Seabed Sediments Research Articles

Oxidation-state distribution of plutonium in surface and subsurface waters at Thule, northwest Greenland

The speciation of plutonium in Arctic waters sampled on the northwest Greenland shelf in August 1997 is discussed in this paper. Specifically, we report the results of analyses carried out on seawater sampled (a) close to the Thule air base where, in 1968, a US military aircraft carrying four nuclear weapons crashed on sea ice, releasing kilogram quantities of plutonium to the snow pack and underlying seabed sediments, and (b) at a reference station (Upernavik) located ∼400 km to the south. The data show that most of the plutonium in the dissolved phase at Thule is in the form of Pu(V, VI) (mean: 68±6%; n=6), with little if any distinction apparent between surface and bottom waters. Further, the oxidation state distribution at stations close to the accident site is similar to that measured at Upernavik, remote from this site. It is also similar to the distribution observed in shelf waters at mid-latitudes, suggesting that the underlying processes controlling plutonium speciation are insensitive to temperature over the range 0–25°C. Measurements using tangential-flow ultrafiltration indicate that virtually all of the plutonium (including the fraction in a reduced chemical form) is present as fully dissolved species. Most of this plutonium would seem to be of weapons fallout origin, as the mean 238Pu/ 239,240Pu activity ratio in the water column (dissolved phase) at Thule (0.06±0.02; n=10) is similar to the global fallout ratio at this latitude (≈0.04). Thus, there is little evidence of weapons-grade plutonium in the water column at Thule at the present time.

The Effect of CO2‐Air Mixture Compositions on the Formation and Dissociation of CO2 Hydrate

Abstract: The disposal of carbon dioxide to the marine and sea bed sediments as CO2 gas hydrate is an innovative technique for solving the global environment issue. Experiments on the formation and dissociation of gas hydrate have been carried out using a pressure vessel to investigate the effect of carbon dioxide concentration in the gas phase. From the experiment results, the following are clarified: (1) There is a strong relationship between the partial pressure of carbon dioxide, concentration, and the temperature of formation and dissociation of gas hydrate. Therefore, the use of this relation enables the estimation of equilibrium conditions of the gas mixture. (2) The initial formation rate varies from 0.1 to 0. 5 ml/(min·g). In terms of average values, the initial formation rate increases as the carbon dioxide concentration of the initial gas mixture increases. (3) From the analysis of component gas of gas hydrate and space gas, it can be assumed that nitrogen and oxygen are also incorporated into the hydrate structure cage as guest molecules. Moreover, it can be seen that the carbon dioxide concentration in the initial space gas is higher than that in the space gas at the time of gas hydrate formation. Therefore, this hydrate technology applies to the concentration of carbon dioxide. In future, we will attempt to carry out tests on the formation and dissociation of CO2 hydrate under a low concentration of CO2. Furthermore, we will analyze the structure of gas hydrate using Raman spectroscopy to clarify that nitrogen and oxygen are incorporated into the gas hydrate cage as guest molecules.

混合ガスにおける二酸化炭素濃度の影響 二酸化炭素ハイドレートの生成解離に関する基礎的研究 Ｉ

The disposal of carbon dioxide to the marine and sea bed sediments as CO2 gas hydrate is an innovative technique for solving the global environment issue. Experiments on the formation and dissociation of gas hydrate have been carried out using a pressure vessel to investigate the effect of carbon dioxide concentration in the gas phase. From the experiment results, the following are clarified: 1 There is a strong relationship between the partial pressure of carbon dioxide concentration and the temperature of formation and dissociation of gas hydrate. Therefore, the use of this relation enables the estimation of equilibrium conditions of mixture gas. 2 The initial formation speed varies from 0.1 to 0.5 ml / (min. g). Considering, the average values of the initial formation speed, the initial formation speed increases as the carbon dioxide concentration of the initial mixture gas also increases. 3 From the analysis of component gas of gas hydrate and space gas, it can be assumed that nitrogen and oxygen are also incorporated into the cage of hydrate structure as guest molecules. Moreover, it can be seen that the carbon dioxide concentration in the initial space gas is of higher level than the carbon dioxide concentration in the space gas at the time when the gas hydrate grew up. Therefore, this hydrate technology applies in the concentration of carbon dioxide. In the near future, we will attempt to carry out tests on the formation and dissociation of CO2 hydrate under a low concentration of CO2. Furthermore, we will analyze the structure of gas hydrate using the Raman spectroscopy to clarify that nitrogen and oxygen are incorporated into the cage of gas hydrate structure as guest molecules.

Study on steel corrosion in different seabed sediments

A series of simulation experiments on carbon steel (A3 steel) and low alloy steel (16 Mn steel) in marine atmosphere (MA), seawater (SW) and seabed sediment (SBS) including rough sea sand, fine sea sand and seabed mud were carried out indoors for a year or so by means of individually hanging plates (IHP) and electrically connected hanging plates (ECHP). The corrosion of steels in SBS was mainly due to the macrogalvanic cell effect. The steel plates at the bottom of SBS, as the anode of a macrogalvanic cell, showed the heaviest corrosion with a corrosion rate of up to 0·12 mm/a, approximately equal to that of steel plates in marine atmosphere.The test results showed that the corrosion rates of A3 and 16 Mn steel in marine environment were in the order: MA>SW>SBS by the IHP method; and MA>SBS>SW by the ECHP method. The corrosion rates of steels in the water/sediment interface were directly proportional to the grain size of the SBS by the ECHP method, but those of steels in the water/sediment interface did not vary with the grain size of SBS by the IHP method. The corrosion rate of low-alloy steel was a little higher than that of carbon steel. The results of this study have important applications for design of offshore steel structures such as oil platform, pier, and port.

Evaluation of the impacts of land-based contaminants on the benthic faunas of Jakarta Bay, Indonesia

To evaluate the impact of land-based contaminants on benthic faunas, as part of the Land-Ocean Contamination Study (LOCS), a study transect was established in Jakarta Bay, Indonesia during the 1996 southeasterly monsoon. The transect extended 72 km between the city's main port, Tanjung Priok, and the Pilau Seribu Island chain in the Java Sea. The dissolved concentrations of Pb, Cu, Zn, Cr and Ni in seawater, in suspended particulate matter (SPM) and surficial sea bed sediments were measured along the transect at nine sites. In addition, metal concentrations were measured in tissues of the corals Goniopora lobata and Lobophyllia hemprichii at five sites, and of the green mussel, Perna viridis at six sites. An assessment of the impact of contaminants on the faunas was made using a biomarker approach, employing coral community analysis and lysosomal stability with Perna viridis. The results of the study showed two main trends. The distributions of metals dissolved in seawater, in SPM, and in the coral and mussel tissues were similar, and failed to show a consistent graded response from inshore to offshore sites. This suggests that the concentration of metals in waters is the primary route for metal uptake by the coral and mussel tissues. By way of contrast, a clear offshore increase in coral generic diversity, coral cover, coral colony numbers and neutral-red retention time (lysosomal stability) was observed, suggesting increasing nearshore stress. Whilst the coral community stucture may reflect the seasonally-averaged metal distributions in the bay (shown by the metal content of sea bed sediments which increase shorewards), it is more likely that the coral community structure and lysosomal biomarker are responding principally to other nearshore stresses, such as sediment and nutrient loading of water (sewage) or, more likely, organic contaminants such as oils and other hydrocarbons.

Determination of caesium and natural radionuclide concentrations in sediments, algae and water on the Bulgarian Black Sea coast

Determination of anthropogenic and natural radionuclide distribution along the Bulgarian Black Sea coastal area has been performed by low level gamma spectrometry. Results were obtained for different types of environmental samples: sea bed sediments, eight types of algae species and sea water. Monitoring for 35 sampling sites has been performed from 1991 to 1996. A comparison between the results of nuclide content in algae and sediments was made. The data obtained show that radionuclide concentrations depend strongly on the sediment nature (sand, sandy, silt, slime). Small variations of nuclide content in sand and sandy sediments are obtained, while the variations for slime and silt sediments are greater. The concentrations of artificial and natural gamma emitters in algae depend mainly on the algae species type. The content of most natural radionuclides in algae is close to the LLD except those for the species Bryopsis plumosa. The results for the nuclide concentration values in Black Sea sediments and algae will enable the creation of a database and assessment of radionuclide transfer and accumulation trends in marine ecosystems.

Determination of caesium and natural radionuclide concentrations in sediments, algae and water on the bulgarian black sea coast

Determination of anthropogenic and natural radionuclide distribution along the Bulgarian Black Sea coastal area has been performed by low level gamma spectrometry. Results were obtained for different types of environmental samples: sea bed sediments, eight types of algae species and sea water. Monitoring fur 35 sampling sites has been performed from 1991 to 1996. A comparison between the results of nuclide content in algae and sediments was made. The data obtained show that radionuclide concentrations depend strongly on the sediment nature (sand, sandy, silt, slime). Small variations of nuclide content in sand and sandy sediments are obtained, while the variations for slime and silt sediments are greater. The concentrations of artificial and natural gamma emitters in algae depend mainly on the algae species type. The content of most natural radionuclides in algae is close to the LLD except those for the species Bryopsis plumosa. The results for the nuclide concentration values in Black Sea sediments and algae will enable the creation of a database and assessment of radionuclide transfer and accumulation trends in marine ecosystems.

Radionuclide accumulation in near-shore sediments along the Bulgarian Black Sea coast

The accumulation of radionuclides in Black Sea marine ecosystems was investigated by low level gamma spectrometry. Artificial as well as natural radionuclides were determined in bottom sediments samples from 35 reference locations along the Bulgarian Black Sea coast, evenly distributed from the Rumanian to the Turkish border including the main Black Sea resorts and rivers. The measurement of radionuclides in sea bed sediments was carried out during six consecutive seasons using a HPGe detector. The data obtained show that the nuclide concentrations depend strongly on the sediment nature. Results for sandy sediments are within close range, while those for slime and silt vary to a much greater extent. The radionuclide content in the sandy sediments of the main Black Sea resorts is at the lowest limit of the determined values. Small seasonal changes of radionuclide concentration in sandy sediments were observed while greater variations in slime and silt occur. From the data obtained 134Cs/ 137Cs and 137Cs meas/ 137Cs Chern ratios are calculated to determine the Chernobyl part of the measured 137Cs. The activities determined in the sediments for natural radionuclides correspond to those cited in the literature for natural levels, showing no additional anthropogenic contamination. A data base for the nuclide concentration values was created which will enable the modeling of radionuclide transfers by estimation of their concentration variations, accumulation and influence on the marine ecosystems.

Consolidation and Permeability Behavior of High Porosity Baltic Seabed Sediments

Abstract The consolidation and permeability (hydraulic conductivity) characteristics of the high-porosity surficial seabed sediments from a test site in the Eckernförde Bay, Baltic Sea, were studied using a constant rate of strain (CRS) consolidation and hydraulic conductivity testing system employing flow pumps. These silty clay sediments are characterized by high void ratios, high organic content, and variable concentration of methane gas. Results from CRS consolidation tests show appreciable apparent overconsolidation of the sediments at the upper 200 cm with a gradual transition to normally consolidated states below 250 cm in depth. The highly compressible nature of the material is reflected in the large values of the compression index. The hydraulic conductivity of the box core samples ranges from 2.5 × 10−7 to 3.0 × 10−6 cm/s for void ratios ranging from 4.5 to 7.0 with a linear relationship between void ratio and log of hydraulic conductivity. The engineering behavior of these sediments is of practical significance in geoacoustic modeling of the seafloor for studies related to mine countermeasure programs.

Aggregation and resuspension of suspended particulate matter at a seasonally stratified site in the southern North Sea: physical and biological controls

The physical and biological characteristics of suspended particulate matter (SPM) have been investigated at a site in the southern North Sea characterised by a seasonally stratified water column and a muddy sand bed. Significant variability in SPM concentration, composition, size and settling velocity was observed over tidal, spring/neap and seasonal time scales. Tidal currents alone are not sufficient to resuspend the bulk of the sea-bed sediment. However, sea-bed erosion does occur when wind/wave action enhances the bed shear stress during winter storms. Also, during winter and summer aggregated organic-rich material with a modal settling velocity of 0.2 mm s -1, originating from the sediment/water interface or from pelagic organic detritus, is subject to spring-tide or wind/wave resuspension. Phytoplankton production in the surface mixed layer (SML) is nutrient limited during much of the summer, but there can be sufficient light penetration to allow production at or below the thermocline. So production in the bottom mixed layer (BML) can be either light or nutrient limited, and BML nutrient levels do not gradually recover during the summer. Aggregation of phytoplankton is an important process at this site, resulting in significant modification of settling behaviour and hence dynamics of SPM. Settling velocity and particle size measurements indicate that both diatom and dinoflagellate blooms form large aggregates towards the end of their growth phase, with modal settling velocities between 2 and 5 mm s -1. Nitszchia pseudoseriata aggregates were relatively stable, remaining as viable sites for primary production over several cycles of tidal resuspension and deposition. Moreover, they efficiently ‘scavenged’ fine inorganic particles from the water column as they settled out. Tidal resuspension of these aggregates results in much higher water-column residence times than would be calculated simply from settling rate and water depth. This allows a greater proportion of remineralisation to be performed in the water column, rather than within the sea bed.

Test of a new surface geochemistry tool for resource prediction in frontier areas

Data from a regional surface geochemistry survey in the North Viking Graben have been studied using principal components analysis, which allows process-specific fingerprints from extremely small amounts of thermogenic hydrocarbons in sea-bed sediments to be isolated and separated from data noise. Although the North Viking Grahem has thick, unfaulted Tertiary sediments with no obvious seepage conduits, the detected signals correlate with proved hydrocarbon accumulations in the area.

Bimodal wave spectra in lower Chesapeake Bay, sea bed energetics and sediment transport during winter storms

The transition zone separating estuarine environments from the coastal ocean is characterized not only by distinctive morphological and sedimentary trends but by unique hydrodynamic forces as well. Lower Chesapeake Bay, a large coastal estuary within the Mid-Atlantic Bight of the U.S. East Coast, experiences complex wave and current-induced forces produced during winter storms. Wave and current measurements made near Thimble Shoal Light over five winter seasons show that most storms simultaneously produce both ocean and bay-generated wave trains that appear as distinct bimodal peaks in directional spectra. Analysis of selected storm wave records reveal that lower-frequency ocean waves, although nominally lower in amplitude than higher-frequency bay waves, are roughly equivalent to bay waves in terms of energy expended on beds of fine- to medium-grained sand at either end of the Thimble Shoal Channel. Grain-friction energy dissipation estimates calculated for waves and currents suggest that waves provide more net energy capable of transporting bottom sediment than currents, although strong barotropic flows briefly encountered during a major storm on 13–14 March 1993, exceeded wave energy expended at the bed by almost an order of magnitude. From analyses of wave orbital velocity spectra, it is shown that dual wave trains characterized by differences in peak frequency and direction may assist each other through interactions that increase their combined contribution to frictional energy dissipation and inferred sediment transport at the bed.

Quantitative analysis of sea-bed sediments from Eastern Black Sea by EDXRF spectrometry

Sea-bed sediments from Eastern Black Sea have been analyzed for their Fe, Ba and Sr contents using the methods of standard addition and energy dispersive X-ray fluorescence (EDXRF). Samples were prepared from powder sifted by a 200 mesh sieve. An annular 100 mCi241Am radioactive source emitting 59.543 keV photons was used for excitation and a Ge(Li) detector, which has a 190 eV FWHM for 5.9 keV photons was used for intensity measurements. 2048 channels of the MCA are employed for 20 or 30 min. Dried and ground sea-bed sediments are found to contain 2.95–7.50% Fe, 0.032–0.268% Ba and about 73–273 ppm Sr. These results agree with those obtained via atomic absorption methods.

Net Sand Transport Direction in a Tidal Inlet, using Foraminiferal Tests as Natural Tracers

The relationship between the flux of exotic benthic foraminiferal tests (i.e. tests which are supplied from open-sea sources alone) in a tidal inlet and that of bulk sediment was analysed, which can be expressed as two first-order linear equations. According to this relationship, in order to determine net sediment transport directions in the entrance, the test concentration in surficial sediments of the tidal basin can be compared against a ‘critical level’. The critical level is determined for the conditions that no net transport of bulk sediment is present within the entrance. If the observed concentration (averaged over the tidal basin) is higher than the simulated critical level, then the net sediment transport is directed to landward. This method is applied to the analysis of net sand transport at Christchurch Harbour, a tidal inlet system located in southern England. In this investigation, concentrations of exotic foraminiferal tests in the surficial sediments of the tidal basin and ebb tidal delta area were obtained from the analysis of sea-bed sediment samples. A series of probable critical levels were calculated based upon the data sets with regard to: (1) sediment discharge from the rivers; (2) magnitude of sediment discharge within the entrance during the ebb; (3) the test concentration outside the harbour; (4) the thickness of the moving layer; and (5) two parameters associated with dispersive processes. The results show that the concentration in the tidal basin sediment is higher than a number of simulated critical concentrations for representative cases. Consequently, the high level of the concentration of exotic benthic foraminiferal tests within the harbour should be explained as a result of landward net transport of sands within the entrance.

Sedimentary processes and the creation of the stratigraphic record in the Late Quaternary North Atlantic Ocean

The primary difficulty in the interpretation of the stratigraphic record is that a multiplicity of sedimentary processes, some producing similar effects, are responsible for it. We seek to unravel the effects of the more important processes through analysis of sedimentary properties. The effects to be unravelled are those relating to pelagic input (vertical flux) due to organic productivity, wind-blown dust, ice-rafting and volcanic ash; to horizontal flux in turbidity currents, debris flows, and nepheloid layers caused by the reworking of sea-bed sediments by internal waves and bottom currents; and to degradation of the record by dissolution, oxidation and mixing of components. Contrasting regions of the North Atlantic are used to show the effects of bottom currents, ice-rafting, wind and productivity on sediments. Applications to estimates of changes in bottom currents, productivity and carbon sequestration in the N.E. Atlantic over the past 30 ka are given.

Extension of the British landmass: evidence from shelf sediment bodies in the English Channel

Abstract Mapping and interpretation of high-resolution shallow seismic and vibrocore sample data from a submerged, infilled valley immediately east of the Owers Bank, at −20 m to −40 m chart datum (CD), eastern English Channel, indicate sediment body formation during multiple cut-and-fill events. Interpretation suggests that these occurred in a combination of diachronous gravel-bed river, peat-land, estuarine and shallow sub-littoral environments with subsequent marine planation (forming ravinement surfaces) and accumulation of a sea-bed sediment veneer. These environments are associated with large-scale relative sea-level and (inferred) climatic changes during the Quaternary which forced subaerial inner continental shelf conditions during cold stages, repeatedly interrupted by interglacial shelf submergence. A model of sediment body formation off the south coast of England, based on the case study findings, shows that subaerial, and not submarine, processes have dominated sedimentary history within present-day, submerged, infilled valleys. Relative to the Quaternary as a whole, global ice-volume is presently unusually low, implying that present eustatic sea-level is unusually high. Subaerial linkage of present-day southern England with the adjacent inner shelf has therefore persisted for most of Quaternary time, with the notion of British insularity applicable to only a relatively small part of the period.

The problem of water-column multiples for processing converted S waves in marine VSP data

SUMMARY Seismic data from marine VSPs contain converted S, waves which can be used to provide information on the internal structure of the subsurface rocks. to make the best analysis of these waves it is necessary to choose a suitable offset to maximize the converted energy. Numerical modelling with a hard sea-bottom shows that even for the moderate offsets which provide the best conversions at the sea-bed in a shallow-water environment, it is not possible to jointly compensate the P and S waves for their effective seismic signatures using a single common scalar function. This is due to a difference in the periodicity of the water-column multiples associated with the two wavetypes. the difference becomes more pronounced for a large Poisson's ratio in the sea-bed sediments. Standard processing on the three-component recordings cannot be adequately performed except for subvertical incidence. Consequently, P and S waves must be separated before signature deconvolution, or else a more sophisticated multicomponent operator must be deployed. In the process of acquisition design, the detailed modelling required to understand the multicomponent response must be weighed up against the offset desired for optimal conversion.

The Diet of Grey Seals in the Inner and Outer Hebrides

1. Faecal material collected in January, June, August and November 1985 was used to investigate seasonal and regional variation in grey seal diet around the Western Isles of Scotland. A total of 511 individual faex was obtained from the Outer and 238 from the Minch/Inner Hebrides; 79% and 67%, respectively, contained fish otoliths and/or cephalopod beaks. 2. Fish otoliths and cephalopod beaks were identified, and measured to estimate diet composition by weight. The degree of otolith erosion due to digestion was accounted for using empirically derived digestion coefficients. 3. Gadids dominated the diet, contributing approximately 40% or greater in percentage by weight. The gadid species most commonly found were ling, cod and whiting, the predominance of which varied by area and season. 4. The main flatfish in the diet were plaice and megrim in the Outer Hebrides, and witch in the Minch/Inner Hebrides. The dominant species again varied by area and season. 5. The contribution of sandeels to the diet in the Hebrides was less than that found in other areas of Britain (Orkney and the east coast). Pelagic schooling fish (herring, mackerel and horse mackerel) contributed more to the diet than in other areas of Britain. 6. The significant variation in sandeel occurrence by area (many more sandeels taken off the Outer than the Minch/Inner Hebrides) can be explained by the distribution of the sandeel's preferred sea bed type (smooth areas of gravelly sand) in the region. However, two species of flatfish, megrim and witch, were most prevalent in the diet in areas which did not correspond to their supposed preferred sea bed depth or sediment type. 7. The greater number of cod and ling taken in summer compared to the rest of the year may compensate for the observed absence of an increase in sandeel consumption, seen at this time of year in other areas of Britain.

Effluent Diffusion and Sediment Dispersion at Microtidal River Mouths, Predicted Using Mathematical Models

Plume characteristics, associated with fluvial discharge into a microtidal environment, are predicted using mathematical models. Such predictions are compared with field measurements from the River Louros, western Greece. Combined frictionally-dominated and buoyant plume expansion models show encouraging agreement (i.e. to within ± 1%) with temperature, salinity and velocity observations. The inclusion of particle settling velocities into the analysis enables surficial sea-bed sediment size distribution to be determined. The predicted patterns show close correspondence with those collected from the subaqueous delta of a small river in Greece. The approach described may form the more general basis for the prediction of riverine sediment deposition on inner continental shelves.

Review of international HGW deep ocean disposal engineering studies : Milloy, C; Nicholson, D; Dutton, T Proc Conference on Disposal of Radioactive Waste in Seabed Sediments, Oxford, 20–21 September 1988P291–303. Publ London: Graham and Trotman, 1989 (Advances in Underwater Technology, Ocean Science and Offshore Engineering, V18)

Review of international HGW deep ocean disposal engineering studies : Milloy, C; Nicholson, D; Dutton, T Proc Conference on Disposal of Radioactive Waste in Seabed Sediments, Oxford, 20–21 September 1988P291–303. Publ London: Graham and Trotman, 1989 (Advances in Underwater Technology, Ocean Science and Offshore Engineering, V18)