Distribution patterns of larval and juvenile fishes at offshore petroleum platforms in the north-central Gulf of Mexico

Vertical and horizontal distribution patterns of larval and juvenile fishes were described from five offshore petroleum platforms in the north-central Gulf of Mexico. Light traps and passively-fished plankton nets were used nocturnally between 1995–2000 to collect fishes in surface and deep (15–23 m depth) waters within the platform structure, and light traps were also used in surface waters directly down-current of the platforms. Light traps fished at the surface, as opposed to at-depth, collected greater CPUEs and greater diversity of larval and juvenile fishes. Of the dominant taxa collected by light traps, clupeids, engraulids, synodontids, and presettlement blenniids were most common in surface waters within the platform, while postflexion scombrids and settlement-size blenniids and pomacentrids were most common in surface waters down-current of the platforms. Deep plankton nets collected greater densities of non-clupeiform larval fishes, although surface plankton nets collected greater numbers of taxa. The vertical distribution patterns described for dominant larval fish collected by plankton nets were generally consistent with those from other studies, i.e. clupeid, carangid, sciaenid and scombrid larvae more abundant in surface waters at platforms, and synodontid, bregmacerotid, gobiid and bothid larvae more abundant in deeper waters. Oil and gas platforms likely impact larval and juvenile fish populations, particularly considering their proliferation in the Gulf (i.e., over 4000 platforms). The results from this study provide valuable baseline information for future research investigating how platforms impact the life history stages of fish populations.     

U–salinity relationships in the Mediterranean: Implications for Th:U particle flux studies

Knowledge of the ²³⁸U concentration in seawater is important for ²³⁴Th:²³⁸U disequilibrium studies of particle fluxes. However, these concentration data are normally obtained through a standard relationship between ²³⁸U and salinity, which has been determined for the open ocean. This study examines ²³⁸U data from both the open Mediterranean and the coastal Thermaikos Gulf, Greece, and compares it to the open ocean. No deviation from the open ocean ²³⁸U–salinity relationship was found for the Thermaikos Gulf, but some enhancement was noted close to Thessaloniki in the vicinity of a phosphate fertiliser plant. The open Mediterranean data showed a small enhancement relative to the open ocean. Although an analytical bias could not be ruled out, a review of ²³⁸U and salinity data in the literature shows that the standard relationship may not be as robust as is often assumed and the 1% uncertainty typically used is not justified at the present time. Nevertheless, salinity-based derivations continue to be the most appropriate means of determining ²³⁸U concentrations for routine applications. We propose a new relationship that accounts for the uncertainties observed, i.e. ²³⁸U (dpm l^− 1) = (0.0713 ± 0.0012) × salinity.     

Interpretation of Repulsive Bifurcation of the Kuroshio and Gulf Stream with Classification of Their Branches

Distinctive features of the repulsive bifurcation of Kuroshio and Gulf Stream jets, induced from various observations, have been examined by equations of motion of a 1 1/2 -layer model in natural coordinates, although causes of the bifurcation are discussed little. From the conservation of volume flux through paths above a main thermocline, we have derived the equation M _n K _In + M _s K _Is = 0 for northern (n) and southern (s), steady-state curved jets, just downstream of the bifurcation from a straight jet, where M is momentum flux through the path and K _I, positive for cyclonic deflection of the streamline, is the curvature of a certain intermediate streamline in the laminar path. Assuming concentric streamlines in the curved path, which is narrower than 1–0.5 times the radius of curvature of the center streamline of the path, and symmetric momentum flux through a path element of unit width with respect to the center streamline, we found that K _I approximates to the curvature averaged across the path, relative errors being <0.25−(0.25)². Bifurcation starts with a ‘Big Bang’ and the leading turbulent front of a split jet may be unsteady, but the succeeding laminar part can be regarded as stationary or as translating at a constant velocity, as if frozen. Thus, such features are described by the equation. A provisional classification is given of various warm extrusions along branches of both Streams, all of which exemplify the repulsive bifurcation, except for the mainstreams with very narrow (< 30–40 km) branches, possibly due to misapplication of the model. This revised version was published online in August 2006 with corrections to the Cover Date.     

Variability of the North Pacific Current and its bifurcation

The North Pacific Current (NPC) bifurcates approaching the west coast of North America into a subpolar branch that forms the Alaska Current, and a subtropical branch that includes the California Current. The variability of this current system is discussed using numerical results from a wind-driven, reduced-gravity model. Indices of the strength of the subpolar and subtropical components of the NPC are examined based on output from multi-decadal simulations with the numerical model. This shows periods of both correlated and anti-correlated variability of the subpolar and subtropical gyres. A decomposition of the gyre transport time series indicates that the dominant mode of variability is a “breathing” mode in which the subpolar and subtropical gyres co-vary in response to fluctuations in the strength of the NPC. This finding is consistent with an analysis of dynamic height data of limited duration from the array of Argo drifting floats.The variability of the NPC is also examined using sea surface height (SSH) data from satellite altimetry over the period 1993-2005. The leading mode of SSH over the northeast Pacific dominates the variability of the NPC and is shown to be associated with in-phase variations in the transport of the subtropical and subpolar gyres. A strong correlation is found between time-dependent fluctuations in SSH across the NPC and variations in the strength of the transport of the NPC in the model. This agreement provides evidence for variability of the NPC occuring in direct response to large-scale atmospheric forcing.     

Lagrangian flow patterns north of Cape Hatteras using near-surface drifters

Lagrangian flow patterns in the vicinity of Cape Hatteras are examined using the tracks of 42 drifters drogued at 10 m depth and initially deployed over Georges Bank. The drifters predominantly move southwestward over the continental shelf and slope. North of Cape Hatteras, the drifters become entrained in the Gulf Stream and are carried eastward into the central Atlantic Ocean. There are two types of entrainment, abrupt and gradual. The first is characterized by a rapid change in drifter speed and an abrupt shift in drifter direction to the east. During such entrainment events, the radius of curvature of the drifter track is less than 30 km. The second type of entrainment is characterized by a gradual change in drifter direction with little change in speed. The radius of curvature of drifter tracks during such entrainment events is large (typically 50 km). The latter type occurs more frequently in summer and fall, when stratification is stronger. The drifter tracks further reveal that entrainment from the shelfbreak front/slope water system into the Gulf Stream may occur a significant distance north of Cape Hatteras, occasionally as far north as 38 °N, 200 km north of Cape Hatteras. Only two drifter tracks extend along the shelf past Diamond Shoals into the South Atlantic Bight. Four drifters are ejected from the Gulf Stream and recirculate over the slope. The observed time scale of recirculation ranges over 1–3 months. These results suggest that there are a variety of processes that determine the maximum southward penetration of Mid-Atlantic Bight shelf water before entrainment into the Gulf Stream as well as the cross-slope speed of entrainment.     

Nutrient fluxes from deep sediment support nutrient budget in the oligotrophic waters of the Gulf of Aqaba

The role of deep sediment in supporting nutrient budget in the Gulf of Aqaba has been investigated by estimating the flux of inorganic nitrogen, phosphate and silicate. Fluxes were calculated directly by pore water profiles and indirectly by chamber incubations carried out onboard the RV Meteor cruise. The results showed that maximum potential fluxes calculated by chamber incubations were higher than those calculated by porewater profiles for all nutrients (6.4–28.5 fold). This has been attributed to the additional flux due to bioturbation and flux from advective porewater exchange in the case of chamber incubation, while porewater fluxes represent diffusive ones. Using a rough estimation considering flux results in addition to the sediment area and water mass of the Gulf of Aqaba, we estimate that 3.3 × 10⁵, 6.4 × 10⁴ and 6.5 × 10⁶ kg year⁻¹ of inorganic nitrogen, phosphate and silicate respectively are effused from deep sediments to the water column. This quanitity would certainly support the primary productivity in the oligotrophic water in the Gulf of Aqaba.     

In situ hydrocarbon concentrations from pressurized cores in surface sediments, Northern Gulf of Mexico

Two newly developed coring devices, the Multi-Autoclave-Corer and the Dynamic Autoclave Piston Corer were deployed in shallow gas hydrate-bearing sediments in the northern Gulf of Mexico during research cruise SO174 (Oct–Nov 2003). For the first time, they enable the retrieval of near-surface sediment cores under ambient pressure. This enables the determination of in situ methane concentrations and amounts of gas hydrate in sediment depths where bottom water temperature and pressure changes most strongly influence gas/hydrate relationships. At seep sites of GC185 (Bush Hill) and the newly discovered sites at GC415, we determined the volume of low-weight hydrocarbons (C₁ through C₅) from nine pressurized cores via controlled degassing. The resulting in situ methane concentrations vary by two orders of magnitudes between 0.031 and 0.985 mol kg^− 1 pore water below the zone of sulfate depletion. This includes dissolved, free, and hydrate-bound CH₄. Combined with results from conventional cores, this establishes a variability of methane concentrations in close proximity to seep sites of five orders of magnitude. In total four out of nine pressure cores had CH₄ concentrations above equilibrium with gas hydrates. Two of them contain gas hydrate volumes of 15% (GC185) and 18% (GC415) of pore space. The measurements prove that the highest methane concentrations are not necessarily related to the highest advection rates. Brine advection inhibits gas hydrate stability a few centimeters below the sediment surface at the depth of anaerobic oxidation of methane and thus inhibits the storage of enhanced methane volumes. Here, computerized tomography (CT) of the pressure cores detected small amounts of free gas. This finding has major implications for methane distribution, possible consumption, and escape into the bottom water in fluid flow systems related to halokinesis.     

Seismic stacking pattern of the Faro-Albufeira contourite system (Gulf of Cadiz): a Quaternary record of paleoceanographic and tectonic influences

A Quaternary stratigraphic stacking pattern on the Faro-Albufeira drift system has been determined by analysing a dense network of high-resolution single-channel seismic reflection profiles. In the northern sector of the system an upslope migrating depositional sequence (elongate separated mounded drift) parallel to the margin has been observed associated with a flanking boundary channel (Alvarez Cabral moat) that depicts the zone of Mediterranean Outflow Water (MOW) acceleration and/or focussing. A consequent erosion along the right hand border and deposition on the left hand flank is produced in this sector. The sheeted aggrading drift is the basinward prolongation of the elongate separated mounded drift, and developed where the MOW is more widely spread out. The overall sheeted contourite system is separated into two sectors due to the Diego Cao deep. This is a recent erosional deep that has steep erosional walls cut into Quaternary sediments. Two major high-order depositional sequences have been recognised in the Quaternary sedimentary record, Q-I and Q-II, composed of eight minor high-order depositional sequences (from A to H). The same trend in every major and minor depositional sequence is observed, especially in the elongate mounded drift within Q-II formed of: A) Transparent units at the base; B) Smooth, parallel reflectors of moderate-high amplitude units in the upper part; and C) An erosional continuous surface of high amplitude on the top of reflective units. This cyclicity in the acoustic response most likely represents cyclic lithological changes showing coarsening- upward sequences. A total of ten minor units has been distinguished within Q-II where the more representative facies in volume are always the more reflective and are prograding upslope with respect to the transparent ones. There is an important change in the overall architectural stacking of the mounded contourite deposits from a more aggrading depositional sequence (Q-I) to a clear progradational body (Q-II). We suggest that Q-I and Q-II constitute high-order depositional sequences related to a 3rd-order cycle at 800 ky separated by the most prominent sea-level fall at the Mid Pleistocene Revolution (MPR), 900–920 ky ago. In more detail the major high-order depositional sequences (from A to H) can be associated with asymmetric 4th-order climatic and sea-level cycles. In the middle slope, the contourite system has a syn-tectonic development with diapiric intrusions and the Guadalquivir Bank uplift. This syn-tectonic evolution affected the overall southern sheeted drift from the A to F depositional sequences, but G and H are not affected. These last two depositional sequences are less affected by these structures with an aggrading stacking pattern that overlaps the older depositional sequences of the Guadalquivir Bank uplift and diapiric intrusions.