A Recent Full-Depth Survey of the Alaskan Stream

We examine results from a cruise in May 1997. CTD casts to near the bottom were made south of the Aleutian Islands, across Amchitka Pass, and north of the islands. We computed a westward geostrophic speed of 123 cm s^–1 at 173.5°W in the Alaskan Stream. The computed volume transport there, referred to the bottom, was 25×10⁶m³s^–1. On other similar sections, transports were 8–15 × 10⁶ m³s^–1. Various complex variations in geopotential height along the Stream apparently altered the cross-stream gradients, and hence the transports. Rotational tendencies were also present. Northward inflow through Amchitka Pass was quite strong (6 × 10⁶ m³s^–1). Data north of the islands supported the existence of a zero-velocity reference level of variable depth.     

Electron microscopic observations of shale diagenesis, offshore Louisiana, USA, Gulf of Mexico

 Scanning and transmission electron microscopic analyses of shale samples from offshore Louisiana, USA, Gulf of Mexico, reveal the relationship between mineralogical and microfabric changes during burial diagenesis. The local geopressured zone begins at 2200-m depth. Above that depth the shales are smectite-rich, generally lack particle orientation, and contain appreciable pores. Below the 2200-m depth, the shales become more illite-rich with increasing burial, more crystalline, and less porous. Microfabric changes are mainly caused by compaction during burial diagenesis; mineralogical changes (smectite-to-illite) and crystal growth also play an important role in fabric alteration during deep burial diagenesis. Received: 12 May 1998 / Revision received: 14 July 1998     

Cenozoic tectonics of Amazon Mouth Basin

 The Cenozoic shelf margin of the Amazon Mouth Basin is characterized by a thick prograding prism of siliciclastic sediments. This prism, composed mainly of Upper Miocene and younger sediments, overlies a Lower Tertiary carbonate shelf. Two tectonic–sedimentary models for the area were developed with the aid of new deep-reflection seismic data. Gravitational tectonics dominate the regional geological framework. Tensional stresses are created near the shelf margin, and compressional features dominate at the base of the slope. The morphology of this compressional zone is closely associated with the St. Paul Fracture Zone and the boundary between continental and oceanic crusts. Received: 20 August 1996 / Revision received: 11 June 1998     

Surfactant components of marine organic matter as agents for biogeochemical fractionation and pollutant transport via marine aerosols

The role of surfactant organic matter in marine aerosol production has been studied under conditions in which there is a large coverage of whitecaps on the sea surface. To improve the knowledge of matter exchange and pollutant recycling from the sea surface into the atmosphere, a spray drop adsorption model (SDAM) was developed and the validity of the proposed model verified by the following experimental results: (1) an increase of surfactant matter on the sea surface during rough sea conditions (‘surface wave concentration'); (2) an (hyperbolic-like) increase of the enrichment ratio (ER) of surfactant fluorescent organic matter (SFOM), made up predominantly by humic substances (HS), as the particle size decrease; (3) a similar behaviour for elements with pollutant properties, and which are known to interact with HS and other surfactant materials, considered pollution tracers. An additional laboratory experiment, based on the adsorption model conditions, gives enrichment ratio greater than unity for K and Ca. The first results on marine aerosols trapped in marine clouds (at 1000 m above sea level and at 100 km from the coast) seem to further support the proposed model and its ability to predict the transition from saline to almost entirely organic particles for the smaller fractions of marine aerosols. The possible contribution of these particles to the recycling and to the long range transport of pollutants via marine aerosols has been considered.     

In situ velocity profiles in gassy sediments: Kiel Bay

Acoustic behavior of gas-bearing sediments is significantly different from that of gas-free sediments. In situ velocity profiles and acoustic signal characteristics in gas-bearing sediments of the upper several meters of the sea floor in Kiel Bay are presented in this study. Observed velocities in gas-bearing sediments are both higher and lower than those of the gas-free sediments. Small amounts of gas appear to cause signal reverberation without much attenuation. whereas large amounts of gas cause substantial attenuation.     

An iterative method for boundary element solution of large offshore structures using the GMRES solver

The GMRES approach is used to solve complex matrix solution arising from boundary element analysis of large offshore structures. This makes it possible to solve problems with large numbers of panels on a workstation with a much smaller memory than typical high performance computers. The speed of the solver is compatible with direct solvers when the enough RAM is available. Otherwise, an iteration procedure can be used. By using an out-of-core treatment, typical RAM requirement is reduced to a size approximately linearly proportional to the panel number n instead of being proportional to n². The code is first verified with direct solver for cases with small number of panels. The applicability to large offshore structure of the model is demonstrated for a TLP case.     

Prediction of hydrodynamic forces on submarine pipelines using an improved Wake II Model

The hydrodynamic force model for prediction of forces on submarine pipelines as described includes flow history effect (wake effects) and time dependence in the force coefficients. The wake velocity correction is derived by using a closed-form solution to the linearized Navier–Stokes equations for oscillatory flow. This is achieved by assuming that the eddy viscosity in the wake is only time dependent and of a harmonic sinusoidal form. The forces predicted by the new Wake (Wake II) Model have been compared to Exxon Production Research Company Wake Model in terms of time histories (force shape) and magnitudes of peak forces. Overall, the model predictions by the Wake II Model are satisfactory and represent a substantial improvement over the predictions of the conventional models. The conventional force models representing adaptations of Morison's equation with ambient velocity and constant coefficients give predictions that are in poor agreement with the measurements especially for the lift force component. The Wake II Force Model can be used for submarine pipeline on-bottom stability design calculations for regular waves with various pipe diameters.