Flood control failures in a Third World City: Benin City,Nigeria — some environmental factors and policy issues

GeoJournal - Inspite of step-up efforts at controlling flood hazards in Third World cities, the frequency and magnitude of the flood problem have more than doubled in recent years. A survey of the...     

Phase transformations and inherited lattice preferred orientations: implications for seismic properties

In phase transitions via either the martensitic (diffusionless shear) or nucleation and growth mechanism a specific orientation relationship may exist between the two phases. In cases where the orientation relationship is known, the lattice preferred orientation (LPO) inherited by the new phase may be calculated from the LPO of the old phase. The method of calculation is presented in a form suitable for the spherical harmonic method of texture analysis using the orientation distribution function (ODF). Examples are presented for the -β-quartz, calcite-aragonite, orthopyroxene-clinopyroxene and olivine-spinel transformations.

The seismic properties of the transformed and untransformed phases are calculated from the ODF and the single crystal elastic constants. In particular the -β quartz transformation is considered in detail. The quartz polycrystal is very anisotropic in the alpha field (V_p anisotropy coefficient, A = 8.1%) and almost isotropic in the β-field (A = 2.1%). The transition is accompanied by V_p velocity increase of 0.6 km/s. In the other example discussed, olivine-β-spinel, there is also a decrease in V_p anisotropy coefficient from 11.1% (olivine) to 4.0% (β-spinel). The estimate of the volume fraction of olivine at the 400 km discontinuity (associated with this phase transition) is shown to depend on the direction of wave propagation.     

Partitioning of siderophile elements in the FeNiS system: 1 bar to 80 kbar

Partition coefficients for Au, Ni, P and Ge between solid FeNi metal and sulfur-bearing metallic liquids have been measured at 7,27 and 80 kbar. To our knowledge, these are the only such data for Au, P and Ge at high pressure. Comparison of our partitioning results to those obtained at 1 bar indicate that only the 80 kbar Ge data differ significantly from the 1-bar experiments. Thus, many low-pressure partitioning experiments in the FeNiSP system may have applicability to the greater portion of the Earth's upper mantle or, alternatively, the entire mantle of Mars.     

Human influence upon sedimentation in Llangorse Lake,Wales

Palaeolimnological studies at Llangorse Lake, Wales, constitute part of the British contribution to IGCP Project 158 (The Palaeohydrology of the Temperate Zone in the Last 15000 Years). The lake has been adopted as the Reference Site for the South Wales region under Subproject 158B (Lake and Mire Environments) and is located in the Reference Area for Britain: the Severn Basin. Results of biological, physical, and multielement chemical analyses on a 12.35 m Livingstone core are presented and compared with previous analyses on short Mackereth cores taken from two troughs in the lake basin. Human impact upon post-glacial sedimentation is assessed with reference to sediment chemistry, physical properties, and sedimentation rates, based on relative pollen, ¹⁴C, and ²¹⁰Pb dating. Vegetational history of the catchment is inferred from a pollen diagram for a section of the core from c. 8 to 6.75 m depth in which evidence for human impact is first apparent. Water content, loss on ignition, and carbonate content are presented for the whole core; density, water content, loss on ignition, carbonates, and elemental analyses of P, Ca, Mg, K, Ti, Al, Fe, and Mn for the interval 6 75 and 8.25 m depth. Sedimentation rates were low during Boreal and Atlantic times (c. 9000- 5000 BP), rose significantly from the time of the elm decline (estimated at 5000 BP) and increased further at c. 1800 BP when nekron muds were succeeded by deposition of silty clay. Inferred rates of erosion of soils from the catchment are discussed and related to sediment chemistry.     

River basin management and food crisis in Nigeria

River Basin Development is one of the key agricultural programmes embarked upon by Nigeria to meet the challenges of escalating import bills on food. This paper takes a critical look at the various efforts made by the management of the River Basin Authorities in responding to the current food crisis. The rationale for the huge financial allocation made to the Authorities is questioned, and the management lapses in their key areas of operation, such as irrigation, mechanization and extension services, are discussed. It is found that despite the huge financial investments made in the river basins, their impact is hardly felt, not only in terms of food production capacity but also in the production performance of the peasant farmers. The paper argues that the problem of food crisis in Nigeria could be solved by strengthening the ability of the peasant farmers and farmers co-operatives through financial aid and extension services, and not by huge financial investments on high technology agricultural projects, characterized by centralized administration and expatriáte management.     

Accretion tectonics and crustal structure in Alaska

The entire width of the North American Cordillera in Alaska is made up of “suspect terranes”. Pre-Late Cretaceous paleogeography is poorly constrained and the ultimate origins of the many fragments which make up the state are unclear. The Prince William and Chugach terranes accreted since Late Cretaceous time and represent the collapse of much of the northeast Pacific Ocean swept into what today is southern Alaska. Greater Wrangellia, a composite terrane now dispersed into fragments scattered from Idaho to southern Alaska, apparently accreted into Alaska in Late Cretaceous time crushing an enormous deep-marine flysch basin on its inboard side. Most of interior eastern Alaska is the Yukon Tanana terrane, a very large entirely fault-bounded metamorphic-plutonic assemblage covering thousands of square kilometers in Canada as well as Alaska. The original stratigraphy and relationship to North America of the Yukon-Tanana terrane are both obscure. A collapsed Mesozoic flysch basin, similar to the one inboard of Wrangellia, lies along the northern margin. Much of Arctic Alaska was apparently a vast expanse of upper Paleozoic to Early Mesozoic deep marine sediments and mafic volcanic and plutonic rocks now scattered widely as large telescoped sheets and Klippen thrust over the Ruby geanticline and the Brooks Range, and probably underlying the Yukon-Koyukuk basin and the Yukon flats. The Brooks Range itself is a stack of north vergent nappes, the telescoping of which began in Early Cretaceous time. Despite compelling evidence for thousands of kilometers of relative displacement between the accreted terranes, and large amounts of telescoping, translation, and rotation since accretion, the resulting new continental crust added to North America in Alaska carries few obvious signatures that allow application of currently popular simple plate tectonic models. Intraplate telescoping and strike-slip translations, delamination at mid-crustal levels, and large-scale lithospheric wedging were important processes in northern Cordilleran tectonic evolution.     

Gold, silver and other elements in aquatic bryophytes from a mineralised area of North Wales, U.K.

Concentrations of Au, Ag, As, Cd and Sb in aquatic bryophytes collected from the Dolgellau Mineral Belt, North Wales, U.K. are reported. One aquatic liverwort, Scapania undulata (L) Dum. and two mosses, Fontinalis squamosa Hedw. and Racomitrium aciculare (Hedw.) Brid. were collected from sites upstream and downstream of the recently reopened Gwynfyndd Au mine. There was little inter-species variation in metal contents for these three bryophytes, but Scapania undulata appeared the most sensitive to changes in water concentrations of Ag, As and Sb. Gold concentrations varied little between the contaminated and control sites. Concentrations in the range < 4–18 ng Aug g⁻¹ D.W. were typical background levels, while bryophytes collected immediately below the mine contained 6–45 ng Au g⁻¹. Silver and Sb both showed more pronounced ( 5–10 fold) elevations above control concentrations in samples collected downstream of the mine. Background concentrations for these elements were 5–85 ng Ag g⁻¹ and 0.15–1.3 μg Sb g⁻¹.Arsenic concentrations downstream of the mine (160–1080 μg g⁻¹) greatly exceeded the background range of 9–32 μg g⁻¹. It is suggested, therefore, that As may be an ideal ‘pathfinder’ element when prospecting for auriferous deposits using aquatic bryophytes.     

A reduced-rank prediction model of stream-channel size response to traditional agricultural land-use practices in southwestern Nigeria

Stream-channel morphologic responses are found to be related to different parameters measuring traditional agricultural land-use patterns and practices in 50 small headwater basins in southwest Nigeria. The problem of intercorrelations among these parameters made it initially difficult to establish their precise channel enlargement effects and to calibrate an impact prediction model. Through factor analysis of the 22 land-use and morphometric parameters, six factors identified as measures of traditional land-use practice, farm size, planting activities, shortened fallow, relief and overland flow, were found to account for 86% of the variance in the data. The factor-defining variables are length of cropping period, areas in short fallow, farm-plot size, length of farm preparation, relief ratio and overland flow. In a multiple regression analysis, only the first three variables were found to be statistically significant in explaining stream-channel morphologic responses. Thus, areas in short fallows, average farm size and length of cropping period adequately described those aspects of the traditional farming practices that affect basin hydrologic and channel responses. Since these variables were orthogonally derived, they formed the basis for the evaluation of the channel impact status of traditional land-use activities. The duplication of information and effects in the original 22-variable full-rank model were removed while utilizing the three-factor reduced model.