A predictive numerical model for potential mapping of the gas hydrate stability zone in the Gulf of Cadiz

This paper presents a computational model for mapping the regional 3D distribution in which seafloor gas hydrates would be stable, that is carried out in a Geographical Information System (GIS) environment. The construction of the model is comprised of three primary steps, namely: (1) the construction of surfaces for the various variables based on available 3D data (seafloor temperature, geothermal gradient and depth-pressure); (2) the calculation of the gas function equilibrium functions for the various hydrocarbon compositions reported from hydrate and sediment samples; and (3) the calculation of the thickness of the hydrate stability zone. The solution is based on a transcendental function, which is solved iteratively in a GIS environment.The model has been applied in the northernmost continental slope of the Gulf of Cadiz, an area where an abundant supply for hydrate formation, such as extensive hydrocarbon seeps, diapirs and fault structures, is combined with deep undercurrents and a complex seafloor morphology. In the Gulf of Cadiz, the model depicts the distribution of the base of the gas hydrate stability zone for both biogenic and thermogenic gas compositions, and explains the geometry and distribution of geological structures derived from gas venting in the Tasyo Field (Gulf of Cadiz) and the generation of BSR levels on the upper continental slope.     

Distribution and spatial variation of hydrothermal faunal assemblages at Lucky Strike (Mid-Atlantic Ridge) revealed by high-resolution video image analysis

Whilst the fauna inhabiting hydrothermal vent structures in the Atlantic Ocean is reasonably well known, less is understood about the spatial distributions of the fauna in relation to abiotic and biotic factors. In this study, a major active hydrothermal edifice (Eiffel Tower, at 1690 m depth) on the Lucky Strike vent field (Mid-Atlantic Ridge (MAR)) was investigated. Video transects were carried out by ROV Victor 6000 and complete image coverage was acquired. Four distinct assemblages, ranging from dense larger-sized Bathymodiolus mussel beds to smaller-sized mussel clumps and alvinocaridid shrimps, and two types of substrata were defined based on high definition photographs and video imagery. To evaluate spatial variation, faunal distribution was mapped in three dimensions. A high degree of patchiness characterizes this 11 m high sulfide structure. The differences observed in assemblage and substratum distribution were related to habitat characteristics (fluid exits, depth and structure orientation). Gradients in community structure were observed, which coincided with an increasing distance from the fluid exits. A biological zonation model for the Eiffel Tower edifice was created in which faunal composition and distribution can be visually explained by the presence/absence of fluid exits.     

Honey, I cooled the cods: Modelling the effect of temperature on the structure of Boreal/Arctic fish ecosystems

Historically colder regions of the North Atlantic had fisheries dominated by only a few fish species; principally cod and capelin. Possible population dynamic mechanisms that lead to such dominance are investigated by considering how a charmingly simple published multispecies model of the North Sea would react if the system operated at a lower temperature. The existing model equations were modified to describe temperature effects on growth, fecundity and recruitment and the model was rerun based on typical temperatures for the North Sea and a colder system. The results suggest that total fish biomass in the colder system increases but the community is more vulnerable to a given rate of fishing mortality. In the colder system, within species density dependence is reduced but relative predation rates are higher. Consequently, intermediate-sized species are vulnerable to relatively high levels of predation throughout their life history and tend to be excluded, leading to a system dominated by small and large species. The model helps to explain how temperature may govern coexistence and competitive exclusion in fish communities and accounts for the observed dominance of small and large species in Boreal/Arctic ecosystems.     

Mineral associations of the Lost Village and Lost City hydrothethermal fields in the North Atlantic

During the 50th cruise of R/V Akademik Mstislav Keldysh, on the south slope of the Atlantis massif (30°07′N; the Middle Atlantic Ridge), an inactive hydrothermal field named the Lost Village was discovered. This new field was formed of light carbonate rock and was located near the active Lost City hydrothermal field. The mineralogical associations of these fields were studied. A conclusion about the participation the ocean water in the changing of the carbonate composition of the inactive hydrothermal field was reached.     

SAR-imaged spiral eddies in Mutsu Bay and their dynamic and kinematic models

A pattern of slick streaks winding into a spiral, known as a spiral eddy, was identified in 5 images taken by the ERS-1/2 synthetic aperture radar (SAR) in Mutsu Bay (Japan); dynamic and kinematic models of these spiral eddies have been proposed. Common characteristics of the five spiral eddies are: 1) an eddy diameter of about 15 km; 2) their location in the western part of the bay; and 3) their cyclonic direction of rotation. Moreover, the wind conditions over the bay were common: prior to acquiring the images, a strong easterly wind continued blowing for more than one day. The wind field on the bay is known to be orographically steered and has strong windstress vorticity, which generates cyclonic circulation. The diameter and location of the circulation simulated with a numerical ocean model corresponded well to those of the identified spiral eddies. Based on these facts, we propose a dynamic model for the movement of a slick streak, and a kinematic model for the formation of a spiral eddy. We have assumed calm air, a microlayer and seawater with a cyclonic circulation in the dynamic model. The balance of forces is established in the microlayer among the frictional force from the seawater, the frictional force from the calm air, the gravitational force, and the Coriolis force. As a result, the velocity vector of the microlayer deflects slightly towards the center of the cyclonic circulation. We have assumed a point source of the microlayer in the kinematic model. The shapes of a slick streak simulated with the models agree well with the identified patterns in the SAR images.     

Laboratory modeling of the propagation of periodic internal waves over bottom slopes

The experimental investigation of the run-up of periodic internal waves in a two-layer fluid on the coastal slope is performed in an open hydrochannel at the Physical Department of the Lomonosov Moscow State University. The waves are produced by a wave generator. We study the transformation of waves, the vertical structure of the field of velocities of mass transfer, and the behavior of the parameters of internal waves propagating over the sloping bottom. It is shown that the run-up and breaking of internal waves are accompanied by periodic emissions of portions of the heavier fluid from the bottom layer upward along the slope. The Stokes drift velocity changes its sign as a function of depth. Moreover, both the wave length (the horizontal distance between the neighboring crests) and the height of waves over the sloping bottom (the elevation of the crest over the slope along the vertical) decrease as the wave approaches the coast.     

Coastal upwelling along the north coast of Papua New Guinea and SST cooling over the pacific warm pool: A case study for the 2002/03 El Niño event

We investigate an overlooked mechanism—coastal upwelling—for sea surface temperature (SST) cooling in the western side of the mean location of the Pacific warm pool (WSWP: 5°S–5°N, 140°E–150°E) prior to El Niño onset. We analyze various observed data such as the TRIangle Trans-Ocean buoy Network (TRITON) moored buoy data, Conductivity-Temperature-Depth (CTD) data, satellite data and a hindcast experiment output by a high-resolution ocean general circulation model (OGCM). We focus on the precondition of the 2002/03 El Niño event, for which many datasets are available. Relatively cool water upwelled along the north coast of Papua New Guinea (PNG) during December 2001, prior to the onset of the 2002/03 El Niño event, and then spread out over a wider area to the northeast. Simultaneously, strong west-northerly surface winds occur along the north coast. Heat budget analysis of TRITON buoy data in the WSWP reveals that negative zonal heat advection due to eastward current is the main factor for cooling the mixed layer in the WSWP in contrast to the warming effect of the surface heat flux during the period. This cooling requires a source of colder water to the west. Similar analysis of OGCM outputs also suggests that the upwelled relatively cool water along the PNG north coast, and its northeastward extension to the equatorial region, contributes to cooling of the surface water over the WSWP mainly via negative zonal heat advection. Similar mechanisms are confirmed also for the 1982/83 and 1997/98 El Niño events by analyses of OGCM outputs and historical SST data. The low SST in the WSWP generated a positive zonal SST gradient together with high SST east of the WSWP. It may contribute to enhancement of the westerly surface wind in this region, leading to the onset of the 2002/03 El Niño event.     

Optimal interpolation of the data of remote sensing of the sea surface

We analyze the statistical structure of the data of remote sensing of the fields of surface temperature and the concentration of chlorophyll in the northwest part of the Black Sea with an aim to apply the method of optimal interpolation to the restoration of the maps of these fields according to the restricted amounts of data. We determine the scales of smoothing for the selection of the random components of the fields and estimate their difference from the statistical model uniform and isotropic over the space correlation coefficients. The functions of spectral densities are constructed along various sections and the best intervals of discretization of the fields required to perform the optimal interpolation with given accuracy are determined. It is shown that the construction of the maps of fields under the conditions of controlled accuracy of reconstruction of their values remains possible even if the amounts of the input satellite data are significantly decreased. We consider a procedure of evaluation of the best possible amounts of data required for the use of the method of optimal interpolation. Some examples of application of this procedure to the data of remote sensing of the fields of surface temperature and the concentration of chlorophyll are presented.     

Stochastic evaluation of fluvial to marginal marine sealing facies

Hydrocarbon top seal analyses are particularly difficult in fluvial and marginal marine settings. Although these settings often contain beds comprised of a low-permeability, clay-rich lithology associated with high quality seals, their presence does not guarantee an effective seal because individual beds within the stratal unit may be laterally discontinuous. Lateral facies changes, truncation of fine-grained beds, or the presence of crevasse splays may provide cross-stratal migration pathways, reducing the effective sealing capacity to a magnitude less than the closure height.     

Modeling an atypical petroleum system: A case study of hydrocarbon generation,migration and accumulation related to igneous intrusions in the Neuquen Basin,Argentina

In the Altiplanicie del Payún area (Neuquen Basin, Argentina), immature source rock sections intruded by up to 600 m thick Tertiary laccoliths show full spectrum maturity aureoles over hundreds of meters from the contacts. Commercial oil accumulations (20–33°API) and oil shows are located along the entire column, both in sandstone/carbonate and fractured igneous reservoirs. A challenging numerical model that included the emplacement of the intrusive bodies, with extreme temperature ranges and unusually short calculation time steps, has been done with the aim to better understand hydrocarbon generation and migration processes related to these thermal anomalies.