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. 相似文献
Sedimentation rates over the last 100 years within two lagoons on the southeast coast of Australia, Lake Illawarra and St
Georges Basin, have been quantified to determine the effects of catchment land use change and native vegetation clearance
on infill rates, and spatial variations in the rate at which the estuaries have filled. Both catchments have similar lake
and catchment area but have experience different degrees of modification due to land clearing for agriculture practices, urbanisation
and industrialisation. Results indicate that in the heavily modified catchment of Lake Illawarra sedimentation rates close
to fluvial deltas can be in excess of 16 mm/year, and between 2 and 4 mm/year in the adjacent central basin. This is approximately
an order of magnitude greater than the pre-European rates. In contrast, at St Georges Basin, where the catchment has experienced
much less modification, sedimentation rates in the central basin appear to have remained close to those prior to European
settlement. However, sedimentation rates in the urbanized margin of St Georges Basin are relatively high (up to 4.4 mm/year).
This rapid modern sedimentation in the margin of the estuarine embayments has been detected in several other estuaries in
the region. However the degree of sedimentation within the bay-head deltas, and more significantly in the central basin appears
proportional to the degree clearance of native vegetation (forest) in the catchment, urban expansion and development of heavy
industry in the respective catchment areas. 相似文献
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. 相似文献
The cross-sectional stability of double inlet systems is investigated using an exploratory model that combines Escoffier’s stability concept for the evolution of the inlet’s cross-sectional area with a two-dimensional, depth-averaged (2DH) hydrodynamic model for tidal flow. The model geometry consists of four rectangular compartments, each with a uniform depth, associated with the ocean, tidal inlets and basin. The water motion, forced by an incoming Kelvin wave at the ocean’s open boundary and satisfying the linear shallow water equations on the f -plane with linearised bottom friction, is in each compartment written as a superposition of eigenmodes, i.e. Kelvin and Poincaré waves. A collocation method is employed to satisfy boundary and matching conditions. The analysis of resulting equilibrium configurations is done using flow diagrams.
Model results show that internally generated spatial variations in the water motion are essential for the existence of stable equilibria with two inlets open. In the hydrodynamic model used in the paper, both radiation damping into the ocean and basin depth effects result in these necessary spatial variations. Coriolis effects trigger an asymmetry in the stable equilibrium cross-sectional areas of the inlets. Furthermore, square basin geometries generally correspond to significantly larger equilibrium values of the inlet cross-sections. These model outcomes result from a competition between a destabilising (caused by inlet bottom friction) and a stabilising mechanism (caused by spatially varying local pressure gradients over the inlets).
Nonlinear development of salinity perturbations in the Atlantic thermohaline circulation (THC) is investigated with a three-dimensional ocean circulation model, using the conditional nonlinear optimal perturbation method. The results show two types of optimal initial perturbations of sea surface salinity, one associated with freshwater and the other with salinity. Both types of perturbations excite decadal variability of the THC. Under the same amplitude of initial perturbation, the decadal variation induced by the freshwater perturbation is much stronger than that by the salinity perturbation, suggesting that the THC is more sensitive to freshwater than salinity perturbation. As the amplitude of initial perturbation increases, the decadal variations become stronger for both perturbations. For salinity perturbations, recovery time of the THC to return to steady state gradually saturates with increasing amplitude, whereas this recovery time increases remarkably for freshwater perturbations. A nonlinear (advective) feedback between density and velocity anomalies is proposed to explain these characteristics of decadal variability excitation. The results are consistent with previous ones from simple box models, and highlight the importance of nonlinear feedback in decadal THC variability. 相似文献
Both geological data and climate model studies indicate that substantially different patterns of the global ocean circulation have existed throughout the Cenozoic. In a climate model study of the late Oligocene [von der Heydt, A., Dijkstra, H.A. (2006). Effect of ocean gateways on the global ocean circulation in the late Oligocene and early Miocene. Paleoceanography, 21, PA1011] a “northern sinking” type of circulation was found, with (shallow) deep water formation in both the North Pacific Ocean and the North Atlantic Ocean. This is in contrast to the present-day “conveyor” circulation, where there is deep water formation in the North Atlantic but not in the North Pacific. In order to explain these differences, we use an ocean general circulation model for idealized two-basin flows and study the effect of asymmetries in the continental geometry on the circulation patterns. Two types of asymmetry are considered: (i) the relative northward extent of the Pacific and the Atlantic basin, and (ii) the existence of a circum-global gateway at low latitudes. The more northward extent of the Pacific basin in the Oligocene makes the Conveyor solution less likely and facilitates deep water formation in the North Pacific compared to the North Atlantic. The low-latitude gateway on the other hand, allows salinity and heat exchange between the two main ocean basins and therefore leads to deep water formation in both the North Atlantic and the North Pacific. 相似文献