Rise and fall of an isolated,unstable carbonate platform: The Cambrian of Southwestern Sardinia

Strong effects of tensional tectonics dominate the Lower and Middle Cambrian sequence of Southwestern Sardinia. A thinning of the continental crust and the possible development of a passive margin in the Cambrian is indicated by different stages of evolution of a mainly carbonatic platform: (a) Clastic-carbonatic homoclinal ramp with algal-archaeocyathan mounds (Epiphyton/Renalcis) in the west, clastic shallow marine to tidal sequences in the east. (b) Carbonatic-clastic ramp or rimmed shelf with an ooid shoal complex, prograding towards the west. The back-shoal area contains peloidal mudstones, algal-archaeocyathan biostromes (Girvanella) and increasingly tidal deposits (clastic and carbonatic) towards the east. (c) Isolated platform, aggraded to sea level, rimmed by slope deposits with slumps and breccia-beds in the southeast and northwest. (d) Isolated, flooded platform; barriers towards the open sea partly broke down. (e) Isolated platform with raised rims and deep interior, often with thick breccia-beds in uppermost parts. (f) Break-down of the platform (late Early Cambrian), marked by nodular limestones and limestoneshale intercalations. (g) Terrigenous clastics cover the former platform.

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Zusammenfassung Starke Auswirkungen von Dehnungstektonik prägen die unter- und mittelkambrische Abfolge Südwest-Sardiniens. Diese belegen eine Ausdünnung der kontinentalen Kruste, die mit der Entwicklung eines passiven Kontinentalrandes während des Kambriums verbunden sein könnte. Hierfür sprechen die beobachteten verschiedenen Entwicklungsstadien einer Plattform: (a) Siliciklastisch-karbonatische, homoklinale Rampe mit Algen-Archaeocyathen-Mounds (Epiphyton/Renalcis) im Westen, flachmarinen bis gezeitenbeeinflu\ten Bereichen im Osten. (b) Karbonatisch-siliciklastische Rampe oder Schelf mit einem randlichen Ooid-Untiefen-Komplex, der sich gegen Westen vorschob; im geschützten östlichen Bereich treten Peloid-Mudstones, Algen-Archaeocyathen Biostrome (Girvanella) und klastisch-karbonatische Gezeitenbildungen auf. (c) Isolierte Karbonatplattform, die sich bis zum Meeresspiegel hochbaute, im Südosten und Nordwesten des Gebietes von Bekkenbereichen mit Slumping und Breccienbildungen begrenzt. (d) Isolierte, überflutete Karbonatplattform; Barrieren zum offenen Meer brachen teilweise zusammen. (e) Isolierte Karbonatplattform mit erhöhten Rändern und tieferem Inneren; zahlreiche Breccienkörper treten in höchsten Teilen dieser Abfolge auf. (f) Zusammenbruch der Karbonatplattform (höchstes Unterkambrium), angezeigt durch Knollenkalke und Kalkschiefer. (g) Siliciklastika überlagern die Plattform.

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Résumé La série stratigraphique du Cambrien inférieur et moyen du sud-ouest de la Sardaigne a été fortement influencée par une tectonique de distension, qui implique un amincissement de la croûte continentale, sans doute lié au développement d'une marge passive au cours du Cambrien. Les arguments à l'appui de ce modèle sont fournis par les stades d'évolution successifs d'une plateforme, essentiellement carbonatée: (a) une rampe homoclinale, détritique-carbonatée, comportant à l'ouest des mounds à algues et archéocyathes (Epiphyton/Renalcis) et à l'est un milieu marin peu profond sous influence tidale; (b) une rampe carbonatéedétritique ou plateforme de haut-fond à ooÏdes qui s'est avancée vers l'ouest. La zone est d'arrière-barre comporte des mudstones à péloÏdes, des biostromes à algues (Girvanella) et archéocyathes et des sédiments carbonatés-détritiques de milieu intertidal; (c) une plateforme carbonatée isolée qui s'est développée jusqu'au niveau de la mer, bordée au sud-est et nord-ouest par des bassins à slumping et brèches; (d) une plateforme carbonatée isolée, inondée, dont les barrières vers la pleine mer se sont en partie détruites; (e) une plateforme carbonatée isolée dont les bords sont élevés et l'intérieur déprimé, recouverte par de nombreux corps bréchiques; (f) destruction de la plateforme carbonatée (partie supérieur de la Cambrien inférieur), indiquée par des calcaires à nodules et des calcschistes; (g) siliciclastes recouvrant la plateforme.

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The seasonal productivity cycle of phytoplankton and controlling factors in Lake Constance

Annual phytoplankton productivity in Lake Constance is about 300 g C m⁻², a value typical for mesoeutrophic lakes. Seasonal variations in phytoplankton biomass and productivity are exceptionally great because of a sequence of factors controlling the production process. During winter productivity is controlled by low energy inputs and high respiratory losses due to deep water column mixing. Biomass is low and water transparancy high. The spring phytoplankton growth is triggered by the thermal stabilization of the water column. The summer phytoplankton biomass maximum mainly depends on phosphorus availability. However, biomass yields comprise only 15–20% of values to be expected from the Redfield ratio because large proportions of POM are detritus and non-algal biota. Moreover, sedimentation during the second half of the year removes biomass from the euphotic zone. Water transparency and thus vertical distribution of algal photosynthesis is highly dependent on phytoplankton biomass. Self-shading causes considerably smaller seasonal variations in areal biomass and photosynthetic rates than in volume-based values. By light-shade adaptation effects of seasonal fluctuations in mean daily surface radiance fluxes on algal photosynthesis can to a significant extent be compensated for. At any given level of biomass daylength is the major determinant of daily production rates. Dedicated to Professor Elster on his 80th birthday.     

A case study of Kelvin-Helmholtz waves within an off-shore stable boundary layer: Observations and linear model

A case study of Kelvin-Helmholtz waves which were observed by two aircraft in a warm off-shore stable boundary-layer flow over the North Sea is presented. During the one-hour flight mission within an area of 40 × 40 km², the waves were intermittent both in space and time. They were centered around two levels, at 90 and 330m, where inflection points in the mean profile of the cross-wave wind component occurred together with Richardson numbers smaller than the critical value of 0.25. Observed wave amplitudes were on the order of 0.1 K for the potential temperature, 0.15ms^-1 for the vertical wind component, 0.3ms^-1 for the cross-wave wind component and 0.15ms^-1 for the along-wave wind component. Horizontally averaged vertical wave transports were down-gradient.Based on the observed wind and temperature profiles, wave simulations with a linear model are performed. Different diffusion coefficient estimates are tested. The model produces two types of Kelvin-Helmholtz waves with maximum amplitudes at the above mentioned two heights. The modeled wavelengths are about 30% shorter than the observed ones. Adjusting the modeled to the observed temperature variations, the modeled vertical wind variance and the vertical transports agree well with the observations, whereas the modeled horizontal wind variances are smaller than the observed ones.     

Modelling the responses of mature forest trees to air pollution

Systems analysis of the responses of forest trees to air pollutants led to mathematical models which represent the essential life processes of mature trees (photosynthesis, respiration, growth etc.) under ‘normal’ conditions and — especially — under pollution strees. Computer simulations using these models have made it possible to test and analyse the consequences of — known or hypothetical — pollution effects. The simulation runs have led to the identification of three distinct behavioural modes (growth, stagnation, breakdown) and corresponding stable and unstable system states. The results have produced new insights and contributed to the understanding of the dieback process of spruce and beech trees. The presentation will concentrate on the most recent and most complex of the hitherto existing models, theBeech model.     

Maximum entropy spectral analysis of volcanic tremor using data from Etna (Sicily) and Merapi (central Java)

The maximum entropy spectral analysis (MESA) method is applied to synthetic and observed tremor time series using autoregressive processes and recordings from the volcanoes Etna (Sicily) and Merapi (central Java). The MESA analysis can be used to estimate power spectra with sharp peaks from short data records. If the tremor source process can be modelled by an autoregressive process, the MESA method is well-suited for determining the coefficients of the underlying difference equations. As in the standard periodogram method of power spectrum estimation, a mesagram estimate using record segmentation and MESA spectrum averaging reduces the variance of the spectral estimator. In combination with periodogram estimates, mesagram estimates confirm that the tremor source may be modelled as an ensemble of randomly excited resonators. Used together, these estimates provide a valuable method for short-term monitoring of volcanic activity. In addition, they can be applied to the determination of new source parameters such as resonator frequencies, damping coefficients, excitation probabilities, correlation of exciting forces, and resonator coupling and in the pattern recognition of source types.     

Fluid evolution,wallrock alteration,and ore mineralization associated with the Rodalquilar epithermal gold-deposit in southeast Spain

At Rodalquilar gold mineralization is found in Late Tertiary volcanic rocks of the Sierra del Cabo de Gata and is related to a caldera collapse. Radial and concentric faults were preferred sites for gold deposition. Hydrothermal activity produced a specific alteration zoning around gold-bearing vein structures, grading from an innermost advanced argillic via an argillic into a more regionally developed propylitic zone. Advanced argillic alteration with silica, pyrophyllite, alunite, and kaolinite extends down to several hundred m indicating a hypogene origin. High-grade gold mineralization in vein structures is confined to the near-surface part of the advanced argillic alteration. Fine-grained gold is associated with hematite, jarosite, limonite, or silica. At a depth of about 120 m, the oxidic ore assemblage grades into sulfide mineralization with pyrite and minor chalcopyrite, covellite, bornite, enargite, and tennantite. Two types of fluids from different sources were involved in the hydrothermal system. Overpressured and hypersaline fluids of presumably magmatic origin initiated the hydrothermal system. Subsequent hydrothermal processes were characterized by the influx of low-salinity solutions of probable marine origin and by interactions between both fluids. Deep-reaching, advanced argillic alteration formed from high-salinity fluids with 20–30 equiv. wt% NaCl at about 225°C. Near-surface gold precipitation and silification are related to fluids with temperatures of about 175°C and 3–4 equiv. wt% NaCl. Gold was transported as Au(HS) ₂ ^– , and precipitation resulted from boiling with a concomitant decrease in temperature, pressure, and pH and an increase in fO₂. All features of the Rodalquilar gold deposit reveal a close relationship to acid-sulfate-type epithermal gold mineralization.     

Dynamics of aircraft exhaust plumes in the jet-regime

A computational model describing the two-dimensional, turbulent mixing of a single jet of exhaust gas from aircraft engines with the ambient atmosphere is presented. The underlying assumptions and governing equations are examined and supplemented by a discussion of analytical solutions. As an application, the jet dynamics of a B747-400 aircraft engine in cruise and its dependence on key parameters is investigated in detail. The computer code for this dynamical model is computationally fast and can easily be coupled to complex chemical and microphysical models in order to perform comprehensive studies of atmospheric effects from aircraft exhaust emissions in the jet regime.     

The Place of Narrative in the Study of Third World Migration: The Case of Spontaneous Rural Migration in Sri Lanka*

Studies of migration in geography have most often been based on quantitative research approaches. Such studies cannot adequately shed light on the contextuality of migration and the individual decision-making processes involved. Personal narratives express individual experiences within a social context. The story of “Amma,” a female rural migrant in Sri Lanka, shows the role of women in migration, the importance of family ties, the contextual causes of migration, and characteristics of the migration flow per se. Personal narratives have an underutilized potential of capturing the variety of migrants' experiences and the complexity of the decision to migrate.     

Rainwater composition over a rural area with special emphasis on the size distribution of insoluble particulate matter

The rainwater composition in the vicinity of Mainz, FRG, has been investigated with special emphasis on insoluble constituents. The number size distribution was determined in the range from 0.1 m up to 100 m radius. For particles with r>0.5 m radius the shape of the size distribution of insoluble particles in rain follows the shape of the average urban and rural aerosol. In this particular size range no major size selective removal processes could be seen. For r<0.5 m the number size distribution tends to flatten compared to the average aerosol. This might be the indication of a size selective removal process (Greenfield Gap).     

The Isthmus of Panama as a Crossroad for Prehistoric Migration of Domesticated Plants

For thousands of years, Indian tribes have gathered, selected, domesticated and spread useful plants over the whole American continent while mostly practicing shifting forms of horti-agriculture. It has been argued that origin and domestication of New World crops could be traced back to essentially two independent, narrowly circumscribed core regions, (i.e. Vavilov's genecentres) one in the highlands of Mexico and the other in the Andes of Peru. Those alleged centers of origin and genetic diversity have been synchronized with the main cradles of American agriculture. Based on long periods of research work in South America, we disagree with this opinion which is maintained primarily by anthropologists, sociologists and ethnologists. Except for marginal desert and mountain environments, there are no real biogeographical restrictions for plant domestication. Apparent geographical barriers, like the Darien gap, did not represent insurmountable hindrances for the diffusion and migration of useful species in the hands of indigenous peoples.