Effect of bottom stress formulation on modelled flow and turbidity maxima in cross-sections of tide-dominated estuaries

A three-dimensional numerical model with a prognostic salinity field is used to investigate the effect of a partial slip bottom boundary condition on lateral flow and sediment distribution in a transect of a tidally dominated channel. The transect has a symmetrical Gaussian cross-channel bottom profile. For a deep, well-mixed, tidally dominated channel, partial slip decreases the relative importance of Coriolis deflection on the generation of cross-channel flow patterns. This has profound implications for the lateral distribution of residual salinity that drives the cross-channel residual circulation pattern. Transverse sediment transport, however, is always found to be governed by a balance between advection of residual sediment concentration by residual lateral flow on the one hand and cross-channel diffusion on the other hand. Hence, the changes in the cross-channel distribution of residual salinity modify the lateral sediment distribution. For no slip, a single turbidity maximum occurs. In contrast, partial slip gives a gradual transition to a symmetrical density distribution with a turbidity maximum near each bank. For a more shallow, partially mixed tidal channel that represents the James River, a single turbidity maximum at the left bank is found irrespective of the near-bed slip condition. In this case, semi-diurnal contributions to sediment distribution and lateral flow play an important role in cross-channel sediment transport. As vertical viscosity and diffusivity are increased, a second maximum at the right bank again exists for partial slip.     

A numerical sandbox: high-resolution distinct element models of halfgraben formation

Basin-scale tectonics and sedimentation are studied using particle flow code (PFC), a special implementation of the distinct-element method (DEM) using circular elements. Special focus is on the development and application of new techniques, which allow for strain weakening and localisation effects and, thus, the formation of discrete fault patterns in high-resolution DE models.Fundamental modelling assumptions and the procedures necessary to define the microproperties of a DE material from given rock mechanical data are first explained. Recent methodical enhancements, consisting of automatic fault detection (AFD) and intelligent crack management (ICM) algorithms are also discussed. Refined DE modelling techniques are then applied to three scenarios of extensional basin formation, i.e. the evolution of halfgrabens above detachments with simple listric and ramp–flat–ramp geometries, respectively. Numerical modelling results compare favourably with the analogue (‘sandbox’) models widely used in this kind of basin studies. Not only do they reproduce the general basin architecture (e.g. roll-over anticlines and crestal collapse grabens), but also detailed fault structure and the sequence of faulting. In addition, numerical models can describe temporal changes in mechanical material properties to model compaction and diagenesis of syntectonic sediments.     

The high-resolution Holocene sea-level curve for Northwest Germany: global signals, local effects or data-artefacts?

A few years ago, a new relative sea-level curve for northwest Germany was constructed for the entire German North Sea coast. It is characterised by several suspected sea-level fluctuations. To test this curve for local effects, it was broken down into five relative sea-level curves representative for five coastal sections. The relative sea-level curves were corrected for tidal effects and also, a rough first correction for compaction was applied. The five curves all differ from the original curve and from each other. Most of the suspected sea-level fluctuations in the original curve cannot be supported and are discussed as data-artefacts or local effects. Around AD 800–1000 all curves show stagnation or drop of sea-level. Thus, this signal is discussed as of over-regional stratigraphic meaning. This study is a first step (1) to show that several curves are needed to demonstrate the Holocene sea-level history of such a big area as the entire German North Sea coast and (2) to establish reliable relative sea-level curves for the German North Sea coast. Further research is necessary to apply detailed corrections especially with respect to compaction-prone data and to improve the individual curves.     

Post-colonial nature conservation in Southern Africa: same emperors, new clothes?

This paper analyses and evaluates the bordering and othering impacts of environmental geopolitical discourse on land conservation in Southern Africa. Through a theoretical in-depth analysis of the use and contents of the term conservation, this paper examines how conservation is determined, instrumentalized and interpreted by the state, international governmental and non-governmental institutions, and specific interest groups including neo-liberal capitalists and local communities especially in the developing world context. In particular, we discuss the impact of current transboundary park-like conservation practices in Southern Africa and how these feed into the continuous attempts to colonise Southern Africa's nature. Native Americans were not mistaken when they accused the Whites of having forked tongues. By separating the relations of political power from the relations of scientific reasoning while continuing to shore up power with reason and reason with power, the moderns have always had two irons in the fire. They have become invincible. (Latour, 1993, p. 38) Since most of us live in a hierarchical society, any discourse on wildlife tends to be about social relationships. Whom can we exclude from our Garden of Eden, and how can we keep `others' from trespassing on valuables that help sustain our life and livelihoods, if not our identities. Marks (1994, p. 120) ...it could be argued that binary divisions are deeply etched into social space and it is a deeper understanding of boundary erection and distancing that is required if we are to provide alternatives to exclusion and conflict (Sibley, 2001, p. 240).     

Glacial and Holocene terrestrial temperature variability in subtropical east Australia as inferred from branched GDGT distributions in a sediment core from Lake McKenzie

Branched glycerol dialkyl glycerol tetraether (GDGT) distributions observed in a sediment core from Lake McKenzie were utilized to quantitatively reconstruct the pattern of mean annual air temperature (MAAT) from coastal subtropical eastern Australia between 37 and 18.3 cal ka BP and 14.0 cal ka BP to present. Both the reconstructed trend and amplitude of MAAT changes from the top of the sediment core were nearly identical to a local instrumental MAAT record from Fraser Island, providing confidence that in this sediment core branched GDGTs could be used to produce a quantitative record of past MAAT. The reconstructed trend of MAAT during 37 to 18.3 cal ka BP and timing of the Last Glacial Maximum (LGM) in the Lake McKenzie record were in agreement with previously published nearby marine climate records. The amplitude of lower-than-present MAAT during the LGM potentially provides information on the latitude of separation of the Tasman Front from the East Australian current in the subtropical western Pacific. The Lake McKenzie record shows an earlier onset of near modern day warm temperatures in the early Holocene compared to marine records and the presence of a warmer than present day period during the mid-Holocene.     

Initial formation of channel–shoal patterns in double–inlet systems

Ocean Dynamics - Channel–shoal patterns are often observed in the back–barrier basins of inlet systems and are important from both an economical and ecological point of view. Focussing...     

Mächtigkeit und Alter der erodierten Sedimente im Saar-Nahe-Becken (SW-Deutschland)

Zusammenfassung Eine umfassende Analyse und Modellierung der Entwicklungsgeschichte des Saar-Nahe-Beckens in SW-Deutschland wird bislang dadurch erschwert, daß erhebliche Teile der ursprünglichen permokarbonen Beckenfüllung bereits erodiert sind. Zur Quantifizierung der maximalen Überlagerung des heutigen Aufschlußniveaus wurde der Kompaktionsgrad von Tonsteinen mit Hilfe geophysikalischer Bohrlochmessungen untersucht. Es kann eine maximale Überlagerung bestimmt werden, die im SW des Saar-Nahe-Beckens 1950 m und im NE 2400 m beträgt (bezogen auf heutiges Meeresspiegelniveau). Aufgrund paläogeographischer Argumente lassen sich diese Überlagerungswerte nur durch eine Sedimentabfolge mit permokarbonem bzw. permischem Alter erklären. Eine maximale Versenkung im Mesozoikum oder Tertiär kann ausgeschlossen werden. Durch die Quantifizierung und zeitliche Einstufung der maximalen Versenkungstiefe kann die Subsidenz- und Hebungsgeschichte des Saar-Nahe-Bekkens seit dem Karbon lückenlos rekonstruiert werden.

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Any detailed analysis and quantitative modelling of the formation of the Saar-Nahe-Becken in SW-Germany is hampered by a number of erosive events which removed major parts of the initial Permo-Carboniferous basin fill. The maximum overburden of the present outcrop level was reconstructed using shale-compaction data derived from sonic logs. The amount of erosion is estimated at 1950 m in the southwestern part of the Saar-Nahe-Basin, and 2400 m in the NE (relative to present sealevel). By comparison with paleogeographic and isopach data these values can only be explained by Permo-Carboniferous and Permian strata. A maximum burial during the Mesocoic or Tertiary can be ruled out. Knowing the amount and age of the maximum overburden, a continuous reconstruction of the burial history of the Saar-Nahe-Basin since the Carboniferous is possible.

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Résumé Une analyse approfondie et une modélisation de l'histoire de la formation du bassin de la Sarre et de la Nahe (Allemagne du sud-ouest) ont été rendues difficiles jusqu'ici par le fait qu'une part considérable du remplissage permocarbonifére primitif du bassin a déjà été érodée. L'épaisseur maximale de la charge qui a surmonté le niveau actuellement en affleurement a été déduite du degré de compaction des argilites, analysé à l'aide de la diagraphie géophysique. Ces estimations ont fourni des valeurs maximales de 1.950 m dans le sud-ouest du bassin et de 2.400 m dans sa partie nord-est (relativement au niveau de la mer). Pour des raisons paléogéographiques il n'est possible de voir dans cette surcharge que des sédiments d'âge permo-carbonifère et permien: on peut montrer, en effet, que la subsidence maximale n'a pas eu lieu au Mésozoïque ou au Tertiaire. Connaissant ainsi l'importancce et l'âge de la surcharge actuellement disparue par érosion, on peut reconstituer l'histoire complète des mouvrements verticaux du bassin de la Sarre et de la Nahe depuis le Carbonifère.

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