Influence of bottom topography on an upwelling current: Generation of long trapped filaments

We investigate the influence of bottom topography on the formation and trapping of long upwelling filaments using a 2-layer shallow water model on the f-plane. A wind forced along-shore current, associated with coastal upwelling along a vertical wall, encounters a promontory of finite width and length, perpendicular to the coast.In the lower layer, topographic eddies form, which are shown to drive the formation of a filament on the front. Indeed, as the upwelling current and front develop along the coast, the along shore flow crosses the promontory, re-arranging the potential vorticity structure and generating intense vortical structures: water columns with high potential vorticity initially localized upon the promontory are advected into the deep ocean, forming cyclonic eddies, while water columns from the deep ocean with low potential vorticity climb on the topography forming a trapped anticyclonic circulation. These topographic eddies interact with the upper layer upwelling front and form an elongated, trapped and narrow filament.Sensitivity tests are then carried out and it is shown that:

• •baroclinic instability of the front does not play a major role on the formation of long trapped filaments;
• •increasing the duration of the wind forcing increases the upwelling current and limits the offshore growth of the filament;
• •modifying the promontory characteristics (width, length, height and slopes) has strong impact on the filament evolution, sometimes leading to a multipolarisation of the potential vorticity anomaly structure which results in much more complicated patterns in the upper layer (numerous shorter and less coherent filaments). This shows that only specific promontory shapes can lead to the formation of well defined filaments;
• •adding bottom friction introduces a slight generation of potential vorticity in the bottom layer over the promontory, but does not significantly alter significantly the formation of the filament along the outcropped front in the present configuration;
• •modifying the stratification characteristics, in particular the density jump between the layers, has only a weak influence on the dynamics of topographic eddies and on filament formation;
• •the influence of capes is also modest in our simulations, showing that topography plays the major role in the formation of long and trapped upwelling filaments.

     

Distribution des sédiments sur la marge du golfe de Tehuantepec (Pacifique Oriental). Exemple d'interaction tectonique-eustatisme

During the PROGEMA I cruise which took place on the south-western Pacific coast of Mexico (between 14° and 16° N, 92° and 96° W), 3.5 kHz echosounding profiles, surface samples and cores as well as hydrological and oxygen minimum measurements down to 2250 m water depth were made in the Gulf of Tehuantepec. Chemical, sedimentological, mineralogical, X-ray and BEM analyses of waters, core and surface samples provided new data which permit a better knowledge of the Gulf of Tehuantepec margin, where tectonic activity is due to an active subduction.On the shelf of the gulf, the distribution pattern of quaternary deposits is affected by a double tectonic forcing and by eustatic oscillations of the sea level. The first tectonic control is linked to the existence of north-south tectonic troughs in the Tehuantepec Isthmus, in which the atmospheric circulation between the Gulfs of Mexico and Tehuantepec is accelerated by the Venturi effect. The resulting winds, called “Tehuanos”, play an important role in the forcing of surface circulation in the Gulf of Tehuantepec, inducing a deviation of the general circulation and thus forcing the distribution pattern of terrigenous sediment supply according to two physiographically and mineralogically distinct provinces, inherited from the past tectonic evolution, which are separated by the Tehuantepec axis.

• •-|A southeastern province related to the Caribbean Plate and fed by: (1) the rivers draining the Chiapas Massif; and (2) the Costa-Rica Current driving tropical and subtropical waters from Central America.
• •-|A northwestern province part of the North American Plate and fed by: (1) the Rio Tehuantepec which drains the Sierra de Oaxaca; and (2) the Californian Current.

The second tectonic control is linked to the tectonic structure of the margin influenced by:

• •-|shallow banks where the products of a present-day phosphatogenesis are concentrated because they are swept by upwellings. The latter are reinforced because the banks are situated south of the wind corridor;
• •-|tectonic depressions acting as sediment traps;
• •-|numerous canyons cutting the slope along brittle accidents, driving weak turbidity currents but which play an important role in the forcing of upwelling currents. Antarctic diatoms are found which are conveyed by the deep circulation from the South-Eastern Pacific and provide evidence that Antarctic Cold Water masses (AABW) rise towards the shelf.

The influence of eustatism is attested by the presence on the shelf of the Gulf of Tehuantepec of three paleoshorelines at 40 , 75 and 130 m water depth, linked to the Würmian sea level and to the postglacial sea level rise as on many passive margins.     

Jurassic subsidence history in the Hebrides,N.W. Scotland

Geohistory analyses have been applied to the generally well-dated Triassic-Jurassic successions of three areas in the Hebrides Basin, north-west Scotland, and the importance of making corrections for decompaction, palaeobathymetry and eustatic changes of sea level are assessed. The thermo-tectonic subsidence patterns in the Skye areas (north) suggest a three-phase history of crustal stretching, though only the first phase is well developed in the Mull area (south).Episodes of relatively rapid subsidence are:

• 1.(1) Late Triassic (c210 Ma) until earliest Toarcian (186 Ma);
• 2.(2) Latest Toarcian (180 Ma) until late Bathonian (c161 Ma);
• 3.(3) Early Oxfordian (154) Ma) until at least early Kimmeridgian (incomplete.).

These are separated by Toarcian (186-180 Ma) and latest Bathonian-Callovian(c161-154 Ma) episodes of either little subsidence or of uplift (but at slower rates than eustatic rise of sea level). The intervals between phases, at 30 Ma and 26 Ma, are remarkably similar but the wider significance of this remains to be established.     

The role of fines during acidizing treatments

Drilling, workover, injection and production operations can all cause physical impairment of the sandface and formation immediately around a wellbore. Matrix acidizing treatments are aimed at removing this type of damage. To obtain maximum damage removal most operators use in general a hydrochloric acid preflush followed by a mud acid main flush. During most acidising treatments large amounts of acids, usually of the order of one hundred gallons per foot of formation, are pumped into the reservoir, where various interactions occur with the matrix constituents, for instance:

• 1.• Cementing material is dissolved and clays and fines are released from their setting.
• 2.• Clays and fines are dissolved and gradually reduced in size.
• 3.• Acid-released clays and fines can be dislodged and plug pore throats.
• 4.• The pH of the spending acid increases and secondary precipitates can be formed.

The relative importance of the above processes varies for each treatment and depends on factors such as the spatial distribution of cement and minerals inside the porous network, the types and concentrations of the acids used, the temperatures at which the reactions take place and the rates at which the acids are displaced into the formation. These effects have been investigated at KSEPL in a series of core flooding experiments, carried out under representative conditions (pressure/rate/temperature), supported with modern petrographic techniques (quantitative SEM analyses).     

The FAO guidelines for the development and use of indicators for sustainable development of marine capture fisheries and an Australian example of their application

Guidelines for developing and implementing sustainability indicators for marine capture fisheries were drafted during a recent expert consultation jointly organised by Australia and FAO in Sydney (Australia). This paper provides an overview of the guidelines and illustrates their application with an Australian example. As a background to the guidelines, the concept of sustainable development (SD) for marine capture fisheries was discussed and an agreed set of definitions and usage of common terms developed. The guidelines then outline the five sequential steps that need to be addressed in developing a meaningful set of indicators in the context of a Sustainable Development Reference System (SDRS). The five steps are:

• 1.specifying the scope of the SDRS;
• 2.developing a framework to agree on components within the system;
• 3.specifying criteria, objectives, potential indicators and reference values;
• 4.choosing the set of indicators and reference values;
• 5.specifying the method of aggregation and visualisation.

These steps are further elaborated in terms of how to scope the SDRS, define the dimensions and hierarchical levels to be included in the system, set multiple objectives and link these to indicators and reference values (e.g. targets, thresholds and/or standards). The guidelines also provide some examples of possible indicators, criteria for selecting some indicators over others and present ways of aggregating and visualising the indicators so that progress towards achieving sustainable development can be communicated easily to decision-makers.     

Heat-flow regimes and the hydrate stability zone of a transient,thermogenic, fault-controlled hydrate system (Woolsey Mound northern Gulf of Mexico)

This study aims to constrain the base of the hydrates stability field in structurally complexsites using the case of Woolsey Mound, a fault-controlled, transient, thermogenic hydrates system, in Mississippi Canyon Block 118, northern Gulf of Mexico. We have computed the base of the hydrates stability field integrating results from a recent heat-flow survey, designed to investigate geothermal anomalies along fault zones which exhibit different fluid flux regimes. An advanced “compositional” simulator was used to model hydrate formation and dissociation at Woolsey Mound and addresses the following hypotheses:

• 1.Migrating thermogenic fluids alter thermal conditions of the Hydrate Stability Zone (HSZ), so heat-flow reflects fault activity;
• 2.Gas hydrate formation and dissociation vary temporally at active faults, temporarily sealing conduits for migration of thermogenic fluids;
• 3.High salinity and inclusion of thermogenic gases with higher molecular weight than methane produce opposite effects on the depth to the bottom of the hydrate stability zone.

Applications of results include identifying and quantifying hydrate deposits in shallow sediments using an interdisciplinary approach that includes multiple resolution seismic data evaluation, geological and geochemical groundtruthing and heat-flow analyses as a proxy for activity along faults.     

Characteristics of deep currents off Cape Shiono-misaki before and after formation of the large meander of the Kuroshio in 1981

Deep currents measured by moored current meters over the shelf-slope off Cape Shiono-misaki, Kii Peninsula during the period from 28 April, 1981 to 4 May, 1982 are analyzed to determine characteristics of the deep current before and after the large meander of the Kuroshio formed. The observed deep currents show some different characteristics between the periods before and after the formation of the large meander of the Kuroshio,i.e.:

1. The mean current direction over the shelf slope changed to westward after the meander was formed, though it was eastward at two offshore stations before the meander was formed.
2. The eddy kinetic energy, \(ke((\overline {u'^2 } + \overline {\upsilon '^2 } )/2)\) became large at all stations after the meander formed.
3. It appears that there were current variations in the period band shorter than 10 days which propagated offshore before the meander formed but inshore after the meander formed.
4. After the meander formed, the current variations with a period of O(25 days) were amplified at two of the three stations. The current variations in this period band showed high coherence among the three stations.

Data from tidal stations showed that sea level variations with a period of O(30 days) were also amplified along the south coast of Japan after the meander was formed. But sea level variations were not coherent with current variations in this period band.     

Distribution of pelagic blue-green algae in the North Pacific Ocean

The distribution of pelagic blue-green algae, especially ofTrichodesmium thiebautii, was investigated on the basis of the collection of theHakuhō Maru Cruise KH-69-4 along 155°W (50°N-15°S) in the North Pacific Ocean from September to November 1969.

1. Five species were identified:Trichodesmium thiebautii (most predominant),T. erythraeum, Oscillatoria sp.,Katagnymene spiralis andRichelia intracellularis.
2. T. thiebautii was most abundant in the western North Pacific central water and abundant next to it in the equatorial water, but it did not occur in the subarctic water.
3. T. thiebautii was ubiquitously distributed in the lower layer of 100–200 m in the equatorial water, though not in a large quantity.
4. T. thiebautii inhabited only the water warmer than 20°C. In its main habitat, nitrate and nitrite were almost zero, but ammonia and phosphate were present. There was not found any correlation between its occurrence and the salinity.
5. Blue-green algae were generally thinly populated in the water rich in diatoms.

     

Marine environmental studies on outbreaks of red tides in neritic waters

The cause of red tides appearing in neritic waters in Japan have been discussed by several workers in relation to the development of eutrophication. The author has been engaged in marine environmental studies on outbreaks of red tides, mainly, in the Seto Inland Sea since 1964 using a combination of the following approaches:

1. surveys of the red tides that have occurred,
2. elucidation of the oceanographical and biological characters of the water areas where red tides occur,
3. isolation of the red tide organisms for physiological studies of their growth,
4. chemical and biological assays of the growth promoting factors, and
5. estimation of the effects of pollutants on the growth of the red tide organisms.

The red tide organisms surveyed wereEutreptiella sp. (Euglenophyceae),Dictyocha fibura (Chrysophyceae),Chattonella marina, C. antiqua, Fibrocapsa japonica, Heterosigma akashiwo (Chloromonadophyceae),Prorocentrum minimum, P. triestinum, Gymnodinium mikimotoi, G. nelsoni, Gymnodinium type '65,Noctiluca miliaris, Protogonyaulax catenella, Gonyaulax polygramma, Peridinium quinquecorne (Dinophyceae) andMesodinium rubrum (Ciliata). Most of these species were isolated and cultured for physiological studies. Algal growth potential in sea water was assayed in Hiuchi Nada and Beppu Bay. Growth promoting activity was found in sea water about 2km off shore from the industrial zone which locates along the coast of Beppu Bay. The effects of industrial wastes were examined forEutreptiella sp.,Heterosigma akashiwo andProrocentrum triestinum. Sulfite pulp waste caused marked promotion of growth in these organisms. The growth promoting activity of the waste was elucidated by chelation of the waste with iron. After the oil spill which occurred at Mizushima Bay in 1974, a heavy red tide ofNoctiluca miliaris appeared, followed byProrocentrum minimum. Polycyclic and polar fractions in heavy oil separated by chromatography were suspected to be the growth promoting substances. Natural growth promoting chemicals, uracil and thymine, were found in interstitial water of bottom mud of Beppu Bay in concentrations of 58–120, and 28–83?g per 100 g dry mud, respectively. The mechanism of fish kills caused by red tides was another subject of study.     

UNEP’s Regional Seas Programme: approach,experience and future plans

The 1972 United Nations Conference on the Human Environment (Stockholm, UN Conference, 1972) outlined a ‘masterplan’ linking environmental assessment, environmental management and supporting measures as basic and inseparable elements of environmental actions plans. It also indicated the advantages of a regional approach in contributing to the solution of global problems. The Regional Seas Programme of UNEP (United Nations Environment Programme) was subsequently initiated in 1974 (UN General Assembly, 1972). Development of UNEP’s Regional Seas Programme demonstrated that the basic concepts formulated at Stockholm can effectively foster regional cooperation among interested States, and may benefit from the support of the United Nations system as a whole.The present Regional Seas Programme includes fourteen regions, comprising over 140 coastal States. It was conceived as an action-oriented programme encompassing a comprehensive, trans-sectoral approach to marine and coastal areas and to environmental problems, considering not only the consequences, but also the causes of environmental degradation. Each Regional Seas Programme is based on the needs of the region concerned. The Regional Seas Programmes promote the parallel development of regional legal agreements, and of action-oriented programme activities, as embodied in the action plans. The overall strategy to be followed, as defined by UNEP’s Governing Council (UNEP, 1974) is:

• 1. Promotion of international and regional conventions, guidelines and actions for the control of marine pollution and the protection and management of aquatic resources.
• 2. Assessment of the state of marine pollution, pollution sources and trends, and of the impact of pollution on human health, marine ecosystems and amenities.
• 3. Coordination of efforts with regard to environmental aspects of the protection, development and management of marine and coastal resources.
• 4. Support for education and training efforts to facilitate the full participation of developing countries in the protection, development and management of marine and coastal resources.

Because the goal of each Regional Seas Programme is to benefit the States of that region, governments are involved from the very beginning in the formulation of the action plan. After acceptance, the implementation of the programme is carried out, under the overall authority of the governments concerned, by national institutions nominated by their governments. Although the Regional Seas Programme is implemented primarily by government-nominated institutions, specialized United Nations bodies, as well as the relevant international and regional organizations, contribute to its formulation, and also provide assistance to these national institutions. UNEP acts as an overall coordinator for the development and implementation of regional action plans. In some cases, this role is limited to the initial phase of the activities.     

Microbiological and Chemical Studies in the Seas of Hiuchi and Bingo—II

The distribution of inorganic nitrogen compounds and the metabolic rates of these compounds by microorganisms as a whole were investigated in the Seas of Hiuchi and Bingo. The results obtained are as follows:

1. Of inorganic nitrogen compounds, the contents in sea water, those in bottom muds, the uptake or liberation rates of microorganisms as a whole in sea water, and the liberation rates from bottom muds to sea water are 0.2～4.0 μg at. N/l, 3～60 μg at.N/100 g, 0.01～0.5 μg at.N//lhr, and 0.3～1.9 μg at.N/100 cm²/hr, respectively, and these contents or rates of ammonia usually are the largest of these inorganic nitrogen compounds.
2. From the above-mentioned results and the others, it is suggested that the nitrogen in the seas circulates mainly in sea water itself and the course of nitrogen cycle, which passes through bottom muds, is not so important, and further that, of the cycle of inorganic nitrogen compounds, the main course is the course which ammonia is liberated from organic nitrogen compounds and it is immediately uptaked by microorganisms, and the course which it is oxidized to nitrate and the others are not so important.

     

A geological field trip to the Côte d'Ivoire-Ghana transform margin

During the Equanaute survey (June 1992), fourteen submersible dives were performed between 4950 and 2250 m water depths across the southern slope of the Côte d'Ivoire-Ghana Marginal Ridge (CIGMR), in the eastern Equatorial Atlantic. The CIGMR, a high-standing topographic marginal ridge along the Côte d'Ivoire-Ghana transform margin, is believed to result from a complex structural evolution due to the specific wrench-related rifting between Western Equatorial Africa and Northeastern Brazil, in Early Cretaceous times. In this paper we report and discuss geological observations made during dives, and sample analyses to resolve the lithology, paleoenvironmental conditions, age and origin of the CIGMR. The data help in better characterizing the different sedimentary and tectonic regimes which successively prevailed during the CIGMR formation and assessing the thermal regime operative during the fabrication and subsequent evolution of the margin. The thick sedimentary pile exposed along the southern CIGMR slope is made up of a repetitive clastic sequence indicative of a deltaic-to-prodeltaic environment. This sedimentary pile, of Early Cretaceous age, has recorded different stages of the transform margin structural evolution.

• 1.(1)|Syn- to post-lithification deformations first record extensional deformations related to the rifting of the adjacent northern divergent margin segment (the Deep Ivorian Basin).
• 2.(2)|Wrench tectonics has, at a later stage, produced intense fracturing and participated in local folding, chiefly detected upslope. The integrated studies of geological samples and in situ observations obtained during the Equanaute survey support models for transform margin evolution proposed mainly from geophysical data.

     

Simultaneous exposure of English sole (Parophrys vetulus) to sediment-associated xenobiotics: Part 2—chronic exposure to an urban estuarine sediment with added 3H-benzo[a]pyrene and 14C-polychlorinated biphenyls

• 1.1. The accumulation of aromatic hydrocarbons (AHs) and polychlorinated biphenyls (PCBs) kv English sole (Parophrys vetulus) exposed for up to 108 days to an urban (test) sediment was compared to that for English sole exposed to sediment from a reference area. The concentrations of identified AHs and PCBs in the test sediment were 32 μg/g dry weight and 2·2 μg/g dry weight, respectively. English sole exposed to test and reference sediments for 108 days had biliary concentrations of aromatic compounds fluorescing at BaP wavelengths of 0·6 μg/g, wet weight and hepatic concentrations of PCBs of 1·4 ± 0·6 μg/g wet weight which were ten and eight times, respectively, as great as those in reference sole. These results show that accumulation of AHs and PCBs from sediment by English sole is a significant route of uptake in contaminated environments.
• 2.2. ³H-benzo(a)pyrene (BaP) and ¹⁴C-PCBs were added to the test sediment to compare the uptake and metabolism of BaP to that of PCBs by sole. Steady-state concentrations of ³H-BaP and ¹⁴C-PCBs in tissues and fluids were reached by 4 days and 14 days, respectively. At steady state, the level of covalent binding of ³H-BaP to hepatic proteins in test sole was six times greater than that of ¹⁴C-PCBs, and there was significantly greater relative formation of BaP-glutathione (GSH) conjugates than PCB-GSH conjugates. Thus, the amount of BaP metabolized to reactive metabolites was greater than that for PCBs, even though the concentration of ¹⁴C-PCBs in liver of test sole was five times as great as that of ³H-BaP.
• 3.3. Trace amounts of ³H-BaP were also added to the reference sediment to determine how simultaneous exposure of English sole to those contaminants already present in the test sediment affected the metabolism and disposition of BaP. The ³H-BaP concentration ratios for liver to sediment and bile to sediment, as well as proportions of biliary BaP-GSH conjugates in test sole, were significantly greater than in reference sole. Thus, the formation and accumulation of potentially toxic metabolites of a carcinogenic AH (BaP) in sole liver were increased by simultaneous exposure to contaminants present in an urban sediment.

     

Abyssal current along the northern periphery of Shikoku Basin

Abyssal currents along the northern periphery of the Shikoku Basin south of Japan were measured by current meters moored off Cape Daio-zaki, Cape Shiono-misaki and Cape Ashizuri-misaki and on the eastern foot of the northernmost part of the Kyushu-Palau Ridge. Total length of observation off Cape Shiono-misaki was about five years including the periods of the Kuroshio large meander and no meander. Analyses of current data show:

1. Mean currents with a magnitude of 5–10 cm sec^?1 were observed during the whole observation period at all of current meters which were set 400 m above the sea bottom that was deeper than 4,500 m. The mean current for each current meter was parallel to the local bottom contour arond each station and was toward a direction looking the Nankai Trough (a trough located along the northern end of the Shikoku Basin) to the left.
2. At each station located above the shelf toe off Cape Daio-zaki and off Cape Shiono-misaki and on the foot of the Kyushu-Palau Ridge, the mean current increases with depth (a bottomward intensification of the mean current), and the vertical extent of the mean current is estimated to be about 2,000 m above the sea bottom.
3. At a station located at 2,600 m depth on the continental slope off Cape Shiono-misaki, no bottom-ward intensification of currents was observed.

These results strongly suggest that a steady abyssal flow exists in the depths deeper than about 3,000 m along the northern and northwestern peripheries of the Shikoku Basin. Existence of the abyssal circulation system is also suggested, at least, in the north of the Shikoku Basin.     

A submersible study in the western Blanco fracture Zone,N.E. Pacific: Structure and evolution during the last 1.6 Ma

During July and August 1991, the French-American Blanconaute dive program used the French submersibleNautile to investigate the West Blanco Depression (WBD), a deep, elongate trough located at the intersection of the Blanco Transform Fault Zone with the southern Juan de Fuca Ridge (JdFR). Twenty dives were carried out along the north wall of the WBD, which exposes the upper oceanic crust over a 65 km distance, from the JdFR axis (to the west) to the oblique trace of an ancient propagator (to the east, crustal age around 2 Ma). Thirteen of these dives were precisely located within a 13 × 7 km zone of the north wall, covered by a high-resolution sonar mapping operation during the Blancotrough cruise in 1987. This series of geological traverses, plus 4 dives across the south wall of the WBD (one dive) and the adjacent Parks Plateau (3 dives), collected 242 rock samples. We report here the main results of the dive program and preliminary laboratory studies:

1. Transform-related tectonic activity has recently abandoned the southern margin of Parks Plateau, and is presently located inside the WBD area, mainly along its northern wall. The tectonic features observed are compatible with a right-lateral strike-slip system, with a NE-SW extensional component.
2. Three main lithological units are exposed along the north wall of the WBD. From top to bottom, they are: (1) a Volcanic Unit, forming a steep upper cliff, made of massive and pillow flows and basaltic dikes, with an estimated average thickness of 800 m; (2) a less steep Transition Zone, about 150 to 400 m thick, largely masked by rubble but exposing both diabase outcrops and pillow flows; and (3) a massive Diabase Unit, exposed over 700–800 m, with a dike complex structure visible from place to place, and cut by a net of hydrothermal veins. Deep crustal rocks such as gabbros were not observed.
3. Spectacular mass-wasting features are visible all along the north wall of the WBD. About 60% of the face of the wall is masked by talus cones, rubble, rock avalanche deposits and slide blocks. Three main landslides, of approximately one km³ in volume each, were tentatively identified. One of them was mapped in detail and consists of an approximately 300 m thick (0.85 km³), coherent slide block detached from a zone where intense hydrothermal alteration and faulting have obviously weakened the bedrock, that is in places entirely altered to blue clays.
4. The basaltic lavas of the WBD north wall show a remarkable evolution with time, from east to west. Around the tip of the ancient propagator, they are restricted to primitive, olivine-rich picritic basalts. Proceeding westward, they exhibit a wide range of differentiation, including highly fractionated, FeTi-rich ferrobasalts at about 35–45 km from the JdFR axis. When approaching the JdFR axis, the FeTi enrichment decreases gradually, and the ferrobasalts evolve towards slightly differentiated MORB-type basalts, typical of the southern JdFR. This magmatic evolution marks the transition from the end of a propagating rift regime to a steady-state accretion regime.
5. The WBD north wall also permits the study of weathering and hydrothermal alteration processes and their evolution in space and time. Vertically, the alteration products evolve from oceanic weathering and zeolite facies (Volcanic Unit) to the greenschist facies (Transition Zone and Diabase Unit). Horizontally, the evolution with time is mainly a general hydration of the crust that is, however, very irregularly distributed.
6. Several vertical magnetic traverses along the north wall of the WBD, using a bottom magnetometer attached to the basket of the submersible, have shown a sharp 5000 to 7000 nT positive anomaly at about 3500 m depth. This anomaly corresponds exactly to the first appearance of extrusive pillow-lava outcrops, and confirms the dramatic decrease in magnetic anomaly amplitude below that depth, detected during the Blancotrough cruise in 1987. The vertical magnetic profiles thus appear to have imaged the base of the magnetic source layer.

     

Hydrographic structure in association with deep boundary current in the north of the Shikoku Basin

High quality CTD data were collected in the north of the Shikoku Basin where an abyssal boundary current has been observed through direct current measurements. Analyses of hydrographic data showed:

1. Colder and saltier water (heavier water) compared to surrounding waters is found above the continental shelf-toe and the eastern flank of the Kyushu-Palau Ridge where the existence of the abyssal boundary current has been expected. The heavier water has a horizontal extent of about 50 km.
2. The heavier water has the vertical scale of 2000 m from the sea bottom, and is associated with a thermal wind shear which enhances a component of the flow toward a direction looking the Nankai Trough (a trough located along the northern end of the Shikoku Basin) to the left in the abyss. The assumed “level of no motion” at about 2500 m depth gives the geostrophically estimated current in a good agreement with the directly measured current.

A volume transport associated with the colder and higher salinity water is estimated to be about 2 Sv off Cape Shiono-misaki which may include a recirculation above the Nankai Trough. This is about twice of the transport estimated in the interior of the Shikoku Basin through a vorticity balance between the stretching term and latitudinal variation of the planetary vorticity.     

On the mean sea levels at various locations along the coasts of Japan

The yearly variations of mean sea levels at various locations along the coasts of Japan have been investigated, based upon the data of observation taken at 55 tide-gauge stations from 1953 to 1970, and some discussions have been made on the relationships between those variations and meteorological and oceanographical conditions, etc. The results obtained are as follows:

1. In cases covering 90% of all the tide-gauge stations, the magnitude of the annual mean rate of variation of ground level is less than 10 mm/year, and the mean value of the magnitude for all the stations is ?3.17 mm/year, while that exclusive of Osaka is ?1.7 mm/year.
2. The effect of atmospheric pressure variation on the height of annual mean sea level is of the order of several centimeters in view from both time and place.
3. In view of the characteristic types of variations, the coasts of Japan may be divided into five regions of similar mean sea level deviations. And in the yearly variations of mean sea levels, there can be seen a kind of variation which corresponds to the variation of oceanographical conditions such as abnormal fall of seawater temperature.
4. The mean sea level deviations at various locations along the coasts of Japan, referred to the standard sea level “T.P.” are different for different places. Namely, (1) on the coast of the Japan Sea, the west coast of Kyushu, the coast of Sanriku and the coast of Hokkaido, the mean sea level is higher than on the Pacific coasts from Southern Kyushu to Southern Honshu. (2) Along the coast of the Seto-Inland Sea, the mean sea level is generally higher.

As for the leading causes of the above deviations, we may safely enumerate the following ones, viz. (a) the effect of the deflecting force of the earth's rotation on currents, and (b) the effect of variation of seawater density. However, as regards the relative importance of these two effects, no decisive conclusion can yet be given for the present.     

Trichodesmium in the East China Sea

A study is made on the surface distribution ofTrichodesmium thiebautii Gomont in the East China Sea during the years 1961–1967.

1. This alga is very scarce or absent in the period from January to March, and begins to increase in May and June, reaching the maximum population density from July to September, and again decreases in October and November.
2. In summerT. thiebautii is predominantly distributed in the area along the Kuroshio current with high densities of 10²–10³ filaments/l, occasionally exceeding 10³ filaments/l. This alga is estimated to occupy the major part or almost all the parts of chlorophylla in situ contained in the surface water of this season.
3. The occurrence ofT. thiebautii off the southern coasts of Central Japan seems to be caused rather by the multiplication in its own habitat than by the transportation by the Kuroshio current from the East China Sea.