A model for Dansgaard–Oeschger events and millennial-scale abrupt climate change without external forcing

Climate Dynamics - We propose a conceptual model which generates abrupt climate changes akin to Dansgaard–Oeschger events. In the model these abrupt climate changes are not triggered by...     

Modeling the buildup of iron oxide encrustations in wells

Ferric iron encrustations are a common problem that seriously affects the performance of wells and drains. Their formation is induced by the mixing of reduced ground water containing ferrous iron with oxic shallow ground water and exposure to air. The process of ferrous iron oxidation is a kinetically controlled reaction. The reaction rate has a quadratic dependency on pH. The precipitating oxides have an autocatalytic effect that further enhances reaction progress. This paper describes the application of kinetic models to the problem of encrustation formation. Influences of pH, residence time, and autocatalysis were modeled. The overall influence of the autocatalytic reaction path is particularly strong when initial amounts of iron oxides are present. Autocatalysis provides a good explanation on the development of well yield commonly measured in the field. Encrustation precipitation is slow at first, but speeds up after a sufficient amount of iron oxide has built up. An incomplete removal of iron oxide encrustations during rehabilitation leads to a renewed increase in catalytic efficiency and encrustation precipitation.     

Strukturelle Skizze von Aegypten

Ohne Zusammenfassung     

Tektonik und Landschaft im Karstgebiet

Ohne Zusammenfassung     

15. Aus dem Sedimentstammbaum eines Trockengebietes

Zusammenfassung Seit dem Ende der Kreide bilden sich als Folgenerscheinung vertikaler Krustenbewegungen im südlichen Afrika Erosionsgürtel aus, die teilweise entlang ortsfremden Entwässerungssystemen liegen, zum anderen sich entlang der Hauptverbiegungszonen am Kontinentrand erstrecken.Aus einem Teilgebiet der Westküste Südafrikas wurden etwa 4500 cbkm Material entfernt, vom gleichzeitig gebildeten Sediment lassen sich unmittelbar etwa 15,5 cbkm, mittelbar etwa 600 cbkm nachweisen.Besondere Erscheinungsformen dieses Sediments sind durch örtliche Faktoren bedingt. Sie werden ausschnittsweise vom Gesichtspunkt der Betrachtungsweise bei der Untersuchung fossiler Sedimente beleuchtet. So besonders die Färbung des neugebildeten Gesteins in ihrer Abhängigkeit von minimalen Klimaunterschieden, das Vorkommen von Schwermineralien, Krustenbildungen und Verwitterungsformen.Zum Schluß ist eine Tabelle des Ausräumungs- und Sedimentationsvorganges im Trockengebiet im Vergleich mit entsprechenden Vorgängen im humiden Gebiet gegeben.     

Granitstöcke und Gneismassive

Ohne ZusammenfassungDas Vorhandensein einer Kontaktmetamorphose am Moltkefels, die auchHerrn Lepsius bekannt war, sucht er als einen besonders begünstigten Fall, durch Einbruch einer Schieferscholle in den Granit zu erklären. Nach neueren Untersuchungen stößt hier aber nur eine Schiefereinlagerung zwischen zwei konkordanten Gneisen diskordant gegen den Granit, und hat genau wie in dem ganz gleichliegenden Falle am sog. Wochenbett am Fuß des Forstkammes eine Kontaktmetamorphose angenommen. In den Nordsudeten finden wir also viele derLepsius-schen Annahmen nicht bestätigt.     

Exploring the thermal cycle of the Northern North Sea area using a 3-D circulation model: the example of PROVESS NNS station

Within the framework of activities of the EC funded Project ‘Processes of Vertical Exchange in Shelf Seas’ (PROVESS), the seasonal thermal behaviour during 1998 at a station in the North Sea has been investigated using COHERENS, a three-dimensional fully non-linear hydrodynamic model. Extensive hydrographic measurements were carried out at the Northern North Sea (NNS) station, located at (59°20′N, 1°E). The collected data are used to validate the model results, showing an acceptable agreement between modelled temperatures and those obtained from CTDs and moored thermistors. This is valid both for surface and bottom temperatures, while the mixed layer thickness appears to be underestimated. A series of 3-D runs, testing different turbulence schemes, an internal wave mixing (IWM) parameterisation and the sensitivity to an increase of the surface stress, have been performed with the aim of assessing the relative importance of the advective and mixing processes. The model comparisons mostly evidenced differences in the behaviour of the bottom layer temperature during the last part of the year, which may be due to advection processes. The adoption of an internal wave mixing parameterisation, though managing to reproduce a deeper thermocline, overestimates the mixing around the period of the thermocline breakdown. The run adopting a wind stress increased by 50% provides a better agreement between observed and modelled thermocline. This applies also for surface velocities when compared to Acoustic Doppler Current Profiler (ADCP) measurements, while the bottom ones appear slightly underestimated only in the U-component. The Northern North Sea site appears to be located close to a gyre induced by thermal fronts. Comparisons with nearby wind data measured by an oil rig and by the ships operating in the area seem to confirm that the wind forcing values adopted during the integration are underestimated.