Age et croissance de la truite lacustre (Salmo trutta lacustris L.) du Léman

The age and growth of adult Lake Geneva trout (Salmo trutta lacustris L.) caught from 1964 to 1974 for mark-and-recapture experiments was studied by scale analysis. The median lengths of 3 years and older fish were fitted to Von Bertalanffy growth curves. Their growth rates are higher than those of the fish younger than 3 years. It is conculded that this increase of growth rate concides with beginning of life in the lake, after the downstream migration of the juvenile trout which probably takes place in the second and third summer. No general increase of growth rates between cohorts could be detected from the beginning to the end of the period studied, in spite of a serious increase of eutrophication of Lake Geneva during the same time.      

The global distribution of long-term total ozone variations during the period 1957–1975

Total ozone observations in the international network have been used as a basis for the analysis of the mean monthly ozone distribution over the globe for the period 1957–75. It has been found that during the period 1961–70 the total ozone amount increased in the Northern Hemisphere by about 12 percent and that this increase seems to be significant at all latitudes. Although the data were sparse for the Southern Hemisphere, there did not appear to be any significant ozone changes during the 10 year period. Relatively large geographic variations were found in the ozone trends and it is suggested that these variations are related to large scale changes in the atmospheric circular pattern.     

Metallogenesis in the southeastern Pacific: A progress report on the IDOE Nazca Plate project

Under the auspices of the International Decade of Ocean Exploration, scientists from Oregon State University and the Hawaii Institute of Geophysics, along with representatives from the several South American countries which border the Nazca Plate, are intensively studying the origin and economic importance of metalliferous sediments which form extensive deposits on and near the East Pacific Rise. The sediments, enriched in iron, manganese, copper, nickel, zinc, silver, molybdenum, and lead are mineralogically and chemically similar to deposits cored by the “Glomar Challenger”, implying a common origin. The metalliferous sediments are especially abundant in an extensive basin, the Bauer Deep, which occurs about 800 km east of the East Pacific Rise. Elemental and isotopic data, supported by geologic information provided by cores and by geophysical instrumentation, suggest that the enriched elements in the sediments originate by the interplay of volcanism and sea water, by precipitation from sea water alone, and perhaps by the influence of deep-ocean physical processes acting on detritus transported from the continents or from the basaltic East Pacific Rise.An additional objective of the project is to define the possible relationship between marine metalliferous sediments of the plate to ore deposits of the Andean Cordillera. As pointed out by several writers, much of the earth's economic mineralization occurs along zones of convergence between subducted oceanic crust, including metalliferous sediments if present, and continental margins. Field and geochemical techniques are being applied to determine if sediments are in fact subducted beneath the Andean margin, and if they are remobilized into the continental ore deposits of the Andes. Isotopic tracers may prove most useful in testing this relation. Before the model can be tested rigorously, one must know not only the important chemical parameters of the ocean sediment that may be subducted, but also those of the pertinent rock and ore bodies of the South American Cordillera.     

Pleistocene climates in central Europe: At least 17 interglacials after the Olduvai event

At least 17 times during the past 1.7 million years, the deposition of loess containing characteristic cold-resistant gastropods was interrupted by the development of temperate interglacial forests. This conclusion was reached in a study of paleomagnetically dated fossiliferous loess sequences in Krems, Austria and Brno, Czechoslovakia. Sequences of windblown loess interlayered with hillwash loams and steppe and forest soils exposed in brickyards around Brno and Praha, Czechoslovakia, revealed eight major depositional cycles within the Brunhes paleomagnetic epoch. We now report nine additional cycles of late and middle Matuyama age bringing the total number of glacial-interglacial cycles to 17, which occurred after the end of the Olduvai. The cycles are separated by marklines, levels of abrupt environmental change correlative with the terminations in deep-sea sediments. They are the boundaries between the windblown loess containing cold-resistant snail assemblages and between the clayey originally decalcified soils, accompanied by warmth loving Helix and Banatica snail faunas of hardwood forests. Because the presence of temperate forests in northwestern and central Europe is instrumental in the definition of an interglacial, each markline represents a glacial-interglacial boundary and each cycle is a glacial-interglacial cycle.     

Hydrothermal alteration and sulfide mineralization in gabbroids of the Markov Deep (Mid-Atlantic Ridge, 6° N)

A new type of sulfide occurrence related to metasomatically altered brecciated gabbroids has been studied at the Sierra Leone site situated in the axial rift valley of the Mid-Atlantic Ridge (Markov Deep, 6° N). Two associations of plutonic, subvolcanic, and volcanic rocks were dredged: (1) mid-ocean ridge basalts (MORB) and their intrusive analogues and (2) rocks of the silicic Fe-Ti-oxide series with dominating gabbronorites and sporadic trondhjemites. Almost all igneous rocks at the Sierra Leone site are enriched in Pb, Cu, U, Ga, Ta, Nb, Cs, and Rb and depleted in Zr, Th, and Hf. The rocks of the Fe-Ti-oxide series are enriched in Zn, Sn, and Mo and depleted in Ni and Cr. The main ore-bearing zone is situated at the foot of the eastern wall of the deep, where it is hosted in cataclastic hornblende gabbro and gabbronorite of the Fe-Ti-oxide series. Ore mineralization in metasomatically altered rocks is composed of quartz-sulfide and prehnite-sulfide veinlets, disseminated sulfide, and veined copper sulfide ore. The ore consists of pyrite, chalcopyrite, sphalerite, pyrrhotite, bornite, chalcocite, and digenite. The δ³⁴S value of sulfides varies from 3.0 to 15.3‰. At the foot of the eastern wall of the Markov Deep, directly downslope from the ore-bearing zone, loose sediments contain grains of native Cu, Pb, Zn, and Sn and intermetallic compounds (isoferroplatinum, tetraferroplatinum, and brass) apparently derived from the ore. Mineral assemblages of ore-bearing metasomatic rocks and fluid inclusions therein were studied. Ore metasomatism developed under a low oxygen potential within a temperature interval from 400 to 160°C, though initial hydrothermal alteration of rocks proceeded at temperatures of 800–450°C. The temperature of stringer-disseminated ore mineralization is estimated at 170–280°C. The hydrothermal fluids are considered to be of magmatic origin; as the hydrothermal system evolved, they became diluted with seawater that was contained in fractured oceanic crust. The ore matter could have been derived from magmatic fluids that were released from water-saturated melts of the Fe-Ti oxide series during their ascent and leached from host gabbroids in the process of metasomatic alteration.     

New hydrothermal sulfide fields of the Mid-Atlantic Ridge: Yubileinoe (20°09′ N) and Surprise (20°45.4′ N)

Two new sulfide fields (Yubileinoe, 20°09′ N, and Surprise, 20°45.4′ N) were discovered between 20°01′ and 20°54′ N within the Russian Application Area of the Mid-Atlantic Ridge (MAR). The Yubileinoe field is located at a depth of 2300–2550 m in the near-top area of the first rift ridge, which is a boundary of the western wall of the rift valley. This new field and the Zenith-Victory field, which was previously discovered in the eastern wall, occur symmetrically relative to the rift valley of this MAR segment. The Surprise field at a depth of 2800–2850 m is situated in the eastern wall of the rift valley, on the slope of the volcanic uplift. After the discovery of these inactive sulfide fields, the number of hydrothermal fields within the Russian Application Area reached ten.     

Hydrocarbon reservoir characterization of “AY” field,deep-water Niger Delta using 3D seismic and well logs

Three-dimensional seismic and well log data from nine wells were used for the characterization of “AY” field in the deep-water, Niger Delta. Result shows that the field has a complex structural arrangement consisting of series of northeast-southwest-trending and northwest-dipping synthetic faults. Petrophysical evaluation of the available well logs helped in identifying 11 hydrocarbon-bearing sands noted as A1000, A1100, A1200, A2000, B2000, B2100, C3000, C3100, D4000, D4100, and E5000. Reservoirs A1000, A1100, A1200, A2000, B2000, C3000, and D4000 are gas-bearing sands while reservoirs B2100, C3100, D4100, and E5000 are oil bearing. The average effective porosity of these reservoirs ranges from 0.168 to 0.292; water saturation is estimated to be between 0.177 and 0.59 and net-to-gross (NTG) ratio from 0.081 to 0.734. Considering the uncertainty in the input petrophysical parameters as well as structural uncertainty particularly in fluid contact, the total hydrocarbon reserves in the field were estimated to vary between 266.942 and 334.457 Bscf and 132.612 and 150.036 MMbbl for gas and oil volumes, respectively.     

The use of multivariate statistical analysis in the assessment of groundwater hydrochemistry in some parts of southwestern Nigeria

Arabian Journal of Geosciences - Turabah area is located in the southwestern Arabian Shield of Saudi Arabia. The present study aims to shed light on the origin and mechanism of formation of the...