Areal extent and first U-Pb age of the Pre-Damaran Abbabis complex in the central Damara belt of South West Africa (Namibia)     

R. E. Jacob  A. Kröner  A. J. Burger 《International Journal of Earth Sciences》1978,67(2):706-718

The Proterozoic Abbabis Complex emerges from beneath the metasediments of the Late Precambrian Damara Supergroup within mantled gneiss domes and anticlinoria over a wide area in the central granite zone of the Pan African Damara belt. U-Pb dating of composite-, size-, and non-magnetic fractions of metamict zircons from two samples of Abbabis granite-gneiss from the farm Abbabis yielded a concordia intercept age of 1925 _–280 ⁺³³⁰ Ma. It is speculated that the Abbabis basement rocks are continuous under the Damaran metasedimentary cover and may link up with the Franzfontein basement inlier of similar age. This conclusion supports the ensialic character of the northeastern branch of the Damara orogenic belt.

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Zusammenfassung Der proterozoische Abbabis-Komplex tritt nicht nur als Grundgebirgsaufwölbung unter den Metasedimenten der Damara-Supergruppe im bisher bekannten Gebiet südlich von Karibib auf, sondern vergleichbare Gesteine wurden auch in Domstrukturen und Antiklinorien nahezu im gesamten Bereich der Zentralzone des Pan-Afrikanischen Damara-Orogens von Südwest-Afrika gefunden. Dieses vorwiegend aus Granitgneis bestehende Grundgebirge wurde während der Damara-Orogenese unterschiedlich stark überprägt, durch Granite intrudiert und zum Teil remobilisiert, so daß sein ursprünglicher Charakter stellenweise nicht mehr erkennbar ist und daher eine Unterscheidung von hochmetamorphen und teilweise anatektischen Damara-Metasedimenten nahezu unmöglich wirdU-Pb-Altersbestimmungen an gemischten Konzentraten metamikter Zirkone von zwei Granitgneis-Proben des Abbabis-Komplexes sowie an verschiedenen Korngrößen- und nicht-magnetischen Fraktionen ergaben ein Concordia-Schnittalter von 1925 _–280 ⁺³³⁰ M. J. Die relativ große Ungenauigkeit dieses Alters ist wahrscheinlich auf die heterogene Natur der analysierten Zirkone und damit auf eine bisher nicht näher bekannte komplexe prä-Damara-Geschichte des Abbabis-Komplexes zurückzuführen. Eine Überprägung während der Damara-Orogenese ist aus den Meßdaten nicht ersichtlich.Ähnlichkeiten der Abbabis-Geochronologie mit der des Franzfontein-Grundgebirgskomplexes im Norden des Damara-Orogens lassen auf eine kontinuierliche sialische Kruste unter den Damara-Deckschichten vom Zentralbereich des Orogens bis zu seinem nördlichen Vorland schließen. Eine direkte Verbindung des Abbabis-Komplexes mit dem Grundgebirge im südlichen Vorland des Orogens wird jedoch von den vorhandenen Altersdaten bisher nicht gestützt, so daß die von einigen Autoren postulierte ensialische Entwicklung des Nordost-Astes der Damara-Geosynklinale, obwohl wahrscheinlich, noch nicht endgültig geklärt ist.

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Résumé Le complexe d'Abbabis (d'âge Protérozoique) affleure en boutonnières au sein des métasédiments du supergroupe du Damara (Précambrien terminal); il forme des anticlinoria et des dômes (mantled gneiss domes) bien développés dans la zone granitique centrale de la chaîne pan-africaine du Damara. Des mesures par la méthode U/Pb effectués sur des fractions composites, granulométriques et non magnétiques de zircons métamictes, extraites de deux échantillons de granito-gneiss d'Abbabis (provenant de la ferme Abbabis) ont fourni un âge de 1925 _–280 ⁺³³⁰ Ma (intersection de la courbe Concordia). La continuité de ce socle d'Abbabis sous les méta-sédiments damariens et sa corrélation avec la boutonnière de socle du même âge de Franzfontein sont proposés. Le caractère ensialique du rameau NE de la chaîne damarienne est ainsi mis en évidence.

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Contribution Nr. 29 of the South African Geodynamics Project, presented at the 9th Colloquium on African Geology, Göttingen, April 1977.  相似文献   

Effects of tropical deforestation on global and regional atmospheric chemistry   总被引：10，自引：0，他引：10  

Michael Keller  Daniel J. Jacob  Steven C. Wofsy  Robert C. Harriss 《Climatic change》1991,19(1-2):139-158

A major portion of tropospheric photochemistry occurs in the tropics. Deforestation, colonization, and development of tropical rain forest areas could provoke significant changes in emissions of radiatively and photochemically active trace gases. A brief review of studies on trace-gas emissions in pristine and disturbed tropical habitats is followed by an effort to model regional tropospheric chemistry under undisturbed and polluted conditions. Model results suggest that changing emissions could stimulate photochemistry leading to enhanced ozone production and greater mineral acidity in rainfall in colonized agricultural regions. Model results agree with measurements made during the NASA ABLE missions. Under agricultural/pastoral development scenarios, tropical rain forest regions could export greater levels of N₂O, CH₄, CO, and photochemical precursors of NO _y and O₃ to the global atmosphere with implications for climatic warming.  相似文献   

Comparison of Downhole and Surface Sampling for the Determination of Volatile Organic Compounds (VOCs) in Ground Water     

Michael E. Rosen  James F. Pankow  Jacob Gibs  Thomas E. Imbrigiotta 《Ground Water Monitoring & Remediation》1992,12(1):126-133

The relative precision and accuracy of sampling and analysis methods for the determination of trace concentrations of volatile organic compounds (VOCs) in ground water were compared. Samples were collected from a well containing nanogram-per-liter (ng/L) to microgram-per-liter (μg/L) levels of VOCs. A Keck helical rotor submersible pump was used to collect samples at the surface for analysis by purge and trap (P&T) and for analysis by adsorption/thermal desorption (ATD). Downhole samples were collected by passing water through an ATD cartridge. Although slight spontaneous bubble outgassing occurred when the water was brought to the surface, the relative precisions and comparabilities of the surface and downhole methods were generally found to be equivalent from a statistical point of view. A main conclusion of this study is that bringing sample water to the surface for placement in VOC vials (and subsequent analysis by P&T) can be done reliably under many circumstances. However, care must still be taken to prevent adsorption losses and cross contamination. Samples subject to strong bubble outgassing will need to be handled in a special fashion (e.g., by downhole ATD) to minimize volatilization losses. Additionally, the higher sensitivity of the ATD method allows lower detection limits than are possible with P&T. For example, several compounds present at the ng/L level could be determined with confidence by ATD, but not by P&T.  相似文献   

Evaluation of210Pb dating in lake sediments using stable Pb,Ambrosia pollen,and137Cs     

Jules M. Blais  Jacob Kalff  R. Jack Cornett  R. Douglas Evans 《Journal of Paleolimnology》1995,13(2):169-178

We used sediment chronology data from fourteen published studies of lake cores across much of North America and Scandinavia in order to make a large scale comparison of the different dating techniques. The uncertainty of²¹⁰Pb derived dates was determined using common sediment event markers: the stable Pb rise, the¹³⁷Cs rise, and theAmbrosia pollen rise. For all data combined, the 95% confidence intervals for the stable Pb rise and theAmbrosia rise, were approximately 30 years. These 95% confidence intervals are slightly higher than those derived by First-Order Error analysis performed by others on²¹⁰Pb derived dates. When comparing the concordance of two²¹⁰Pb models (CRS and CIC) against markers of known history, we found that the CRS model dates (constant rate of supply) had consistently better agreement than the CIC model dates (constant initial concentration). Major discrepancies between¹³⁷Cs and²¹⁰Pb were common, but were consistently more severe in sediments of soft water lakes pointing to an inability of sediments with low mineral content to immobilize Cs.This is the ninth of a series of papers to be published by this journal following the 20 ^th anniversary of the first application of²¹⁰Pb dating of lake sediments, Dr. P. G. Appleby is guest editing this series.  相似文献   

Tidal effects on groundwater dynamics in shallow coastal aquifers—southeast coast of Tamilnadu,India     

C.?SingarajaEmail author  S.?Chidambaram  Noble?Jacob  S.?Selvam  M.?V.?Prasanna 《Arabian Journal of Geosciences》2016,9(6):467

Tides play a significant role in the coastal environment; the mangroves, back waters, creeks, and the coastal groundwater interface. Tidal range has been calculated by using TIDECAL software. This study attempts to find the relation between water table condition and tides in the shallow coastal aquifers. The study was conducted by selecting 12 open wells along the coast of the southern part of Cuddalore to the northern part of Nagapattinam district of Tamilnadu. Observations were made in situ for water table fluctuation, dissolved oxygen (DO), and electrical conductivity (EC) for 24 h during full moon (FM) and new moon (NM) of every 2 h. The present study shows the relationship between tidal variations with respect to water table fluctuations and helps to understand the behavior of DO and EC. An interpolation technique, inverse distance weighted (IDW) method was used to obtain the spatial distribution map. The temporal and the spatial relationships between water table fluctuation, EC, and DO were also used in order to understand the coastal ecosystem in the natural environment.  相似文献   

To Ease or Not to Ease: Interest in Conservation Easements Among Landowners in Brown County,Indiana     

James R. Farmer  Charles Chancellor  Jacob Brenner  James Whitacre  Eric G. Knackmuhs 《The Professional geographer》2016,68(4):584-594

Clearly understanding the connections among land use, demographics, and sensitivity and engagement with natural areas can provide insight into landowner decision making. Pertinent findings from our mailed survey of landowners in Brown County, Indiana, include that (1) individuals already engaged in land conservation or environmental activities are most likely to consider a conservation easement (CE), (2) hunters and anglers are less likely to have interest in CEs, and (3) few differences exist between seasonal and permanent residents. Among other things, this research highlights the significance for landowner willingness to grant a CE of involving landowners in conservation activities and environmental organizations.  相似文献   

Confined fluids and their role in pressure solution     

Alessandro Anzalone  James Boles  George Greene  Kevin Young  Jacob Israelachvili  Norma Alcantar   《Chemical Geology》2006,230(3-4):220-231

The process of pressure solution is defined as the dissolution of materials under high stress at grain-to-grain contacts and precipitation at interfaces under low stress. The kinetics of this process are still poorly understood mainly because of the large timescales involved. In this research, the Surface Forces Apparatus (SFA) technique was coupled with an optical interference technique for in situ visualization of the nanoscale deformations and thickness changes. The SFA was used to measure the forces (or pressures) and distances between two solid surfaces pressed together with a thin film between them. Using the SFA, combined with geological observations, we are studying the short-range colloidal forces between surfaces of mica and silica at the nanoscale such as van der Waals, electrostatic, and hydration forces.

This study involves two cases, the symmetric case of mica in contact with mica and the asymmetric case of a quartz surface in contact with mica. Our results reveal highly subtle effects depending on the nature and concentration of the counterions present in the solution either of Na⁺, Ca²⁺, or mixtures of these ions, as well as on the pH. For the symmetric case, the equilibrium interactions of force F or pressure P versus fluid film thickness T have been measured between the mica surfaces across aqueous films in the thickness range from T = 25 Å down to contact separations around T = 0 Å, and depend on the solution conditions and applied lithostatic pressure. Measurements have also been made of the rates of diffusion of ions through such ultra-thin films and on the precipitation and growth of ionic crystallite layers on the surfaces. Our results show that the diffusion coefficient of hydrated sodium is two orders of magnitude lower than the diffusion of water into mica–mica cleavage and a factor of 40 lower than the coefficient of sodium ions in bulk water.

For the asymmetric case, the dissolution of the quartz surface was observed to be dependent on the interfacial fluid composition and pH, the externally applied ‘lithostatic’ pressure, and the type of crystalline structure exposed to the mica surface. Our experiments also show that there is an initial stage after fresh solution is added in which the spacing between the surfaces increases, however, the thickness started decreasing steadily after approximately 4 h of exposure independently of the crystallinity of the quartz surface. For a particular set of conditions, the process eventually slows down and reaches equilibrium after some time, but a further increase in pressure restarts the dissolution process. This is also true for the addition of fresh interfacial solution during the experiment after a period of thickness fluctuation. These results are consistent with the observation that pressure solution of quartz is greatly enhanced with the presence of mica.  相似文献   

Interactive Effects of UV-B Radiation and Chemical Contamination on Physiological Parameters in the Lichen Ramalina lacera     

Jacob Garty  Tal Levin  Haya Lehr  Sharon Tomer  Ayala Hochman 《Journal of Atmospheric Chemistry》2004,49(1-3):267-289

To assess the impact of ultraviolet B (UV-B) radiation under laboratory conditions on lichens from different biomonitoring sites, thalli of the lichen Ramalina lacera were transplanted from a clean air site to nine different polluted sites in Israel. Our objective was to determine the probability of synergistic/antagonistic effects on physiological parameters by considering two stressors: chemical contamination and UV-B. Exposure to both airborne pollutants in the field and subsequent UV-B radiation in the laboratory which led to a severe disintegration of cell membranes, followed by electrolyte leakage, indicate the probability of a synergistic effect. The impairment of the potential quantum yield of electron transfer through photosystem II (PSII) of photosynthesis indicated by a significant decrease of the ratio F_v/F_m, further demonstrated the synergistic effect of both chemical contamination and UV-B radiation. Besides enlarged amounts of stress-ethylene in lichens at contaminated sites, the reduced levels of ethylene production upon UV-B radiation in the present study suggest the likelihood of an antagonistic effect of xenobiotic elements and UV-B radiation as a result of an impaired ethylene-producing system.  相似文献   

Nature and origin of eclogite xenoliths from kimberlites   总被引：16，自引：0，他引：16  

D.E. Jacob   《Lithos》2004,77(1-4):295-316

Eclogites from the Earth's mantle found in kimberlites provide important information on craton formation and ancient geodynamic processes because such eclogites are mostly Archean in age. They have equilibrated over a range of temperatures and pressures throughout the subcratonic mantle and some are diamond-bearing. Most mantle eclogites are bimineralic (omphacite and garnet) rarely with accessory rutiles. Contrary to their overall mineralogical simplicity, their broadly basaltic-picritic bulk compositions cover a large range and overlap with (but are not identical to) much younger lower grade eclogites from orogenic massifs. The majority of mantle eclogites have trace element geochemical features that require an origin from plagioclase-bearing protoliths and oxygen isotopic characteristics consistent with seawater alteration of oceanic crust. Therefore, most suites of eclogite xenoliths from kimberlites can be satisfactorily explained as samples of subducted oceanic crust. In contrast, eclogite xenoliths from Kuruman, South Africa and Koidu, Sierra Leone stem from protoliths that were picritic cumulates from intermediate pressures (1–2 Ga) and were subsequently transposed to higher pressures within the subcratonic mantle, consistent with craton growth via island arc collisions. None of the eclogite suites can be satisfactorily explained by an origin as high pressure cumulates from primary melts from garnet peridotite.  相似文献   

Effects of Sample Isolation and Handling on the Recovery of Purgeable Organic Compounds     

Jacob Gibs  Thomas E. Imbrigiotta  James H. Ficken  James F. Pankow  Michael E. Rosen 《Ground Water Monitoring & Remediation》1994,14(2):142-152

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