Bifurcating and wandering ocean ridges: A progress report

First conclusions of a study of the fault pattern in the North Atlantic ocean are briefly outlined. Correlation of fracture zones in deep ocean basins, even over short distances (30 miles) is difficult when using only topographic and seismic profiler data, and requires the additional criterion that the magnetic pattern is shifted along the fracture zones. This implies that each track must be paralleled by at least one more track at close distance to detect breaks in the magnetic pattern.Transform faults are regarded as complementary to the median rift, which, as a tension fault, should be normal to the direction of maximum strain according to laboratory experiments by Gramberg. Median rift segments and transform faults together form an orthogonal system approximating an originally irregular but continuous fissure.Mantle convection currents are thought to be the agents of continental drift. In view of the dimensions of convection cells, a direct coupling between ridges (or ridge segments) and the upwelling branch of a convection current seems impossible. Thus the known bifurcations of mid-ocean ridges in the Indian and in the Pacific Ocean may be interpreted as the result of a divergence of the horizontal branch of a convection cell or a divergence of the movements of crustal plates through collision of continents. Secondary fissures would lead to the development of secondary ridges that cannot be distinguished from primary ridges. In this concept mid-ocean ridges can partake in the horizontal movement by asymmetrical spreading. This might explain the disappearance of the East Pacific Rise under the North American continent.The resulting convection current system is simple: one upwelling and one downgoing current as two complementary helices winding round the earth. Ways are indicated to check several aspects of the proposed hypotheses.     

Studies on the determination of inorganic iodine in seawater

Existing methods for the determination of the various forms of iodine present in seawater are laborious or unwieldy in use. This paper describes modified forms of the iodate and total iodine methods described by Barkley and Thompson (1960), a spectrophotometric procedure for iodate determination derived from Johannesson's (1958) work, and an automatic method for total iodine determination. Procedures for iodate and total-iodine determination which are suitable for use aboard ship, are recommended. Both procedures for iodate determination returned a standard deviation close to 0.5 μg/l when several replicate samples of a seawater containing approximately 30 μg/l of iodate-iodine were analysed. The automatic method for total iodine determination yielded a standard deviation of 0.8 μg/l by the repeated analysis of a seawater containing a total of approximately 50 μg/l of iodine. Suitable methods for the filtration and storage of seawater are also described.     

Plate boundaries and evolution of the Solomon Sea region

The Solomon Sea Plate was widely developed during late Oligocene, separating the proto-West Melanesian Arc from the proto-Trobriand Arc. Spreading in the Bismarck Sea and in the Woodlark Basin resulted from interaction between the Pacific and Australian Plates, specifically from the collision of the proto-West Melanesian Arc with north New Guinea, which occurred after arc reversal. This model explains the extensive Miocene, Pliocene, and Quaternary volcanism of the Papua New Guinea mainland as it related to southward subduction of the Trobriand Trough. Our interpreted plate motions are concordant with the geological evidence onshore and also with complex tectonic features in the Solomon Sea Basin Region.     

A random walk model for simulating simultaneous currents at several depths

Empirical orthogonal function (EOF) analysis is used in conjunction with Markov chains to generate simultaneous current time series at several depths with a random walk model. Duration statistics and probability distributions for EOF modes obtained from real data are used to generate transition probability matrices for the random walk model. New time series of EOF modes are generated, and then combined to produce current time series. The technique has been tested on data from Haltenbanken. Statistical properties of real and simulated time series were compared. Results indicate that the model is acceptable as a tool in simulating current time series to be used in offshore operation models.     

The Reproductive Cycle of Ophioderma brevispinum (Echinodermata: Ophiuroidea)

Abstract. .The reproductive cycle of the brittlestar Ophioderma brevispinum is described using histological and organ index data for a population in Massachusetts, U.S.A. The cycle consists of a one month mid-summer spawning phase followed by gametogenesis and gradual gonadal growth during the winter, and greatly accelerated gonadal growth from May to June. At the end of the spawning season, oogonia proliferate near the base of the ovary, and a continuous layer of spermatogonia lines the testis. As oocytes grow to a maximum diameter of 350 um, yolk granules accumulate and the cytoplasm becomes less basophilic. Prior to spawning, the testis becomes branched and sulcate, and a whorl of spermatozoa produced by columns of spermatids accumulates in the lumen. Comparisons between the reproductive cycles of different populations of O. brevispinum and its congeners support the hypothesis that temperature may be a critical exogenous factor, but definitely not the only factor, in the initiation and duration of the growth and spawning phases of the ophiuroid reproductive cycle.     

Kinetic studies of the rate of dissolution of silica and diatom tests in sea water

Kinetic studies have been carried out on the dissolution of 10μm beads of silica and of the siliceous tests of two species of diatoms (Thalassiosira fluviatus andSkeletonema costatum) in sea water at 10–50°C and over the pH range 6–9. At all temperatures dissolution of the biogenous silica occurred most rapidly at pH 8 and most slowly at pH 6. First order kinetics were closely followed when the silica was present in a considerable excess over that required for saturation, the rate being proportional to the area of the silica. Apparent deviation from this type of kinetics occurred when there was insufficient silica to bring about saturation. This was undoubtedly due to the progressive decrease in the surface area which takes place as the particle dissolves. Application of the rather simplistic model developed by Kamataniet al. (1980), which makes allowance for this decrease, gave a close correspondence with the data until ∼ 90% of the silica had dissolved.     

Gravity anomalies over inactive fracture zones in the central North Atlantic

The basement topography and the free-air gravity along two profiles in the central North Atlantic between 16° and 25° N, crossing a number of fracture zones, were divided in three wavelength intervals. Two-dimensional modelling shows that the short wavelength (>50 km) gravity is well explained by uncompensated topography (mainly spreading topography). For the long wavelengths (>200 km) there is no correlation of topography and gravity. In principle this topography is compensated. Residual anomalies comprise the Ridge effect as well as regional anomalies related to depth anomalies. The 50 to 200km band-pass filtered topography and gravity contain relevant information on fracture zones. Models require a base of the crust that parallels the topography rather than a form of regional compensation. For an explanation of this crustal model that has the appearance of frozen in normal faults we have to consider the typical morphology as created in the transform domain. The geophysical processes that cause this morphology are still an object of study.