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1.
We sampled hydrothermal plumes over the N. Gorda Ridge four times between March and August 1996 to document Mn and Fe discharge resulting from a magmatic intrusion/seafloor eruption. Two separate event plumes, EP96A and B, and chronic hydrothermal emissions lasting 6 months were characterized. Shipboard time-series measurements of an event plume sample were used to calibrate an Fe phase clock useful for estimating sample age up to 6 days after fluid discharge. Samples collected from EP96A and B had Mn/heat (<0.15 nmol J-1) and Fe/Mn (>2 mol mol-1) ratios similar to historical event plume observations. We suggest these “signature” ratio values are generally characteristic of event plumes and hypothesize that Mn and Fe may be supplied to event plumes by different processes: Mn by entrainment of fluids from an extant shallow subseafloor reservoir, and Fe by short-lived, high-temperature water-rock reaction coincident with dike emplacement. Calculations based on the Fe phase clock indicate that the two event plumes were released more than a month apart. The largest event plume, EP96A (2.3×106 M Mn and 13×106 M Fe), formed 7 March soon after seismic activity began. The smaller EP96B (0.49×106 M Mn and 3.5×106 M Fe) was not discharged until 11 April, 3 weeks after the cessation of seismic activity detectable by SOSUS T-phase monitoring. We hypothesize that the subseafloor disturbance that triggered EP96B also resulted in the episodic flushing of a reservoir of chronic-plume-like fluids. Total event plume inventories of Mn and Fe at N. Gorda Ridge are much smaller than those associated with the 1986 event at N. Cleft segment of the Juan de Fuca Ridge, but comparable to event plume inventories at N. Cleft segment in 1987 and CoAxial segment in 1993. Mn/heat values for chronic plumes over the eruption site underlying EP96A evolved from moderate (0.25 oonmol J-1, reflecting probable admixture with event plume formation fluids) to high (0.7 nmol J-1, typical of chronic plumes) to low (0.1 nmol J-1, similar to diffuse vent fluid values), marking a complete episode of intrusion/eruption-induced hydrothermal discharge.  相似文献   

2.
The rapid field response in March and April 1996 to diking-eruptive events on the Northern Gorda Ridge allowed the first detailed co-measurement of 3He, H2, and CH4 in plumes associated with a submarine eruption, and examination of the chemical transformation of these volatile species during aging of the plumes. The diking-eruptive event generated extensive event plumes with a low 3He/heat ratio of ∼0.4×10-12 cm3 STP gm-1 °C-1, while chronic plumes had 3He/heat ratios of 2.1–6.2×10-12 cm3 STP gm-1 °C-1. Hydrogen concentrations in the initial event plume (EP96A) reached a maximum value of 47 nM, 235 times that of ocean background concentration (0.2 nM). In contrast, roughly one month later H2 concentrations, although still elevated, had drastically dropped to 40 times background values. Methane concentrations in both event and chronic plume fluids were also elevated and similar to stable hydrothermal systems in unsedimented ridge environments, reaching a maximum value of 7 nM. The intense rapid interaction at very high temperatures of hydrothermal fluids with the host rock during dike intrusion or with seawater during extrusion of the basalt likely generated high, localized concentrations of H2. The data indicate, however, that maximum rates of H2 production during event plume formation could have lasted a few days at most.  相似文献   

3.
A low-temperature diffuse flow site associated with abundant vent fauna was found by submersible observations on the southern East Pacific Rise at 17°25′ S in 1997. This site was characterized by thin sediment covered pillow and sheet lavas with collapsed pits up to ∼15 m in diameter. There were three warm water vents (temperature: 6.5 to 10.5 °C) within the site above which the vented fluids rise as plumes. To estimate heat flux of the warm water vents, a temperature logger array was deployed and the vertical temperature distribution in the water column up to 38 m above the seafloor was monitored. A stationary deep seafloor observatory system was also deployed to monitor hydrothermal activity in this site. The temperature logger array measured temperature anomalies, while the plumes from the vents passed through the array. Because the temperature anomalies were measured in only specific current directions, we identified one of the vents as the source. Heat flux from the vent was estimated by applying a plume model in crossflow in a density-stratified environment. The average heat flux from September 13 to October 18, 1997 was 39 MW. This heat flux is as same order as those of high-temperature black smokers, indicating that a large volume flux was discharged from the vent (1.9 m3/s). Previous observations found many similar warm water flow vents along the spreading axis between 17°20′ S–30′ S. The total heat flux was estimated to be at least a few hundred mega-watts. This venting style would contribute to form effluent hydrothermal plumes extended above the spreading axis.  相似文献   

4.
The dynamics of the Lesina coastal lagoon (Italy) in terms of nutrients, phytoplankton and chemical–physical parameters were evaluated, together with their functional relationships with freshwater inputs, in order to identify ecosystem responses to changes in driving forces in a Mediterranean non-tidal lentic environment. Lesina Lagoon is a shallow coastal environment characterised by limited exchange with coastal waters, which favours enrichment of nutrients and organic matter and benthic fluxes within the system. Lagoon–sea exchanges are influenced by human management. There is a steep salinity gradient from East to West. High nitrogen and silica values were found close to freshwater inputs, indicating wastewater discharges and agricultural runoff, especially in winter. Dissolved oxygen was well below saturation (65%) near sewage and runoff inputs in the western part of the lagoon during summer. Classification in accordance with EEA (2001) guidelines suggests the system is of “poor” or “bad” quality in terms of nitrogen concentrations in the eastern zone during the winter rainy period. In terms of phosphate concentrations, the majority of the stations fall into the “good” category, with only two stations (close to the sewage and runoff inputs) classed as “bad”. In both cases, the raw nitrogen levels make the lagoon a P-limited system, especially in the eastern part. There was wide space–time variability in chlorophyll a concentrations, which ranged from 0.25 to 56 μg l−1. No relationships between chlorophyll a and nutrients were found, suggesting that autotrophic biomass may be controlled by a large number of internal and external forcing factors driving eutrophication processes. Water quality for this type of environment depends heavily on pressure from human activities but also on the management of sewage treatment plants, agricultural practices and the channels connecting the lagoon with the sea.  相似文献   

5.
The amount of metabolic energy available for primary production by chemolithoautotrophic microorganisms in a submarine hydrothermal plume is evaluated using geochemical models. Oxidation of elemental sulfur and metal sulfides precipitated in the hydrothermal plume represent the largest potential sources of metabolic energy in the plume (∼600 cal/kg vent fluid from each source). Among dissolved substrates, oxidation of H2 potentially provides the greatest amount of energy (∼160 cal/kg). Smaller, but still significant, amounts of energy are also available from sulfate reduction (54 cal/kg), methanogenesis (17 cal/kg), and methanotrophy (13 cal/kg). Only negligible amounts of energy are available from oxidation of Fe(II) or Mn(II) compounds or Fe3+ reduction (<1 cal/kg vent fluid). The models suggest that most primary production in the plume should occur in the early stages of plume development from sulfur- and H2-oxidizers entrained in the plume or colonizing the surfaces of minerals settling from the plume. The total primary productivity potential in the plume is estimated to be about 50 mg dry wt biomass/kg vent fluid. This translates to a global annual biomass production in hydrothermal plumes on the order of 1012 g dry wt/yr, which represents only a small fraction of the total photosynthetic biomass production in the oceans (∼1017 g dry wt/yr). Nevertheless, biomass generated in hydrothermal plumes may represent a significant fraction of the organic matter in the deep ocean as well as that deposited in sediments in ocean basins.  相似文献   

6.
The diets of breeding seabirds can be a good monitor of marine environmental changes. From 1984 to 2001 we monitored the diets of black-tailed gulls (Larus crassirostris) (“surface foragers”), rhinoceros auklets (Cerorhinca monocerata) (“epipelagic divers”), and Japanese cormorants (Phalacrocorax filamentotus) (“bottom divers”) that breed on Teuri Island at the northern boundary of the Tsushima Warm current in the Sea of Japan/East Sea. Between 1984 and 1987, both the gulls and the auklets foraged on the sardine (Sardinops melanostictus), but after 1992, they switched to the anchovy (Engraulis japonica). This change might reflect the collapse of the sardine stock in the late 1980s. In the 1990s, the year-to-year variations of the percentage of anchovy in the diets of the three seabird species showed similar trends: High in 1994 and 1998–2001; and low in 1992–1993 and 1995–1997. The estimated stock size of the anchovy population in the Tsushima Current area was positively correlated with the percentage of mass of anchovy in the seabirds’ diets. Thus, the short-term annual changes of the total anchovy availability, which might reflect SST or the volume transport of Tsushima Current, possibly affected the seabirds diets on this island.  相似文献   

7.
Stormwater plumes in the southern California coastal ocean were detected by MODIS-Aqua satellite imagery and compared to ship-based data on surface salinity and fecal indicator bacterial (FIB) counts collected during the Bight'03 Regional Water Quality Program surveys in February–March of 2004 and 2005. MODIS imagery was processed using a combined near-infrared/shortwave-infrared (NIR-SWIR) atmospheric correction method, which substantially improved normalized water-leaving radiation (nLw) optical spectra in coastal waters with high turbidity. Plumes were detected using a minimum-distance supervised classification method based on nLw spectra averaged within the training areas, defined as circular zones of 1.5–5.0-km radii around field stations with a surface salinity of S < 32.0 (“plume”) and S > 33.0 (“ocean”). The plume optical signatures (i.e., the nLw differences between “plume” and “ocean”) were most evident during the first 2 days after the rainstorms. To assess the accuracy of plume detection, stations were classified into “plume” and “ocean” using two criteria: (1) “plume” included the stations with salinity below a certain threshold estimated from the maximum accuracy of plume detection; and (2) FIB counts in “plume” exceeded the California State Water Board standards. The salinity threshold between “plume” and “ocean” was estimated as 32.2. The total accuracy of plume detection in terms of surface salinity was not high (68% on average), seemingly because of imperfect correlation between plume salinity and ocean color. The accuracy of plume detection in terms of FIB exceedances was even lower (64% on average), resulting from low correlation between ocean color and bacterial contamination. Nevertheless, satellite imagery was shown to be a useful tool for the estimation of the extent of potentially polluted plumes, which was hardly achievable by direct sampling methods (in particular, because the grids of ship-based stations covered only small parts of the plumes detected via synoptic MODIS imagery). In most southern California coastal areas, the zones of bacterial contamination were much smaller than the areas of turbid plumes; an exception was the plume of the Tijuana River, where the zone of bacterial contamination was comparable with the zone of plume detected by ocean color.  相似文献   

8.
Seafloor geomorphology and surficial stratigraphy of the New Jersey middle continental shelf provide a detailed record of sea-level change during the last advance and retreat of the Laurentide ice sheet (120 kyr B.P. to Present). A NW–SE-oriented corridor on the middle shelf between water depths of 40 m (the mid-shelf “paleo-shore”) and 100 m (the Franklin “paleo-shore”) encompasses 500 line-km of 2D Huntec boomer profiles (500–3500 Hz), an embedded 4.6 km2 3D volume, and a 490 km2 swath bathymetry map. We use these data to develop a relative stratigraphy. Core samples from published studies also provide some chronological and sedimentological constraints on the upper <5 m of the stratigraphic succession.The following stratigraphic units and surfaces occur (from bottom to top): (1) “R”, a high-amplitude reflection that separates sediment >46.5 kyr old (by AMS 14C dating) from overlying sediment wedges; (2) the outer shelf wedge, a marine unit up to 50 m thick that onlaps “R”; (3) “Channels”, a reflection sub-parallel to the seafloor that incises “R”, and appears as a dendritic system of channels in map view; (4) “Channels” fill, the upper portion of which is sampled and known to represent deepening-upward marine sediments 12.3 kyr in age; (5) the “T” horizon, a seismically discontinuous surface that caps “Channels” fill; (6) oblique ridge deposits, coarse-grained shelly units comprised of km-scale, shallow shelf bedforms; and (7) ribbon-floored swales, bathymetric depressions parallel to modern shelf currents that truncate the oblique ridges and cut into surficial deposits.We interpret this succession of features in light of a global eustatic sea-level curve and the consequent migration of the coastline across the middle shelf during the last 120 kyr. The morphology of the New Jersey middle shelf shows a discrete sequence of stratigraphic elements, and reflects the pulsed episodicity of the last sea-level cycle. “R” is a complicated marine/non-marine erosional surface formed during the last regression, while the outer shelf wedge represents a shelf wedge emplaced during a minor glacial retreat before maximum Wisconsin lowstand (i.e., marine oxygen isotope stage 3.1). “Channels” is a widespread fluvial subarial erosion surface formed at the late Wisconsin glacial maximum 22 kyr B.P. The shoreline migrated back across the mid-shelf corridor non-uniformly during the period represented by “Channels” fill. Oblique ridges are relict features on the New Jersey middle shelf, while the ribbon-floored swales represent modern shelf erosion. There is no systematic relationship between modern seafloor morphology and the very shallowly buried stratigraphic succession.  相似文献   

9.
This study presents first-time observations of bacterial and viral abundances in hydrothermal event plumes. Two water-column event plumes were formed in conjunction with seismic events and seafloor volcanic eruptions on the northern Gorda Ridge in February–March 1996. Epifluorescence counts of bacteria and viruses were performed on water samples from 3 successive cruises staged in the 10–90 days that followed the onset of seismicity. Relative to background seawater at these 1800–3200 m depths, bacterial abundance was enhanced by 2–3 fold within both event plumes. In contrast, viral numbers were below background seawater values in the younger and more intense of the two event plumes (EP96A), and enhanced in the other (EP96B). Changes in viral abundance may be a secondary response to that of plume bacteria as well as being influenced by particle formation and precipitation within the plumes. Lower bacteria/heat, virus/heat and virus/bacteria ratios in EP96A versus EP96B confirm distinct differences in the microbial response to event plume formation, possibly related to observed differences in plume chemistry.  相似文献   

10.
Differences in the biological and ecological strategies of two tropical flatfishes, Etropus crossotus (fringed flounder) and Citharichthys spilopterus (bay whiff) are discussed. The comparative analysis was based on the seasonal distribution of relative abundance of the two species and flatfish movements utilizing estuarine plume influenced areas as part of their life cycles. Growth parameters of the von Bertalanffy equation and recruitment were estimated (FISAT software) to compare life cycle patterns. The recruitment patterns illustrate the key difference between “estuarine-dependent” and “estuarine-related” nekton strategies. Both species have a short life cycle with a continuous recruitment activity. E. crossotus is an estuarine-related species, with two recruitment pulses (winter frontal season), in the estuarine plume on the shelf. C. spilopterus is an estuarine-dependent species, with a main recruitment pulse during the rainy season in the estuarine plume, and a residual secondary pulse inside the adjacent estuarine system (Terminos Lagoon). The results show that the ecological success of these tropical coastal marine flatfish, with similar biological patterns, is based upon the sequential use (in time and space) of estuarine plume influenced habitats, suggesting that fish migration to shallow waters is related to food availability changes as a strategy towards optimum recruitment.  相似文献   

11.
Most of the existing relevant materials have been obtained from experiments, in which evaluating the added mass at the resonant frequency corresponding to the peak of a frequency-response curve obtained from the “forced” vibration analysis is the most popular technique. In this paper, a simple experimental method was presented where the “free” vibration responses instead of the “forced” ones were used to determine the values of mah and Iap. The main part of the experimental system is composed of a floating body (model) and a spring–shaft shaker. The “free” vibration of this main part was induced by imposing on it an initial displacement (and/or an initial velocity), and from the time histories of displacements information such as the “damped” natural frequencies, damping ratios, sectional added mass coefficients (CV and CP) were obtained. Since the displacements of the spring–shaft shaker are “translational” and those of the floating body due to pitch motions are “angular”, a technique for the transformation between the associated parameters of the two components of the main part was presented.  相似文献   

12.
A study on the bulk distributions and molecular structures of n-alkanes and polycyclic aromatic hydrocarbons (PAH) in organic matter of the sediments from the Bay of Bengal and the Eastern and Central Indian Basins was undertaken. The former two regions represent areas characterised by “normal” sedimentation while the third one mainly represents a region of “active tectonism”. Content of the hydrocarbons in the sediments of “normal” sedimentation ranges between 4.6 and 10.5 μg/g and aromatic hydrocarbons ranges between 0 and 0.38 μg/g. n-Alkanes in the sediments of the northern deep part of the Bay of Bengal consist mostly of long-chain structures (total C25–C33 up to 70%) with a high carbon preference index (CPI=3.01–3.43), indicating a large contribution of organic matter from terrigenous sources. The sediments from the Eastern Indian Basin have n-alkane distributions in which the long-chain components did not exceed 52.5% and the CPI was 1.7–1.90, indicating that the hydrocarbons are mostly derived from marine sources. Sharp increases of hydrocarbons are found in the vicinity of the tectonically active region of the Central Indian Basin, particularly in the sediments collected from the fracture zone. The total concentration of hydrocarbons increase to 170 μg/g and the aromatic hydrocarbons fraction to 156.3 μg/g. The proportion of short-chain n-alkanes increases up to 70%, CPI decreases to 0.76–1.12, and high concentrations of n-C16 (16–40%) occur, all of which are absent in the other samples. The molecular content of PAH includes the unsubstituted individual structures: biphenyl, fluorene, pyrene, perylene, benzo(ghi)perylene, and the groups of homologues of naphthalene, benzofluorene, phenanthrene and chrysene. The association of the PAH and composition of paraffin hydrocarbons in the surficial sediments of deformation zone indicate that these are the resultant products of hydrothermal processes. It is, therefore, suggested that the association and composition of the hydrocarbons in sediments can be utilised as a paleoceanographic parameter to decipher the history of tectonism of an area.  相似文献   

13.
Transparent exopolymer particles (TEP) have recently been recognized as a class of sticky particles that often stimulate macroaggregate formation. Until now, TEP studies were focused on surface waters, particularly in association with diatom blooms. This is the first report of TEP concentrations in an open ocean deep-sea hydrothermal plume environment. The occurrence of TEP in deep water demonstrates its existence in an environment that is devoid of growing phytoplankton, the organisms generally regarded as TEP's primary source. The main objective of this study was to investigate the presence and potential abundance of TEP within hydrothermal plumes over the Juan de Fuca Ridge. Samples for TEP and bacteria were collected from CTD-rosette casts through hydrothermal plumes over the Juan de Fuca Ridge during the summers of 1999 and 2000. Concentrations of TEP and bacteria within the hydrothermal plumes were significantly greater than in background seawater. TEP number and volume concentrations reported here are low compared to reported values for shallow environments, but are substantially higher than published reports of TEP in other deep water environments. Ratios of TEP anomalies to temperature anomalies (a conservative hydrothermal tracer) increased with distance from the apparent plume source, suggesting that TEP are produced within the aging hydrothermal plumes. Potential TEP sources associated with hydrothermal plumes are discussed. TEP in hydrothermal plumes could stimulate in situ aggregate formation, support populations of attached bacteria, and serve as a potential food source to zooplankton.  相似文献   

14.
Measurements of bromoform (CHBr3), diiodomethane (CH2I2), chloroiodomethane (CH2ICl) and bromoiodomethane (CH2IBr) were made in the water column (5–100 m depth) of the Southern Ocean within 0–40 km of the Antarctic sea ice during the ANTXX1/2 transect of the German R/V Polarstern, at five locations between 70–72°S and 9–11°W in the Antarctic spring/summer of 2003–2004. Some of the profiles exhibited a very pronounced layer of surface sea-ice meltwater, as evidenced by salinity minima and temperature maxima, along with surface maxima in concentrations of CHBr3, CH2I2, CH2ICl and CH2IBr. These results are consistent with in situ surface halocarbon production by ice algae liberated from the sea ice, although production within the sea ice followed by transport cannot be entirely ruled out. Additional sub-surface maxima in halocarbons occurred between 20 and 80 m. At a station further from shore and not affected by surface sea-ice meltwater, surface concentrations of CH2I2 were decreased whereas CH2ICl concentrations were increased compared to the stations influenced by meltwater, consistent with photochemical conversion of CH2I2 to CH2ICl, perhaps during upward mixing from a layer at  70 m enhanced in iodocarbons. Mean surface (5–10 m) water concentrations of halocarbons in these coastal Antarctic waters were 57 pmol l− 1 CHBr3 (range 44–78 pmol l− 1), 4.2 pmol l− 1 CH2I2 (range 1.7–8.2 pmol l− 1), 0.8 pmol l− 1 CH2IBr (range 0.2–1.4 pmol l− 1), and 0.7 pmol l− 1 CH2ICl (range 0.2–2.4 pmol l− 1). Concurrent measurements in air suggested a sea-air flux of bromoform near the Antarctic coast of between 1 and 100 (mean 32.3, median 10.4) nmol m− 2 day− 1 and saturation anomalies of 557–1082% (mean 783%, median 733%), similar in magnitude to global shelf values. In surface samples affected by meltwater, CH2I2 fluxes ranged from 0.02 to 6.1 nmol m− 2 day− 1, with mean and median values of 1.9 and 1.1 nmol m− 2 day− 1, respectively.  相似文献   

15.
The Mussel Watch program conducted along the French coasts for the last 20 years indicates that the highest mercury concentrations in the soft tissue of the blue mussel (Mytilus edulis) occur in animals from the eastern part of Seine Bay on the south coast of the English Channel, the “Pays de Caux”. This region is characterized by the presence of intertidal and submarine groundwater discharges, and no particular mercury effluent has been reported in its vicinity. Two groundwater emergence systems in the karstic coastal zone of the Pays de Caux (Etretat and Yport with slow and fast water percolation pathways respectively) were seasonally sampled to study mercury distribution, partitioning and speciation in water. Samples were also collected in the freshwater–seawater mixing zones in order to compare mercury concentrations and speciation between these “subterranean” or “groundwater” estuaries and the adjacent macrotidal Seine estuary, characterized by a high turbidity zone (HTZ). The mercury concentrations in the soft tissue of mussels from the same areas were monitored at the same time.The means of the “dissolved” (< 0.45 μm) mercury concentrations (HgTD) in the groundwater springs were 0.99 ± 0.15 ng l− 1 (n = 18) and 0.44 ± 0.17 ng l− 1 (n = 17) at Etretat and Yport respectively. High HgTD concentrations were associated with strong runoff over short water pathways during storm periods, while low concentrations were associated with long groundwater pathways. Mean particulate mercury concentrations were 0.22 ± 0.05 ng mg− 1 (n = 16) and 0.16 ± 0.10 ng mg− 1 (n = 17) at Etretat and Yport respectively, and decreased with increasing particle concentration probably as a result of dilution by particles from soil erosion. Groundwater mercury speciation was characterized by high reactive-to-total mercury ratios in the dissolved phase (HgRD/HgTD: 44–95%), and very low total monomethylmercury concentrations (MMHg < 8 pg l− 1). The HgTD distributions in the Yport and Etretat mixing zones were similar (overall mean concentration of 0.73 ± 0.21 ng l− 1, n = 43), but higher than those measured in the adjacent industrialized Seine estuary (mean: 0.31 ± 0.11 ng l− 1, n = 67). In the coastal waters along the Pays de Caux dissolved monomethylmercury (MMHgD) concentrations varied from 9.5 to 13.5 pg l− 1 (2 to 8% of the HgTD). Comparable levels were measured in the Seine estuary (range: 12.2– 21.1 pg l−1; 6–12% of the HgTD). These groundwater karstic estuaries seem to be mostly characterized by the higher HgTD and HgRD concentrations than in the adjacent HTZ Seine estuary. While the HTZ of the Seine estuary acts as a dissolved mercury removal system, the low turbid mixing zone of the Pays de Caux receives the dissolved mercury inputs from the groundwater seepage with an apparent Hg transfer from the particulate phase to the “dissolved” phase (< 0.45 μm). In parallel, the soft tissue of mussels collected near the groundwater discharges, at Etretat and Yport, exhibited significantly higher values than those found in the mussel from the mouth of the Seine estuary. We observe that this difference mimics the differences found in the mercury distribution in the water, and argue that the dissolved phase of the groundwater estuaries and coastal particles are significant sources of bioavailable mercury for mussels.  相似文献   

16.
To address the relative importance of shrimp trawling on seabed resuspension and bottom characteristics in shallow estuaries, a series of disturbance and monitoring experiments were conducted at a bay bottom mud site (2.5 m depth) in Galveston Bay, Texas in July 1998 and May 1999. Based on pre- and post-trawl sediment profiles of 7Be; pore water dissolved oxygen and sulfide concentration; and bulk sediment properties, it was estimated that the trawl rig, including the net, trawl doors, and “tickler chain,” excavate the seabed to a maximum depth of approximately 1.5 cm, with most areas displaying considerably less disturbance. Water column profile data in the turbid plume left by the trawl in these underconsolidated muds (85–90% porosity; <0.25 kPa undrained shear strength) demonstrate that suspended sediment inventories of up to 85–90 mg/cm2 are produced immediately behind the trawl net; an order of magnitude higher than pre-trawl inventories and comparable to those observed during a 9–10 m/s wind event at the study site. Plume settling and dispersion caused suspended sediment inventories to return to pre-trawl values about 14 min after trawl passage in two separate experiments, indicating particles re-settle primarily as flocs before they can be widely dispersed by local currents. As a result of the passage of the trawl rig across the seabed, shear strength of the sediment surface showed no significant increase, suggesting that bed armoring is not taking place and the trawled areas will not show an increase in critical shear stress.  相似文献   

17.
Unstructured-grid models grounded on semi-implicit, finite-volume, Eulerian–Lagrangian algorithms, such as UnTRIM and ELCIRC, have enjoyed considerable success recently in simulating 3D estuarine and coastal circulation. However, opportunities for improving the accuracy of this type of models were identified during extensive simulations of a tightly coupled estuary–plume–shelf system in the Columbia River system. Efforts to improve numerical accuracy resulted in SELFE, a new finite-element model for cross-scale ocean modeling. SELFE retains key benefits, including computational efficiency of existing semi-implicit Eulerian–Lagrangian finite-volume models, but relaxes restrictions on grids, uses higher-order shape functions for elevation, and enables superior flexibility in representing the bathymetry. Better representation of the bathymetry is enabled by a novel, “localized” vertical grid that resembles unstructured grids. At a particular horizontal location, SELFE uses either S coordinates or SZ coordinates, but the equations are consistently solved in Z space. SELFE also performs well relative to volume conservation and spurious oscillations, two problems that plague some finite-element models. This paper introduces SELFE as an open-source code available for community use and enhancement. The main focus here is on describing the formulation of the model and on showing results for a range of progressively demanding benchmark tests. While leaving details to separate publications, we also briefly illustrate the superior performance of SELFE over ELCIRC in a field application to the Columbia River estuary and plume.  相似文献   

18.
In 1984, on a transect covering the whole Baltic Sea and parts of the adjacent North Sea, 160 water samples were taken and analysed for their concentrations of particulate and dissolved metals. In addition, the suspended materials were investigated for their elemental bulk composition.The particulate fractions represented from about 5% (Cd, Cu and Ni) to 50% (Fe and Pb) of the total (particulate plus dissolved) concentrations. For some elements (Ba, Cd, Cu, Pb and Zn), the particulate matter from the surface microlayer was enriched with respect to those suspended materials taken from 0.2 m depth. This could reflect the atmospheric input of metal-rich aerosols. In anoxic deep waters, maximum contents of Zn (6400 μg g−1), Cu (1330 μg g−1) and Cd (12 μg g−1) were observed in the particulate matter, indicating sulphidic forms. On the other hand, under oxic conditions the distribution coefficients (Kd) decreased with the water depth (Cd, Fe and Pb).Relative to global background levels, the particulate matter contained metal “excesses” amounting to more than 90% of the total contents (Cd, Mn, Pb and Zn). Automated electron probe X-ray microanalysis (EPXMA) revealed that the elemental composition of sediments is mainly governed by post-depositional processes of early diagenesis and is only weakly related to the composition of suspended matter in the overlying water body. For instance, in relation to surface mud sediments of the central Baltic net-sedimentation basins, Zn, Cd, Cu and Mn had 30–100% higher levels in the suspended materials. The general pattern of metal contents of particulate matter taken from 10 m depth on a transect between the Bothnian Bay and the North Sea were—possibly as a result of anthropogenic inputs—rather similar for Pb, Zn and Cu. For Fe and Mn, the distribution patterns along the transect were probably governed by the natural loading characteristics and by the biogeochemistry of those elements.  相似文献   

19.
Pore water aliquots were taken with an in situ close interval sampler: the “Peeper”.We report here the pore water concentration profiles of TCO2, SO4, TH2S, Ca and the trace metals Mn, Cu, Pb, Cd and Cr from sediments of a relatively polluted area, the Villefranche Bay, on the French Riviera (close to Nice).We investigated the major ion concentrations in order to establish geochemical mass balances of organic matter oxidation. ΔTCO2/ΔSO4 was <−2.0, reflecting the precipitation of calcite as confirmed by the calcium profile. Reduction of sulfate led to increasing sulfide concentrations with depth.Trace metal interstitial water concentrations decreased from 63 to 5 nM, 18 to 4 nM and 6.6 to less than 2 nM for Cu, Pb and Cr, respectively. Cd showed a different pattern with top and deep values of 0.7 nM and a minimum of 0.27 nM.Thermodynamic calculations were performed which suggest the potential formation of mineral phases such as sulfides.  相似文献   

20.
Event plumes form as episodic discharges of large volumes of hydrothermal solutions in response to magmatic diking/eruptive events. In consequence, event plumes represent the sudden injection of exploitable reduced chemical substrates, as well as inhibitory constituents, and likely induce successional changes in the microbial community structure and activity within event plume waters. In response to a major seismic event detected beginning 28 February 1996 at the northern Gorda Ridge, a series of three rapid response and follow-up cruises (GREAT 1, 2 and 3) were mounted over a period of three months. This report focuses on time-series measurements of manganese geomicrobial parameters in the two event plumes found in association with this seismic event.Scanning transmission electron microscopy, elemental microanalysis, and radioisotope (54Mn) uptake experiments were employed on samples collected from vertical and tow-yo casts from the three cruises. Numbers of bacteria and ratios of metal precipitating capsuled bacteria to total bacteria were greatest in the youngest (days old) plume, EP96A, found during GREAT 1; however, when normalized to the hydrothermal temperature anomaly, the greatest values were found in a second event plume, EP96B, discovered during GREAT 2 (up to 1 month old). Early capsule bacteria and particulate Mn distributions may have been influenced by entrainment of resuspended sediment, while those of the oldest (2–3 months) plume sample may have been subjected to preferential aggregation and particle settling.  相似文献   

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