This study evaluates sediment accumulation rates in relation to inlet migration, storm events and sea-level rise. The marker horizon technique displayed strong temporal and spatial variability in response to storm events and proximity to the inlet. Sediment accumulation rates of up to 24mm year−1were recorded in the immediate vicinity of the inlet during a period that included several major coastal storms, while feldspar sites remote from the inlet had substantially lower rates (trace accumulation to 2·2mm year−1). During storm-free periods, accumulation rates did not exceed 6·7mm year−1, but remained quite variable among sites. Based on137Cs (3·8 to 4·5mm year−1) and210Pb (2·6 to 4·2mm year−1) radiometric techniques, integrating sediment accumulation over decadal time scales, the marsh appeared to be keeping pace with the relative rate of sea-level rise from 1921 to 1993 of 2·4mm year−1. At one site, the210Pb-based sedimentation rate and rate of relative sea-level rise were nearly similar and peat rhizome analysis revealed thatDistichlis spicatarecently replaced this onceS. patenssite, suggesting that this portion of Nauset Marsh may be getting wetter, thus representing an initial response to wetland submergence. Horizon markers are useful in evaluating the role of short-term events, such as storms or inlet migration, influencing marsh sedimentation processes. However, sampling methods that integrate marsh sedimentation over decadal time scales are preferable when evaluating a systems response to sea-level rise.

Morphologic development, relative sea level rise and sustainable management of water and sediment in the Ebre Delta, Spain

The Ebre (Ebro) Delta is one of the most important wetland areas in the western Mediterranean. Ca. 40 % of the delta plain is less than 0.5 m above mean sea level and part of the southern margin of the delta is at mean sea level in an area protected by dikes. Both mean rates of secular subsidence in the Ebre Delta and eustatic sea level rise are ca. 1 – 2 mm/yr. Thus, the present annual relative sea level rise (RSLR) rate in the Ebre Delta may be at least 3 mm/yr. Measured accretion rates in the delta range from 4 mm/yr in the wetlands surrounding the river mouth to 99 % of the riverine sediments are retained in the reservoirs and rice fields are losing ca. 0.2 mm/yr.

Accelerated relative sea-level rise and rapid coastal erosion:: testing a causal relationship for the Louisiana barrier islands

The role of relative sea-level rise as a cause for the rapid erosion of Louisiana's barrier island coast is investigated through a numerical implementation of a modified Bruun rule that accounts for the low percentage of sand-sized sediment in the eroding Louisiana shoreface. Shore-normal profiles from 150 km of coastline west of the Mississippi delta are derived from bathymetric surveys conducted during the 1880s, 1930s and 1980s. An RMS difference criterion is employed to test whether an equilibrium profile form is maintained between survey years. Only about half the studied profiles meet the equilibrium criterion; this represents a significant limitation on the potential applicability of the Bruun rule. The profiles meeting the equilibrium criterion, along with measured rates of relative sea-level rise, are used to hindcast shoreline retreat rates at 37 locations within the study area. Modeled and observed shoreline retreat rates show no significant correlation. Thus, in terms of the Bruun approach, relative sea-level rise has no power for hindcasting (and presumably forecasting) rates of coastal erosion for the Louisiana barrier islands.

Growth Dynamics of Neoproterozoic Calcimicrobial Reefs, Mackenzie Mountains, Northwest Canada

ABSTRACT Neoproterozoic buildups of the Little Dal Group grew in a deep-water epicratonic basin. These kilometer-scale reefs display aggradational and progradational geometries comparable to those described from Phanerozoic reefs. Four phases of reef growth correspond to four regional shale-to-carbonate packages in laterally equivalent off-reef strata. The lower, shaly part of each package is interpreted to reflect transgressive to highstand deposition. The upper carbonate part of each package reflects carbonate precipitation in the water column as a result of postulated basin restriction and increased salinity during sea-level lowstand. Reefs nucleated at the beginning of the first major transgressive event. Reefs typically aggraded during transgressive intervals, but could also prograde or contract, likely depending on the rate of relative sea-level rise versus reef growth rate. Reefs prograded during regressions, probably owing to reduction of accommodation space. They shed talus at lowstand in response to increased erosion and/or progradation over unstable substrates. Reef growth ended with return of shallow-water conditions. The overall architecture of aggradation and progradation demonstrates that the Neoproterozoic calcimicrobial reef ecosystem was capable of responding to environmental changes in the same way as the more extensively studied, ecologically complex, and faunally diverse buildups of the Phanerozoi .

Distal Orogenic Effects on Peripheral Bulge Sedimentation: Middle and Upper Ordovician of the Nashville Dome

ABSTRACT A major switch in depositional style in the Ordovician carbonates of the Nashville Dome corresponds closely with the onset of the late Middle Ordovician Taconic orogeny. This time marks a shift from tropical-type to temperate-type carbonates, the initiation of widespread major phosphate deposition, the introduction of large amounts of terrigenous silt and clay, the occurrence of widespread seismically induced soft-sediment deformation, and a change from a low-energy flat-topped carbonate shelf to a high-energy doubly dipping carbonate ramp. Soft-sediment deformation and the introduction of siliciclastics are direct effects of the Taconic orogeny; the switch from tropical-type to temperate-type carbonates, the initiation of phosphate deposition, and the switch in carbonate ramp are lar ely oceanographic effects triggered by the orogeny. In particular, phosphate deposition and the switch to temperate-type limestones appears to have been driven by upwelling along the eastern side of the Nashville Dome within the newly deepened Taconic foreland basin. A fourfold decrease in the rate of relative sea-level rise occurred on the Nashville Dome nearly 3 m.y. following the onset of thrusting and foreland basin initiation. Subsidence rates were constant before and after this decrease, and no evidence of a change in subsidence rates is seen to coincide with the onset of thrusting. The slowing of subsidence may reflect viscoelastic uplift of the Nashville Dome, but the abrupt change from one constant subsidence rate to another is not predicted by existing foreland basin models.

Fluvial Response in a Sequence Stratigraphic Framework: Example from the Montserrat Fan Delta, Spain

ABSTRACT Exceptional exposure of the Montserrat fan-delta system (Eocene) in northeastern Spain provides an excellent framework to evaluate the alluvial response to sea-level changes over two different time scales. The alluvial system contains multiple fifth-order cycles ( 104 yr) and eight fourth-order cycles ( 105 yr). Fifth-order cycles are characterized by long-distance shoreline migrations and, occasionally, by incised basal scour surfaces but not by changes in fluvial style, lithofacies, or channel stacking pattern. Fourth-order cycles are composed of stacked fifth-order cycles and have non-erosional basal boundaries. Vertical sedimentation rates and channel-stacking patterns change significantly within fourth-order cycles. The lower parts of these cycles, referred to as the transgressive facies tract, show overall shoreline transgression, and associated alluvial deposits contain abundant overbank materials with isolated (ribbon) channel bodies. During this time the supply of terrigenous material to the shoreline was reduced, as indicated by sediment starvation offshore. The overlying middle part of fourth-order cycles, the lower regressive facies tract, differs only in that shoreline is overall regressive, and there is increased clastic supply to the offshore. In the upper part of hese cycles, the upper regressive facies tract, channel-stacking geometries become denser and more sheet-like, shoreline regression is more pronounced, and vertical aggradation rates are inferred to be reduced. Changes in the alluvial system during fourth-order cycles are most pronounced adjacent to shoreline and die away upstream over just a few kilometers--indicating that the base-level signal decays away over the distance of a few backwater lengths (channel flow depth/slope). Higher-frequency (fifth-order) changes in relative sea level appear to produce the largest shoreline migrations, but lower-frequency (fourth-order) changes have more impact on the channel stacking architecture of the alluvial systems. Observed changes in alluvial stacking pattern may be most commonly found in tectonically active, rapidly subsiding, foreland basins because of their back-tilted geometry. We propose a model in which sediment is trapped in the proximal basin during times of rapid tectonic subsidence and attendant relative sea-level rise. Progradation occurs as erosion rates in the mountain belt increase, and rates of subsidence and relative sea-level rise diminish. Changes in alluvial architecture reflect an increase in sediment flux towards the shoreline as less sediment is trapped upstream. Hence, changes in channel-stacking pattern coincident with transgressions and regressions likely reflect the interplay between subsidence and sediment supply in the proximal part of the basin and are not necessari y driven by eustatic sea-level changes.

A sea-level rise curve from Guilford, Connecticut, USA

High-resolution stratigraphic studies based on sediment chemistry, lithology, macroflora, and benthic foraminiferal assemblages in three peat cores from coastal salt marshes at Guilford, Connecticut, show that coastal marshes are ephemeral environments. Marsh-wide environmental variations were common, and century-long episodes of relative submergence alternated with emergence. Despite about 2 m of relative sea-level rise (RSLR) in Connecticut over the last 1500 years, the marshes have expanded both landwards and seawards, and marsh accretion has been outpaced only marginally by RSLR. We used radiocarbon dating and the level of anthropogenic markers (metal pollution, as dated with 210Pb) in the cores for age control. For most of the last 1000 years the rate of RSLR was between 1.3 and 1.8 mm/yr, but over the last 300–400 years it increased to 2.9–3.3 mm/yr, and has been faster than the accretion rate, especially in the middle marsh. The net-submergence rate or “submergence index” (ratio of the rate of RSLR and marsh-accretion rate) averaged about 1.15 over the last 1000 years, and increased to about 1.5 over the last 200 years. The rate of RSLR was very sluggish during the early part of the Little Ice Age, but we found a slightly higher rate during the Little Climate Optimum; this excursion is close to the noise level, however. The most significant observation is that RSLR increased strongly around A.D. 1650. The onset of this acceleration falls in the middle to end of the Little Ice Age, and thus preceded the period of modern global warming that started late last century and that has been tentatively correlated with anthropogenic emissions of greenhouse gases.

The evolution of Buctouche Spit, New Brunswick, Canada

Buctouche Spit is an 11-km long sandy spit on the northeast shore of New Brunswick, Canada. It is a typical flying spit, having a narrow proximal section which is characterized by a single foredune that is prone to overwash and transgression, and a much wider prograding distal section which is characterized by a well-developed foredune backed by a series of relict foredunes. An optical-luminescence dating method in which infrared stimulation is used on potassium feldspars was developed for this study. It is a very promising method for dating Holocene deposits, yielding good resolution and having simplicity of measurement. The luminescence ages obtained indicate that Buctouche Spit extended at a rate of ~4 m yr −1 between 715 ± 45 years ago and 240 ± 25 years ago. This extension rate equates to a distal end accumulation rate of ~ 56,000 m 3 yr −1 for this period. At this rate of accretion, the present Buctouche Spit would have formed during the past 2000 years. The core data indicate that the spit is ⩾2200 years old, which is consistent with the preceding estimate. Since 240 ± 25 years ago, the rate of sediment accumulation at Buctouche Spit has apparently been falling. The accumulation rate between 1839 and 1945 was ~23,000 m 3 yr −1, less than half of the historic rate of ~56,000 m 3 yr −1. Since 1945, the distal end of Buctouche Spit has not prograded/extended measurably. This is because Buctouche Spit's present vertical growth rate is, at most, equal to the rate of relative sea-level rise at the spit and may be as little as half of the rate of relative sea-level rise. Present sediment supply to Buctouche Spit is probably < 16,000 m 3 yr −1. It is likely that the much larger sediment supply required to sustain an historic accretion rate of ~ 56,000 m 3 yr −1 was derived from the reworking of a former sandy barrier located seaward, and possibly updrift of, the present spit. Buctouche Spit is now in a limited sediment supply situation and it has become a constrained spit. Sediment is being eroded from the centre section of the spit, transported to the distal end, and then carried away by ebb-tidal currents. The centre section of the spit will likely breach within the next hundred years. If a breach (or breaches) becomes permanently established in the centre section of the spit, the distal end will suffer significant erosion and may be destroyed. After this, the cycle of rebuilding and landward migration would likely start again at a new location landward of the present spit as relative sea level continues to rise. It is concluded that the key to understanding Buctouche Spit's long-term evolution is recognizing that it is migrating landward both by continuous processes like overwash, and by the more discrete and catastrophic process of overstepping.

Holocene evolution of relative sea level and local mean high water spring tides in Belgium—a first assessment

Holocene age-depth data from the Belgian coastal plain, basal peat data particularly, are assessed in terms of local water and tide levels. Basal peat formation was generally induced by sea-level rise; effects of local seepage were limited and significant river-gradient effects were absent. No relation was found between the timing of basal peat inception and substrate permeability. In general, edaphic (moisture) conditions were unfavourable for organic accumulation prior to sea-level related rise of the groundwater table. Error envelopes for the minimum level of local mean high water spring tides and for upper and extreme lower relative mean sea-level limits during the last 9400 solar years (8400 yrs B.P.) are presented. These indicate that the rate of relative sea-level rise showed marked retardations at ca. 7500-7000 yrs cal B.P. (± 6650-6100 yrs B.P.) and ca. 5500-5000 yrs cal B.P. (4400 yrs B.P.). A eustatic explanation appears to be plausible, particularly for the former. Prior to ca. 4000 yrs cal B.P. (ca. 3600 yrs B.P.), the relative sea-level rise in Belgium differs noticeably from that established for the western Netherlands, with both diverging progressively back in time. This implies that considerable differential crustal movement occurred between the two areas in the early Holocene.

The response of coastal marshes to sea‐level rise: Survival or submergence?

AbstractIn order to maintain an elevation in the intertidal zone at which marsh vegetation can survive, vertical accretion of the marsh surface must take place at a rate at least equal to the rate of relative sea‐level rise. Net vertical accretion of coastal marshes is a result of interactions between tidal imports, vegetation and depositional processes. All of these factors are affected, directly or indirectly, by alterations in marsh hydrology which might occur as a result of sea‐level rise. The overall response of coastal marshes to relative sea‐level rise depends upon the relative importance of the inorganic and organic components of the marsh soil and the impact of increased hydroperiod on net accumulation. The varied combination of factors contributing to sediment supply, and their complexity at the scale of individual marshes, means that predicting the response of suspended sediment concentration in marsh floodwater to any changes which may occur as a result of sea‐level rise, at anything other than the local scale is unlikely to be accurate. The impact of sea‐level rise on net below‐ground production is also complex. The sensitivity of certain species to waterlogging and soil chemical changes could result in a change in species composition or the migration of vegetation zones. Consequently, predicting the net impact of sea‐level rise on organic matter accumulation is fraught with difficulties and requires improved understanding of interactions between vegetation, soil and hydrologic processes.

Holocene environments in the Hawkesbury valley, new South Wales: A comparison of terrestrial and marine records

Rapid sedimentation of sands, silts and muds associated with seaward prograding tidal delta and central basin deposits, characterise the early Holocene sediment infill of the Hawkesbury River's estuary. From about 6 ka BP fluvial muds and sands dominate the estuary and sedimentation rates are much reduced. Micro-fossil records contained in sediments from the middle Hawkesbury-Colo Rivers region support the sedimentological-lithostratigraphic interpretation of estuary development. Information from fossil diatoms indicates that for a period of about 300 years around 6 ka BP, there was an increased marine influence in this part of the present estuary. This may have resulted from the probable peak in the rate of relative sea-level rise, although there may also have been a contemporaneous increase in freshwater flow down the river. Pollen and charcoal records from Mill Creek, downstream from the Colo sequence, show that sclerophyll forest, occasionally with rainforest elements, became more dominant from 8 ka BP. In contrast, the upper tidal Casuarina glauca forest decreased in importance during the phases of sea-level rise and the subsequent ‘still-stand’. Together the palaeoenvironmental records show broad agreement in defining the patterns of regional environmental changes and linked phases of estuary development during the Holocene. Differences in records appear to relate to local site factors and to the limitations in provenance, persistence and resolving power of the data types. The rapid accumulation rates provide an opportunity to investigate high magnitude but low frequency events in estuarine environments, including floods and fire.