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1.
Data from array for real-time geostrophic oceanography (ARGO) profiling floats, oil tanker thermosalinographs, shipboard ADCP and towed-CTD surveys, and satellite altimetry are used to examine properties of two ∼200 km diameter, anticyclonic Yakutat Eddies that propagated westward at ∼1.5 km day−1 along the continental slope of the northern Gulf of Alaska (GOA) in 2001 and 2003. The eddies had lifetimes of up to 5 years, remained close to the shelfbreak, and had relatively constant size and strength until they encountered the Alaskan Stream where they appeared to spawn smaller, shorter-lived anticyclones. The azimuthal velocity field was vigorous (25–40 cm s−1) and in gradient wind balance with Rossby numbers of ∼0.05. Conservation of salt and azimuthal mass transports (between 20 m and the depth of the 32.2 isohaline) from shipboard surveys in May and August 2003 suggest little mass exchange occurred between the surface layers of the eddy and ambient waters. Chlorophyll concentrations were greater in the eddy than in ambient waters in both May and August. In May, the chlorophyll was patchily distributed, while in August dense chlorophyll concentrations occurred in and beneath the seasonal thermocline within 50 km of the eddy center. The high August chlorophyll concentrations might have been fostered by a broad and shallow (<∼150 m) upwelling of the eddy center between May and August.It appears likely that as Yakutat Eddies approach the shelfbreak non-linear processes will modify the slope flow field (and the stability and structure of the shelfbreak front), leading to cross-slope flows and flow reversals. This interaction may induce ∼30 km-wide streamers of shelf water to flow around the trailing edge of the anticyclone. The role of streamers in the freshwater and nutrient budgets of the GOA shelf and basin is unknown, but their contribution to these budgets will depend on the trajectory of a Yakutat Eddy, especially its proximity to the shelfbreak as the eddy propagates along the GOA continental slope.  相似文献   

2.
A process-oriented, quasi-geostrophic, barotropic model has been developed with the aim of studying the relative importance of wind and topographic forcing on oceanic eddy generation by tall, deep water islands. As a case study, we chose the island of Gran Canaria. Topographic forcing was established using different intensities (weak, medium, strong, and very strong) for the oceanic current incident to the island. Wind forcing was introduced to simulate the mean wind curl observed in atmospheric tall island wakes. As observed from in situ data, the resulting wind curl consists of two cells of opposite sign which become a complementary source of vorticity at the island lee. The intensity and the shape of the two cells depend on the strength of the incident wind against the obstacle. The oceanic model was forced at three different wind (trade winds) speeds which correspond to weak, medium and strong wind intensities. Results from several numerical experiments show that in those periods where the incident wind is in the medium–strong range and the incident current speed is low (low Reynolds number), the wind forcing is the trigger mechanism for oceanic eddy generation. Eddies are spun off from the island for a lower Reynolds number (Re)/intensity of the oceanic flow (Re = 20) when compared with only topographic forcing (Re > 60). However, when the current speed is strong (high Reynolds number), the vorticity input by the wind is quickly advected by the oceanic flow and does not contribute to oceanic eddy generation. When only wind forcing is considered, only two stationary eddies are generated in the island wake. In this case, eddies of opposite sign are not sequentially spun off by the island and a Von-Kármán-like eddy street is not developed downstream of the island. Therefore, the main mechanism responsible for the development of an eddy street is the topographic perturbation of the oceanic flow by the island flanks. The wind over the island wake acts only as an additional source of vorticity, promoting the generation of an eddy street at a lower intensity of the incident oceanic flow, but not being capable of generating an eddy street without the topographic forcing.  相似文献   

3.
The suspended particulate organic matter (sPOM) around two isolated NE Atlantic seamounts, Seine (33°46′N 14°21′W; summit at ∼170 m) and Sedlo (40°19′N 26°40′W; summit at ∼780 m), was studied over a period of 2 years during four 2–4-week oceanographic surveys. Elemental (C and N), chlorophyll a and lipid biomarker concentrations and N stable isotopic values were variable close to the surface (40–90 m), although some chlorophyll a enrichment above the summits was discerned sporadically. Results from near-surface waters showed a generally “fresh”, mainly phytoplankton signature in sPOM with some seasonality, which was more pronounced around Sedlo. sPOM concentrations and composition changed with depth, apparently controlled by seasonality and proximity to the seamounts. A few metres above the Seine summit, the suspended particulate organic carbon (sPOC) concentrations and labile polyunsaturated fatty acids (% of lipids) were higher than elsewhere at similar depths (∼200 m) in summer 2004. In the same season at Sedlo, polyunsaturated fatty acids were also relatively more abundant (up to 43% of total lipids) around the topographic feature throughout the water column, indicating supply of more labile sPOM, perhaps by advection, downwelling or passive sinking of locally produced phytoplankton and/or in situ production. The high-quality sPOM that seems to be present around the seamounts could provide an important food source to the biological community.  相似文献   

4.
The biomass and production of phytoplankton and bacterioplankton was investigated in relation to the mesoscale structures found in the Algerian Current during the ALGERS'96 cruise (October 1996). Biological determinations were carried out in three transects between 0° and 2°E aimed at crossing a so-called event, formed by a coastal anticyclonic eddy associated with an offshore cyclonic eddy to the west. The concentration of chlorophyll a (Chl) was maximum (>1.2 mg m−3) within the cyclonic eddy and at the frontal zones between the Modified Atlantic Water (MAW) of the Algerian Current and the Mediterranean waters further north. Chl (total and >2 μm) was significantly correlated with proxies of nutrient flux into the upper layers. Autotrophic picoplankton and heterotrophic bacterial abundance and production presented clear differences between MAW and Mediterranean water, with higher values at those stations under the influence of the Algerian Current. In general, greater differences were observed in production than in biomass variables. The photosynthetic parameters (derived from P–E relationships) and integrated primary production (range 189–645 mg m−2 d−1) responded greatly to the different hydrological conditions. The mesoscale phenomena inducing fertilization caused a 2 to 3-fold increase in primary production rates. The relatively high values found within the cyclonic eddy suggest that, although short-lived in comparison with anticyclonic eddies, these eddies may produce episodic increases of biological production not accounted for in previous surveys in the region.  相似文献   

5.
CTD and ADCP measurements together with a sequence of satellite images indicate pronounced current meandering and eddy activity in the western Black Sea during April 1993. The Rim Current is identified as a well-defined meandering jet stream confined over the steepest topographic slope and associated cyclonic–anticyclonic eddy pairs located on both its sides. It has a form of highly energetic and unstable flow system, which, as it propagates cyclonically along the periphery of the basin, is modified in character. It possesses a two-layer vertical structure with uniform upper layer speed in excess of 50 cm/s (maximum value ∼100 cm/s), followed by a relatively sharp change across the pycnocline (between 100 and 200 m) and the uniform sub-pycnocline currents of 20 cm/s (maximum value ∼40 cm/s) observed up to the depth of ∼350 dbar, being the approximate limit of ADCP measurements. The cross-stream velocity structure exhibits a narrow core region (∼30 km), flanked by a narrow zone of anticyclonic shear on its coastal side and a broader region of cyclonic shear on its offshore side. The northwestern shelf circulation is generally decoupled from the influence of the basinwide circulation and is characterized by much weaker currents, less than 10 cm/s. The southward coastal flow associated with the Danube and Dinepr Rivers is weak during the measurement period and is restricted to a very narrow coastal zone.The data suggest the presence of temperature-induced overturning prior to the measurements, and subsequent formation of the Cold Intermediate Water mass (CIW) within the Northwestern Shelf (NWS) and interior of the western basin. The newly formed shelf CIW is transported in part along the shelf by the coastal current system, and in part it flows downslope across the shelf and intrudes into the Rim Current convergence zone. A major part of the cold water mass, however, seems to be trapped within the northwestern shelf. The CIW mass, injected into the Rim Current zone from the shelf and the interior region, is then circulated around the basin.  相似文献   

6.
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7.
Variations in the distribution of chemotaxonomic pigments were monitored in the Arabian Sea and the Gulf of Oman at the end of the SW monsoon in September 1994 and during the inter-monsoon period in November/December 1994 to determine the seasonal changes in phytoplankton composition. The Gulf of Oman was characterized by sub-surface chlorophyll maxima at 20-40 m during both seasons, and low levels of divinyl chlorophyll a indicated that prochlorophytes did not contribute significantly to the total chlorophyll a. Prymnesiophytes (19′-hexanoyloxyfucoxanthin), diatoms (fucoxanthin) and chlorophyll b containing organisms accounted for most of the phytoplankton biomass in September, while prymnesiophytes dominated in November/December. In the Arabian Sea in September, high total chlorophyll a concentrations up to 1742 ng l-1 were measured in the coastal upwelling region and a progressive decline was monitored along the 1670 km offshore transect to oligotrophic waters at 8°N. Divinyl chlorophyll a was not detected along this transect except at the two most southerly stations where prochlorophytes were estimated to contribute 25–30% to the total chlorophyll a. Inshore, the dominance of fucoxanthin and/or hexanoyloxyfucoxanthin indicated that diatoms and prymnesiophytes generally dominated the patchy phytoplankton community, with zeaxanthin-containing Synechococcus also being important, especially in surface waters. At the southern oligotrophic localities, Synechococcus and prochlorophytes dominated the upper 40 m and prymnesiophytes were the most prominent at the deep chlorophyll maximum. During the inter-monsoon season, total chlorophyll a concentrations were generally half those measured in September and highest levels were found on the shelf (1170 ng l-1). Divinyl chlorophyll a was detected at all stations along the Arabian Sea transect, and we estimated that prochlorophytes contributed between 3 and 28% to the total chlorophyll a, while at the two oligotrophic stations this proportion increased to 51–52%. While procaryotes were more important in November/December than September, eucaryotes still accounted for >50% of the total chlorophyll a. Pigment/total chlorophyll a ratios indicated that 19′-hexanoyloxyfucoxanthin-containing prymnesiophytes were the dominant group, although procaryotes accounted for 65% at the two southerly oligotrophic stations.  相似文献   

8.
A synoptic spatial examination of the eddy Haulani (17–20 November 2000) revealed a structure typical of Hawaiian cyclonic eddies with divergent surface flow forcing the upward displacement of deep waters. Hydrographic surveys revealed that surface water in the eddy center was ca. 3.5°C cooler, 0.5 saltier, and 1.4 kg m−3 denser than surface waters outside the eddy. Vertically integrated concentrations of nitrate+nitrite, phosphate and silicate were enhanced over out-eddy values by about 2-fold, and nitrate+nitrite concentrations were ca. 8× greater within the euphotic zone inside the eddy than outside. Si:N ratios were lower within the upper mixed layer of the eddy, indicating an enhanced Si uptake relative to nitrate+nitrite. Chlorophyll a concentrations were higher within the eddy compared to control stations outside, when integrated over the upper 150 m, but were not significantly different when integrated over the depth of the euphotic zone. Photosynthetic competency, assessed using fast repetition-rate fluorometry, varied with the doming of the isopycnals and the supply of macro-nutrients to the euphotic zone. The physical and chemical environment of the eddy selected for the accumulation of larger phytoplankton species. Photosynthetic bacteria (Prochlorococcus and Synechococcus) and small (<3 μm diameter) photosynthetic eukaryotes were 3.6-fold more numerically abundant outside the eddy as compared to inside. Large photosynthetic eukaryotes (>3 μm diameter) were more abundant inside the eddy than outside. Diatoms of the genera Rhizosolenia and Hemiaulus outside the eddy contained diazotrophic endosymbiontic cyanobacteria, but these endosymbionts were absent from the cells of these species inside the eddy. The increase in cell numbers of large photosynthetic eukaryotes with hard silica or calcite cell walls is likely to have a profound impact on the proportion of the organic carbon production that is exported to deep water by sinking of senescent cells and cells grazed by herbivorous zooplankton and repackaged as large fecal pellets.  相似文献   

9.
The vertical distribution (0–900 m) of zooplankton biomass and indices of feeding (gut fluorescence, GF) and metabolism (electron transfer system, ETS) were studied across an anticyclonic eddy south of Gran Canaria Island (Canary Islands). Two dense layers of organisms were clearly observed during the day, one above 200 m and the other at about 500 m, coincident with the deep scattering layer (DSL). The biomass displacement due to interzonal migrants in the euphotic zone was more than 2-fold higher than that previously reported for the southern area of this archipelago. The gut flux estimated (0.14–0.44 mgC m−2 d−1) was similar to the values previously found in the Canaries. The respiratory flux outside the eddy (1.85 mgC m−2 d−1) was in the lower range of values reported for this area. Inside the eddy, migrant biomass and respiration rates were 2- and 4- fold higher than in the surrounding waters. Active flux mediated by diel vertical migrants inside the eddy (8.28 mgC m−2 d−1) was up to 53% of the passive carbon flux to the mesopelagic zone (15.8 mgC m−2 d−1). It is, therefore, suggested that the anticyclonic eddy enhanced both migration from deep waters and active flux.  相似文献   

10.
Spatial and temporal variability of the island mass effect (IME; defined as local increases of phytoplankton associated with the presence of islands) at the Juan Fernández Archipelago (JFA) is analyzed using chlorophyll-a (Chl-a) satellite data, altimetry, sea surface temperature, wind, geostrophic currents and net heat flux over a ten year period (2002–2012). The the JFA islands (Robinson Crusoe-Santa Clara (RC-SC) and Alejandro Selkirk (AS)) present wakes with significant Chl-a increases, mainly during spring time. These wakes can reach Chl-a values of one order of magnitude higher (~1 mg m−3) than the surrounding oligotrophic waters (<0.1 mg m−3). The wakes are similar to von Kármán vortex streets which have been used to explain the impact of IME on Chl-a increases in numerical models. The wakes are formed from a high productivity area in the lee of the island, extending to the oceanic region as high Chl-a patches associated with submesoscale eddies that are detached from the islands and connected by less-productive zones. This pattern coincides with previous models that predict the effects of island-generated flow perturbations on biological production variability. The IME is a recurrent feature of islands that has even been observed in decadal average fields. In such average fields, the Chl-a values in RC-SC and AS islands can exceed values found in a Control Zone (a zone without islands) by ~50% and 30%, respectively. Seasonal and interannual variability reveals that, as a consequence of the IME, the winter Chl-a maximum associated with the development of winter convection and mesoscale eddies that propagate from the continental zone, promote that the Chl-a maximum extends towards spring. The IME has an impact on the island on both a local as well as a more regional scale that affects an area of ~40,000 km2 (1°Latitude×4°Longitude) centered on the islands. The transport of high productivity patches associated with submesoscale eddies may be responsible for IME propagation at a regional scale. Around the islands, the presence of a weak oceanic incident flow and strong and recurrent wind-wakes, suggest that the generation of Chl-a wakes result from a combined effect between both forcings.  相似文献   

11.
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12.
The response of an oscillating circular cylinder at the wake of an upstream fixed circular cylinder was classified by different researchers as galloping, wake induced galloping or wake induced vibration. Furthermore it is already known that a sharp edge square cylinder would undergo galloping if it is subjected to uniform flow. In this study the influence of the wake of a fixed circular cylinder on the response of a downstream square cylinder at different spacing ratios (S/D = 4, 8, 11) is experimentally investigated. The subject appears not to have received previous attention. The lateral displacements, lift forces and the pressure data from gauges mounted in the wake of the oscillating cylinder are recorded and analyzed. The single degree of freedom vibrating system has a low mass-damping parameter and the Reynolds number ranges from 7.7 × 102 to 3.7 × 104.In contrast to that for two circular cylinders in tandem arrangement, the freely mounted downstream square cylinder displays a VIV type of response at all spacing ratios tested. There is no sign of galloping or wake induced galloping with the square cylinder. With increase at the spacing ratio the cross-flow oscillations decrease. It is shown that the vortices arriving from the upstream fixed circular cylinder play a major role on the shedding mechanism behind the downstream square cylinder and cause the square cylinder to shed vortices with frequencies above Strouhal frequency of the fixed square cylinder (St = 0.13). The VIV type of oscillations in the downstream square cylinder is most probably caused by the vortices newly generated behind the square cylinder.  相似文献   

13.
In October 1991, mesozooplankton biomass and ichthyoplankton were studied in the waters surrounding the island of Gran Canaria (Canary Islands). The average dry weight obtained for mesozooplankton biomass (4.5 mg m−3) is typical of the area. Average fish egg abundance (94 eggs per 10 m2) was similar to that found previously for the Canary Current. However, the average fish larva abundance (904 individuals per 10 m2) was higher than previously recorded for the Canary Current and similar regions. The horizontal distributions of the planktonic components studied appear strongly related to the mesoscale oceanographic structures in the area. These included an area of weak flow around the stagnation point upstream of the island, where higher concentrations of neritic ichthyoplankton occurred, a warm lee region downstream, where mesozooplankton biomass and neritic ichthyoplankton were increased, particularly on the convergent anticyclonic boundary, and the offshore boundary of an upwelling filament from the NW African coast, which acted as a barrier for neritic ichthyoplankton. These concentrations suggest that the stagnation point and the lee are areas of retention for neritic fish eggs and larvae. Simple Lagrangian simulations of particle trajectories in the observed field of flow demonstrate the potential for retention of organisms, both passive and with limited swimming ability, in these areas. On the flanks of the island and in the filament, the simulation suggests even swimming organisms will be largely swept away. The various oceanographic structures, by increasing the planktonic production, are partially responsible for the relatively high values of abundance obtained for fish larvae.  相似文献   

14.
This study deals with the inflow of warm and saline Atlantic water to the Nordic Seas, an important factor for climate, ecology and biological production in Northern Europe. The investigations are carried out along the Svinøy standard hydrographic section, which cuts through the Atlantic inflow to the Norwegian Sea just to the north of the Faroe–Shetland Channel. In the Svinøy section, we consider the Atlantic inflow as water with salinity above 35.0, corresponding to temperatures above 5°C. Current measurements for the period April 1995 to February 1999, positioned on the continental slope in water depths between 490 and 990 m, are combined with VM-ADCP, SeaSoar-CTD and CTD transects to estimate long-term transports and spatial features of the Atlantic inflow. A well-defined two-branched Norwegian Atlantic Current was revealed with an eastern and a western branch. The eastern branch appears as a narrow, topographically trapped, near barotropic, 30–50 km wide current, with a maximum speed of 117 cm/s. The western branch is also about 30–50 km wide, and appears as an unstable frontal jet about 400 m deep with a maximum speed of 87 cm/s. Between these two prominent branches, the observations show an average eddy field with a recirculation to the southwest. Transport estimates from the current records in the eastern branch show an annual mean inflow of 4.2 Sv (1 Sv=106 m3/s) with variation on a 25 h time scale ranging from −2.2 to 11.8 Sv, and between 2.0 and 8.0 Sv on a monthly time scale. The current record in the core of the eastern branch mirrors the estimated transport on a monthly time scale with a correlation coefficient of 0.86. Except for the year 1995–1996, this nearly four-year current record shows evidence of a systematic annual cycle with summer to winter variations in the proportion of 1 to 2. Comparison between the North Atlantic Oscillation (NAO) index and the current record on a three-month time scale shows a strong connection for most of the period. This reflects the strong coupling between the westerly winds and the inflow. The baroclinic transport west of the eastern branch, including the frontal jet, is inferred from hydrography in combination with VM-ADCP transects, and has a total mean of 3.4 Sv. Thus, investigations to date indicate a yearly mean Atlantic inflow of 7.6 Sv in the Svinøy section.  相似文献   

15.
Surface distribution (0–100 m) of zooplankton biomass and specific aminoacyl-tRNA synthetases (AARS) activity, as a proxy of structural growth, were assessed during winter 2002 and spring 2004 in the Labrador Sea. Two fronts formed by strong boundary currents, several anticyclonic eddies and a cyclonic eddy were studied. The spatial contrasts observed in seawater temperature, salinity and fluorescence, associated with those mesoscale structures, affected the distributions of both zooplankton biomass and specific AARS activity, particularly those of the smaller individuals. Production rates of large organisms (200–1000 μm) were significantly related to microzooplankton biomass (63–200 μm), suggesting a cascade effect from hydrography through microzooplankton to large zooplankton. Water masses defined the biomass distribution of the three dominant species: Calanus glacialis was restricted to cold waters on the shelves while Calanus hyperboreus and Calanus finmarchicus were widespread from Canada to Greenland. Zooplankton production was up to ten-fold higher inside anticyclonic eddies than in the surrounding waters. The recent warming tendency observed in the Labrador Sea will likely generate weaker convection and less energetic mesoscale eddies. This may lead to a decrease in zooplankton growth and production in the Labrador basin.  相似文献   

16.
We describe here the results of an interdisciplinary study conducted off the coast of northern and central California during September 1993 in which we deployed an Optical Plankton Counter. This instrument counted and measured particles in the size range between 0.27 and 9.8 mm equivalent spheric diameter (ESD) occurring between the surface and 240 m depth. The survey region was characterized by the presence of the California Current jet and a cyclonic and an anticyclonic eddy. We analyzed the spatial (horizontal and vertical) distribution of planktonic particles and their relation to this hydrodynamic structure. We used specific analytical methods that take into account spatial constraints, i.e. autocorrelation analysis, constrained agglomerative clustering and contiguity constraints permutation analysis of variance. Horizontal spatial organization of particles was revealed at three different spatial scales (5, 18 and 100 km), while vertical patterns were described at a much smaller scale (20 m). We could detect some degree of similarity between particle size category spatial organization and hydrodynamic structure both by size category association independent of current movements and by comparison of dynamically differentiated areas. Five groups of similar size composition were detected that had some relation to the dynamic structure. Four sub-regions were determined a priori by their different hydrodynamic heights. We could describe a variability of particle abundance among these regions, both for total particles and for some size categories. Particles were more abundant inside the cyclonic eddy and less abundant inside the anticyclonic eddy. We also found deep concentration maxima inside the anticyclonic eddy and shallower concentration maxima inside the cyclonic eddy, with particles >2 mm ESD at deeper levels, for both daytime and nighttime sampling. No systematic difference was detected between daytime and nighttime samples in 0–240 m integrated total particle abundance. However, at night particles appeared to be concentrated into three depth strata (10–50, 70–90 and 90–230 m) of different size-abundance composition, while during the day particles were distributed into one shallow (10–50 m) and one deeper stratum (70–240 m). Smaller particles always occupied the most shallow depths  相似文献   

17.
Observations from a five-mooring array deployed in the vicinity of Sedlo Seamount over a 4-month period, together with supporting hydrographic and underway ADCP measurements, are described. Sedlo Seamount is an elongated, intermediate depth seamount with three separate peaks, rising from 2200 m water depth to summit peaks between 950 and 780 m depth, located at 40°20′N, 26°40W. Currents measured in depth range 750 and 820 m – the layer close to the summit depth of the shallowest southeast peak – showed a mean anti-cyclonic flow around the seamount, with residual current velocities of 2–5 cm s−1. Significant mesoscale variability was present at this level, and this is attributed to the weak and variable background impinging flow. Stronger, more persistent currents were found at the summit mooring as a result of tidal rectification and some weak amplification. Below 1300 m, currents were extremely weak, even close to the seabed. Time series of relative vorticity for the depth layer 750–820 m showed persistent anti-cyclonic vorticity except for two periods of cyclonic vorticity. A mean relative vorticity of −0.06f (f=the local Coriolis frequency) was calculated from a triangle of current meters located at the flanks of the seamount. Modelling results confirmed that anti-cyclonic flow above the seamount was likely due to Taylor Cone generation driven by a combination of steady impinging and tidally rectified flow. The closed circulation pattern over the seamount was found to extend to ∼150 m above the summit level, consistent with simple idealised theory and the supporting hydrographic observations. At shallower depths (<500 m) model simulations predicted a predominantly cyclonic recirculation most likely controlled by topographic steering along the zonal axis of the seamount. There was some indication of flow reversal at these depths from Acoustic Doppler Current Profiler (ADCP) measurements carried out at one hydrographic survey. The model results were in good agreement with observations at the seamount summit, but were unable to reproduce the mesoscale variability patterns recorded in shallower layers. Kinetic energy patterns derived from the model revealed high variability in the oceanic far field downstream of the seamount summit probably as a result of complex flow interaction along the chain of seamount peaks. Possible impacts of the flow dynamics on the biological functioning at Sedlo Seamount and its surroundings are discussed.  相似文献   

18.
A new hydrothermal vent site in the Southern Mariana Trough has been discovered using acoustic and magnetic surveys conducted by the Japan Agency for Marine-Earth Science and Technology's (JAMSTEC) autonomous underwater vehicle (AUV), Urashima. The high-resolution magnetic survey, part of a near-bottom geophysical mapping around a previously known hydrothermal vent site, the Pika site, during the YK09-08 cruise in June–July 2009, found that a clear magnetization low extends ∼500 m north from the Pika site. Acoustic signals, suggesting hydrothermal plumes, and 10 m-scale chimney-like topographic highs were detected within this low magnetization zone by a 120 kHz side-scan sonar and a 400 kHz multibeam echo sounder. In order to confirm the seafloor sources of the geophysical signals, seafloor observations were carried out using the deep-sea manned submersible Shinkai 6500 during the YK 10-10 cruise in August 2010. This discovered a new hydrothermal vent site (12°55.30′N, 143°38.89′E; at a depth of 2922 m), which we have named the Urashima site. This hydrothermal vent site covers an area of approximately 300 m×300 m and consists of black and clear smoker chimneys, brownish-colored shimmering chimneys, and inactive chimneys. All of the fluids sampled from the Urashima and Pika sites have chlorinity greater than local ambient seawater, suggesting subseafloor phase separation or leaching from rocks in the hydrothermal reaction zone. End-member compositions of the Urashima and Pika fluids suggest that fluids from two different sources feed the two sites, even though they are located on the same knoll and separated by only ∼500 m. We demonstrate that investigations on hydrothermal vent sites located in close proximity to one another can provide important insights into subseafloor hydrothermal fluid flow, and also that, while such hydrothermal sites are difficult to detect by conventional plume survey methods, high-resolution underwater geophysical surveys provide an effective means.  相似文献   

19.
Sulfur hexafluoride (SF6) tracer release experiments were carried out to trace the iron-fertilized water mass during the iron-fertilization experiments in the western North Pacific of Subarctic Pacific Iron Experiment for Ecosystem Dynamics Study II (SEEDS II) in 2004. A solution of Fe and SF6 tracer was released into the surface mixed layer over an 8×8 km area, and the fertilized patch was traced by onboard SF6 analysis for 12 days during each experiment. A Lagrangian frame of reference was maintained by the use of a drogued GPS buoy released at the center of the patch to reduce the advection effect on observations. The patch moved along the contour of sea-surface height (SSH) of a clockwise mesoscale eddy for 4 days after release. Then strong easterly winds dragged the patch across the contour of SSH. The patch behavior was affected by both the mesoscale eddy and surface winds. Apparent horizontal diffusivities were determined by the change of the distribution of SF6 concentrations. The averaged apparent horizontal diffusivity was about 49 m2 s−1 during SEEDS II. It was larger than the one in SEEDS. Mixed-layer depth (MLD) was 8.5–18 m during SEEDS, and 12–33 m during SEEDS II. The larger horizontal diffusivity and deeper MLD in SEEDS II were disadvantages to maintain a high iron concentration in the surface layer compared to SEEDS. Temporal change of the MLD corresponded to the temporal change of chlorophyll-a concentration. Temporal change in the surface MLD was also important for the response of phytoplankton by iron fertilization.  相似文献   

20.
The abundance and distribution of microphytobenthic pigments determined by HPLC (chlorophylls and carotenoids) were compared between muddy and sandy sediments of the Tagus estuary (Portugal). In the two types of sediment, with similar periods of illuminated emersion, chlorophyll a concentrations on a per area basis (mg m−2) were comparable (down to 2 mm). Pigment analysis also revealed similar microphytobenthic communities in terms of algal classes. Diatoms were the dominant microalgae, but cyanophytes, euglenophytes and phanerogam debris were also present. For both muddy and sandy sediments, microphytobenthic biomass showed a high level of variability both within and between two consecutive years. Microphytobenthos was highly stratified in the mud, with most of the chlorophyll a occurring in the top 500 μm. In the sand, relatively constant concentrations were found throughout the sediment profile down to 3 mm. This is probably related to deeper light penetration in sandy sediment and/or increased physical mixing caused by invertebrate activity or overlying currents, leading to the burial of an important fraction of the microphytobenthic cells. Differences observed in the intensity of sediment coloration of muddy and sandy sediments might have resulted from the different vertical distribution of benthic biomass.  相似文献   

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