The mineralogical composition of precursor sediments of calcareous rhythmites: a new approach

Previous studies in Silurian carbonates from Gotland (Sweden) have led to a model for the development of limestone-marl alternations. This model postulates that early diagenesis of precursor sediments without strong primary differences can result in a differentiation by selective dissolution of aragonite in marl beds and reprecipitation of calcite cement in limestone beds. This model is described as a set of mathematical equations that quantify the diagenetic processes (aragonite dissolution and calcite reprecipitation) that occur during the formation of limestone-marl interbeds from a hypothetical homogeneous precursor sediment. The calculations demonstrate that resulting hypothetical limestone-marl alternations show characteristic mathematical relationships between the ratios of the bed thicknesses of limestones and marls on one side, and the carbonate contents, on the other. By reversing this model, the original mineralogical composition of the precursor sediment of real-world rhythmic successions can be determined. In this study, alternations from the Silurian of Gotland, the Cambrian, Devonian, and Mississippian of North America, the Jurassic of France and Germany, and the Cretaceous of France are shown to exhibit mathematical relationships similar to those calculated for hypothetical precursor sediments without primary differences. Therefore, the mineralogical composition of their precursor sediments can be estimated. In contrast, the clear mismatch shown by the Lower Jurassic Belemnite Marls from Dorset indicates that these rhythms did not suffer an early diagenetic overprint. Our model helps to differentiate between rhythmites with strong depositional variations and those without; however, it cannot indicate whether a given alternation is the product of rhythmic diagenesis of a homogeneous precursor sediment or the result of diagenetic enhancement of subtle underlying sedimentary rhythms. For horizontally correlated patterns, such as laterally extensive beds and layers of nodules, an a priori unknown external signal has to be assumed.     

Performance of small-scale irrigation schemes in Lake Tana Basin of Ethiopia: technical and socio-political attributes

This study was conducted to investigate technical and socio-political attributes that lead to the underperformance of two selected irrigation schemes (Shina and Bebeks) in the Lake Tana floodplains, Ethiopia. Irrigation application efficiency (AE) at nine experimental fields showed a wide range, from 20 to 80%, but was mostly between 40 and 60%. Irrigation water-use efficiency (IWUE) varied from 1.9 to 7.2 kg m^?3 for onion and 0.9 to 1.2 kg m^?3 for maize. The lined and earthen canal conveyance losses in Bebeks were 0.037 and 0.047 l s^?1 m^?1, whereas in Shina they were 0.033 and 0.044 l s^?1 m^?1, respectively. The overall consumed ratio (OCR) of water was 0.58 for Bebeks and varied from 0.73 to 1.2 in Shina. Both schemes are performing below the standard based on technical performance indicators. Irrigation water user associations (WUAs) were not implemented, but irrigation committees (ICs), composed of local political leaders, are managing both schemes. Canal and reservoir sedimentation from erosion of upstream catchment areas during the rainy season was the major problem.     

VERTICAL SEISMIC PROFILING IN COAL*

The intellection of seismic wave propagation in coal measures demands direct observation of the wavefield progression. Two vertical seismic profiles with high spatial and temporal sampling, were recently recorded in the Sydney Basin coalfields as part of an experimental coal seismic program. Static corrections and interval velocities were obtained by an automated system to determine first kicks and pulse rise times. Upgoing and downgoing waves were separated in the f—k-plane using a novel technique of contour slice filtering. The isolated upgoing waves clearly display reflections from the major coal seams within the stratigraphic sequence. The downgoing wave spectra were subjected to attenuation analysis. The deduced specific quality factor Q for Permian coal measure rocks lies in the range 20–70. Similar estimates were obtained in the time domain from measurements of pulse broadening. Synthetic VSP seismograms, computed using an exact recursive formulation, are an indispensable aid to interpretation. They illustrate the filtering effects of coal seams and sequences, and the effects of the contribution of internal and free-surface multiple reflections in the recorded wavetrains.     

PROBLEMS IN CONSTRUCTING A BATHYMETRIC BASE MAP FOR THE GULF OF FINLAND

The various steps required for compilation of a bathymetric chart of the Gulf of Finland are outlined, taking into account the specific conditions characteristic of that water body. The use of available initial source materials and special survey work required for supplementing these sources is discussed. Translated by Edward Torrey, Alexandria, VA 22308 from: Geodeziya i kartografiya, 1988, No. 8, pp. 40-42.     

Manganese concentration in the silts of the Mozhaisk Reservoir

The causes of the high Mn concentration in the solid phase and pore solution of bottom sediments in the Mozhaisk Reservoir are discussed. Regularities are identified in variations the concentrations of Mn forms in reservoir deposits from its upper part to the dam.     

Pastoralists’ perceptions and realities of vegetation change and browse consumption in the northern Kalahari, Namibia

Pastoral production around artificial watering points in semi-arid environments affects the density and composition of plant communities. In the Kalahari desert of southwestern Africa, bush encroachment is often regarded as the major form of land degradation resulting from pastoral production around watering points. We investigated the OvaHerero pastoralists’ perceptions of the extent of vegetation change since the establishment of artificial watering points in the northern Kalahari desert of Namibia, and related this to ecological data on vegetation change. We determined the utility of woody vegetation to pastoralists’ livelihoods in terms of provision of construction material, fuel wood and browse. We quantified local knowledge of cattle browse consumption and correlated this with field data. We also assessed the purposes for which major livestock types were used. Our results showed that: (1) pastoral knowledge of bush encroachment and browse consumption was consistent with concurrently collected field data, (2) the current level of bush encroachment was perceived to be beneficial for pastoral production, and (3) cattle played an important role in the production of milk and milk by-products for domestic use, and served as a source of cash income, while sheep and goats were primarily kept for meat consumption. This result contrasts with historical studies that mainly portray cattle as a symbol of social status among OvaHerero pastoralists.     

A ∼6100 14C yr record of El Niño activity from the Galápagos Islands

Lithostratigrahic and mineralogic analyses of sediments from hypersaline Bainbridge Crater Lake, Galápagos Islands, provide evidence of past El Niño frequency and intensity. Laminated sediments indicate that at least 435 moderate to very strong El Niño events have occurred since 6100 ¹⁴C yr BP (7130 cal yr BP), and that frequency and intensity of events increased at about 3000 ¹⁴C yr BP (3100 cal yr BP). El Niño activity was present between 6100 and 4000 ¹⁴C yr BP (4600 cal yr BP) but infrequent. The Bainbridge record indicates that there has been considerable millennial-scale variability in El Niño since the mid-Holocene.