Petroleum hydrocarbons in sediments of Southampton water estuary

The sediments of Southampton Water were analysed for petroleum hydrocarbons to determine the fate of petrochemical refinery waste in the estuary. Much of the petroleum hydrocarbons appears to be removed by adsorption onto estuarine sediments close to source. Surface sediment concentrations range from 3·1mg/g dry weight near a refinery to 0·5 mg/g dry weight at locations distant from the refinery.The absence of any change with depth in total hydrocarbon concentrations from some sediment cores and the presence of distinct ‘oil’ horizons in others suggests that mixing of sediments in the area is very random. Sedimentation rates in areas where a distinct oil horizon was observed ranged from 1 to 3cm a year.     

The environmental management of shipping

The environmental management of shipping requires systematic consideration of the geography of shipping routes, ship types and cargoes, and environments which can form the basis of a classificatory approach to Environmental Impact Assessment (EIA). Beyond this, effective environmental management depends in the first instance upon sound information management including monitoring of the environment, surveillance of shipping and information technology, followed by information assessment including, in particular, risk and environmental impact. The general management dimension includes co-ordination of technical management and organisational aspects related to shipping. It is suggested that effective environmental management of shipping may depend upon a regionalisation of the marine environment for EIA purposes akin to that found in the loadline rules, together with rationalisation of the increasing number of special areas in existence or proposed for this purpose. EIA will have to become an integral part of the overall management system in the shipping industry.     

Hypothesis for a submarine landslide and cohesionless sediment flows resulting from a 17th century earthquake-triggered landslide in Quebec,Canada

Textural isotopic and microfossil data from two gravity cores obtained in Saguenay Fjord, Quebec, suggest that a distinctive sandy clay bed was deposited as the result of a major landslide in the Saguenay River basin. Pb-210 dating of the cores indicate that the bed is of similar age to the magnitude 7 earthquake of February 5, 1663. The slide involved sensitive marine clays and may have occurred in two stages. Slide sediments carried into the Saguenay River channel were probably reworked and subsequently transported down the Fjord basin as two distinct cohesionless mass flows. Fine clay laminae that overlie the older mass flow bed record the modulation of depositional processes by tidal currents for several weeks after this event.     

Morphodynamics of intertidal bars on a megatidal beach, Merlimont, Northern France

The morphology, bedforms and hydrodynamics of Merlimont beach, in northern France, characterised by intertidal bars and a spring tidal range of 8.3 m, were surveyed over a 10-day experiment with variable wave conditions that included a 2-day storm with significant wave heights of up to 2.8 m. The beach exhibited two pronounced bar-trough systems located between the mean sea level and low neap tide level. Waves showed a cross-shore depth modulation, attaining maximum heights at high tide. The mean current was characterised dominantly by strong tide-induced longshore flows significantly reinforced by wind forcing during the storm, and by weaker, dominantly offshore, wave-induced flows. Vertical tidal water-level variations (tidal excursion rates) showed a bimodal distribution with a peak towards the mid-tide position and low rates near low and high water. The two bar-trough systems in the mid-tide zone remained stable in position during the experiment but showed significant local change. The absence of bar migration in spite of the relatively energetic context of this beach reflects high macro-scale bar morphological lag due to a combination of the large vertical tidal excursion rates in the mid-tide zone, the cross-shore wave structure, and the pronounced dual bar-trough system. The profile exhibited a highly variable pattern of local morphological change that showed poor correlation with wave energy levels and tidal excursion rates. Profile change reflected marked local morphodynamic feedback effects due mainly to breaks in slope associated with the bar-trough topography and with trough activity. Change was as important during low wave-energy conditions as during the storm. Strong flows in the entrenched troughs hindered cross-shore bar mobility while inducing longshore migration of medium-sized bedforms that contributed in generating short-term profile change. The large size and location of the two pronounced bars in the mid-tide zone of the beach are tentatively attributed respectively to the relatively high wave-energy levels affecting Merlimont beach, and to the cross-shore increase in wave height hinged on tidal modulation of water depths. These two large quasi-permanent bars probably originated as essentially breakpoint bars and are different from a small bar formed by swash and surf processes in the course of the experiment at the mean high water neap tide level, which is characterised by a certain degree of tidal stationarity and larger high-tide waves.     

Cycling the representer algorithm for variational data assimilation with a nonlinear reduced gravity ocean model

The representer method was used by [Ngodock, H.E., Jacobs, G.A., Chen, M., 2006. The representer method, the ensemble Kalman filter and the ensemble Kalman smoother: a comparison study using a nonlinear reduced gravity ocean model. Ocean Modelling 12, 378–400] in a comparison study with the ensemble Kalman filter and smoother involving a 1.5 nonlinear reduced gravity idealized ocean model simulating the Loop Current (LC) and the Loop Current eddies (LCE) in the Gulf of Mexico. It was reported that the representer method was more accurate than its ensemble counterparts, yet it had difficulties fitting the data in the last month of the 4-month assimilation window when the data density was significantly decreased. The authors attributed this failure to increased advective nonlinearities in the presence of an eddy shedding causing the tangent linear model (TLM) to become inaccurate. In a separate study [Ngodock, H.E., Smith, S.R., Jacobs, G.A., 2007. Cycling the representer algorithm for variational data assimilation with the Lorenz attractor. Monthly Weather Review 135 (2), 373–386] applied the cycling representer algorithm to the Lorenz attractor and demonstrated that the cycling solution was able to accurately fit the data within each cycle and beyond the range of accuracy of the TLM, once adjustments were made in the early cycles, thus overcoming the difficulties of the non-cycling solution. The cycling algorithm is used here in assimilation experiments with the nonlinear reduced gravity model. It is shown that the cycling solution overcomes the difficulties encountered by the non-cycling solution due to a limited time range of accuracy of the TLM. Thus, for variational assimilation applications where the TLM accuracy is limited in time, the cycling representer becomes a very powerful and attractive alternative, given that its computational cost is significantly lower than that of the non-cycling algorithm.     

Cross-shelf exchange in a model of the Ross Sea circulation and biogeochemistry

Transport of warm, nutrient-rich Circumpolar Deep Water (CDW) onto Antarctic continental shelves and coastal seas has important effects on physical and biological processes. The present study investigates the locations of this transport and its dynamics in the Ross Sea with a high-resolution three-dimensional numerical model. The model circulation is forced by daily wind stress along with heat and salt fluxes calculated from atmospheric climatologies by bulk formulae. All surface fluxes are modified by an imposed climatological ice cover. Waters under the Ross Ice Shelf are not included explicitly, but their effect on temperature and salinity is imposed in a buffer zone at the southern end of the model domain. A simple nutrient uptake is calculated based on the climatological chlorophyll distribution and Monod uptake kinetics.Model circulation is strongly affected by bottom topography, due to weak stratification, and agrees with schematics of the general flow and long-term current measurements except near the southern boundary. The sea-surface temperature is similar to satellite estimates except that the warmest simulated temperatures are slightly higher than observations. There is a significant correlation between the curvature of the shelf break and the transport across the shelf break. A momentum term balance shows that momentum advection helps to force flow across the shelf break in specific locations due to the curvature of the bathymetry (that is, the isobaths curve in front of the flow). For the model to create a strong intrusion of CDW onto the shelf, it appears two mechanisms are necessary. First, CDW is driven onto the shelf at least partially due to momentum advection and the curvature of the shelf break; then, the general circulation on the shelf takes the CDW into the interior.     

Barotropic tidal mixing effects in a coupled climate model: Oceanic conditions in the Northern Atlantic

Impacts of mixing driven by barotropic tides in a coupled climate model are investigated by using an atmosphere–ocean–ice–land coupled climate model, the GFDL CM2.0. We focus on oceanic conditions of the Northern Atlantic. Barotropic tidal mixing effects increase the surface salinity and density in the Northern Atlantic and decrease the RMS error of the model surface salinity and temperature fields related to the observational data.     

Usefulness of classification and index properties of clay-rich sediments in evaluating sea-bed environmental conditions

A comprehensive study of undisturbed, mostly silty clay samples, taken from large-diameter surface cores collected in the North Atlantic and its bordering seas, shows that measurement of the Atterberg limits, water content, specific gravity of grains and grain-size distribution can provide the basis for predicting many seabed geotechnical and geophysical properties which can lead to a quick assessment of environmental conditions. Thus, the sediment compression curve can be reconstructed, acoustic properties assessed, electrical and thermal resistivities deduced and an indication of the shearing forces, which have affected the sediment, inferred.