Geomagnetic multipoles, 1829 to 1960

Summary Using the results of spherical harmonic analyses of the geomagnetic field for some fourteen different epochs, includingGauss' first analysis for epoch 1835, and theErman-Petersen analysis for epoch 1829, the strength and axes of geomagnetic multipoles have been computed. In particular, a dipole from the three first order spherical harmonic coefficients, a quadrupole from five second order coefficients, and an octupole from seven third order coefficients. The axes of the quadrupole and octupole have moved quite rapidly when compared with movements of the dipole axis, and show a general movement westwards. Although the strength of the dipole has generally diminished, the strengths of the quadrupole and octupole have generally increased.On leave National University of La Plata, Argentina     

IDENTIFYING AND ASSESSING PROBLEM AREAS IN SOIL EROSION SURVEYS USING AERIAL PHOTOGRAPHS

Gully erosion has for many years been a problem in Rhodesia1s Tribal Trust Lands. This paper describes how Soil Conservation Service staff, with no previous ground knowledge of these areas, used good quality 1/25,000 scale aerial photographs to measure the extent of the gully erosion and to pinpoint erosion “black spots.” This enabled quick and reliable estimates of the cost of reclamation work to be made without the need for time-consuming field survey. The survey method discussed is capable of wide application wherever gully erosion occurs and needs to be brought under control.     

Isotopic variation of fishes in freshwater and estuarine zones of a large subtropical coastal lagoon

We used stable C and N isotope ratios of tissues from 29 fish species from a large subtropical lagoon in southern Brazil to examine spatial variability in isotopic composition and vertical trophic structure across freshwater and estuarine habitats. Nitrogen isotope ratios indicated a smooth gradation in trophic positions among species, with most fishes occupying the secondary and tertiary consumer level. Fish assemblages showed a significant shift in their carbon isotopic signatures between freshwater and estuarine sites. Depleted carbon signatures (from −24.7‰ to −17.8‰) were found in freshwater, whereas more enriched signatures (from −19.1‰ to −12.3‰) were obtained within the estuarine zone downstream. Based on our survey of the C₃ and C₄ plants and isotopic values for phytoplankton and benthic microalgae reported for ecosystems elsewhere, we hypothesized that the observed δ¹³C differences in the fish assemblage between freshwater and estuarine sites is due to a shift from assimilating organic matter ultimately derived from C₃ freshwater marsh vegetation and phytoplankton at the freshwater site (δ¹³C ranging from −25‰ to −19‰), to C₄ salt-marsh (e.g. Spartina) and widgeon grass (Ruppia maritima), benthic microalgae and marine phytoplankton at the estuarine sites (from −18‰ to −12‰). Our results suggested that fish assemblages are generally supported by autochthonous primary production. Freshwater fishes that likely were displaced downstream into the estuary during periods of high freshwater discharge had depleted δ¹³C values that were characteristic of the upper lagoon. These results suggest that spatial foodweb subsidies can occur within the lagoon.     

Diurnal changes in the bio-optical properties of the phytoplankton in the Alborán Sea (Mediterranean Sea)

The changes in the phytoplankton absorption properties during a diurnal cycle were investigated at one station located in the north-western area of the Alborán Sea. The experiment was performed in spring when the water column was strongly stratified. This hydrological situation permitted the establishment of a deep chlorophyll a (chl a) fluorescence maximum (DFM) which was located on average close to the lower limit of the mixed layer and the nutricline. The relative abundance of pico-phytoplankton (estimated as its contribution to the total chl a) was higher in the surface, however, micro-phytoplankton dominated the community at the DFM level. Chl a specific absorption coefficient (a*(λ)) also varied with optical depth, with a* (the spectrally average specific absorption coefficient) decreasing by 30% at the DFM depth with respect to the surface. A significant negative correlation between the contribution of the micro-phytoplankton to the total chl a and a* was obtained indicating that a* reduction was due to changes in the packaging effect. Below the euphotic layer, a* increased three-fold with respect to the DFM, which agrees with the expected accumulation of accessory pigments relative to chl a as an acclimation response to the low available irradiance. The most conspicuous change during the diurnal cycle was produced in the euphotic layer where the chl a concentration decreased significantly in the afternoon (from a mean concentration of 1.1 μg L⁻¹ to 0.7 μg L⁻¹) and increased at dusk when it averaged 1.4 μg L⁻¹. In addition, a* and the blue-to-red absorption band ratio increased in the afternoon. These results suggest that a*(λ) diurnal variability was due to increase in photo-protective and accessory pigments relative to chl a. The variation ranges of a*(λ) at 675 and 440 nm (the absorption peaks in the red and blue spectral bands, respectively) in the euphotic layer were 0.01–0.04 and 0.02–0.10 m² mg⁻¹ chl a, respectively. Approximately 30% out of this variability can be attributed to the diurnal cycle. This factor should therefore be taken into account in refining primary production models based on phytoplankton light absorption.     

Nonlinear effects of the distribution of amplitudes of sea waves

The effect of nonlinearities, such as wave-breaking and vertical asymmetry associated with sea waves, on the distribution of wave amplitudes is explored. Semiclosed theoretical expressions are derived to describe the distributions of breaking-limited crest and trough amplitudes for Stokes-type nonlinear sea waves. These are compared with the conventional Rayleigh distribution appropriate to linear wave amplitudes. The construction of nonlinear wave envelopes with the fast Fourier transform technique is described. The technique can be utilized to enlarge the data base in empirical analyses of field records which typically contain limited information on amplitude characteristics. The theoretical distributions and the proposed data enlargement technique are demonstrated with the analysis of a nonlinear wave record.     

Numerical simulation of undersea cable dynamics

A fully three-dimensional code has been written to compute the motion of a towed cable. The code is based on a robust and stable finite difference approximation to the differential equations derived from basic dynamics. A 3500-ft (1.07 km) cable pulled at 18.5 knots (34.3 km hr⁻¹) through a circular turn of 700 yd (0.64 km) radius has been computed in about half of the real time of the maneuver. The computed displacements are close to the measured ones; the changes in depth are within 2%.     

A sonic well log of the basement complex of the Walvis Ridge

A sonic well log was obtained within the basement complex of the Walvis Ridge during Deep Sea Drilling Project Leg 74. The top of the basement complex is characterized by smooth acoustic reflectors. The rocks recovered within the basement complex consist of basalts with intercalated sediments. According to the log ～-50% of the upper 75 m of basement are igneous rocks and the other 50% sedimentary. Sonobuoy results indicate that the ratio of sediments to basalt increases with depth for an additional 225 m until a typical oceanic velocity structure is observed. Paleontological results suggest that the processes forming this upper 300 m of the basement complex was accomplished within a short time interval.     

Interpolation and extrapolation of hydrodynamic coefficients

The solution to the problem of calculating the hydrodynamic forces on multiple floating bodies is summarised. The complex coefficients which characterise the forces at each frequency of oscillation are expensive to compute; interpolation of intermediate values and extrapolation to high frequencies appear difficult since the coefficients seem to vary arbitrarily with frequency. Interpolation and extrapolation schemes based on a novel representation of the coefficients are proposed and tested. The excellent numerical results indicate that the values of the coefficients at a few frequency points can characterise the hydrodynamics at all frequencies. These techniques can save economically significant amounts of computer time and permit investigations which would otherwise be prohibitively expensive.