Muddy Tidal Flats Research Articles

Remediation of Coastal Sediments by Addition of Calcium Nitrate and Presence of Benthos in a Muddy Tidal Flat

This study investigated the change in denitrification rate in muddy tidal flats sediment by chemical and biological remediation. Column experiments were conducted to determine the dependence of denitrification rate on the redox potential. Filtered seawater was fed continuously to the pipe filled with sediment from the muddy tidal flats. An increase in the redox potential from -198 mV to 21 mV with bioturbation led to increases in the denitrification rate from 24.2 mg/m2/day to 29.8 mg/m2/day. The increase in denitrification was explained by the acceleration of mass transfer in the sediment. Redox-potential increased from -400 mV to -200 mV by the addition of 10 g calcium nitrate, acid volatile sulfide was below the detection limit to 20 cm depth, and denitrification rate was increased from 21.3 mg/m2/day to 36.4 mg/m2/day. It is considered that the increased nitrogen concentration led to an increase in microbial activity.

Sedimentology, ichnology, ecology and anthropogenic modification of muddy tidal flats in a cold-temperate environment: Chignecto Bay, Canada

Abstract In Chignecto Bay, upper Bay of Fundy, Canada, the muddy tidal flats exhibit distinctive sedimentological and ichnological characteristics indicative of winter conditions and the development of ice. From late spring to autumn (May–October: mean temperature +13.6 °C), the mud flats sustain a high infaunal biomass and sediment deposition is dominated by tidal processes. Neap–spring tidal rhythmites, fluid-mud deposition and high levels of bioturbation are all characteristic of summer deposits. During the winter, temperatures remain below zero (December–March: mean temperature −6.3 °C), and ice forms in the bay and periodically on the mud flats. Ice rafts and blocks are common on the tidal-flat surface, and these blocks deform the muddy sediment, cut deep scours and deposit allochthonous sediment (including gravel) across the flats. Intermittent storms in the autumn and winter also contribute to sediment scouring and erosion. Annual die-offs of infauna are reflected by reduced bioturbation in winter deposits. Each spring, renewed larval recruitment and opportunistic colonization results in increasing levels of bioturbation, and the trace suite is dominated by a low-diversity assemblage of diminutive, vertical burrows. The summer–winter cyclicity in infaunal colonization manifests, ichnologically, as a distinctive bioturbated–non-bioturbated bedset character. Infaunal populations in Chignecto Bay peak in the late spring and in the late summer–early autumn. Both population peaks are exploited by vertebrates, including migrating Atlantic sturgeon (late spring) and migratory shorebirds (late summer–early autumn). In particular, the upper Bay of Fundy, including Chignecto Bay, is a staging ground for approximately 42–74% of the world’s Semipalmated sandpipers ( Calidris pusilla ) on their annual migration from the Arctic to South America. Fortunately, anthropogenic modification of Chignecto Bay, and its associated bays and rivers (e.g. construction and dismantling of a causeway over the Petitcodiac River), has not had a significant effect, to date, on the mud-flat habitat or the infaunal biomass.

Remediation of muddy tidal flat sediments using hot air-dried crushed oyster shells

In order to prove that hot air-dried crushed oyster shells (HACOS) are effective in reducing hydrogen sulfide in muddy tidal flat sediments and increasing the biomass, field experiments were carried out. The concentration of hydrogen sulfide in the interstitial water, which was 16mgSL−1 before the application of HACOS, decreased sharply and maintained almost zero in the experimental sites (HACOS application sites) for one year, whereas it was remained at ca. 5mgSL−1 in the control sites. The number of macrobenthos individuals increased to 2–4.5 times higher than that in the control site. Using a simple numerical model, the effective periods for suppression of hydrogen sulfide were estimated to be 3.2–7.6 and 6.4–15.2years for the experimental sites with 4 and 8tons per 10×10×0.2m area, respectively. From these results, it is concluded that HACOS is an effective material to remediate muddy tidal flats.

Water and sediment transport of channel-flat systems in a mesotidal mudflat: Willapa Bay, Washington

The muddy tidal flats of southern Willapa Bay, Washington are tidally dominated and receive little direct freshwater input. We use data from instruments deployed in channels of different size and on their adjacent flats to investigate the hydrodynamics and sediment dynamics of each morphological setting under a range of seasonal and meteorological conditions, including rain and wind events. Interaction between the morphology of the channel/flat complex and tidal water-level variations produces well-defined velocity pulses during both flooding and ebbing tides. These pulses represent about 27% of the total along-channel water transport and 35% of the suspended-sediment transport of the system. Maintenance of continuity produces the velocity pulse, and pulse magnitude is determined by tidal range. Wind alters the flow regime in channels and on the flat, enhancing over-flat ebb flow in this study location while decreasing ebb-pulse intensity. Wind speed was positively correlated with minimum suspended-sediment concentration. Precipitation falling directly on flats was found to erode flat sediment, which subsequently formed a temporary deposit in the adjacent channel. Residual along-channel water transport in channels and on nearby flats was flood dominant under all seasonal conditions sampled, and sediment flux was flood dominant during winter and spring deployments.

A snapshot of suspended sediment and fluid mud occurrence in a mixed-energy embayment, Tijucas Bay, Brazil

Along the southern Brazilian coast, Tijucas Bay is known for its unique muddy tidal flats associated with chenier plains. Previous field observations pointed to very high suspended sediment concentrations (SSCs) in the inner parts of the bay, and in the estuary of the Tijucas River, suggesting the presence of fluid mud. In this study, the occurrences of suspended sediments and fluid mud were examined during a larger-scale, high-resolution 2-day field campaign on 1–2 May 2007, encompassing survey lines spanning nearly 80 km, 75 water sampling stations for near-bottom density estimates, and ten sediment sampling stations. Wave refraction modeling provided qualitative wave energy estimates as a function of different incidence directions. The results show that SSC increases toward the inner bay near the water surface, but seaward near the bottom. This suggests that suspended sediment is supplied by the local rivers, in particular the Tijucas. Near-surface SSCs were of the order of 50 mg l−1 close to the shore, but exceeded 100 mg l−1 near the bottom in the deeper parts of the bay. Fluid mud thickness and location given by densimetry and echo-sounding agreed in some places, although being mostly discordant. The best agreement was observed where wave energy was high during the campaign. The discrepancy between the two methods may be an indication for the existence of fluid mud, which is recorded by one method but not the other. Agreement is considered to be an indication of fluidization, whereas disagreement indicates more consolidation. Wave modeling suggests that waves from the ENE and SE are the most effective in supplying energy to the inner bay, which may induce the liquefaction of mud deposits to form fluid mud. Nearshore mud resuspension and weak horizontal currents result in sediment-laden offshore flow, which explains the higher SSCs measured in the deeper parts of the bay, besides providing a mechanism for fine-sediment export to the inner shelf.

THE GULF WAR OIL SPILL TWELVE YEARS LATER: CONSEQUENCES OF ECO-TERRORISM

ABSTRACT The intentional release of an estimated 11 million barrels of oil during the 1991 Gulf War was the largest oil spill in history. An assessment of the physical, chemical, and ecological impacts of this spill shows that, 12 years later, oil residues and habitat modifications continue to have toxic effects on intertidal communities. As of 2003, there are an estimated 8 million cubic meters of oiled sediment remaining along the 803 km of impacted shoreline in Saudi Arabia. Of this volume, 45% occurs in muddy tidal flats and 23% in salt marshes and mangroves. Much of the oil in these sheltered habitats occurs as oiled crab burrows, with liquid oil remaining in the burrows to depths that exceed 50 cm. These habitats show the lowest degree of ecological recovery since the spill, with 87% of the upper intertidal zones of mangroves and marshes and 71 % of muddy tidal flats having reduced species richness and a disturbed community structure. Those habitats exposed to the greatest amount of wave activity contain the smallest amount of residual oil; however, on outer sand beaches, the oil is commonly buried to depths exceeding 1 m. The factors that affect the ecological recovery of the intertidal habitats include: 1) The chemical toxicity of the oil residues; 2) the physical toxicity of heavy and hardened oil residues; 3) other physical barriers that affect seed germination of plants, settlement of larvae, and burrowing; 4) limited sources for recruitment of biota; 5) reduced hydrological functioning of tidal channels. This study shows the importance of oil removal as the first phase of habitat recovery. It also indicates the potential for large-scale damage by blatant acts of eco-terrorism.

Micro-craters in muddy tidal flats of cold regions

Meltwaters dripping from overhanging ice cakes, stranded at low tide on muddy tidal flats of the St.Lawrence Estuary, near Québec City, cause micro-craters to form on soft mud and silty-sand sediments. The shallow depressions are circular, symmetrical, and with rims a few mm high, some have a tiny hole centered in the bottom. Craters range from 7 to 15 cm in diameter, and 20 to 50 mm in depth ; they characterize tidal flats of cold regions.

A review of Asian Hediste species complex (Nereididae, Polychaeta) with descriptions of two new species and a redescription of Hediste japonica (Izuka, 1908)

Three Asian species of the genus Hediste (Nereididae, Polychaeta, Annelida), including two new species, are reviewed. Hediste japonica (Izuka, 1908) was redescribed based on recently discovered type material and on additional specimens newly collected from the Ariake Sea in Japan and the west coast of Korea. Two new species, H. diadroma and H. atoka, were described based on specimens collected from Japan. These three species are morphologically similar, but distinguishable by some differences in parapodial and chaetal morphology, at least at the sexually mature stage. The presence of homogomph falcigers and the absence of heterogomph spinigers are unique characteristics of H. japonica among this genus. Inconspicuous epitokous metamorphosis occurred as a slight enlargement of the eyes and parapodial ligules in mature adults of H. diadroma sp. nov. and H. japonica in combination with their reproductive swarming and pelagic larval development. The epitoke-specific sesquigomph spinigers were added in mature adults of H. diadroma sp. nov., while the number of ordinary homogomph spinigers increased in some adults of H. japonica. Epitokous metamorphosis in the Hediste species was compared with that in other nereidid genera. No epitokous metamorphosis was observed in H. atoka sp. nov., corresponding to its reproduction without swarming and to its development without a planktonic larval stage. Significant differences in paragnath numbers in the maxillary ring of the proboscis were found among the three species, suggesting different feeding habits. In Japan, the present distribution of H. japonica is restricted to muddy tidal flats in the inner part of the Ariake Sea, probably due to recent human impact, while the other two species are widespread.

Benthic microalgal communities in the inner part of the Bahía Blanca estuary (Argentina): a preliminary qualitative study

The intertidal zone of the inner part of the Bahía Blanca estuary, Argentina is characterised by the presence of muddy tidal flats and salt marshes, habitats for burrowing crabs and halophytic plants. The surface layer of the sediments is composed of silt and clay. The sediments are substrate for the proliferation of benthic microalgal communities. The aim of this study was to analyse qualitatively the species composition of these communities and their role in the stabilisation of the sediments. Samples came from two stations: Puerto Cuatreros and Villarino Viejo. Puerto Cuatreros is an old pier without harbour activities closer than Villarino Viejo to the harbour zone affected by dredging. Both stations showed similar physical and chemical parameters. In addition, the benthic microalgal communities at both stations were comprised of cyanophytes and diatoms. The main difference between the two stations was the relative frequency of the algae—cyanophytes were predominant in Villarino Viejo while diatoms predominated in Puerto Cuatreros. Presumably, the difference was due to the major disturbance of the first due to deposition of particles of the fluid mud layer produced by the nearby dredging. Major species of cyanophytes were Microcoleus chtonoplastes and Oscillatoria limosa. Diatom assemblages were very diverse since the estuarine characteristics of the system allow the coexistence of marine, freshwater and estuarine species; also some of these species may be present in the plankton community.

The geometry, flow patterns and sedimentary processes of Devonian rivers and coasts, New York and Pennsylvania, USA

Abstract The Middle-Upper Devonian strata of New York and Pennsylvania were deposited in a foreland basin adjacent to the Acadian fold-thrust belt. Fluvial (Catskill) strata in the east thin to the west into marine (Chemung) strata. The overall sequence is marine regressive, but there numerous superimposed marine transgressions. The Catskill river channels changed character with distance from the palaeoshoreline. Sinuous, single-channel rivers (widths tens of metres, maximum bankfull depths 4–5 m, sinuosity 1.1–1.3, mean bankfull flow velocity 0.4–0.7 m s −1 ) occurred near the coast. With increasing distance from the coast, rivers increased in slope, became wider (up to hundreds of metres) and deeper (up to 15 m), coarser grained, and possibly braided. This suggests that rivers were distributive near the coast. Furthermore, muddy floodplain areas were more extensive near the coast than up-valley, such that the proportion of channel deposits increased up-valley. Coastal areas comprised tide-influenced channels and mouth bars, wave-influenced sandy shoals and washovers, sandy and muddy tidal flats, and muddy interdistributary bays and lakes. Major marine transgressions are thus represented in wholly fluvial successions by a decrease in the proportion of channel sandstone bodies, and a decrease in channel size and grain size. Marine transgressions in near-coastal successions are represented in places by an increase in sandstone proportion associated with progradation of coastal sands over muddy backswamps and interdistributary areas. Small-scale marine transgressions and regressions in coastal deposits (i.e. metres-thick parasequences) are probably associated with switching of coastal channels or delta lobes. There is no evidence for the preservation of incised valleys, nor for any changes in coastal channel patterns associated with these parasequences: therefore, major regional sea-level changes were not responsible. However, large-scale (tens to hundreds of metres) transgressive-regressive sequences were due partly to eustatic sea-level change (e.g. in mid-Givetian time) but also due to changes in the balance between sediment supply and subsidence associated with tectonic uplift of the hinterland and peripheral bulge. Such uplift may have induced some kind of climate change. However, climate changes in the depositional basin are not supported by sedimentological evidence. Further understanding of the origin of these strata will require higher precision in the biostratigraphic correlation of marine and non-marine strata (using miospores), and quantitative interpretation of depositional environments from other regions.

Facies and geometry of tidal channel-fill deposits (Arcachon Lagoon, SW France)

The Arcachon Lagoon has an important network of tidal channels and well developed tidal flats covered by the marine grass Zostera marina. Based on 66 piston cores taken from the Graveyron tidal channel, and observations on the neighbouring channels, this paper documents the facies and geometry of the channel-fill deposits. In the inner lagoon (studied area) the tidal channels are 80 to 150 m wide and have a meandering morphology with sandy point bars 2 to 5 m thick. The channel-fill does not consist of the classic inclined heterolithic bedding typical of many channel-fills ( Reineck, 1958), but of cross-stratified sandy deposits characterized by the absence of slack-water clay-drapes. These unusual facies characteristics are due to the low turbidity of the lagoonal waters which is caused by the lack of significant river inflow and the dense coverage of Zostera marina on the tidal flats. The overall geometry of the channel-fill deposits is characterized by a narrow sand-ribbon shape, a few kilometres long, 80 to 150 m wide and 1 to 5 m thick. This sand ribbon is made of elliptical sand bodies, deposited as point bars, that coalesce longitudinally along the channel axis. This narrow shape is due to the fact that the lateral migration of the channel is virtually nil (reduced to a few metres). In spite of their characteristic meandering morphology, these channels do not deposit extensive tabular sand sheets of amalgamated point bars like the tidal creeks on the North Sea tidal flats. Two factors are thought to control this lack of channel migration. (1) The tidal flats adjacent to the tidal channels are made of 3- to 5-m-thick cohesive muddy sediments covered by Zostera marina that prevents the erosion of the channel banks. This first mechanism is supported by the observation that the tidal creeks that drain the muddy tidal flats covered by Zostera marina do not migrate laterally, whereas those that drain the sandy tidal flats devoid of a dense coverage of marine grass do have active lateral migration. (2) The tidal channels are not fed by any river and therefore do not receive any fluvial sand influx during the winter floods. Their morphology is in equilibrium with the tidal discharge and represents a stable stage in the development of the channel. This second mechanism is supported by the fact that the only tidal channels that actively migrate laterally in the lagoon receive sandy fluvial influx from the River Leyre located in the southeastern corner of the lagoon.

Coastal modification due to human influence in south-western Taiwan

The coastal zone of southwest Taiwan is characterised by muddy tidal flats, offshore bars, spits, lagoons and coastal sand dunes. These geomorphic features from an essential part of the natural defences against sea-level change and coastal erosion. Between 1955 and 1995 the area bounded by the Pei-Kang and Tseng-Wen rivers underwent considerable land use and topographic changes. Two examples were studied using time-series aerial photographs, remote sensing images and geomorphological field mapping. The result of this investigation demonstrates the influence of human activities and the importance of the natural coastal landscape in acting as a first-line of coastal defence and protection in Taiwan. Attention is drawn to: (1) inappropriately designed or misplaced coastal engineering structures which may destroy or reduce the effectiveness of neighbouring natural and engineered structures leading to storm damage, flooding and encroachment by the sea; (2) the need for a co-ordinated coastal management program to regulate on-shore activities, in particular the removal of ground water within the coastal zone. This research demonstrates that there is significant land subsidence and inland penetration of sea water into the ground water system in the coastal zone. Human activities also lead to considerable financial expenditure on the prevention of coastal erosion in Taiwan.

Marine transgressions and regressions recorded in Middle Devonian shore-zone deposits of the Catskill clastic wedge

The shore zone of the Middle Devonian Catskill clastic wedge in New York State consists of (1) a storm-wave-dominated muddy marine shelf with sandy shoals; (2) sandy, tide-influenced channels with wave-and tide-influenced mouth bars; (3) sandy and muddy tidal flats, including channels, mouth bars, and washovers; and (4) muddy brackish bays, lakes, and flood plains. The spatial organization of these subenvironments suggests a wave- and tide-influenced deltaic shoreline. Meter-scale sequences can be related to processes such as lateral migration and filling of tidal channels, progradation of channel-mouth bars and tidal flats, and filling of coastal bays; these may be related to channel switching. Asymmetrical regressive-transgressive sequences that range from tens of meters to >100 m thick can be correlated over many kilometers with fully marine and fluvial deposits. Such sequences record a combination of eustatic sea-level changes and tectonically induced changes in sediment supply and subsidence rate. However, at present it is very difficult to determine the relative importance of these controls.

Mud-Flat Cycles, Incised Channels, and Relative Sea-Level Changes on a Paleocene Mud-Dominated Coast, Ellesmere Island, Arctic Canada

ABSTRACT The Upper Paleocene Cape Pillsbury Member on west-central Ellesmere Island, consisting of thick (600+ m), cyclical, calcareous mud-flat and laterally associated mixed mud-sand shelf deposits, provides an example of deposition on a mud-dominated coast. Mud-flat cycles are characterized by laminated mudstone. Cross-bedding is conspicuously rare. Normal grading of mudstone laminae suggests that deposition from suspension was significant. Storms also were common on this mud-dominated coast, as evidenced by beds of locally derived mud-chip conglomerate that commonly are intercalated with the laminated mudrocks, and by hummocky cross-stratification in the deeper shelf facies. Soft-sediment folding and microfaulting is ubiquitous in the mudrock facies. The deformation is analogous to flowsli es in muddy tidal flats now accumulating on the exposed northeast coast of South America. The flowslides are the only potential indicators of ebb-flood tidal reversals in the entire mudstone succession. Most of the mudrock cycles are incised by sand-filled channels. The channels were excavated during a drop in relative sea level (that terminated the preceding cycle). Subsequent filling of channels began when sediment flux in the channels exceeded the rate of sea-level fall. Thin coal seams capping some of the sand-filled channels may be the first indications of early transgression. The next cycle of laminated mudstone accumulated during the succeeding transgressive/progradational phase, but the distinction between these two components is not clear. The mud-flat and shelf facies were mostly progradational, accumulating during successive rises and high stands of relative sea level.

Macrotidal Mudflats of the Southwestern Korean Coast: A Model for Interpretation of Intertidal Deposits

ABSTRACT Extensive muddy tidal flats along the western coast of Korea are distinctly different from other intertidal environments used as standard models for intertidal sedimentation (e.g., the North Sea). The muddy Korean tidal flats lack extensive, dendritic drainage systems, seaward barriers, landward salt marshes, and most sedimentary structures considered diagnostic of intertidal environments (i.e., flaser, wavy, and ripple-laminated bedding). Fifty-one cores were collected from intertidal and subtidal areas of Nam Yang Bay to investigate the distribution of texture, composition, structure, and accumulation rate of these muddy sediments. Sediments coarsen seaward (from about 8.8 phi to 4.6 phi) and downcore, indicating that the tidal flats are actively accreting, with finer-grained, high-tidal- and mid-tidal-flat sediments prograding over the coarser-grained, low-tidal-flat sediments. Sedimentary structures show a trend of increasing preservation of physical stratification (mm-scale parallel to slightly wavy-bedded laminae) toward the mid-tidal flat. Pb-210 accumulation rates are highest on the mid-tidal flat (5-9 mm/yr) and decrease both seaward and landward to 1-2 mm/yr. Upper portions of intertidal cores are laminated and lower portions are mottled; the opposite is true of subtidal cores. Different processes dominate these two environments on both short and long timescales. The most significant difference between the muddy Korean tidal flats and the sandier North Sea setting is the lack of intricate drainage networks. Drainage channels on the Korean tidal flats seem to be relatively stable. Thus, channel-fill sequences do not compose a major part of muddy intertidal deposits. Muddy intertidal deposits would be more difficult to identify in the rock record compared to sandy intertidal deposits because of the lack of channel-fill sequences (which compose the bulk of sandy intertidal deposits) and because the gradients in sedimentological properties are gradual and subtle.

Pleistocene glacial tectonism and sedimentation on a macrotidal piedmont coast, Ekuk Bluffs, southwestern Alaska

Extensive wave-cut exposures at Ekuk Bluffs in the Nushagak lowland of southwestern Alaska provide a 7-km longitudinal section through deposits related to advance of Pleistocene glaciers from the Ahklun Mountains onto the adjoining piedmont. Facies analysis of the proglacial sediments reveals the following successive depositional environments: (1) lower sandy tidal flats, (2) upper muddy tidal flats, (3) upper foreshore/beach face, and (4) braided sandur. Horizontal compression at the toe of the advancing glacier induced large-scale imbricate thrusting and folding of the proglacial sediments, producing a distally accreting wedge of deformed sediment beyond the ice margin. The presence of this topographic wedge subsequently dictated the over-all pattern of sediment dispersal, forward during ice advance and rearward during retreat, and led to extensive gravity-driven resedimentation. Previous studies of deposition in glacial-tectonic settings generally have not emphasized these consequences of glacial-tectonically produced relief. The major stratigraphic implication is that sediment flows related to a single ice advance can generate multiple till-like diamicton units, some of which are separated by angular unconformities.

Use of organochlorine contaminants to measure sedimentation rates in estuaries: a case study from the Manukau Harbour

Abstract Organochlorine contaminants are widely distributed in sediments of the Manukau Harbour. The first appearance of DDT in cores is a potential stratigraphic marker because it can have appeared only since extensive applications of DDT on New Zealand pastures for grass grub control began in c. 1950. The technique permits calculation of average annual sedimentation rates, but precision is limited by bioturbation. A nett sedimentation rate of about 5 mm yr-1 was determined from measurements of DDT for muddy tidal flats in Drury Creek. This compared well with a rate of 5 mm yr-1 based upon a pollen dating technique. The past widespread use of DDT may enable comparisons to be made of very recent sedimentation rates in other New Zealand estuaries. Other contaminants identified in the Manukau Harbour sediments include dieldrin, lindane, PCB's and chlordane.

Factors affecting the burrowing behaviour of Helice tridens (Grapsidae) and Macrophthalmus japonicus (Ocypodidae) in an estuary of northeast Japan

Helice tridens (De Haan) (Grapsidae) and Macrophthalmus japonicus (De Haan) (Ocypodidae) build separate burrows in reed marshes and muddy tidal flats, respectively, in the brackish-water estuaries of northeast Japan (38°11′N, 141°48′E). Habitat segregation between burrows of these two species was analysed by comparing the density of burrows of both species in their natural habitats, and on tidal flats subjected to various types of artificial treatment, in the summer of 1981 and 1982. Baskets, containing many stones, were placed on the ground in the reed marsh, tidal flat and creek and attracted many individuals of H. tridens, but not M. japonicus. When stones were placed on an area of tidal flat, H. tridens frequently formed burrows at the border between the stones and mud, suggesting that burrowing of H. tridens was related to the presence of solid substances lying on the ground, such as stones and shoots of the reed Phragmites australis (Cav.) Trin. In closed systems on the tidal flat and reed marsh, H. tridens and M. japonicus were able to construct burrows in both substrata regardless of high or low frequency. Moreover, it was recognized that H. tridens prevented the burrowing of M. japonicus in these closed systems. These results suggested that habitat segregation between H. tridens and M. japonicus burrows was caused primarily by an exclusive interaction between individuals of these two species.

Accumulation rates of estuarine sediment in the dollard area: comparison of 210P b and pollen influx methods

Profiles of excess 210Pb activity in sediment cores collected on the muddy tidal flats of the Dollard indicate deposition rates ranging from 0.14 to 0.27 cm·y −1. One profile was also analysed for pollen: the pollen profile in the core provided a record of the sedimentation rate because two independent historical events were found recorded in the profile. The first, a sudden abundance of an Aster-type pollen, reflected, at a depth of 33 cm in the sediment, the large land reclamation of 1862 A.D. The second, the appearance of Zea mays (corn) at a depth of 6 cm, reflected the increase of the area in which corn was cultivated: from 44 ha in 1970 up to 552 ha in 1973. The pollen data gave an estimated sedimentation rate of 0.25 cm·y −1. Excess 210Pb analysis of the same core yielded a sedimentation rate of 0.27 cm·y −1.

Evolution and stratigraphy of a regressive barrier/backbarrier complex: Kiawah Island, South Carolina

ABSTRACTAnalysis of 75 vibracores from the backbarrier region of Kiawah Island, South Carolina reveals a complex association of three distinct stratigraphic sequences. Beach ridge progradation and orientation‐controlled backbarrier development during the evolution of Kiawah Island, and resulted in deposition of: (1) a mud‐rich central backbarrier sequence consisting of low marsh overlying fine‐grained, tidal flat/lagoonal mud; (2) a sandy beach‐ridge swale sequence consisting of high and low marsh overlying tidal creek channel and point bar sand, and foreshore/shoreface; and (3) a regressive sequence of sandy, mixed, and muddy tidal flats capped by salt marsh that occurs on the updrift end of the island.Central backbarrier deposits formed as a result of the development of the initial beach ridge on Kiawah Island. Formation of this beach ridge created a backbarrier lagoon in which fine‐grained estuarine and tidal flat mud accumulated. Washovers, oyster mounds, and tidal creek deposits form isolated sand and/or shell‐rich lenses in the lagoon. Spartina alterniflora low marsh prograded into the lagoon as the tidal flats aggraded.Barrier progradation and sediment bar‐bypassing at Stono Inlet created digitate beach ridges on the northeast end of Kiawah Island. Within the beach‐ridge swales, tidal flats were disconformably deposited on shoreface and foreshore sand of the older beach ridges. Tidal creek drainage systems evolved to drain the swales. These rapidly migrating creeks reworked the tidal flat, foreshore, and shoreface sediments while redepositing a fining‐upward sequence of channel lag and point bar deposits, which served as a substrate for salt marsh colonization.This resultant regressive sedimentary package marks the culmination of barrier island development and estuary infilling. Given enough time and sedimentation, the backbarrier sequence will ultimately prograde over the barrier island, reworking dune, beach, and foreshore sediments to form the upper sand‐rich bounding surface of the barrier lithosome. Preservation of the regressive sequence is dependent upon sediment supply and the relative rate of sea‐level rise, but the reworking of barrier islands by tidal inlets and migrating tidal creeks greatly alter and complicate the stratigraphic sequence.