The process of sedimentation on the surface of a salt marsh

An unditched salt marsh-creek drainage basin (Holland Glade Marsh, Lewes, Delaware) has a sedimentation rate of 0·5 cm year^?1. During normal, storm-free conditions, the creek carries negligible amounts of sand and coarse silt. Of the material in the waters flooding the marsh surface, over 80% disappears from the floodwaters within 12 m of the creek. About one-half of the lost material is theoretically too fine to settle, even if flow were not turbulent; however, sediment found on Spartina stems can account for the loss.The quantity of suspended sediment that does reach the back marsh during these normal tides is inadequate to maintain the marsh surface against local sea level rise. This suspended sediment is also much finer than the deposited sediments. Additionally, remote sections of low marsh, sections flooded by only the highest spring tides, have 15–30 cm of highly inorganic marsh muds.This evidence indicates that normal tidal flooding does not produce sedimentation in Holland Glade. Study of the effects of two severe storms, of a frequency of once per year, suggests that such storms can deposit sufficient sediment to maintain the marsh.The actual deposition of fine-grained sediments (fine silt and clay) appears to result primarily from biological trapping rather than from settling. In addition, this study proposes that the total sedimentation on mature marshes results from a balance between tidal and storm sedimentation. Storms will control sediment supply and movement on micro- and meso-tidal marshes, and will have less influence on macro-tidal marshes.     

Similarities between Mn,Ni and Co contents of deep-sea clays and manganese nodules from the south-west region of the North Atlantic

Recently published distribution patterns have shown that in the Atlantic Ocean Mn, Ni and Co tend to be concentrated in those deep-sea sediments deposited in a central zone which extends down the length of the ocean. The present paper shows that these trace elements are also concentrated in certain clays from the south-west region of the North Atlantic. This region is rich in manganese nodules, and analyses reveal that the non-lithogenous fractions of some of the clays approximate to those of the nodules in composition. It is suggested that these clays have acquired their relatively high concentration of non-lithogenous trace elements from sea water by processes associated with the formation of manganese nodules.     

Sedimentary facies and paleoenvironmental interpretation of a Holocene marsh in the Gironde Estuary in France

1Introduction Since the last glacial maximum, the Holocenehas been marked by a rapid rise in sea level. After6 000 a BP, the present-day level was reached andthe rate of sea-level rise (SLR) decreased rapidly(Morzadec -Kerfourn, 1974; Kidson, 1986 ).These…     

Arsenic incorporation in a salt marsh ecosystem

Replicate portions of a Delaware salt marsh were enclosed in cylindrical microcosms and exposed to elevated levels of inorganic arsenic (arsenate). All biotic and abiotic components in dosed cylinders rapidly incorporated arsenic. Spartina blades showed the greatest arsenic enrichment, with dosed plants incorporating arsenic concentrations an order of magnitude higher than controls. Spartina detritus and sediments also exhibited greatly elevated arsenic concentrations. Virtually all of the arsenic was incorporated into plant tissue or strongly sorbed to cell surfaces. Thus, elevated arsenic concentrations in estuarine waters will be reflected in living and non-living components of a salt marsh ecosystem, implying that increased arsenic will be available to organisms within the marsh ecosystem.     

中印度洋海盆富稀土沉积地球化学特征及富集机制研究

本文对中印度洋海盆深海富稀土沉积区两根柱样GC02和GC06开展了沉积物涂片观察,X-射线衍射分析,主量、微量和稀土元素分析,以及单矿物原位微区地球化学分析等,探讨了其地球化学特征、物质来源及稀土元素(REY)的富集机制。结果表明,GC02柱状沉积物类型为钙质黏土和沸石黏土,GC06柱状沉积物类型为钙质黏土、含沸石黏土和沸石黏土。稀土元素主要在含沸石黏土和沸石黏土中富集。北美页岩标准化(NASC)配分模式指示沉积物的REY主要来源于海水,矿物学和地球化学等特征表明该地区沉积物陆源组分可能主要源于澳大利亚的风尘物质。元素相关性和CaO/P₂O₅比值等指示了深海富稀土沉积中REY的主要赋存矿物为生物磷灰石(鱼牙/骨等),其次为铁锰微结核。本文总结和探讨了深海富稀土沉积的形成机制,完善了深海富稀土沉积形成过程的概念模型。     

Temporal variations of sedimentary sulfur in a Delaware salt marsh

The cycling of sedimentary sulfur was examined over a one year period in the Great Marsh, Delaware (U.S.A.) using newly developed analytical procedures. Iron monosulfide (FeS) and elemental sulfur both display large seasonal changes in concentration and distribution with depth, indicating a coupling with marsh redox conditions. In contrast, the depth distribution and concentration of greigite (Fe₃S₄) did not show appreciable changes with season. Pyrite (FeS₂) underwent large concentration changes in the upper 15 cm of sediment during the spring, but remained relatively constant with respect to concentration and distribution below this zone. Using a mass balance approach in the upper marsh sediment, sulfur needed for rapid pyritization is found to be derived from elemental sulfur, iron monosulfide and sulfate reduction. In the deeper sediments, pyritization occurs through a greigite intermediate, and diagenetic modeling indicates that pyrite formation is limited by the synthesis of greigite, and not by the conversion of greigite to pyrite.     

Behaviour of iron,manganese, phosphate and humic acid during mixing in a Delaware salt marsh creek

The mixing behaviour of iron, manganese, phosphate and humic acid in a Delaware salt marsh creek was studied using field data, laboratory mixing experiments, and geochemical mass balance equations. Property-salinity diagrams for field data indicated that the removal of iron is 56–70% in the 0–10‰ salinity range. A proposed mechanism of removal is the flocculation of colloidal iron, perhaps with humic acid. Phosphate, however, undergoes 195% addition in the same low salinity region, which may be due to release of phosphate from resuspended sediments. Dissolved manganese is conserved, as is humic acid throughout the salt marsh mixing zone. Within the uncertainty of the data the maximum possible removal of humic acid is 23%.Laboratory mixing experiments that simulated salt marsh mixing along the same salinity gradient as observed in the field (5–25‰) showed only small-scale additions and removals compared to the field results. Such small-scale changes occurred largely at salinities >10‰ in the laboratory experiments, whereas most removals and additions occurred at salinities <10‰ in the field. Mixing studies also showed little difference between prefiltered and unfiltered mixes. The studies suggest that simple mixing of salt marsh waters, with or without suspended material, does not strongly influence the observed behaviour of dissolved constituents in salt marshes, and that other processes (e.g. sediment or intertidal exchange) must dominate their behaviour.     

Temporal and spatial variability of sea level and volume flux in the Murderkill Estuary

The instantaneous sea level determined at two sites in the Murderkill Estuary, a tributary of Delaware Bay, results from the superposition of temporal variability operating over different time and spatial scales. Over the relatively short tidal time scales, the semidiurnal tides that represent the dominant tidal constituents in lower Delaware Bay show a modest increase in tidal amplitudes from the bay mouth (Lewes, Delaware), up to Bowers Beach (the mouth of the Murderkill Estuary). However, as the tides propagate into the Murderkill Estuary, the semidiurnal constituents undergo heavy attenuation, resulting in a 48% reduction in tidal amplitude from Bowers to Frederica (approximately the extent of saline intrusion). The diurnal tide, on the other hand, experiences only a 25% reduction in amplitude. The limited tidal asymmetry that is observed may be a result of interaction between flows in the tidal channel and the adjacent salt marsh. At longer time scales, the subtidal sea level experiences no attenuation. The Murderkill Estuary thus behaves like a low pass filter to preferentially damp out high frequency sea level forcing from lower Delaware Bay. The subtidal volume flux in the Murderkill is highly coherent with the time rate of change of sea level, indicating that the Murderkill basically co-oscillates with Delaware Bay in a standing wave fashion over the subtidal time scale. This remote coupling controls more than 90% of the variance in subtidal sea level in the estuary. The surface slopes in the lower bay and the Murderkill Estuary are closely correlated with winds along the orientation of the two waterways, consistent with the effect of local wind on subtidal sea level.     

Sea level induced variations in clay mineral composition in the southwestern South China Sea over the past 17,000 yr

Variations in clay mineral composition of sediment cores from the margin and continental slope of the Sunda Shelf (southern South China Sea, SE Asia) covering the past 17,000 yr reflect changing influences of sediment sources together with clay mineral partitioning processes in shallow waters. We identify the deglacial sea level rise as the principal factor driving these changes. During the late glacial, high values of kaolinite are interpreted to reflect a higher contribution of clays from soils that have formed on the exposed Sunda Shelf and in the southern archipelagos of Indonesia. At this time core sites were located in close proximity to the mouths of the Sunda Shelf palaeo-drainage systems on the emerged shelf (“Sundaland”). The progressive landward displacement of the coastline and breakdown of these vast drainage systems during deglaciation led to a decrease in influence of the kaolinite-rich southern sources. When the coastline had retreated closely to its present-day position in mid-Holocene times, the former dominance of southern sources was replaced by a stronger influence of illite-rich sources (e.g. Borneo). The overriding control of sea level changes on the clay mineral distribution patterns precludes a definite climatic interpretation of clay mineral data in terms of climatic/monsoonal changes in such highly dynamic sedimentary environment.     

Pb,Zn, Cs,Sc and rare earth elements as tracers of the Loire and Gironde particles on the Biscay shelf (SW France)

The western Atlantic continental margin of the Bay of Biscay is characterised by a combined macro-tidal and a high energy environment. These conditions are responsible for the essentially sandy sedimentary cover along with well-defined fine deposit zones observed on the shelf. The main continental sources are the Loire, Gironde and Adour Rivers. It is estimated that 2.4 to 2.5 million tons of fine sediments are supplied annually to the ocean from these rivers of the French Atlantic coast. The Gironde and the Loire contribute with 84 % and the Gironde with 60 % of this amount. Previous studies of the distribution of the clay mineral assemblages and of oligo-elements have distinguished two provinces (north and south Biscay areas) according to these riverine sources. The use of an inductively coupled plasma-mass spectrometry (ICP-MS) technique allowed us to analyse both heavy metals and rare earth elements (REE), thus permitting an effective characterisation of continental sources. On the basis of these results, it has been possible to better identify the respective contributions of the Loire and the Gironde Rivers to the fine-grained sedimentation on the inner shelf.     

长江口外缺氧区柱样沉积物元素的分布及其百年沉积环境效应

在210Pb定年的基础上,对取自长江口外缺氧区内外的柱样沉积物开展了10种常量元素、13种微量元素和粒度的测定分析,研究了其物源及分布特征。结果表明,缺氧区外柱样沉积物主要来源于老黄河口海岸泥沙,大部分元素具有"粒度控制"规律。缺氧区内沉积物主要来源于夏季长江陆源的输入和海洋自生生物死亡后的沉降,部分氧化还原敏感元素(RSE)和亲生物元素不受控于"粒控效应",其中Mo、Cd、As等氧化还原敏感元素自20世纪70年代以来明显富集,分别增加了83%、73%和50%,而Mn出现贫化,指示了缺氧区水体富营养化加剧和底层水体季节性缺氧,引起底层水-沉积物界面氧化还原环境变化;亲生物元素Ca、Sr、P含量自20世纪70年代起分别增加了129%、65%和38%,反映了受化肥使用等人类活动影响,近40年来长江口外水体生产力提高和生物量增加。     

Sedimentation processes on the continental rise of northeastern South America

A section of the continental rise of northeastern South America northeast of the Orinoco delta contains physiographic features built by the interaction of southward-flowing North Atlantic Deep Water and turbidity currents generated in the Orinoco region during the last Pleistocene glacials. A sedimentary outer ridge of low relief (Demerara Outer Ridge) trends northeast along the rise and a field of westward-migrating sediment waves trending north-northwest is superimposed on the outer ridge. The sediment waves have a maximum amplitude and wavelength of 20 m and 4 km, respectively. Seismic profiler records indicate that the outer ridge was probably built during the Pleistocene. A major turbidity-current pathway adjacent to the outer ridge on the north supplied sediment to the southward-flowing North Atlantic Deep Water which then deposited this sediment down-stream on the outer ridge and formed the sediment waves. Piston cores from the outer ridge contain numerous silt—sand beds and appear to be contourites. The cores consist primarily of gray hemipelagic clay of a Late Wisconsin age and have high ($\text{>}\text{10}\text{cm}\text{1000}\text{yrs}$) sedimentation rates. In contrast, cores from the continental rise north of the turbidite channel are brown clays with relatively low sedimentation rates ($\text{3.0}\text{cm}\text{1000}\text{yrs}$) and do not contain silt—sand contourites.     

Sources and timing of trace metal contamination to sediments in remote sealochs,N.W. Scotland

Sediment cores from a transect of sealochs, Etive, Sunart, Nevis, Duich, Broom and Torridon in N.W. Scotland were analysed for the trace metals lead, copper, and zinc. In addition, sediment cores from the Clyde Sea Basin (Loch Fyne, the Gareloch, Clyde dumpsite) and from the Irish and Celtic Seas were analysed as possible source areas for contaminated particles. The sediment cores were dated using excess ²¹⁰Pb and ¹³⁷Cs, and Al normalisation was used to correct for sediment compositional effects. All cores showed increased concentrations of all trace metals (and M/Al) towards the surface, with a maximum of 280–500% above background being observed in Loch Etive. Only cores from Duich and Torridon (Zn/Al and Pb/Al) and Etive (Zn/Al) did not continue to show an increase in trace metals to the uppermost samples. The burden of atmospherically derived trace metals accumulated over the past 100 years was compared with similar published burdens from a series of freshwater lochs adjacent to the sealochs. Plots of excess ²¹⁰Pb (used to correct for sediment focusing) against atmospherically derived Pb showed a series of linear relationships of systematically decreasing gradient towards the north-west, away from potential urban sources of lead. It was calculated that 49% (15–77%) of the Pb but only 3% (1–4%) of the Zn and 2% (0–3%) of the Cu reached the sealochs from atmospheric sources. The remaining trace metals deposited in the sealochs came from marine particles. Using trace metal ratios as an indicator of pollutant provenance, it was shown that the trace metals came predominantly from the Irish Sea. There was no evidence of trace metals derived from the Firth of Clyde basin. It was concluded that trace metal contamination from the Clyde and the surrounding industrialised urban area was mainly deposited in the estuary and the Clyde Sea basin with its adjoining sealochs, such as the Gareloch and Loch Fyne.     

Geochemistry of eastern Mediterranean sediments: Primary sediment composition and diagenetic alterations

Two cores recovered in the eastern Mediterranean were analysed for major, minor and trace elements. The primary chemical composition of the sediment is different at each location, probably because the lithological sources and the relative biogenic contributions differ.

Carbonates are important for the concentration of Ca, Mg and Sr, whereas aluminosilicates determine the concentration of Si, Al, K, Li, Y and Be, and to a lesser extent that of Fe, Cr, Ti, Mg, Zn and Zr. In sapropels, organic carbon and sulphur seem to be closely related. Bromine, Mo, P, Fe, V, Cu, Zn, Co, Ni and Cr are closely associated with organic and sulphidic compounds. The concentration versus depth profile for organic carbon in two sapropels points to a rapid establishment of conditions that gave rise to sapropel formation, followed by a gradual transition back to “normal” conditions.

The primary composition is overprinted by diagenetic processes. Sulphate-reducing conditions occurred during and just after sapropel deposition. A progressive oxidation front mechanism, which became active after sapropel deposition, is responsible for additional major geochemical changes. This diagenetic phenomenon has strong implications for the chemistry of Fe, Mn, Ni, Co, Zn, Cu, Cr, V, U, As and Sb.     

A study of indication of trace metal pollution of marine areas by analysis of salt marsh soils

Salt marsh soils from the mean water level and from different levels above the mean water level at two Danish coastal lagoons, which differ in water pollution due to differences in the character of the surrounding land use, were analysed for EDTA extractable Pb, Cu, Zn and Ni and for organic matter and cation exchange capacity. From the results a positive correlation was apparent between the trace metal loading of the soils, expressed by the trace metal concentration in relation to the content of soil organic matter, and the mean trace metal concentration of the lagoon water, submerging the salt marsh at high water. This opens up the possibility of using analyses of salt marsh soils in the indication of trace metal pollution of protected coastal waters.     

Trace-element distribution in the continental-shelf sediments off the east coast of India

Thirty continental-shelf clays sampled off the east coast of India have been analysed spectrographically to determine the concentrations of Ge, Ga, Pb, Cu, Mo, Sn, V, Ni, Co, Zr, Cr, Ba, Sr and B. Most of these elements are detrital in origin and are primarily held up in clay minerals or adsorbed onto the clays in the nearshore mixing zone. Organic matter influences the concentrations of Ge, Sn, and to a lesser extent, Ga and Pb in the shelf.     

Biogeochemical factors influencing the residence time of microconstituents in a large tidal estuary, Delaware Bay

Generally the large tidal estuaries of the eastern United States, such as Delaware Bay, are characterized by rather high suspended particle concentrations at the landward end and high biological activity at the seaward end. As such, these estuaries can be conceptualized as geochemical and biochemical “reactors” for those processes controlling the transmission of trace elements from fresh to the coastal shelf waters. The efficiency of these reaction processes relative to estuarine flushing will control the residence times of microconstituents in such estuaries.Evidence is drawn from the Delaware estuary to illustrate biogeochemical estuarine reaction processes using salinity distribution data and mass balance calculations. The Delaware retains some of its estuarine trace elements as sedimented estuarine particles, while others are more conservative and largely exported. Those retained by sedimenting processes include trace elements in primarily geochemical (particle reactive) chemistries, while those exported appear recycled by biochemical (nutrient reactive) chemistries. Often, the behavior of trace elements (e.g., Fe, Cd) and nutrients (e.g., PO₄) appear biogeochemically linked. Other examples are drawn from mixing studies to illustrate particle interaction, and benthic flux measurements to illustrate limited diagenetic reflux.The residence time of estuarine microconstituents should depend seasonally on the relative turbidity, flushing rate, and primary production of tidally dominated estuaries such as the Delaware. Thus, residence times of the more biogeochemically reactive microconstituents must be substantially shorter (days to weeks) than the average flushing times of these larger estuaries, while the residence times of the less reactive ones should approach such flushing times (weeks to months). True estuarine residence times of microconstituents can only be modeled after using large data sets averaged over time (season, tides) and space (salinity).     

Seasonal differences in Spartina recoverable underground reserves in the Great Sippewissett marsh in Massachusetts

Recoverable underground reserves (RUR) were collected from cores taken from stands of three growth forms of Spartina alterniflora and one of Spartina patens in a Massachusetts salt marsh. Collections were timed to sample reserves before spring growth, following the flush of early season growth, and following senescence in the fall. In the spring, streamside Spartina alterniflora had more RUR than did the short plants further back from the streams. When compared to reserves of Spartina alterniflora in back marsh stands in Delaware and Georgia, those at the higher latitude were greater. Reserves were very low during summer, but were restored by fall. Unlike Spartina alterniflora, Spartina patens RUR remained high during the summer.     

Physico-chemical and mineralogical studies on cochin marine clays

Large deposits of marine clays are encountered all along the Indian coastal belt. These clays are pleistocene to recent in origin, are considered to be young, and were deposited in a salt or brackish environment. These clays are very soft in consistency with low in-situ strength and high compressibility. The properties of these soil deposits depend mainly on the clay minerals present. In the present investigation, the mineralogical studies of some Cochin marine clays were carried out using XRD technique. The physical and chemical properties of these deposits were also reported. The test results were compared with some earlier reported works on marine clays.     

Hypothesis for a submarine landslide and cohesionless sediment flows resulting from a 17th century earthquake-triggered landslide in Quebec,Canada

Textural isotopic and microfossil data from two gravity cores obtained in Saguenay Fjord, Quebec, suggest that a distinctive sandy clay bed was deposited as the result of a major landslide in the Saguenay River basin. Pb-210 dating of the cores indicate that the bed is of similar age to the magnitude 7 earthquake of February 5, 1663. The slide involved sensitive marine clays and may have occurred in two stages. Slide sediments carried into the Saguenay River channel were probably reworked and subsequently transported down the Fjord basin as two distinct cohesionless mass flows. Fine clay laminae that overlie the older mass flow bed record the modulation of depositional processes by tidal currents for several weeks after this event.