Imbalance in the carbonate budget of surficial sediments in the North Atlantic Ocean: variations over the last millenium?

Fluxes contributing to the particulate carbonate system in deep-sea sediments were investigated at the BENGAL site in the Porcupine Abyssal Plain (Northeast Atlantic). Deposition fluxes were estimated using sediment traps at a nominal depth of 3000 m and amounted to 0.37±0.1 mmol C m⁻² d⁻¹. Dissolution of carbonate was determined using flux of total alkalinity from in situ benthic chambers, is 0.4±0.1 mmol C m⁻² d⁻¹. Burial of carbonate was calculated from data on the carbonate content of the sediment and sedimentation rates from a model age based on ¹⁴C dating on foraminifera (0.66±0.1 mmol C m⁻² d⁻¹). Burial plus dissolution was three times larger than particle deposition flux which indicates that steady-state is not achieved in these sediments. Mass balances for other components (BSi, ²¹⁰Pb), and calculations of the focusing factor using ²³⁰Th, show that lateral inputs play only a minor role in this imbalance. Decadal variations of annual particle fluxes are also within the uncertainty of our average. Long-term change in dissolution may contribute to the imbalance, but can not be the main reason because burial alone is greater than the input flux. The observed imbalance is thus the consequence of a large change of carbonate input flux which has occured in the recent past. A box model is used to check the response time of the solid carbonate system in these sediments and the time to reach a new steady-state is in the order of 3 kyr. Thus it is likely that the system has been perturbed recently and that large dissolution and burial rates reflect the previously larger particulate carbonate deposition rates. We estimate that particulate carbonate fluxes have certainly decreased by a factor of at least 3 and that this change has occurred during the last few centuries.     

A New Methodology for Incorporating Tide Gauge Data in Sea Surface Topography Models

As part of the Vertical Offshore Reference Frames (VORF) project sponsored by the U. K. Hydrographic Office, a new model for Sea Surface Topography (SST) around the British Isles has been developed. For offshore areas (greater than 30 km from the coast), this model is largely derived from satellite altimetry. However, its accuracy and level of detail have been enhanced in coastal areas by the inclusion of not only the 60 PSMSL tide gauges with long-term records around the coasts of the United Kingdom and Ireland but also some 385 gauges established at different epochs and for different observation spans by the U. K. Admiralty. All tide gauge data were brought into a common reference frame by a combination of datum models and direct GPS observations, but a more significant challenge was to bring all short-term sea level observations to an unbiased value at a common epoch. This was achieved through developing a spatial-temporal correlation model for the variations in mean sea level around the British Isles, which in turn meant that gauges with long-term observation spans could be used as control points to improve the accuracy of Admiralty gauges. It is demonstrated that the latter can contribute point observations of mean sea level (MSL) with a precision of 0.078 m. A combination of least squares collocation and interpolation was developed to merge the coastal point and offshore gridded data sets, with particular algorithms having to be developed for different configurations of coastal topology. The resulting model of sea surface topography is shown to present a smooth transition from inshore coastal areas to offshore zones. Further benefits of the techniques developed include an enhanced methodology for detecting datum discontinuities at permanent tide gauges.     

Tectonic core of a sedimentary drift: a potential ridge propagation feature beneath the Blake Outer Ridge

The Blake Outer Ridge is a 480–kilometer long linear sedimentary drift ridge striking perpendicular to the North American coastline. By modeling free-air gravity anomalies we tested for the presence of a crustal feature that may control the location and orientation of the Blake Outer Ridge. Most of our crustal density models that match observed gravity anomalies require an increase in oceanic crustal thickness of 1–3 km on the southwest side of the Blake Outer Ridge relative to the northeast side. Most of these models also require 1–4 km of crustal thinning in zone 20–30 km southwest of the crest of the Blake Outer Ridge. Although these features are consistent with the structure of oceanic fracture zones, the Blake Outer Ridge is not parallel to adjacent known fracture zones. Magnetic anomalies suggest that the ocean crust beneath this feature formed during a period of mid-ocean ridge reorganization, and that the Blake Outer Ridge may be built upon the bathymetric expression of an oblique extensional feature associated with ridge propagation. It is likely that the orientation of this trough acted as a catalyst for sediment deposition with the start of the Western Boundary Undercurrent in the mid-Oligocene.     

Three-parameter model of turbulence for the atmospheric boundary layer over an urbanized surface

A modified three-parameter model of turbulence for a thermally stratified atmospheric boundary layer (ABL) is presented. The model is based on tensor-invariant parametrizations for the pressure-strain and pressure-temperature correlations that are more complete than the parametrizations used in the Mellor-Yamada model of level 3.0. The turbulent momentum and heat fluxes are calculated with explicit algebraic models obtained with the aid of symbol algebra from the transport equations for momentum and heat fluxes in the approximation of weakly equilibrium turbulence. The turbulent transport of heat and momentum fluxes is assumed to be negligibly small in this approximation. The three-parameter E ? ε ? 2> model of thermally stratified turbulence is employed to obtain closed-form algebraic expressions for the fluxes. A computational test of a 24-h ABL evolution is implemented for an idealized two-dimensional region. Comparison of the computed results with the available observational data and other numerical models shows that the proposed model is able to reproduce both the most important structural features of the turbulence in an urban canopy layer near the urbanized ABL surface and the effect of urban roughness on a global structure of the fields of wind and temperature over a city. The results of the computational test for the new model indicate that the motion of air in the urban canopy layer is strongly influenced by mechanical factors (buildings) and thermal stratification.     

Target echo enhancement using a single-element time reversal mirror

Iterative Time Reversal (TR) procedures have been demonstrated to be successful in detecting scatterers even in a highly reverberant environment. The success of the method is attributable to a Time Reversal Mirror (TRM) consisting of an array of transceivers that allows one to reconstruct and reverse-propagate wavefronts of interest. This paper deals with the limiting case of a TRM reduced to a single transceiver, tested using a tank experiment with three scattering objects in free-field conditions. It is shown that it is possible to excite a resonance between the TR operator and the strongest mode of the target in the bandwidth of insonification. A simple free-field scattering model is used to study the mechanisms of convergence and to drive the physics-based analysis of data.     

Surface circulation of the Levantine Basin: Comparison of model results with observations

The eastern Mediterranean (Levantine Basin) hydrography and circulation are investigated by comparing the results of a high-resolution primitive equation model with observations. After a 10-year integration, the model is able to reproduce the major water masses and the circulation patterns of the eastern Mediterranean. Comparisons with the POEM hydrographical observations show good agreement. The vertical distribution of the water masses matches that of the observations quite well in terms of monthly mean. The model surface circulation is in agreement with circulation schemes derived from recent observations. Some well-known mesoscale features of the upper thermocline circulation are also realistically reproduced. In agreement with satellite observations, the model shows that high-energy mesoscale eddies dominate the upper thermocline circulation in the southern and the central parts of the Levantine Basin. Most of the Atlantic Water follows the north African coast and forms a strong coastal jet near the Libyan coast rather than forming the Mid-Mediterranean Jet described by several authors. The sub-basin circulation shows a strong seasonal signal. A strong and stable current flows along the isobaths in winter, becoming weaker and with more meanders in summer. The mesoscale eddies throughout the whole basin are more energetic in summer than in winter.     

Modified mild slope equation and open boundary conditions

A numerical model to compute wave field is developed. It is based on the Berkhoff diffraction-refraction equation, in which an energy dissipation term is added, to take into account the breaking and the bottom friction phenomena. The energy dissipation function, by breaking and by bottom friction, is introduced in the Berkhoff equation to obtain a new equation of propagation.The resolution is done with the hybrid finite element method, where lagrangians elements are used.     

An experimental study on non-linear progressive wave-induced dynamic stresses in seabed

In this study, the dynamic stresses within the seabed induced by non-linear progressive waves were explored through a series of hydraulic model tests on a movable bed within a wave flume. By comparing Stokes’ 2nd-order wave theory with the theory of wave-induced dynamic stresses within the seabed as proposed by Yamamoto et al. [1978. On the response of a poro-elastic bed to water waves. Journal of Fluid Mechanics 87 (1), 193–206.] and Hsu and Jeng [1994. Wane-induced soil response in an unsaturated anisotropic seabed of finite thickness. International Journal for Numerical and Analytical Methods in Geomechanics 18, 785–807], the experimental results show that the pressure on the seabed surface, the pore water pressure within the seabed as well as the vertical and the horizontal stresses are all smaller than their theoretical values. If we were to obtain the characteristics of seabed soil, the analytical solution of Hsu and Jeng [1994. Wane-induced soil response in an unsaturated anisotropic seabed of finite thickness. International Journal for Numerical and Analytical Methods in Geomechanics 18, 785–807] might agree to the simulation of the wave-induced effective stresses and shear stress in the sandy seabed. A different phase shift exists among all the three soil stresses. Their influences on the three dynamic stresses within seabed soil are important for seabed stability, and can be used in the verification of numerical models. In the whole, the non-linear progressive waves and the naturally deposited seabed are found to have a strong interaction, and the behavior of the induced dynamic stresses within the seabed is very complicated, and should be investigated integrally.