Geochemical characterisation of organic matter in Keg River Formation (Elk point group, Middle Devonian), La Crete Basin, Western Canada

Carbonates from the Keg River Formation, La Crete Basin, Alberta, western Canada were examined in order to define: (a) oil source rock potential; (b) bulk maceral composition; (c) extract yield; and finally (d) facies variations using PY-GC-MS. Thirty samples from 6 different wells were examined from the lower Keg River member and 4 from the upper Keg River member using conventional geochemical methods. As maturity differences are absent within the sample set, variations in TOC, T_max, hydrogen index, organic petrography and extract yields are caused by variability in organic matter input, which is revealed by molecular characterisation using PY-GC-MS. Lower Keg River member bituminous wackestones are excellent potential source rocks containing Types I–II and Type II organic matter. Types I–II organic matter contains large well preserved (up to 200μm in diameter) thick-walled Tasmanites (10–15% of sample) and akinete algal cells indicative of algal blooms within an amorphous bituminite. Type II organic matter contains a higher proportion of degraded alginites/bituminite relative to well-preserved alginites. Extract yields (mg/g TOC) were seen to increase from Types I–II to Type II organic matter. PY-GC-MS revealed that 1,2,3,4-tetramethylbenzene was a major peak in most samples. This is a pyrolysis product arising from β-cleavage of C₄₀ diaromatic carotenoids incorporated within the kerogen during diagenesis. The source of this compound is thought to be from an unknown diaromatic compound with a 2,3,6-/3,4,5-trimethyl substitution pattern and isorenieratene, which is specific to photosynthetic green sulphur bacteria (Chlorobiaceae) suggesting that the photic zone was at least partially anoxic during deposition of these samples. The relative abundance of this compound/n-C11-alkene and organic sulphur (calculated from the thiophene ratio) both increase from Types I–II to Type II organic matter. This trend was grossly similar to the trend seen in the variability of extract yield with hydrogen index. A similar trend for HI and T_max indicates samples containing a higher proportion of degraded alginites/bituminite relative to well-preserved alginite are more labile than Type I–II organic matter. Upper Keg River member marls contain Type II organic matter, which is characterised by heavily degraded algal material within a bituminous groundmass. Pyrolysates of two of the marl samples contain only low amounts of 1,2,3,4-tetramethylbenzene, in contrast to the bituminous wackestones, indicating that the depositional environment/source input was different during deposition of the marl samples. Although both marls contain similar organic matter, their pyrolysates were significantly different. One marl (1141.9 m) was highly paraffinic containing dominantly short-chain alkene/alkane doublets, while the other marl (1137.6 m) contained a bimodal n-alkane/alkene distribution and high amounts of alkylphenols, which may be derived from preservation of resistant algal polyphenolic molecules or suggest a terrestrial input.     

Measurements of positive ion conversion and removal reactions relating to the Jovian ionosphere

Measured rates are presented for the reaction of He⁺ ions with H₂ (and D₂) molecules to form H⁺, H₂⁺, and HeH⁺ ions, as well as for the subsequent reactions of H⁺ and HeH⁺ ions with H₂ to form H₃⁺. The neutralization of H₃⁺ (and H₅⁺) ions by dissociative recombination with electrons is shown to be fast. The reaction He⁺ + H₂ is slow (k = 1.1 × 10^?13 cm³/sec at300°K) and produces principally H⁺ by the dissociative charge transfer branch. It is concluded that there may be a serious bottleneck in the conversion of two of the primary ions of the upper Jovian ionosphere, H⁺ and He⁺ (which recombine slowly), to the rapidly recombining H₃⁺ ion (α[H₃⁺]?3.4 × 10^?7 cm³/sec at 150°K).     

Analysis of processes leading to localized electron enhancements in the outer radiation belt

New information obtained about >500 keV electron intensity enhancements, which have been observed intermittently close to the outer edge of the electron radiation belt, is used in conjunction with an earlier statistical study by Brown and Stone (1972) to investigate processes which could lead to such structures. The enhancements are typically of ～20 sec duration and occur in a very narrow invariant latitude band, maximally 2° wide. The intensity increase relative to the “normal” background level is up to a factor 10, and the “spike” frequency of occurrence is strongly local time dependent, with more spikes observed in the night and dusk-noon sectors than in the noon-dawn sector. The processes investigated quantitatively are distortions of the magnetospheric topology in the equatorial region, wave-particle interactions and the effects of ionospheric currents. It is shown that the various processes which contribute to equatorial field disturbances can explain the observations.     

Distribution and Migrations of two Cerithid Snails on a Sand Flat in Bermuda

Abstract. Two species of cerithid prosobranchs live on a sand flat in Bermuda. At low tide during day light Batillaria minima is found on the surface whereas Cerithium lutosum remains buried in the sediment. This correlates with much higher lethal and coma temperatures, as well as much greater resistance to desiccation, of the former species. However, at other diurnal or tidal stages the two species co-occur in the same layer of sand. Despite greater resistance to physical factors the population of B. minima seems to be declining since in 1980 juveniles of this species were rare on Tuckers Town Flat whereas the population of C. lutosum was thriving.     

A SF6 tracer study of horizontal mixing in Lake Constance

Horizontal mixing processes in the hypolimnion of the western part of Lake Constance are studied by measuring the dispersion of a sulfur hexafluoride (SF₆) tracer plume. Only 1 liter gaseous SF₆ (STP) was released at a central hypolimnic depth of 60 m in August 1990. Over a period of 3 months the horizontal dispersion of the tracer plume was measured by 19 surveys using a new, vertically integrating sampling device. The observed horizontal dispersion is marked by strong storm-induced stirring events. Nevertheless mean turbulent diffusion coefficients for the whole period can be computed. They rise about linear from 0.7 10⁵ cm²/s to 3.0 10⁵ cm²/s with the distance from the western end of the lake. For the hypolimnion of Überlingersee, a sill-separated basin in the western part of Lake Constance, a simple budget model gives an exchange time of 67 ± 6 days with the main basin (Obersee).     

Plate tectonics and intracratonic mountain ranges in Morocco - The mesozoic-cenozoic development of the Central High Atlas and the Middle Atlas

Plate tectonic processes in the Atlantic and western Tethyan realm directed the post-Variscan sedimentary and structural evolution of the High Atlas and Middle Atlas intracontinental mountain ranges of Morocco. Plate movements caused a reactivation of an inherited pan-African or Hercynian fault pattern by the variation of stress regimes through time. This resulted in strike-slip as well as vertical tectonics. During times of relative tectonic quiescence eustatic sea-level changes governed the sedimentary development. The most important, often interacting, global tectonic determinants are: taphrogenesis of the NW-African continental margin lasting until the Early Cretaceous (Triassic rifting and subsequent mid-Atlantic spreading), strike-slip-faulting at the Newfoundland-Gibraltar fault zone (Liassic — earliest Eocene), and continental convergence between Europe (Iberia) and Africa which started in the Late Cretaceous and reached its acme in the Neogene. In the realm of the Central High Atlas and the Middle Atlas the interaction of these processes triggered continental rifting (Triassic) and subsequent marine flooding of the intergrown riftgrabens prograding from the Tethys realm (Early Jurassic — earliest Middle Jurassic). After its abortion, the former Atlas rift was filled up with marine sediments (Bajocian — Bathonian), followed by continental redbeds and final uplift (late Mid Jurassic — late Early Cretaceous). Eustatic sea-level changes mostly governed the sedimentary evolution from Aptian to latest Mid Eocene. After a first weak uplift of the central High Atlas during the Senonian major uplift of the intracontinental chains commenced at the Mid/ Late Eocene transition. Diastrophism of the Atlas ranges during the Miocene and Pliocene coincided with the main orogenic movements of the Betico-Rifean arc.

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Zusammenfassung Die post-variskische sedimentäre und strukturelle Entwicklung der intrakontinentalen Gebirgsketten des Hohen und des Mittleren Atlas wurde durch plattentektonische Prozesse im atlantischen und westmediterranen Raum gesteuert, die auf ein ererbtes panafrikanisches bzw. variskisches Störungsmuster einwirkten. Zu Zeiten relativer tektonischer Ruhe bestimmten eustatische Meeresspiegelschwankungen die sedimentäre Entwicklung. Die von horizontaler und vertikaler Bruchtektonik beherrschte Atlas-Tektonik wurde durch in der Zeit wechselnde Streßfelder bestimmt. Wichtigste großtektonische, oft zusammenwirkende Steuerungsfaktoren waren: die bis in die Unterkreide wirkende Taphrogenese im Bereich des NW-afrikanischen Kontinentalrandes (triadisches Rifting und anschließendes Spreading im Atlantik), Transform-Bewegungen an der Neufundland-Gibraltar-Störungszone zwischen Lias und ältestem Eozän sowie die Konvergenz zwischen Europa (Iberia) und Afrika, welche nach ihrem Beginn in der späten Kreide ihren Höhepunkt im Neogen erreichte. Das Zusammenspiel dieser Faktoren bewirkte im zentralen Hohen Atlas und im Mittleren Atlas die Entstehung kontinentaler Riftgräben während der Trias, die von der Tethys ausgehende Flutung der zusammengewachsenen Riftgräben im Lias und untersten Dogger (Sinemurium — Aalenium) sowie die zuerst marine (Bajocium — Bathonium), dann kontinentale Füllung und Heraushebung der stillgelegten Riftzone zwischen spätem Mitteljura und später Unterkreide. Zwischen Apt und Ende des Mitteleozän kontrollierten vor allem eustatische Meeresspiegelschwankungen die sedimentäre Entwicklung. Nach ersten Hebungen im zentralen Hohen Atlas während des Senon begann die eigentliche Heraushebung der intrakontinentalen Ketten an der Wende Mittel-/Obereozän. Die mio-/pliozäne Hauptphase der Atlas-Tektogenese war zeitgleich mit der Orogenese im Betico-Rif-Bogen.

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Résumé L'évolution sédimentaire et structurale postvarisque des chaînes intracontinentales du Haut et du Moyen Atlas a été régie par les processus de la tectonique des plaques qui se déroulaient dans l'Atlantique et dans la Méditerrannée occidentale. Ces mouvements de plaques ont réactivé un ensemble de failles héritées des phases pan-africaine et hercynienne, en y produisant des déplacements verticaux et horizontaux en réponse aux fluctuations temporelles des champs de contraintes. Au cours des périodes de calme tectonique relatif, les changements eustatiques du niveau de la mer ont déterminé le régime de la sédimentation. Les facteurs de la tectonique les plus importants, à action d'ailleurs souvent combinée, ont été: la traphrogenèse, active jusqu'au Crétacé inférieur, de la marge nord-ouest du continent africain (rifting triasique et ouverture subséquente de l'Atlantique), les mouvements de décrochement le long de la zone transformante de Terre Neuve-Gibraltar depuis le Lias jusqu'au début de l'Eocène, ainis que la convergence entre l'Europe (Ibérie) et l'Afrique qui débuta au Crétacé supérieur pour culminer au Néogène. Dans le Haut Atlas central et dans le Moyen Atlas, le jeu combiné de ces facteurs a provoqué la formation de grabens continentaux (Trias) puis leur envahissement par la mer à partir de la Téthys (Lias-Dogger inférieur). Après la fin de son activité, le rift atlasique a été comblé par des sédiments, d'abord marins (Bajocien-Bathonien), puis continentaux jusqu'au soulèvement final (fin du Jurassique moyen — fin du Crétacé inférieur). De l'Aptien à la fin de l'Eocène moyen, l'évolution sédimentaire a été régie par les changements eustatiques du niveau de la mer. Après un premier soulèvement modéré du Haut Atlas central au cours du Sénonien, le soulèvement majeur des chaînes intracontinentales débuta à la limite Eocène moyen-Eocène supérieur. Le diastrophisme de l'Atlas au cours du Mio-Pliocène a coïncidé avec les mouvements orogéniques de l'arc bético-rifain.

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Recent changes of the thermohaline circulation in the subpolar North Atlantic

Time series of hydrographic sections in the northern North Atlantic from the period 1990 to 2004 are analyzed for changes in the characteristics and distribution of water masses that are involved in the thermohaline circulation (THC). During the 1990s, the North Atlantic Oscillation (NAO) alternates from a positive phase (strong westerlies) to a negative phase (weak westerlies). The reduced ocean heat loss confined the convection in the Labrador Sea to the upper 1,200 m, generating a new salinity minimum layer characterizing the Upper Labrador Sea Water (ULSW), and led to a warming and salinization of the older LSW below due to lateral mixing. The Lower LSW, formed in the first half of the 1990s, spread in the subpolar gyre and reached the Newfoundland and Irminger basins after about 1 to 2 years, where the associated isopycnal doming contributed to eastward frontal shifts in the upper layer. After 5 and 6 years, it arrived in the Iceland and West European basins, respectively. The collapse of the isopycnal dome in the Labrador Sea, associated with the drainage of the Lower LSW, resulted in a slowing of the cyclonic circulation of the subpolar gyre. This was accompanied in the upper layer by a westward shift of the southeastern extension of the gyre and a northward advection of warm and saline subtropical water in its eastern part, which finally reached the Labrador Sea after about 7 years. In the upper layer of the Labrador Sea, the advection of warm and saline water dominated over the heat loss to the atmosphere and the freshwater gain from melting ice and precipitation in the NAO-low period, so that no accumulation of freshwater but an increase of the heat and salt contents were observed, as in the whole eastern part of the subpolar gyre. Within 1 to 2 years after the drop of the NAO in the winter of 1995/1996, the Subarctic (Subpolar) Front shifted northward and westward north of about 50°N, favored by the retreat of the low-salinity tongue extending eastward from the southern Labrador Sea, and it shifted southward and eastward in the Newfoundland Basin. Therefore, the enhanced northward advection of subtropical waters in the northeastern North Atlantic is balanced by the enhanced southward advection of subarctic waters, including Lower LSW in the Newfoundland Basin, indicating a strong response of the gyre component of the THC.     

Intercomparison of Two Hydroxyl Radical Measurement Techniques at the Atmosphere Simulation Chamber SAPHIR

At the atmosphere simulation chamber SAPHIR in Jülich both Laser-Induced Fluorescence Spectroscopy (LIF) and Long-Path Differential Optical Laser Absorption Spectroscopy (DOAS) are operational for the detection of OH radicals at tropospheric levels. The two different spectroscopic techniques were compared within the controlled environment of SAPHIR based on all simultaneous measurements acquired in 2003 (13 days). Hydroxyl radicals were scavenged by added CO during four of these days in order to experimentally check the calculated precisions at the detection limit. LIF measurements have a higher precision (σ= 0.88×10⁶ cm^–3) and better time resolution (Δt = 60 s), but the DOAS method (σ= 1.24×10⁶ cm^–3, Δt = 135 s) is regarded as primary standard for comparisons because of its good accuracy. A high correlation coefficient of r = 0.95 was found for the whole data set highlighting the advantage of using a simulation chamber. The data set consists of two groups. The first one includes 3 days, where the LIF measurements yield (1 – 2) ×10⁶ cm^–3 higher OH concentrations than observed by the DOAS instrument. The experimental conditions during these days are characterized by increased NO_x concentration and a small dynamic range in OH. Excellent agreement is found within the other group of 6 days. The regression to the combined data of this large group yields unity slope without a significant offset.     

Neural Networks: A General Framework for Non-Linear Function Approximation

The focus of this paper is on the neural network modelling approach that has gained increasing recognition in GIScience in recent years. The novelty about neural networks lies in their ability to model non‐linear processes with few, if any, a priori assumptions about the nature of the data‐generating process. The paper discusses some important issues that are central for successful application development. The scope is limited to feedforward neural networks, the leading example of neural networks. It is argued that failures in applications can usually be attributed to inadequate learning and/or inadequate complexity of the network model. Parameter estimation and a suitably chosen number of hidden units are, thus, of crucial importance for the success of real world neural network applications. The paper views network learning as an optimization problem, reviews two alternative approaches to network learning, and provides insights into current best practice to optimize complexity so to perform well on generalization tasks.