Sedimentation in a fluvially infilling, barrier-bound estuary on a wave-dominated, microtidal coast: the Ouémé River estuary, Benin, west Africa

The Ouémé River estuary is located on the seasonally humid tropical coast of Benin, west Africa. A striking feature of this microtidal estuary is the presence of a large sand barrier bounding a 120 km² circular central basin, Lake Nokoué, that is being infilled by heterogeneous fluvial deposits supplied by a relatively large catchment (50 000 km²). Borehole cores from the lower estuary show basal Pleistocene lowstand alluvial sediments overlain by Holocene transgressive–highstand lagoonal mud and by transgressive to probably early highstand tidal inlet and flood‐tidal delta sand deposited in association with non‐preserved transgressive sand barriers. The change in estuary‐mouth sedimentation from a transgressive barrier‐inlet system to a regressive highstand barrier reflects regional modifications in marine sand supply and in the cross‐barrier tidal flux associated with barrier‐inlet systems. As barrier formation west of the Ouémé River led to an increasingly rectilinear shoreline, the longshore drift cell matured, ensuring voluminous eastward transport of sand from the Volta Delta in Ghana, the major purveyor of sand, to the Ouémé embayment, 200 km east. Concomitantly, the number of tidal inlets, and the tidal flux associated with a hitherto interlinked lagoonal system on this coast, diminished. Complete sealing of Lake Nokoué has produced a large, permanently closed estuary, where tidal intrusion is assured through the interconnected coastal lagoon via an inlet located 60 km east. Since 1885, tides have entered the estuary directly through an artificial outlet cut across the sand barrier. Although precluding the seaward loss of fluvial sediments, permanent estuary‐mouth closure has especially deprived the highstand estuary of marine sand, a potentially important component in estuarine infill on wave‐dominated coasts. In spite of a significant fluvial sediment supply, estuarine infill has been moderate, because of the size of the central basin. Estuarine closure has resulted in two co‐existing highstand sediment suites, with limited admixture, the marine‐derived, estuary‐mouth barrier and upland‐derived back‐barrier sediments. This situation differs from that of mature barrier estuaries characterized by active fluvial‐marine sediment mixing and facies interfingering.     

Secondary turbidity maximum in a partially mixed microtidal estuary

Data from a two-year period of monthly slackwater surveys reveal that in addition to the classical estuary turbidity maximum (ETM), another peak of bottom total suspended sediment (TSS) concentration, or a so-called secondary turbidity maximum (STM), often exists in the middle part of the York River estuary, Virginia. This STM, observed in most (but not all) of the slackwater surveys, moves back and forth in the region of about 20 to 40 km from the York River mouth where the mud percentage of bottom sediment is very high. The distribution of the potential energy anomaly, which was calculated using salinity data, indicates that the STM usually resides in the transition zone between the upstream well mixed and the downstream more stratified water columns. An analysis using the conservation equation of suspended sediment concentration in the water column reveals that four processes may contribute to the formation of the STM: convergence of bottom residual flow, tidal asymmetry, inhibition of turbulent diffusion by stratification, and bottom resuspension. The along-channel variations of the strength of bottom residual flow, the effect of tidal asymmetry, and the stratification patterns are probably due to the geometric features of the York River estuary.     

Sedimentation in a river dominated estuary

The Mgeni Estuary on the wave dominated east coast of South Africa occupies a narrow, bedrock confined, alluvial valley and is partially blocked at the coast by an elongate sandy barrier. Fluvial sediment extends to the barrier and marine deposition is restricted to a small flood tidal delta. Sequential aerial photography, sediment sampling and topographical surveys reveal a cyclical pattern of sedimentation that is mediated by severe fluvial floods which exceed normal energy thresholds. During severe floods (up to 10x 10³ m³ s^?1), lateral channel confinement promotes vertical erosion ofbed material. Eroded material is deposited as an ephemeral delta in the sea. After floods the river gradient is restored within a few months through rapid fluvial deposition and formation of a shallow, braided channel. Over an extended period (approximately 70 years) the estuary banks and bars are stabilised by vegetation and mud deposition. Subsequent downcutting in marginal areas transforms the channel to an anastomosing pattern which represents a stable morphology which adjusts to the normal range of hydrodynamic conditions. This cyclical pattern of deposition produces multiple fill sequences in such estuaries under conditions of stable sea level. The barrier and adjacent coastline prograde temporarily after major floods as the eroded barrier is reformed by wave action, but excess sediment is ultimately eroded as waves adjust the barrier to an equilibrium plan form morphology. Deltaic progradation is prevented by a steep nearshore slope, and rapid sediment dispersal by wave action and shelf currents. During transgression, estuarine sedimentation patterns are controlled by the balance between sedimentation rates and receiving basin volume. If fluvial sedimentation keeps pace with the volume increase of a basin an estuary may remain shallow and river dominated throughout its evolution and excess fluvial sediments pass through the estuary into the sea. Only if the rate of volume increase of the drowned river valley exceeds the volume of sediment supply are deep water environments formed. Under such conditions an estuary becomes a sediment sink and infills by deltaic progradation and lateral accretion as predicted by evolutionary models for microtidal estuaries. Bedrock valley geometry may exert an important control on this rate of volume increase independently of variations in the rate of relative sea level change. If estuarine morphology is viewed as a function of the balance of wave, tidal and fluvial processes, the Mgeni Estuary may be defined as a river dominated estuary in which deltaic progradation at the coast is limited by high wave energy. It is broadly representative of other river dominated estuaries along the Natal coast and a conceptual regional depositional model is proposed. Refinement of a globally applicable model will require further comparative studies of river dominated estuaries in this and other settings, but it is proposed that river dominated estuaries represent a distinct type of estuarine morphology.     

Seasonal cycling of arsenic species in a stratified,microtidal estuary (Huon River,Tasmania)

A detailed study of arsenic cycling in the Huon estuary, south-east Tasmania, was undertaken October 1996–September 1998. Arsenic species data were obtained during eight 3-monthly spatial surveys, and a time-series study at a single site in the estuary over a 7-mo period. The data have been correlated with other chemical measurements, including nutrients, salinity, and dissolved oxygen, and also with biological information about the microalgal species present. In the Huon estuary, arsenic cycling is almost entirely biologically influenced. The seasonal cycle of reduced, methylated, and hydride refractory arsenic species was similar to that found in other temperate estuaries of the Northern hemisphere, with greatest production occurring during summer months. Inorganic arsenic concentrations in the Huon River were very low [As(V+III): 0.023–0.057 μg l⁻¹], even when compared with other pristine systems. Concentrations at the seaward end of the estuary were typical of those found in unpolluted coastal seawater. The behavior of As(V+III) in the estuary was nearly conservative in all surveys except those conducted during summer (February), when significant depletion was observed at higher salinity. During these surveys, up to 60% of inorganic arsenic was apparently depleted from the water column with only a small proportion (20–25%) recycled as methylated and UV-labile arsenic species. This was particularly the case in a high salinity side-arm of the estuary, Port Cygnet. The extent of inorganic arsenic depletion correlated with cell numbers of the phytoplanktonPseudo-nitzschia. The fate of the missing inorganic arsenic is unclear, but the co-existence ofPseudo-nitzschia blooms with commercial mussel leases in Port Cygnet could provide one explanation for the loss.     

Sedimentation on a wave-dominated, open-coast tidal flat, south-western Korea: summer tidal flat – winter shoreface

Sedimentation on the open-coast tidal flats of south-western Korea is controlled by seasonal variation in the intensity of onshore-directed winds and waves. As a result, an environmental oscillation takes place between tide-dominated conditions in summer and wave-dominated conditions in winter. In summer, thick muddy deposits, including sporadic storm deposits, accumulate in response to low wave energy, weak currents, and intense solar insolation that promotes consolidation of the mud at low tide. Bioturbation is minimal because of rapid sedimentation and soft substrate. During the autumn, the summer mud deposits experience erosion due to increasingly strong onshore winds and waves, until only small mud patches and mud pebbles remain. The concentration of ebb runoff between the mud patches produces small, ephemeral tidal creeks. In winter, storm waves occur frequently (ca 10 days a month) and dominate sedimentation in the intertidal zone, producing extensive wave-generated parallel lamination and short-wavelength (0·3–2 m) hummocky cross-stratification. The prevalence of strong onshore winds decreases in spring, allowing longer and more frequent intervals of calm weather, during which time muddy sediments are deposited by tidal processes. Over the long term, winter storm waves dominate sedimentation and the preserved deposits consist of amalgamated storm beds that resemble those generally associated with shorefaces. This raises the question of how many ancient ‘shorefaces’ are, in fact, open-coast tidal flats.     

Distribution of dissolved carbohydrates and uronic acids in a tropical estuary,India

Carbohydrates including uronic acids are among the active components of dissolved organic carbon, and play an important role in biogeochemical cycling of organic carbon in marine environments. In order to understand their distribution, concentrations of total dissolved carbohydrate (TCHO), dissolved polysaccharide (PCHO), dissolved monosaccharide (MCHO), and dissolved uronic acid (URA) were measured in the Mandovi estuary, west coast of India during the monsoon and premonsoon seasons. The estuary experienced nearly fresh water condition during the monsoon season and marine condition during the pre-monsoon season. Concentrations of TCHO, MCHO and URA ranged from 17.7 to 67.3 μM C, 4.1 to 15.5 μM C and 2.3 to 10.8 μM C, and their contribution to dissolved organic carbon (DOC) varied from ∼11 to 60%, 2.5 to 9.7%, and 1.8 to 5.3%, respectively. PCHO accounted for ∼52 to 92% of the TCHO. Generally, concentrations and yields of TCHO species were greater during the monsoon season. Phytoplankton abundance and bacterial cell numbers influenced the distribution of TCHO in the pre-monsoon season but not during the monsoon season. Transport of TCHO rich (11 to 60%) dissolved organic matter from the Mandovi estuary to the coastal waters during the monsoon season may affect ecosystem function by fueling biological activity of heterotrophic micro-organisms.     

Speciation of trace metals in sediments of a tropical estuary

Axial surveys were performed in the two river tributaries of the Cochin estuary, SW India during November 1988. Surficial sediments were subjected to sequential chemical extractions to delineate five metal fractions, namely, exchangeable, carbonate bound, easily reducible, organic/sulfide bound, and residual. The results indicated selective accumulation of Mn and Ni in carbonate bound and organic/sulfide forms, along with marginal amounts of Co in the exchangeable fraction. Large portions of Fe and Cr occurred in the residual fraction, whereas composite fractionation of Zn species was noticed. The exchangeable fractions of Fe and Cr as well as of easily reducible cobalt were below detection limits. The levels of Cr and Zn indicate anthropogenic inputs in this estuary, whereas Co and Ni show regional contamination exceeding natural levels. The analytical speciation procedure helps to deduce the sedimental diagenetic processes in the estuarine environment.     

Fractionation of phosphorus in the sediments of a tropical estuary

Fractionation of phosphorus in the sediments of the Cochin estuary situated along the southwest coast of India was studied by applying sequential chemical extraction. The different forms of phosphorus were estimated seasonally (premonsoon, monsoon, and postmonsoon) under eight different schemes. The major forms of phosphorus analyzed were exchangeable P, anion exchangeable P, carbonate-bound P, labile and resistant organic P, Fe and Al P, calcium-bound P, and hydrolyzable surplus P. Quantitatively, the above fractions in isolation or in combination vary in content due to chemoestuarine variability and seasonal fluctuations. Changes in speciation have been noted in association with salinity variations in the waterway, especially following enhanced river runoff during the monsoon. The chemical forms of the sediment-bound phosphorus in the northern parts of this estuary have been shown to be modified by nonpoint sources. Sediment P fractionation defines the role of chemical speciation of phosphates (as nutrients) and is indicative of the processes controlling the pathways of P into the coastal waters. The changes in the exchangeable P, together with marked regional variations in calcium-bound P, exemplify the complex estuarine variability of phosphorus. Enhanced amounts of exchangeable P mark its appearence in high saline waters, signifying the presence of biologically available nutrient phosphorus. The calcium-bound P and hydrolyzable surplus P show significant relation with sediment organic carbon and Fe whereas other forms do not exhibit any marked covariation. The Ca and Na NTA extraction scheme is very specific in its selectivity.     

Sedimentation cycle of a freshwater tidal flat in the St. Lawrence estuary

Cap-Tourmente tidal flat is located on the north shore of the St. Lawrence middle estuary, 50 km downstream from Quebec city, Canada. Seasonal variations in sedient accretion were studied during the period without ice from May to December, using horizontal plates in conjunction with vertical stakes. The sedimentological regime which is characterized by very rapid changes, can be divided into four periods. There are two periods of intense erosion in May and October–November; during these months, erosion is initiated by walking and beak probing of very large flocks of snow geese. During the summer, very rapid accretion occurs for approximately 100 days; during this period, the sedimentation rate is 2 mm per day in the lower section of the tidal marsh which is covered with vegetation. Among the numerous factors influencing the sedimentation cycle, the very gradual slope of the upper part of the flat (0.25%) seems to be one of the most important because it results in a very large area (550 m wide) which is protected from strong tidal currents. This latter area is covered with dense and high vegetation which provides an effective protection against erosion unitl the snow geese arrive in October and raze it in a few days. The presence of vegetation reduces the velocities of tidal currents, particularly during ebb and modifies their direction for both flood and ebb, orienting them perpendicularly to the main tidal flow.     

Anthropogenic impacts recorded in the sediments of Lunawa, a small tropical estuary, Sri Lanka

The Lunawa Estuary has been selected as a model for studying the long-term diagnoss of tropical estuarine pollution and its impact on ecosystems. Surface and short-core sediments from the Lunawa Estuary were collected, and analyzed for total organic carbon (TOC), nitrogen, sulfur and hydrocarbon contents, hydrogen index and other related sediment properties. Water quality and surface sediment data indicate that the Lunawa Estuary is presently an anoxic water body. The upper part of the sediment cores, characterized by very high TOC values of more than 10%, and absence of bioturbation along with strong laminations indicate anoxic water since 1970 AD, determined by ¹⁴ C dating. However, before 1970 AD, lower TOC content and presence of bioturbation and homogenous sediments with animal traces suggest the presence of oxic bottom water and thus a much better physicochemical condition. The lower part of the cores may also suggest the influence of gradual global warming. Regular laminations in the upper portions of the cores probably resulted from monsoon pulses with material preserved under anoxic condition.     

Recent sedimentary development of Tampa Bay, Florida: A microtidal estuary incised into tertiary platform carbonates

Tampa Bay, a large, microtidal, clastic-filled estuary incised into Tertiary carbonate strata, is the largest estuary on Florida’s west coast. A total of 250 surface sediment samples and 17 cores were collected in Tampa Bay in order to determine the patterns and controlling factors governing the recent infilling and modern sediment distribution, and to examine the results in terms of current models of estuarine sedimentation and development. Surficial sediments in Tampa Bay consist of three facies types, each occurring in a distinct zone: modern terrigenous clastic muds occurring in the upper bay and around the bay periphery; relict, reworked-fluvial, quartz-rich sands occupying the open portion of the middle bay; and modern carbonate-rich, marine-derived sands and gravels occupying the lower bay. Factors controlling sediment distribution include: sediment source and supply rate; bathymetry, which is a function of the antecedent topography; and the winnowing effect of wind-generated waves that prohibits modern accumulation in the shallow middle bay. These factors also play a major role in the recent infilling history of Tampa Bay, which has progressed in four stages during the Holocene sea-level rise. Recently developed models of estuarine sedimentation are based primarily on mesotidal to macrotidal estuaries in terrigenous clastic settings in which sedimentation patterns and infilling history are a result of the relative contribution of marine and fluvial processes. Tampa Bay differs in that it was originally incised into carbonate strata, and neither fluvial or marine processes are interpreted to be major contributors to modern sediment distribution. Tampa Bay, therefore, provides an example of an unusual estuary type, which should be considered in future modeling efforts. *** DIRECT SUPPORT *** A01BY083 00004     

Spatial and temporal variation of heavy metals in a tropical estuary

The lower part of the Coatzacoalcos River has great industrial development in which high pollution has been reported. Surface sediments of this area were studied over a year to observe spatial and seasonal sedimentological and chemical variations. Higher metal concentrations were found during the dry season, Pajaritos Dock and Teapa sampling points showing the highest values. Ni (182 ppm) and Co (37 ppm) concentrations are elevated relative to other estuarine areas. Metal normalization against Al₂O₃ content shows contaminant input in areas where urban and industrial discharges exist. A cluster analysis of the data separates sediment samples from the different sampling trips, suggesting the dynamic nature of the sediments. Through the statistical t-test, metal enrichment of Cd, Cu, Cr, Ni, Pb, and Zn was evaluated and compared with metal concentration in sediments of the river higher parts. The percentage of bioavailable metals is a function of the physicochemical characteristics of the system.     

沉积分异作用与河口海洋沉积

沉积分异作用在解释表层沉积现象时具有重要意义。在水流辐聚或辐散区域,表层沉积物将因辐聚和辐散运移而分别呈细化和粗化状态。河口区细颗粒沉积区的形成,是因为河口区存在底层余流辐聚的滞流区现象;东中国海涡旋区泥质沉积现象的形成,是因为涡旋区存在指向涡旋中心的底层辐聚流;浙闽沿岸泥质沉积现象是近岸的浙闽沿岸流和远岸的台湾暖流之间存在余流辐聚带;英吉利海峡宽度最窄处的沉积物粗化现象,是因该处沉积物在水流作用下呈分离运移趋势所引起。     

Distribution of carbon in a tropical hypersaline estuary,the Casamance (Senegal,West Africa)

The Casamance estuary, on the coast of Senegal, is an inverse hypersaline estuary: salinity increases landward, and dry season salinity values are up to 172 psu due to the evaporation of seawater. Dissolved inorganic carbon (DIC) concentrations decreased landward as a negative linear function of salinity. Thermodynamic modelling and the absence of CaCO₃ in the sediments indicate that this loss of DIC was not due to calcite precipitation in the main water body. The innermost, almost landlocked, waters contained high phytoplankton biomass (50–300 μg chl I^?1) and high concentrations of allochthonous dissolved organic carbon. Photosynthetic uptake of DIC and subsequent particulate organic carbon sedimentation is proposed as hypothetical explanation of the relationship between DIC and salinity; localized overheating in shallow waters might also be involved.     

Sedimentation in a disequilibrium river-dominated estuary: the Raša River Estuary (Adriatic Sea, Croatia)

This paper examines sediment transport, sedimentation and properties of suspended matter and sediments in the Ra?a River estuary, a small, rock-bounded, microtidal, low-wave-energy karstic estuary in the north-eastern Adriatic. The Ra?a River is characterized by large variation in water flow and variable load of mineral particles. More than 90% of this load is brought into the estuary as fine-grained suspended matter, consisting of only 24–36% of carbonates, the rest being clays. Sedimentation occurs at the salt wedge, resulting in a prograding estuarine delta. Salt-induced flocculation is the predominant process of sediment deposition. The Ra?a estuary is infilling with sediment, and classifies as a disequilibrium estuary. We propose a modification of Cooper's (1993) classification scheme to include river-dominated, disequilibrium estuaries, with the Ra?a River as an example.     

Mobilization of trace metals in a tropical turbid estuary influenced by a monsoon season

The selective removal of trace metals by suspended matter in high turbidity zones plays a major role in the fluvial transport of terrigenous metals to the marine environment. The seasonal longitudinal variability of trace elements (Cu, Zn, Cd, Ni, Pb, Fe, and Mn) in Cochin estuary, a tropical positive estuary, was studied and the results were compared with the prevailing situation in other subtropical waterways. The hydrodynamical features showed increasing turbidity downstream with increasing salinities during both the seasons. In contrast with the temperate estuaries where the development of turbidity maxima causes the removal of metals, the estuaries of tropics modify the fluvial transport of metals by the way of redistribution between the dissolved and particulate fractions in the intermediate salinities. In Cochin estuary, the distributional features of trace metals are primarily influenced by the variations in salinities and river discharges. Consequently, this gives rise to two different types of distributional patterns: (1) during premonsoon, the estuarine reactivity is more pronounced and hence, mid-estuarine solubilization of the particulate metal appears to play a prominent role in controlling the fluxes of trace metals studied and (2) but during monsoon, the hydrological conditions influence the downstream transport of the metals more by physical dilution than chemical reactivity.     

The transport of Osmium and Strontium isotopes through a tropical estuary

We have determined the concentration and isotopic composition of Os and Sr in the estuarine waters from the Godavari delta in Peninsular India. Additionally, we have obtained the concentration and isotopic composition of Os and Al concentration in selected suspended particulate matter recovered on 0.45 μm filters. The Na, K, Mg, and Ca concentrations of water samples obtained along salinity gradients from two distributary channels in the delta display a general two component mixing between river- and sea-water. The data also reveal that Al behaves non-conservatively and is affected by interactions with suspended particulates. The ⁸⁷Sr/⁸⁶Sr ratio of the riverine end member is 0.716303 and shows a linear decrease with salinity to seawater value and Sr isotope systematics indicate that its behavior is conservative in the estuary.The ¹⁸⁷Os/¹⁸⁸Os ratio of the Godavari river end-member is 1.24 and within error of the average eroding upper continental crust. The concentration and isotopic composition of Os through the two salinity transects shows that its behavior in the Godavari estuary is complex and non-conservative. By comparing the Al/Os ratios and Os isotopes in the waters with those of the suspended particulate we find that both Os gains and losses occur in the water column. However, in one of the distributaries (Vasishta) the Os concentration of suspended load increases and that of dissolved load decreases with increasing salinity towards the Bay of Bengal end-member. We infer that there is removal of seawater Os at higher salinities. The estimated mean residence time of Os in the oceans is 37 ± 14 (2σ) kyr. A comparison of the Os concentration of the Bay of Bengal and the Indian Ocean waters indicates that the rainout rate of Os in Bay of Bengal is 30% faster than that in the open ocean and suggests that the observed discrepancy between the mean residence time calculated from mass balance considerations and that estimated from the relaxation of the Os isotopic ratio in marine record may not be real as the relaxation time experiments likely estimate the residence time for a basin/sub-basin and not for the entire ocean.     

Spatial sediment variability in a tropical tide dominated estuary: Sources and drivers

Surficial sediment composition and spatial variability reflect the dynamics and level of natural and anthropogenic impact in estuarine systems. The aim of this study is to evaluate some key environmental and sedimentary variables, as well as to evaluate the current environmental quality in the tide-dominated Caravelas estuarine system (Brazil). Sixty-five surficial sediment samples were studied for grain size as well as calcium carbonate, total organic carbon, total nitrogen, metals, semi-metals and rare earth elements and for organic matter δ¹³C. Based on the spatial distribution of these data and summarizing our findings, three sectors can be individualized in the Caravelas estuarine system: i. Barra Velha inlet; ii. the main channel of the Caravelas estuary, and iii. the channel interconnecting Caravelas and Nova Viçosa estuaries. In the inlet the sedimentary organic matter is provided essentially by marine sources, which corroborates the small continental input. Freshwater dissolved organic carbon and particulate organic carbon occur in the interconnecting channel, proving the occurrence of the residual transport towards the Caravelas estuary, with continental contribution towards Caravelas River. The geochemical fingerprint indicates that the Barreiras Group, composed by Neogene terrigenous deposits, located west of the study area, as an important source of terrigenous material to the entire Caravelas estuary system. The higher values of heavy rare earth elements (between 20 and 30 mg kg⁻¹) in the northern sector of the study area may be related to the high degree of chemical weathering in magmatic or metamorphic rocks, which occur to the north of the study area.