Prediction of impact pressure induced by breaking waves on vertical cylinders in random seas

This paper presents a method to statistically predict the magnitude of impact pressure (including extreme values) produced by deep water waves breaking on a circular cylinder representing a column of an ocean structure. Breaking waves defined here are not those whose tops are blown off by the wind but those whose breaking is associated with steepness. The probability density function of wave period associated with breaking waves is derived for a specified wave spectrum, and then converted to the probability density function of impact pressure. Impacts caused by two different breaking conditions are considered; one is the impact associated with waves breaking in close proximity to the column, the other is an impact caused by waves approaching the column after they have broken. As an example of the application of the present method, numerical computations are carried out for a wave spectrum obtained from measured data in the North Atlantic.     

Evidence for slope instability and current-induced sediment transport,the RMSTitanic wreck search area,Newfoundland rise

The first map of the sea bed morphology and sedimentary features within the RMSTitanic search area is proposed from the interpretation of SAR side-scan sonar images. Downslope sedimentary features such as erosional furrows and crown scarps constitute a 7 km wide instability corridor. A large field (15 km²) of asymmetrical sediment waves indicating a downslope transport is identified. Current-induced features corresponding to associated sand ribbons and barchan dunes resulting from the Western Boundary Undercurrent action are mapped. The morphology of theTitanic Canyon is also precised from the SAR images. Finally, the origin of the sea bed features is discussed in an attempt to link each bed form to a sedimentary process.     

Geophysical and geochemical observations on actively seeping hydrocarbon gases on the south-eastern Yellow Sea continental shelf

In the southeastern Yellow Sea, active seepage of hydrocarbon gases has been observed by high-resolution (3.5 kHz) seismic profiling both in 1987 and 2001, occurring through a large number of plumes from the topmost pre-Holocene sedimentary layer. It is strong enough to compensate for current speed, extending vertically up to the sea surface. The gas seepage often appears to be explosive to form craters and diapirs, although pockmarks are rare due to the redistribution of mobile palimpsest sands. In core-top seawater and sediments, the gases are characterized by high amounts of C₂, homogenous ¹³C₁ values and a large difference (19.7 on average) between ¹³C₁ (–55.2 to –53.6 PDB) and ¹³C₂ (–36.8 to –32.5 PDB) values. The gases are considered to be generated with a smaller amount of C₁ at the early thermal cracking stage of labile source materials, after which the C₂ gas is enriched in ¹³C by diffusion or biological alternation at the generation or accumulation site. The homogenous ¹³C₁ values may be one of the geochemical characteristics of gases acquired at depth which are less altered in the case of rapid diffusive gas migration to the seafloor.     

A map series of the Southern East Pacific Rise and its flanks, 15° S to 19° S

Four large-scale bathymetric maps of the Southern East Pacific Rise and its flanks between 15° S and 19° S display many of the unique features of this superfast spreading environment including abundant seamounts (the Rano Rahi Field), axial discontinuities, discontinuity migration, and abyssal hill variation. Along with a summary of the regional geology, these maps will provide a valuable reference for other sea-going programs on-and off-axis in this area, including the Mantle ELectromagnetic and Tomography (MELT) experiment.     

Diet shifts of Caribbean grunts (Haemulidae) and snappers (Lutjanidae) and the relation with nursery-to-coral reef migrations

The spatial size distribution of grunts and snappers have previously indicated the separation of juveniles in nursery habitats from the adults on the coral reef. This implies life cycle migrations from nursery habitats (such as seagrass beds and mangroves) to the coral reef. If diet shifts are related to such migrations, then the diets of these fish must change before or around the fish size at which such migrations take place. A wide size range of juveniles of two grunt species (Haemulon sciurus and Haemulon flavolineatum) and of two snapper species (Lutjanus apodus and Ocyurus chrysurus) were caught in seagrass beds and mangroves, and their gut contents identified and quantified. Regression analysis between fish size and dietary importance of small crustaceans showed a negative relationship in all four species. Positive relations were found for H. sciurus, L. apodus and O. chrysurus between fish length and the dietary importance of decapods, and for L. apodusand O. chrysurus between fish length and prey fish importance. Critical changes in the fish diets with fish size were examined by application of a Canonical Correspondence Analysis (CCA). The CCA yielded three clusters of size-classes of fishes with similar diets, and application of a Mantel test showed that each of these clusters had significantly different diets, and that each cluster diet was significantly specialised. The size at which a fish species ‘switched’ from one cluster to another was compared with size-at-maturity data and with the typical size at which these species migrate from the nursery habitats to the coral reef. H. sciurus and H. flavolineatum may be prompted to migrate from the nursery habitats to coral reef habitats because of dietary changes, or because of the development of the gonads. For L. apodus and O. chrysurus, a dietary changeover forms a more likely explanation for nursery-to-reef migrations than does sexual maturation because these species reach maturity at sizes much larger than the maximum size of individuals found in nursery habitats. Although other factors may theoretically initiate or promote the migration patterns, the results of this study indicate that ontogenetic dietary changes may crucially influence the nursery-to-coral reef migrations of these reef fish species.     

Variability in oceanographic and ecological processes in the Canadian Arctic Archipelago

This paper brings together unpublished historical data sets and published literature to review the role of climatic, oceanographic and ecological processes in the marine ecosystem of the eastern Canadian Archipelago. Physical data include characteristics of the water masses, circulation patterns, sea ice conditions, and climatic records from 1950s onward. Biological data include unpublished data sets on nutrients, primary and secondary production, and sedimentation, which were collected during the 1980–1990s in the eastern Canadian Archipelago. These results show high year-to-year variability in nutrient inventories and ratios, the magnitude of the ice algae and phytoplankton bloom, the timing of ice algae sedimentation in the spring, and the composition of the zooplankton community. The significance of this high interannual variability and its effect on pelagic–benthic coupling processes is discussed in the context of climatic and oceanographic forcing, with emphasis on recent (past decade) Arctic changes. An estimate of total primary production in the Archipelago is also presented, along with published production estimates for other Arctic shelves, showing that the Archipelago may support up to 32% of the total primary production of Arctic shelves. The high year-to-year variability in production and carbon transfer pathways (e.g. pelagic versus benthic) in the Archipelago suggest that the system might be resilient to the increased variability in climatic conditions occurring in the past decade. However, this increased variability combined with directional change in climatic and oceanographic conditions might also modify the existing balance of ecological processes. For example, shifts in the timing of events appear to have already occurred in the past decade, with potential cascading effects throughout the ecosystem.     

Respiratory Rate and Assessment of Secondary Production in the Holothuroid Holothuria tubulosa (Echinodermata) from Mediterranean Seagrass Beds

Abstract. Respiration in Holothuria tubulosa was investigated in individuals from the Posidonia oceanica meadow off Lacco Ameno (Ischia Island, Italy). Respiratory rates increase with increasing body weight and increasing sea water temperature. Oxygen consumption of an average individual (7g dw body wall) ranges from 0.409 (14 °C) to 1.300 (26 °C) mg O₂· h^-1. Data on population density, mean size of individuals, and annual sea water temperature variations allow an assessment of holothuroid production. Values of 45.65 and 13.75 kJ · m^-2· y^-1 were calculated for shallow (3 to 10 m) and deep (25 to 33 m) areas of the Posidonia meadow, respectively. Holothuroid production shows a bathymetric pattern similar to primary production of the Posidonia -epiphytes complex and the production of Posidonia litter.     

Active spreading and hydrothermalism in North Fiji Basin (SW Pacific). Results of Japanese French Cruise Kaiyo 87

The aim of the Japanese-French Kaiyo 87 cruise was the study of the spreading axis in the North Fiji Basin (SW Pacific). A Seabeam and geophysical survey allowed us to define the detailed structure of the active NS spreading axis between 16° and 22° S and its relationships with the left lateral motion of the North Fiji Fracture Zone. Between 21° S and 18°10′ S, the spreading axis trends NS. From 18°10 S to 16°40 S the orientation of the spreading axis changes from NS to 015°. North of 16°40′ S the spreading axis trends 160°. These two 015° and 160° branches converge with the left lateral North Fiji fracture zone around 16°40′ S to define an RRFZ triple junction. Water sampling, dredging and photo TV deep towing give new information concerning the hydrothermal activity along the spreading axis. The discovery of hydrothermal deposits associated with living communities confirms this activity.     

Sediment transport pattern at the Barra Nova inlet, south Portugal: a conceptual model

The Barra Nova inlet, in south Portugal, is known to migrate progressively southeastwards under wave action. The morphodynamics of this system during a representative year suggests that this long-term evolution is dependent on a seasonal behavior of the tidal inlet which can be described through a three-stage model of post-storm, transition and extended fair-weather conditions. Processes involved in this evolution indicate that the historical migration of the Barra Nova is not dependent on the longshore drift forcing constricting the channel on the updrift coast, but rather on the adjustment of the system to a major erosion of the downdrift coast during short storm events.