The Mid-atlantic Ridge (31°S–34°30′S): Temporal and spatial variations of accretionary processes

The ridge located between 31° S and 34°30′S is spreading at a rate of 35 mm yr⁻¹, a transitional velocity between the very slow (≤20 mm yr⁻¹) opening rates of the North Atlantic and Southwest Indian Oceans, and the intermediate rates (60 mm yr⁻¹) of the northern limb of the East Pacific Rise, and the Galapagos and Juan de Fuca Ridges. A synthesis of multi-narrow beam, magnetics and gravity data document that in this area the ridge represents a dynamically evolving system. Here the ridge is partitioned into an ensemble of six distinct segments of variable lengths (12 to 100 km) by two transform faults (first-order discontinuities) and three small offset (< 30 km) discontinuities (second-order discontinuities) that behave non-rigidly creating complex and heterogeneous morphotectonic patterns that are not parallel to flow lines. The offset magnitudes of both the first and second-order discontinuities change in response to differential asymmetric spreading. In addition, along the fossil trace of second-order discontinuities, the lengths of abyssal hills located to either side of a discordant zone are observed to lengthen and shorten creating a saw-toothed pattern. Although the spreading rate remains the same along the length of the ridge studied, the morphology of the spreading segments varies from a deep median valley with characteristics analogous to the rift segments of the North Atlantic to a gently rifted axial bulge that is indistinguishable from the shape and relief of the intermediate rate spreading centers of the East Pacific Rise (i.e., 21°N). Like other carefully surveyed ridge segments at slow and fast rates of accretion, the along-axis profiles of each ridge segment are distinctly convex upwards, and exhibit along-strike changes in relief of 500m to 1500 between the shallowest portion of the segment (approximate center) and the segment ends. Such spatial variations create marked along-axis changes in the morphology and relief of each segment. A relatively low mantle Bouguer anomaly is known to be associated with the ridge segment characterized by a gently rifted axial bulge and is interpreted to indicate the presence of focused mantle upwelling (Kuo and Forsyth, 1988). Moreover, the terrain at the ends of each segment are known to be highly magnetized compared to the centers of each segment (Carbotte et al, 1990). Taken together, these data clearly establish that these profound spatial variations in ridge segment properties between adjoining segments, and along and across each segment, indicate that the upper mantle processes responsible for the formation of this contrasting architecture are not solely related to passive upwelling of the asthenosphere beneath the ridge axis. Rather, there must be differences in the thermal and mechanical structure of the crust and upper mantle between and along the ridge segments to explain these spatial variations in axial topography, crustal structure and magnetization. These results are consistent with the results of investigations from other parts of the ridge and suggest that the emplacement of magma is highly focused along segments and positioned beneath the depth minimum of a given segment. The profound differences between segments indicate that the processes governing the behavior of upwelling mantle are decoupled and the variations in the patterns of axis flanking morphology and rate of accretion indicate that processes controlling upwelling and melt production vary markedly in time as well. At this spreading rate and in this area, the accretionary processes are clearly three-dimensional. In addition, the morphology of a ridge segment is not governed so much by opening rate as by the thermal structure of the mantle which underlies the segment.     

Large-scale vertical hydroacoustic structure of the North Atlantic and its seasonal variability

A classification based on the number and types of large-scale acoustic waveguides is proposed for the mean seasonal profiles of sound speed propagation. A scheme for North Atlantic zoning, using typical curves of the sound speed vertical distribution, is given. The channel axis's position is shown not to depend on the water mass haline properties, being controlled by the temperature field vertical stratification.Translated by Vladimir A. Puchkin.     

High-resolution seafloor mapping using the DSL-120 sonar system: Quantitative assessment of sidescan and phase-bathymetry data from the Lucky Strike segment of the Mid-Atlantic Ridge

High-resolution, side-looking sonar data collected near the seafloor (100 m altitude) provide important structural and topographic information for defining the geological history and current tectonic framework of seafloor terrains. DSL-120 kHz sonar data collected in the rift valley of the Lucky Strike segment of the Mid-Atlantic Ridge near 37° N provide the ability to quantitatively assess the effective resolution limits of both the sidescan imagery and the computed phase-bathymetry of this sonar system. While the theoretical, vertical and horizontal pixel resolutions of the DSL-120 system are <1 m, statistical analysis of DSL-120 sonar data collected from the Lucky Strike segment indicates that the effective spatial resolution of features is 1–2 m for sidescan imagery and 4 m for phase-bathymetry in the seafloor terrain of the Mid-Atlantic Ridge rift valley. Comparison of multibeam bathymetry data collected at the sea-surface with deep-tow DSL-120 bathymetry indicates that depth differences are on the order of the resolution of the multibeam system (10–30 m). Much of this residual can be accounted for by navigational mismatches and the higher resolving ability of the DSL-120 data, which has a bathymetric footprint on the seafloor that is 20 times smaller than that of hull-mounted multibeam at these seafloor depths (2000 m). Comparison of DSL-120 bathymetry with itself on crossing lines indicates that residual depth values are ±20 m, with much of that variation being accounted for by navigational errors. A DSL-120 survey conducted in 1998 on the Juan de Fuca Ridge with better navigation and less complex seafloor terrain had residual depth values half those of the Lucky Strike survey. The quality of the bathymetry data varies as a function of position within the swath, with poorer data directly beneath the tow vehicle and also towards the swath edges.Variations in sidescan amplitude observed across the rift valley and on Lucky Strike Seamount correlate well with changes in seafloor roughness caused by transitions from sedimented seafloor to bare rock outcrops. Distinct changes in sonar backscatter amplitude were also observed between areas covered with hydrothermal pavement that grade into lava flows and the collapsed surface of the lava lake in the summit depression of Lucky Strike Seamount. Small features on the seafloor, including volcanic constructional features (e.g., small cones, haystacks, fissures and collapse features) and hydrothermal vent chimneys or mounds taller than 2 m and greater than 9 m² in surface area, can easily be resolved and mapped using this system. These features at Lucky Strike have been confirmed visually using the submersible Alvin, the remotely operated vehicle Jason, and the towed optical/acoustic mapping system Argo II.     

An evaluation of systems for the integrated assessment of capture fisheries

Integrated fishery assessments are intended to take an integrated and wholistic view of fisheries management, to provide comprehensive systems-based evaluations of the sustainability of individual fisheries and their products. Using guidance derived from decision theory and management science, we develop 18 criteria to evaluate the likely effectiveness of nine integrated fishery assessment systems. The assessment systems do not consistently consider four key criteria: the socio-economic impacts of fisheries, the provision of food security, the local relevance of the assessment to the fishery under assessment, and the independent peer review of assessment outcomes. Other important areas of weakness include the lack of appropriate consultation processes relating to both the design of the assessment system and the outcomes of fishery assessments, and limitations in the quality of data and information used in assessments. Improvements will be needed in these areas across all the systems we evaluated if fishers, consumers, managers and other stakeholders are to be provided with consistently reliable assessment outcomes, and an assurance of the sustainability of capture fisheries.     

Shallow-Water Gasohydrothermal Vents of Ushishir Volcano and the Ecosystem of Kraternaya Bight (The Kurile Islands)

Abstract. A marine ecosystem in the crater of the Ushishir Volcano (Kraternaya Bight, Yankich Island, the Kuriles) showing gasothcrmal activity was studied for hydrographical, hydrochemical, and biological characteristics. Maximal changes in chemical and biological characteristics were observed in the surface water layer of 0–5 m. This layer had higher water temperature, was saturated with volcanogcnic carbon dioxide (up to 2000 10^-6 torr), ovcrsaturatcd with oxygen (up to 200 %), and contained high concentrations of chlorophyll a. Hydrogen sulfide was found in the surface water layer and at a depth of 15 m in the area of maximal effect of volcanic effluents.
The planktonic community is characterized by high rates of production and destruction of organic matter only in the 0–5 m layer. Daily vertical migrations of the ciliatc Mesodinium rubrum were observed, which caused "red tides".
Bacterial, algobacterial, and diatom mats developed on the bottom of the bight in the zone of gasohydrothermal vents and in areas of volcanic water seeping. The rate of organic matter production in algobacterial mats reached 33.4g C-rrr²-d^-l, chcmolithotrophy predominating. Bottom settlements had high population density and great animal biomass, which reached 10 kg m^-2 in gasohydrothermal fields. They obtain sufficient energy from primary production of microphy-tobenthos, algobacterial, and bacterial mats. Bcnthic species dominant in the bight were not found off the Ushishir Islands.
Some species of macrobenthos inhabiting the Kraternaya Bight differ markedly in size and biomass from the same species found beyond the bight.     

Validation of a time-domain strip method to calculate the motions and loads on a fast monohull

The paper presents a comparison between experimental data and numerical results of the hydrodynamic coefficients and also of the wave induced motions and loads on a fast monohull model. The model with 4.52 m length was constructed in Fibre Reinforced Plastic (FRP), and made up of 4 segments connected by a backbone in order to measure sectional loads. The objective of the investigation was to assess the capability of a nonlinear time domain strip method to represent the nonlinear and also the forward speed effects on a displacement high speed vessel advancing in large amplitude waves. With this objective in mind the experimental program included forced oscillation tests in heaving and pitching, for a range of periods, three different amplitudes and several speeds of advance. In head regular waves comprehensive ranges of wave periods, wave steepness and speeds, were tested in order to measure heave, pitch and loads in three cross sections.

The numerical method assumes that the radiation and diffraction hydrodynamic forces are linear and the nonlinear contributions arise from the hydrostatics and Froude–Krilov forces and the effects of green water on deck. The assumption of linearity of the radiation forces is validated by comparing calculated hydrodynamic coefficients with experimental data for three different amplitudes of the forced oscillations. Both global coefficients and sectional coefficients are compared. The motions and loads in waves are compared in terms of first and higher harmonic amplitudes and also in terms of sagging and hogging peaks.     

On the use of grate-turbulators for tests of microstructural probes

Highly sensitive microstructural probes subjected to noise disturbances are free of these in laboratory tests. This leads necessarily to the working out of methods for complex testsin situ. The paper reports on the successful use for this purpose of a grate (net) suspended from a free-falling probe. The grate sets up turbulence with reproducible structure in the area of sensor location.UDK 551.46.085