Physiography and deposition on a distal deep-sea system: The Valencia Fan (Northwestern Mediterranean)

The Valencia Fan developed as the distal fill of a deep-sea valley, detached from the continental slope and the main sedimentary source. A survey of side-scan sonar, Sea Beam and reflection seismics shows that the sediment is largely fed through the Valencia Valley. The upper fan comprises large channels with low-relief levees, and the middle fan has sinuous distributary channels. Depositional bedforms predominate on the valley floor and levees, and erosional bedforms are common in the valley walls. A change to slope on the fan apex and the presence of volcanoes on the upper fan are the main factors influencing fan-growth pattern.     

Zooplanktonic Communities of the Divergence Zone in the Northwestern Alboran Sea

Abstract. Using principal component analysis, the zooplanktonic communities of the divergence zone in the northwestern sector of the Alboran Sea were studied. The study was based on 35 vertical trawls in the 200 metres superficial level. The two first components account for 65 % of the total variance. The first component is a contrast between a warm water coastal community (characterized by low specific diversity and the abundance of Cladocera and Copepoda such as Acartia clausi, Centropages chierchiae and Temora stylifera) and a community of cold subsuperficial water distinguished by high specific diversity and a low number of individuals, one of its most characteristic elements being the eggs and larval stages of the Conostomasidae Maurolicus muelleri. The second component appears to be associated with an ecotone between the communities previously cited, and with a specific grouping (Rhincalanus nasutus, Eucalanus monachiis, Pseudocalanus elongatus, Temora longicornis , etc.) resulting from the peripheral displacement of subsuperficial elements brought to the surface by the cyclonic circulation of water masses between the Atlantic current and the Spanish coast.     

Stone mobility and longshore transport at reshaping breakwaters

Physical model tests have been performed in two different wave flumes to analyse the threshold of stone movement and quantify the frequency and length of displacements due to head-on wave attacks at a reshaping breakwater. Data on stone movements were obtained from the observation of cumulative displacements at the end of each wave attack and from video records during the attack. Threshold conditions, frequency of movement and displacement length are expressed as function of a suitably modified stability number.A simple model is defined relating longshore transport due to oblique wave attack to stone mobility. The transport model is based on the assumption that movement statistics is affected by obliquity only through the appropriate mobility index and that stones move during up- and down-rush in the direction of incident and reflected waves. Without any calibration, results compare favourably with experimental data available in literature in the range of low mobility conditions where movement statistics was observed. A calibration is provided in order to obtain an accurate transport formula valid in a wide mobility range i.e. for reshaping breakwaters and up to gravel beaches.     

Importance of tropical coastal environments

On a global basis, a very large fraction (>50%) of the mass flux from land to the marine environment enters from tropical rivers. A broad range of processes active in the adjacent coastal ocean determines the fate of this material. The tropical setting causes many of the coastal processes to be fundamentally distinct from those operating in temperate and polar regions. Therefore, their operation cannot necessarily be understood by extrapolation from empirical observations at higher latitudes. Other coastal processes are influenced by the extremely large water, particulate, and solute discharges from tropical rivers. Further complexity is added by the diverse range of geographic settings through which tropical rivers flow and into which they empty. In summary, coastal—ocean processes in the wet tropics are: globally important, fundamentally different than at higher latitudes, and diverse in operation. Many recent advances have been made in the understanding of the tropical coastal ocean, and it is hoped these will continue into the future. Such understanding is important for predicting processes of river—ocean interaction and terrestrial fluxes to the global ocean, which affect and are affected by human populations. It also can be combined with knowledge of temperate and polar settings to provide a comprehensive understanding of the coastal ocean.     

Spatial and vertical distributions of elements in sediments of the Colorado River delta and Upper Gulf of California

The abundance of major components (Fe, Ca, K, and organic carbon) and trace elements was analyzed in surface sediments and core samples from the Colorado River delta (CRD) and the Upper Gulf of California (UGC) using instrumental neutron activation analysis.The spatial distribution patterns of the elements studied are consistent with the model of sedimentary dynamics proposed for this area [Mar. Geol. 158 (1999) 125]: intense tidal resuspension of sediments in the delta with subsequent transport of suspended particulate matter in a southerly direction, followed by sedimentation of fine-grained material in a depocenter near the southwestern margin of the UGC. Concentrations of most of the elements are higher in the surface sediments of this depocenter. The gradual mixing of terrigenous and marine biogenic materials, normally expected for the estuarine sediments, was not detected in the CRD–UGC system because of homogenization of the sediments by tides and wind.Vertical profiles of element contents in samples of the sediment core collected in the depocenter area revealed (i) almost no anthropogenic contamination of the area by environmentally important trace elements such as Cr, Co, Sb, and As; (ii) a twofold decrease of Fe, Sc, Cr, and Co in upper core sediments; and (iii) the enrichment of the sediments at 60–62 cm depth in the core, in calcium carbonate, Ca, Sr, and the Eu_n/Sm_n shale-normalized ratio along with a depletion in this layer of Fe, Sc, Cr, Co, light rare-earth elements (REEs), and some other elements of terrigenous origin, presumably caused by the dilution of fluvial terrigenous material by biogenic carbonates, which were probably introduced at this level in the sediments by the action of a strong episodic winter storm, followed by the advective transport of shell fragments from the coastal clam banks or as a result of strong planktonic bloom.     

Subharmonic oscillations in nonlinear rolling

In this paper the rolling motion of a ship is examined with particular regard to the possibility of obtaining oscillations which are subharmonic to the excitation frequency. Three different mechanisms are found to be responsible for this phenomenon, the importance of which has already been recognized in the context of ship stability. The first is related to a strong symmetric or non-symmetric nonlinearity in the righting arm. The second is linked to the harmonic composition of sea waves and the third to the well known parametric excitation caused by coupling between different ship motions in a following sea.The onset of subharmonics is related to a threshold value for the excitation strongly depending on damping. The more appropriate analytical methods for a theoretical study of each mechanism are suggested.