Geological interpretation of combined SEABEAM,GLORIA and seismic data from Porto and Vigo Seamounts,Iberian continental margin

Long-range sidescan sonar (GLORIA) data over Porto and Vigo Seamounts collected in 1978 has been re-interpreted in the light of SEABEAM bathymetric surveys conducted in 1982. The application of quantitative bathymetric information enables the interpreter to allow for artefacts inherent in the GLORIA data and to separate topography-related primary backscattering variations on the sonographs from more subtle changes that result from textural, slope and outcrop effects. The distinctions are made easier when slant-range corrected GLORIA data are available.Use of the combined survey data to precisely locate seismic profiling tracks and to identify likely areas of outcrop has allowed refined geological maps of the seamounts to be drawn and regional fault trends detected. The overall outline of the seamounts appears strongly fault-controlled.Porto and Vigo Seamounts are made up of the same geological formations and have had a similar structural history since their uplift as continental fault blocks in the Late Cretaceous to Middle Eocene period. Ravines that dissect the presumably lithified scarps bounding the seamounts may be relict features but still appear to control sediment input to gulley and channel systems in the surrounding topography. Sedimentary ridges associated with the seamounts represent anomalously thick sequences of post-Eocene material and probably result from interaction of downslope sedimentary processes and contour-following boundary currents.Contribution No. 274 from the Groupe d'Etude de la Marge Continentale (ERA 605).     

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.     

Interference fringes on GLORIA side-scan sonar images from the Bering Sea and their implications

GLORIA side-scan sonographs from the Bering Sea Basin show a complex pattern of interference fringes sub-parallel to the ship's track. Surveys along the same trackline made in 1986 and 1987 show nearly identical patterns. It is concluded from this that the interference patterns are caused by features in the shallow subsurface rather than in the water column. The fringes are interpreted as a thin-layer interference effect that occurs when some of the sound reaching the seafloor passes through it and is reflected off a subsurface layer. The backscattered sound interferes (constructively or desctructively) with the reflected sound. Constructive/destructive interference occurs when the difference in the length of the two soundpaths is a whole/half multiple of GLORIA's 25 cm wavelength. Thus as range from the ship increases, sound moves in and out of phase causing bands of greater and lesser intensity on the GLORIA sonograph. Fluctuations (or wiggles) of the fringes on the GLORIA sonographs relate to changes in layer thickness. In principle, a simple three dimensional image of the subsurface layer may be obtained using GLORIA and bathymetric data from adjacent (parallel) ship's tracks. These patterns have also been identified in images from two other systems; SeaMARC II (12 kHz) long-range sonar, and TOBI (30 kHz) deep-towed sonar. In these, and other cases world-wide, the fringes do not appear with the same persistence as those seen in the Bering Sea.     

Intrusive Stratification of the Gulf-Stream Frontal Zone According to the Results of Investigations Performed during Cruise 43 of the R/V Akademik Vernadskii

We analyze the data of investigation of the intrusive structure of the Gulf-Stream frontal zone obtained in making frequent drift sections with the help of an MGI-8102 probing complex, study the regularities of variation of temperature, salinity, and density along separate intrusions, and present a series of results connected with the specific features of initiation and development of intrusions and the types of exchange processes determining their transformations. It is shown that the T-diagrams of all intrusions are well separated into segments with different slopes. Moreover, by comparing the slopes and locations of these segments with each other and with the T-diagrams of the Gulf Stream and slope waters, we can fairly reliably attribute the corresponding segments of intrusions to one of the following four types: initial dynamic folds of the frontal zone, layers of domestic water separating the intrusive segments of foreign water from each other, intrusive segments characterized by the penetration of ambient water, and segments not intrusive initially but getting the required slope as a result of interaction with upper and lower intrusive segments. For segments of the last two types, it is possible to specify the predominant type of exchange.     

Heat balance of the upper ocean under a land and sea breeze in Sagami Bay in summer

The heat balance of the upper ocean under a land and sea breeze was investigated based on observations of sea water temperature in the upper 300 m layer and heat flux across the sea surface at a fixed station in Sagami Bay (3510N, 13925E) during two periods of two days in August 1980 and three days in August 1981. During both periods, a typical land and sea breeze of 4–6 m sec^–1 at maximum prevailed in the observation area. Large diurnal variation of sea surface temperature with a maximum peak around noon LST was observed during both periods (the daily value of the range was 0.9C and 2.5C in 1980, and 1.2C, 1.5C and 1.7C in 1981). It was found that these large temperature variations were caused by diurnal variation of the wind speed which dropped to 0–3 m sec^–1 at noon when the strongest insolation (–270 Wm^–2) penetrated the sea and at midnight in association with alternations of the land breeze and the sea breeze. On the other hand, vertical mixing of the sea water caused by the wind stress and/or convection due to cooling at night extended down only to the surface 10 m layer. Horizontal heat advection was negligibly small. Therefore the local time change of the heat content in the upper 10 m water column was affected mainly by the heat flux across the air-sea interface which was estimated from data on radiation fluxes measured directly on board and latent and sensible heat fluxes calculated by the aerodynamic bulk method. The water temperature below the 10 m layer also varied with time and the temperature variation in the thermocline (20–50 m depth) was frequently larger than that of the sea surface temperature. However, the variation in the upper 10 m layer was little influenced by that below the layer.     

The status of geological dredging techniques

Scientific sea-floor dredging is currently used in marine geology primarily by the hard-rock community interested in the recovery of basement rock samples from the unsedimented deep ocean floor. The technique has generally been eclipsed by ocean drilling for recovery of sedimentary rocks, because of perceived uncertainties in the location of sampling and in the representativeness of recovered material. This contribution reviews dredging equipment currently in use by marine geological institutions and refers to pinger attachments that allow precise information on the behaviour of the dredge to be telemetered back to the ship. We argue that improvements in ship navigation and transponder navigation at the seafloor, when used in conjunction with surface and/or deeply towed sidescan and swathemapping surveys, now allow for considerably less uncertainty on the location of dredge sampling. Refined sorting criteria for dredge hauls are now also available. Recent comparisons of regional sample recovery by ocean drilling and by dredge sampling indicate that the dredge hauls can usefully supplement the drilling data in the construction of sedimentary and tectonic histories of seafloor areas.     

Seamount gravity anomaly modelling with variably thick sediment cover

Inversion modelling of marine gravity anomalies to derive predicted seafloor topography has provided significant advance in delineating deep-ocean bathymetry where the seafloor both conforms to the half-space cooling model of seafloor spreading, and largely sediment-free. Similar modelling for elevated ridges and seamounts, that are formed by processes other than seafloor spreading and/or have proximal sediment sources (e.g., continental margins and volcanic arcs), have significantly higher errors when validated against modern shipborne echo-sounding data. A three-dimensional, five-layer gravity model is emulated for the cases of both synthetic and real seamounts, with varying degrees of sediment burial, to establish the sensitivity of variable sediment cover as a source of error. A simple `Gaussian' seamount with base radius of 30 km, 2000 m of relief, has a maximum 140–160 mGal anomaly, that decreases to 50 mGal with the addition of 1 km of sediment cover with simple `flood' geometry. Complete burial, with a typical sediment density of 2300 kg m^–3, results in a 120 mGal difference from a sediment-free seamount model. Increasing sediment density results in an exponential decay of the seamount anomaly. More complex synthetic geometries of varying basement relief and sediment thickness show that the anomaly amplitude remains significant, especially where the latter is >700–800 m thick. For the real case, seamounts of the Three Kings Ridge (northern New Zealand) imaged with seismic reflection data, with varying degrees of sediment cover of up to 1 km, when modelled both with and with-out the inclusion of a sediment layer, typically have rms differences of 30 mGal between observed and modelled gravity anomalies. Significantly, the rms errors are reduced by 50% with the inclusion of a sediment layer that corresponds to a reduction of predicted seafloor topography rms errors of 192–684 m to 78–360 m.     

The mechanism of the development of wind-wave spectra

The mechanism of the development of wind-waves will be proposed on the basis of the observed wave spectra in the wind tunnels and at Lake Biwa (Imasato, 1976). It consists of two aspects: One is that the air flow over the wind-waves transfers momentum concentratively to the steepest component waves and the other is that the upper limit of the growth of a wave spectral density is given by the ultimate value in the slope spectral density. The first aspect means that the wave field has the momentum transfer filter on receiving the momentum from the air flow. Wind-waves in the stage of sea-waves receive the necessary amount of momentum by the form drag,e.g. according to the Miles' (1960) inviscid mechanism, through a very narrow frequency region around a dominant spectral peak. On the other hand, wind-waves in the stage of initial-wavelets receive it according to the Miles' (1962a) viscous model through a fairly broad frequency region around the peak. The upper limit ofS _max developing according to viscous mechanism is given byS _max =6.40×10^–4 k _max ^–2cm²s andS _max =2.03C(f _max )^–2cm²s(S _max is the power density of the wave spectral peak with the frequencyf _max ,k _max is the wave number corresponding to the frequencyf _max andC is the phase velocity).From the second aspect, the upper limit of the growth of wave spectral density is given by 33.3f ^–4cm²s in the frequency region of late stage of sea-waves. Therefore, the spectral peak, which has the largest value in the slope spectral density in the component waves of the wave spectrum, rises high over the line 4.15f ^–5cm²s. The energy is transported from the spectral peak to the high frequency part and to the forward face of a wave spectrum by nonlinear wave-wave interaction. This nonlinearity is confirmed by the bispectra calculated from the observed wind-wave data. In the stage of sea-waves, nonlinear rearrangement of the wave energy comes from a narrow momentum transfer filter, and, in the stage of initial-wavelets, it comes mainly from small corrugations and small steepness of the wave field.     

Reflection and breaking of internal waves on a sloping beach

The reflection and breaking of internal waves on a sloping beach were studied in a small wavetank filled with water and petroleum. The dependence of the reflection coefficient of the internal waves on wave steepness and on beach slope is found to be very similar to that of surface waves. The reflection coefficient is small for the very gentle slope, increases rapidly as the slope increases, and becomes almost constant for the steep slope. The reflection coefficient decreases with increase of the wave steepness. Also, the transition slope at which the coefficient curve has the maximum gradient increases with increase of the wave steepness. Breaking pattern of the internal waves is classified into four types; breaking, semi-breaking, wrinkle-generating, and non-breaking. Their dependence on beach slope and wave steepness is examined. The regular sequence of the four breaking types from breaking to non-breaking is observed with decrease of wave steepness or with increase of beach slope.     

A stratigraphic and tectonofacies framework of the “calcari aLucina” in the Apennine Chain,Italy

The Apennine Chain provides the first example of stratigraphic (time) and synsedimentary tectonic (space) distribution of the calcari aLucina Miocene equivalents of modern cold-vent carbonates. Chemosynthetic faunal assemblages and related carbonate deposits are found at different stratigraphic levels, with peaks during Langhian-Serravallian and late Tortonian-early Messinian times. A general increase in frequency and volume occurs with time. A genetic link between venting and the Messinian Evaporite event is difficult to demonstrate. However,Lucina limestones are limited to preevaporitic times, and their maximum abundance is reached just before the onset of the Messinian Evaporite accumulation.Lucina limestones occur in almost all tectofacies of the orogen, from backland to foreland.     

Deposition Rates of Terrestrial and Marine Organic Carbon in the Osaka Bay, Seto Inland Sea, Japan, Determined Using Carbon and Nitrogen Stable Isotope Ratios in the Sediment

Carbon and nitrogen stable isotope ratios (¹³C and ¹⁵N) of surface sediments were measured within Osaka Bay, in the Seto Inland Sea in Japan, in order to better understand the sedimentation processes operating on both terrestrial and marine organic matter in the Bay. The ¹³C and ¹⁵N of surface sediments in the estuary of the Yodo River were less than –23 and 5 respectively, but increased in the area up to about 10 km from the river mouth. At greater distances they became constant (giving ¹³C of about –20 and ¹⁵N about 6). It can be concluded that large amounts of terrestrial organic matter exist near the mouth of the Yodo River. Stable isotope ratios in the estuary of the Yodo River within 10 km of the river mouth were useful indicators allowing study of the movement of terrestrial organic matter. Deposition rates for total organic carbon (TOC) and total nitrogen (TN) over the whole of the Bay were estimated to be 63,100 ton C/year and 7,590 ton N/year, respectively. The deposition rate of terrestrial organic carbon was estimated to be 13,200 (range 2,000–21,500) ton C/year for the whole of Osaka Bay, and terrestrial organic carbon was estimated to be about 21% (range 3–34) of the TOC deposition rate. The ratio of the deposition rate of terrestrial organic carbon to the rate inflow of riverine TOC and particulate organic carbon (POC) were estimated to be 19% (range 3–31) and 76% (range 12–100), respectively.     

Dissolved carbon and 33-133-133-1anomalies in the water column caused by hydrocarbon seeps on the northwestern Gulf of Mexico slope

Alkalinity, dissolved inorganic carbon (DIC), and ¹³C profiles from seep sites on the northwestern Gulf of Mexico upper slope show anomalously negative ¹³C values of up to –4.5 PDB, increased levels of DIC of up to 2.45 mmole/liter, and slight alkalinity rises of up to 2.54 meq/liter, relative to water column profiles from a seep-free site (0.63, 2.04 mmole/liter, and 2.39 meq/liter). The observed DIC enrichments coupled with the¹³C-depletions are attributed to the release of CO₂ by microbial oxidation of crude oil in the seep environment, and its migration into the water column. The ¹³C composition of the migrating CO₂ is estimated to be –26.0 on the basis of dissolved carbon inventory. Manifestation of DIC and ¹³C anomalies in the water column caused by hydrocarbon seepage holds promise to be useful for hydrocarbon reconnaissance surveys over large offshore tracts on account of the simplicity of sampling acquisition, and rapidity of analytical techniques in the laboratory.     

Practical Formulas for Intercoupling of Pressure and Depth in the Black Sea

On the basis of the generalization of the concept of standard ocean to the Black Sea, we deduce practical formulas for the conversion of pressure into depth and vice versa depending on the latitude of the place with regard for the equation of state for 1980. The error of these relations for the standard Black Sea (whose salinity is equal to 22.2 at a temperature of +9°C from the surface to the bottom) does not exceed ±0.2m and ±0.2dbar. The difference between the practical and actual depths in winter and summer periods does not exceed ±0.35m for depths varying within range 0–2000m. The proposed practical formulas enable one to simplify the procedure of rapid evaluation of depth (or pressure) in real time by excluding the procedure of integration over a specific volume.     

A note on small organism diffusion around an attractive center: A mathematical model

A mathematical model of small organism diffusion around an attractive center is presented. The diffusion equation includes a forcing term which creates a virtual flow of organisms toward the center. Owing to the attractive force, the organisms tend to collect in swarms or patches notwithstanding the diffusive motion of surrounding water.Contribution No. 176 of the Chesapeake Bay Institute, Department of Earth and Planetary Sciences, The Johns Hopkins University. This work was supported by the Office of Naval Research Contract N00014-67-A-0163-0006, Research Project NR083-016, by the U.S. Atomic Energy Commission under Contract AT (30-1) 3109, Document COO-3109-2, and by National Science Foundation Grant, GA-16603.     

The evolution of isolated vortices interacted with steep slope

Numerical solutions are examined for isolated, intense vortices as influenced by western bounding bottom topography through the use of a rigid-lid, two-layer primitive -plane numerical model. Systematic studies are made of the sense of rotation (cyclonic/anticyclonic), the consequence of varying the gradient of bottom slope, and the different vertical shear in a two layer ocean. In the basin with a bottom slope, the nearly barotropic anticyclonic vortex forms a modon-like vortex for S with fixedRo ₂<O(1) (where is the ratio between the variation of the Coriolis parameter across the eddy to the Coriolis parameter in the center, S the topographic effect and,Ro ₂ the Rossby number in the lower layer) and its generation is due to a compound effect of the planetary beta, topographic beta, avvection, and mirror image. The formation of the modon-like vortex and the propagation of the original vortex onto the bottom slope depends on the strength of slope gradient and the baroclinicity of the vortex. The nearly barotropic anticyclonic vortex evolves into the stronger upper ocean one with increasing S: the gradient of the bottom slope becomes steeper. Then the original vortex lives longer because the barotropic component of the energy is converted to the baroclinic one and it moves toward southeast in forming a modon-like vortex in the lower layer. The evolution of a vortex in the model results are compared to observational results of a Kuroshio warm core ring (KWCR) obtained from hydrographic data (June, 1985) and from NOAA satellite infrared images (April, 1985 to July, 1985). It is shown that a KWCR (June, 1985) is influenced by the western continental slope/shelf of the East Japan.     

Classification of “siome”, streaks and fronts

Siome, which was firstly defined by Uda (1938); is a line of convergence on the sea surface. There are many kinds of siome in the sea. I propose in this paper that siome should be classified into streak and front. Streak is defined as the convergence within the same water mass and front the convergence between two different water masses. Streaks and fronts are classified into more details on the basis of their mechanisms of generation. The proposed classification will be useful for understanding the dynamics of siome and should help to promote interdisciplinary studies around siome regions.     

Vertical distribution of smaller macrobenthos and larger meiobenthos in the sediment profile in the deep-sea system of Suruga Bay (central Japan)

The vertical distribution of benthic organisms in the sediment profile was studied using horizontally sliced sediments collected at five stations at depths from 115 to 472 m in Suruga Bay, central Japan. Using sieves of 1.0 and 0.5 mm mesh, benthic organisms were divided into two size classes, smaller macrobenthos (>1mm, <1g wet weight) and larger meiobenthos (1.0 mm0.5 mm). The maximum depth of vertical distribution of organisms in the sediment profile was expressed by the 95 % intercept of the cumulative % curve of the number of individuals drawn with respect to depth in the sediment. It has long been supposed that benthic animals are concentrated in the surface centimeters of sediment in the deep-sea system, and the present study clearly substantiated this. Most benthic organisms of both of these two size classes were concentrated in the upper 5 cm of sediment. The vertical distribution was almost always deeper in the case of smaller macrobenthos than for larger meiobenthos. However the difference could not be substantiated statistically since the number of samples was insufficient. The maximum depth indices of polychaetes were found to be significantly larger than those of crustaceans in the case of macrobenthos, while in the case of meiobenthos, the difference was not significant. The maximum depth index of all benthic organisms was positively and significantly correlated with water-depth and the possible cause for this relationship is discussed.     

Some remarks on the spectrum of rapid-changing random data in the ocean

Spectral characteristics of rapid-changing random data in the ocean are discussed. Analysis shows that, if records are approximated by saw-toothed random series ₁ and step series ₂, ₁ and ₂ generally have spectral ranges of the –4th power and –2nd power, respectively, with respect to frequency (or wave number).     

Magnetic basement in the central Bay of Bengal

Analyses of about 6000 km of processed magnetic data in the central Bay of Bengal using Analytical Signal Processing and Werner Deconvolution techniques revealed that the depth to top of the magnetic basement varies between 5 and 12 km from the sea surface, where the water column thickness is about 3.4 km. These inferred depths are comparable to the reported acoustic basement depths. The basement map derived from magnetic interpretation defines the general configuration of the central Bay of Bengal. The N10–12° W trending subsurface 85° E Ridge buried under 2 to 3 km thick sediments is a prominent tectonic feature. Offshore basins characterised by deeper magnetic basement (9 km) and 100–200 km wide are present on either sides of the ridge. These basins were filled with 6–8 km thick lower Cretaceous to recent sediments. Integrated geophysical study depicts that the magnetic basement is characterised by NW-SE, NE-SW, NNE-SSW, N10-12° W and E-W trending structural features that are associated with the lower Cretaceous ocean floor. The Analytical Signal Processing and Werner Deconvolution techniques proved to be effective in determining the depth to the basement in areas covered by thick sediment overburden and characterized by a complex geologic/tectonic framework.     

Parameterization of the dispersion relation of internal waves using the data of hydrological soundings in the Tropical Atlantic

Dependences have been determined which connect the parameters of the dispersion relation of the lowest mode of internal waves with the integral characteristics of the seasonal thermocline when 10 min30 min, 20 mh150 m, and 0·4 m²/s² Q5·2 m²/s².Translated by Mikhail M. Trufanov.