Formation and distribution of manganese nodule deposit in the northwestern margin of Clarion-Clipperton fracture zones,northeast equatorial Pacific

In the northwestern margin of the Clarion and Clipperton fracture zones, manganese nodules are latitudinally variable in character, resulting from the progressive (re)generation of nodules since the Oligocene as the Pacific Plate migrates northwestward beneath the equatorial production zone. Complemented by the surface water productivity, bottom current activities have concentrated nodules in the Oligocene to Miocene siliceous clay. Resedimentation processes on nodules form three types of nodules with concentric structures and 10 Å-manganaterich bottoms, by periodically causing the diagenetic growth of nodules within the topmost sediment layer.     

A formal derivation and numerical modelling of the improved Boussinesq equations for varying depth

A formal derivation of the improved Boussinesq equations of Madsen and Sørens (1992) is presented to provide the correct forms of the depth-gradient related terms. Linear shoaling characteristics of the new equations are investigated by the method of Madsen and Sørensen (1992) and by the energy flux concept separately and found to agree perfectly, whereas these approaches give conflicting results for the equations derived by Madsen and Sørensen (1992). Furthermore, Nwogu's (1993) modified Boussinesq model is found to produce a linear shoaling-gradient identical with the present work. Numerical modelling of the derived equations for directional waves is carried out by three-time-level finite-difference approximations. A higher-order radiation condition is implemented for effective absorption of the outgoing waves. Several test cases are included to demonstrate the performance of the model.     

Some statistical methods of the objective classification of oceanographic data

Algorithms for the compression and objective classification of low-volume samplings according to the test of their belonging to different statistical populations are given. Samplings are discriminated by thex ² test. The distributions of the temperature patch horizontal scales in the upper ocean are analysed. It is shown that optimum empirical thresholds exist when the abstract classification mechanism goes over to the objective natural classification.Translated by Mikhail M. Trufanov.     

Wave climate in a groin field

Wave records at seven different locations within a groin field have been analysed by both statistical and spectral approaches to study the general wave climate. The wave heights and periods from the wave records were obtained by both upcross and downcross methods for the statistical approach. The variation of different wave height and wave period parameters with respect to the time of measurement at a particular location and its variation at different locations within the groin field at a particular instant of time are presented and discussed in detail in this paper. It is generally found that the wave heights follow the Weibull distribution.     

Interaction of internal waves with a horizontally inhomogeneous density field area overlying a ridge

In the context of the general linear theory, we consider the propagation of an internal tide across a frontal zone overyling an oceanic ridge. For a uniformly stratified ocean, the solution was derived using Riemann's technique. The dependences of the generated internal wave amplitudes on the stratification parameters and bottom topography were determined. We have found that wave disturbances of high intensity inside and in the neighbourhood of the ridge may be concentrated in raytype areas. An increase of the horizontal density gradient in the frontal zone results in a perceptible deformation of these areas.Translated by Vladimir A. Puchkin.     

Sea-floor observations in the tongue of the ocean,Bahamas: An Argo/SeaMARC survey

SeaMARC side-scan sonographs and Argo video and photographic data suggest that the recent sedimentary environment of the floor of the Tongue of the Ocean is controlled by an interplay of turbidity current flow from the south, sediment spill-over from the carbonate platform to the east (windward side), and rock falls from the west carbonate escarpment (lee side). The spill-over forms a sandy sedimentary deposit that acts as a topographic obstruction to the turbidity current flow from the south. This obstruction is expressed by the westward migration of a northwest-southeast oriented turbidity-current-cut channel.     

Mediterranean outflow through the Strait of Gibraltar since 18,000 Years B.P.: Mineralogical and geochemical arguments

Data obtained from mineralogical and geochemical analyses of piston and gravity cores, recovered from an area off Lisbon (Portugal) to the Alboran Sea (Mediterranean), serve as a basis for better understanding the past 18,000 years of hydrological exchanges at Gibraltar. Tracers used in this study are smectite, kaolinite, Ta, Th, La. One of the primary sources of particles both into and out of the Mediterranean is the Guadalquivir River. These particles are transported back into the Atlantic in the Mediterranean outflow water, and deposited along the Iberian slope. No evidence for reversal of this outflow current was found in those cores, since 18,000 years B.P.     

Oceanic thermohaline intrusions: observations

Intrusions are commonly observed in the upper, deep and coastal oceans, and are closely linked to lateral fluxes of heat, salt and momentum. This is a review of observations of intrusions and the results of comparisons of properties such as scale, slopes, microstructure activity, and fluxes with theoretical models. A summary of estimates of lateral heat fluxes indicates a wide range of lateral diffusivities. We conclude by noting that our present knowledge is insufficient to predict the structure, length-scales and lateral fluxes of thermohaline intrusions with confidence, and list a number of unresolved questions. Suggestions are made for compilation of existing data into a database for exploratory analysis and testing of theoretical hypotheses. An outline is given of a potential collaborative field experiment using CTD, fluorescent dye, and microstructure observations.