ALBERT

Description: The asymmetry of the continental margin around southern Africa can be related to Mesozoic sediment thicknesses, which were in turn controlled by the local structural setting. On the west coast, the Orange Basin sediments were built out as a thick wedge over the margin of the continent by discharge from the Orange River, whereas on the Agulhas Bank, sedimentation was confined to continental areas. Off the east coast the extremely narrow margin of the continent did not form an effective trap for sediments, which were readily carried beyond it. Cainozoic sediments are thin, and modify the Mesozoic sediment pile only locally on the outer shelf and slope.

Description: Two thousand and twenty well-characterized coral specimens from 17 localities have been analyzed for Sr. Seventy-three genera and subgenera, mostly hermatypic scleractinians, are represented. For some genera, specimens living in surface reef environments are compared with those from 18.3 m depths on the same reefs. Growth rates for some species have also been measured at these depths at one of the sampling sites. Skeletal strontium for a given genus decreases with increasing water temperature, a relationship which previously eluded detection. Aragonite deposited by corals living on the reef at a depth of 18.3 m contains more strontium than the skeletal aragonite of the same coral genera from shallow-water, surface environments. Quantitative treatment of the data for Acropora, one of the most abundant and widely distributed of the reef-building corals, suggests that the observed strontium variations may reflect variations in the rate of skeletal calcification, rather than direct dependence upon temperature or water depth. There is evidence for ‘species effects’, apparently unrelated to growth rate differences, in that certain coral genera are consistently enriched or depleted in skeletal strontium content relative to other genera living in the same reef environments under identical ambient conditions. Temperature, salinity, water depth, seawater composition, and/or other such parameters may in part determine the levels of trace element concentration in carbonates deposited by corals and other marine invertebrates, but it would appear that these variables more directly affect physiological processes which in turn control skeletal chemistry.

Description: Volcanic ash layers, which represent the products of volcanic activity within the ocean basins, are common in sedimentary cores taken near Cobb Seamount and on the actively spreading Gorda and Juan de Fuca Ridges. Petrographic and chemical analyses of the glass shards from these deposits have revealed that they are unaltered and are as chemically representative of local volcanic events as are the glassy margins of fresh pillow basalts recovered from the same areas. The presence of unhydrated glass shards in samples as old as 3.8 my is in direct conflict with published hydration rates of both terrestrial and submarine volcanic glasses. A study of a sequence of ash layers from Cobb Seamount, which spans in time much of the Seamount's history, indicates that the volcanic products from Cobb Seamount have had alkaline affinities and that its eruptions have been becoming progressively enriched in Al2O3. Recent experimental petrological evidence and the data on the chemical compositions of Cobb Seamount and the adjacent Juan de Fuca Ridge magmas are in agreement with the hypothesis that magmas are being generated at progressively greater depths beneath Cobb Seamount as it migrates away from the Juan de Fuca Ridge.

Description: The Agulhas Bank, which forms the continental margin on the southern tip of Africa, consists of a thick Meso-Cainozoic sedimentary sequence (up to 6.2 km) resting on and behind pre-Mesozoic continental acoustic basement. The stratigraphy of this sequence is outlined and its history and facies variations mentioned where they are known. Refraction seismic velocity and bottom sample data indicate a basic three-fold subdivision of the Mesozoic sequence, which can be correlated with the onshore succession in the Algoa Basin. It is separated by a major hiatus from the Cainozoic sediments, which consist of a Palaeogene and Neogene sequence subdivided by another well-defined level of erosion. Various formations within the Cainozoic are defined and named. An outline of the bathymetry of the eastern Agulhas Bank is also given.

Description: Four palaeogeographical reconstructions are presented for the southern Cape covering the period Late Permian to Late Cretaceous. This time spans the commencement to an advanced stage of breakup of Gondwanaland, during which the area moved from a mid-continental, high latitude, to an ocean-dominated, middle latitude position. These movements can be traced in facies changes and erosional cycles associated with the rift between West Gondwana and Antarctica (proto southwest Indian Ocean) and the later rift between South America and Africa (proto southeast Atlantic Ocean).

Description: Surface area measurements as well as organic carbon, nitrogen and phosphorus analyses on various grain size fractions of carbonate mud samples confirm that in natural environments of carbonate deposition, surface sorption processes take place which are similar to those described earlier for dissolved organics and artificially suspended calcite particles in both seawater and synthetic solutions. The specific surface area of the sediment increases from 1.8m2/g for the coarse-grained fraction to 12.5 m2/g for the fine material; likewise organic carbon, nitrogen and phosphorus increase with increasing surface area so that there are 1.20 mg C, 0.175 mg N and 0.06–0.20 mg P associated with every square meter of carbonate surface irrespective of the mineralogy of the sediment particles. It appears that the organic matter in these sediments is similar in composition, structure and quantity to the organic layers produced in sorption experiments. With their apparently defined structure and ubiquitous nature, these layers could determine the mineralogy and orientation of submarine carbonate cement or could even be a prerequisite to calcification in general.

Description: This is a study of turbulence which results from Kelvin—Helmholtz instability at the interface between two miscible fluids in a two-dimensional shear flow in the laboratory. The growth of two-dimensional ‘billows’, their disruption by turbulence, and the eventual decay of this turbulence and the re-establishment of a gravitationally and kinematically stable interface are described. Continuous measurements of density and horizontal velocity from both fixed and vertically moving probes have been made, and the records obtained are presented, together with photographs showing the simultaneous appearance of the flow, which serve to identify the physical nature of events seen in the records. The measurements show how the fine-structure of the density field described in earlier experiments is related to velocity fluctuations. The vertical length scales of the final mean velocity and density structure are found to be different, and to depend on the Richardson number at which instability first occurred. The eventual Richardson number at the centre of layer is, however, not dependent on the initial Richardson number and has a value of about one third. The implications of these results to the eddy diffusion coefficients, to the energy exchange, and to turbulence in the ocean and the atmosphere are discussed. © 1973, Cambridge University Press. All rights reserved.

Description: An exact analytic solution is derived for the perturbation of a magnetic field while exposed to an immersed, axisymmetric, azimuthal, steady current source of arbitrary distribution in the presence of a slender, electrically conducting, independently permeated, compressible jet threading the axis of symmetry, subject to an equilibrium pressure balance. A further influence is the enclosure of the magnetic field by a coaxial cylindrical wall. The steady-state result invariably exhibits an infinite discrete superposition of axially decaying terms. In addition, there arise two admissible alternatives involving a fluid parameter λ (dependent on the flow speed, sound speed and both Alfvén speeds pertaining to the jet) together with a scale parameter χ(0) (equal to twice the ratio of the cross-sectional area of the jet to that of its externally enveloping field). Provided that λ exceeds χ(0), each element constituting the current distribution induces a stationary-wave contribution confined, as a consequence of an applied radiation condition, to the upstream domain, corresponding to an upstream-directed group velocity. However, if λ is exceeded by χ(0), this upstream wave is replaced by another decaying term, acting on both sides of every current constituent, like all other decaying terms. © 1973, Cambridge University Press. All rights reserved.

Description: By generalizing the theory of ‘rapid distortion’ of turbulence developed by Batchelor & Proudman (1954) it is shown in this paper that the turbulent velocity around a bluff body placed in a turbulent flow can be calculated outside and upstream of the regions of separated flow, if the incident turbulent flow satisfies the following conditions: (i) if [formula omitted] and Re ≫ (a/Lx)2, where [formula omitted], is the mean uniform incident velocity, [formula omitted] is the r.m.s. velocity of the homogeneous incident turbulence, a is a transverse dimension of the body (the radius in the case of a circular cylinder), Lx is the integral scale of the incident turbulence and v is the kinematic viscosity. Detailed calculations are given for the flow around a circular cylinder with particular emphasis on the turbulence very close to the surface. (The results can be generalized to other cylindrical bodies.) Mean-square values and spectra of velocity have been found only in the limiting situations where the turbulence scale is very much larger or smaller than the size of the body, i.e. Lx ≫ a or Lx ≪ a. But, whatever the value of a/Lx, if the frequency is sufficiently large the results for spectra tend to those of the limiting situation where Lx ≪ a. The reason why the turbulence velocities have not been calculated for intermediate values of a/Lx is that closed-form solutions cannot be found and that the computing time then required is quite excessive. However, some computed results are used in the paper to suggest the qualitative behaviour of the turbulence when Lx is of order a. An important result of the theory is that it illuminates and distinguishes between the governing physical processes of distortion of the turbulence by the mean flow, the direct ‘blocking’ of the turbulence by the body, and concentration of vortex lines at the body's surface. The results of the theory have many applications, for example in calculating turbulent dispersion and fluctuating pressures on the body, as shown elsewhere by Hunt & Mulhearn (1973) and Hunt (1973). In conclusion the theoretical results are briefly compared with experimental measurements of turbulent flows round non-circular cylinders. A detailed comparison with measurements round circular cylinders will be published later by Petty (1974). © 1973, Cambridge University Press. All rights reserved.