Geometry and migration characteristics of bedforms under waves and currents: Part 2: Ripples superimposed on sandwaves

This paper presents results of a series of detailed measurements of geometric and migrating characteristics of ripples superimposed upon sandwaves under the action of combined waves and currents. Velocity measurements within the fluid, surface wave characteristics and 3D mapping of the bottom were recorded with an Acoustic Doppler Velocimeter (ADV), an acoustic water level sensor and a 32 composite element array of sub-aquatic acoustic sensors, respectively. Bottom records were statistically analyzed to obtain height, length and migration rates of ripples. Experiments examined ripple heights and wavelengths for the mobility factors (as defined in Eq. (4)) and the Reynolds wave number within the ranges 10 < ψ < 88 and 16 × 10³ < Re_w < 5 × 10⁵, respectively. Measured values were compared with laboratory and field data together with semi-empirical and analytical formulae from the literature. Good correlation was obtained when plotting measured ripple length and length in dimensionless form as a function of the Reynolds wave number Re_w. Under a given hydraulic condition, it was observed that ripples with different geometric characteristics may coexist at different locations over the sandwave. Ripple steepness is presented as a function of the Shields parameter although characterized with rather large scatter. Finally, average ripple migration speed is presented as a function of the Shields parameter and the mobility number.     

Force and flow characteristics of a circular cylinder with uniform surface roughness at subcritical Reynolds numbers

The main purpose of this study is to establish a better understanding of the relationship between drag reduction and surface roughness. Experiments were conducted to measure the force and flow characteristics of a circular cylinder with different types of artificial surface roughness over the range 6 × 10³ < Re < 8 × 10⁴ (Re is based on the cylinder diameter D). The roughness cylinder was formed by covering the exterior surface of the cylinder with uniformly distributed (1) sandpaper, (2) netting, and (3) dimples. The roughness coefficient ranged from k/D = 0.0028 to 0.025 (k is the roughness height). A detailed quantitative measurement of the flow field around the cylinder using Particle Imaging Velocimetry (PIV) was carried out. The hydrodynamic force coefficients (drag and lift) of the rough cylinders are compared against those of a smooth cylinder measured under the same flow conditions. It is found that certain configuration of surface roughness significantly reduces the mean drag coefficient of the cylinder, particularly at large Reynolds numbers. In addition, the root-mean-square (r.m.s.) lift coefficient of the rough cylinders is considerably lower than that of a smooth cylinder.     

Analysis of temporal and spatial characteristics of waves in the Indian Ocean based on ERA-40 wave reanalysis

Based on the 45-year (09/1957-08/2008) European Centre for Medium-Range Weather Forecasts (ECMWF) Reanalysis (ERA-40) wave reanalysis dataset, this study analyzes interannual and interdecadal variabilities and intraseasonal oscillations of sea surface wind speed (WS), wind sea wave height (H^w), swell wave height (H^s) and significant wave height (H_s) in the Roaring Forties and tropical waters of the Indian Ocean, to determine swell propagation characteristics. The results show: (1) monthly variabilities of H^s in the Roaring Forties are in good agreement with those in tropical waters of the Indian Ocean; swell plays a dominant role in mixed waves throughout most of the Indian Ocean; and WS, H^w, H^s, and H_s exhibit a significant increasing trend over the 45-year study period. (2) H^s in the Roaring Forties and tropical waters of the Indian Ocean share a common period of 9.8–10.4 years on an interdecadal scale; and WS and H^s in the Roaring Forties and H^s in the tropical waters of the Indian Ocean share a common period of approximately 8 days (weekly oscillation) on an intraseasonal scale. (3) Swell of the Roaring Forties needs approximately 30 h to fully respond to the wind in this region. Approximately 84 h are required for H^s to propagate from the Roaring Forties to the tropical waters of the south Indian Ocean, while it takes approximately 132–138 h for H^s to propagate from the Roaring Forties to the tropical waters of the north Indian Ocean.     

Fluid flow reconstruction in karstified Panormide platform limestones (north-central Sicily): Implications for hydrocarbon prospectivity in the Sicilian fold and thrust belt

Diagenetic analysis based on field and petrographic observations, isotope and microthermometric data was used to reconstruct the fluid flow history of the Cretaceous shallow water limestones from the Panormide platform exposed in north-central Sicily. Analysis focused on diagenetic products in cavities and dissolution enlarged fractures of the karstified limestones that occur just below a regional unconformity. The fluid flow history could be broken down into five stages that were linked to the kinematic and burial history of the region. (1) Petrography (zoned cathodoluminescence and speleothem textures) and stable isotopes (6.5 < δ¹⁸O_V-PDB < ?3.5‰ and 0 < δ¹³C_V-PDB < ?14‰) indicate that the earliest calcite phase was associated with karstification during emergence of the platform. Limestone dissolution at this stage is important with regard to possible reservoir creation in the Panormide palaeogeographic domain. (2) Fine-grained micrite sedimentation, dated as latest Cretaceous by nannopalaeontology and its ⁸⁷Sr/⁸⁶Sr isotope ratio (0.7078), marks replacement by marine fluids during subsequent submergence of the karstified platform. (3) The following calcite cement was still precipitated by marine-derived fluids (?7.0 < δ¹⁸O_V-PDB < ?5.0‰ and ?3.0 < δ¹³C_V-PDB < 0.5‰/T_m = ?2 to ?5 °C), but at increasingly higher temperatures (T_h = 60–120 °C). This has been interpreted as precipitation during Oligocene foredeep burial. (4) Hot (T_h = 130–180 °C), low saline (T_m < ?2.5 °C) fluids with increasingly higher calculated δ¹⁸O_SMOW signatures (+6 to +14‰) subsequently invaded the karst system. These fluids most likely migrated during fold and thrust belt development. The low salinity and relatively high δ¹⁸O_SMOW signatures of the fluids are interpreted to be the result of clay dewatering reactions. The presence of bitumen and associated fluorite with hydrocarbon inclusions at this stage in the paragenesis constrains the timing of oil migration in the region. (5) Finally, high saline fluids with elevated ⁸⁷Sr/⁸⁶Sr (0.7095–0.7105) signatures invaded the karst system. This last fluid flow event was possibly coeval with localized dolomitization and calcite cementation along high-angle faults of Pliocene age, as suggested by identical radiogenic signatures of these diagenetic products.     

Offshore wind farm impacts on surface waves and circulation in Eastern Lake Ontario

A coupled wave and hydrodynamic model was applied to the Kingston Basin of eastern Lake Ontario, a region with bathymetric variability due to channels and shoals, to assess the potential impacts on surface waves and wind-driven circulation of an offshore wind farm. The model was used to simulate a series of storm events with time-varying wind forcing and validated against wave, current and water level observations. The wind farm was simulated by adding semi-permeable structures in the surface wave model to represent the turbine monopiles, and by adding an energy loss term to the fluid momentum equations in the hydrodynamic model to represent the added drag of the monopiles on the flow. The results suggest that the wind farm would have a small influence on waves and circulation throughout the wind farm area, with spatial variability due to focussing of wave energy and re-direction of the flow. Overall, the results indicate that the wave height in coastal areas will be minimally affected with changes in significant wave height predicted to be < 3%. Larger changes to the strength of circulation occur inside the wind farm region with localized changes in current magnitude of up to 8 cm s^− 1. The results of this study may help to understand the impacts of future offshore wind farms and other offshore structures in the Great Lakes.     

Trade-wind waves and mud dynamics on the French Guiana coast,South America: Input from ERA-40 wave data and field investigations

The South American coast between Brazil and Venezuela is affected by longshore migrating mud banks derived from the fine-grained Amazon sediment discharge. Onshore mud migration prevails over shallow ‘bank’ areas alternating alongshore with deeper ‘inter-bank’ areas. The transport on the inner shelf, and attachment to the shoreline, of this migrating mud has been attributed mainly to wind waves. However, the lack of in situ data on waves hampers understanding of the relationship between waves and mud dynamics. A 44-yr record (1960–2004) of the ERA-40 wave dataset generated by the European Centre for Medium-Range Weather Forecasts (ECMWF) was used, in conjunction with field investigations in French Guiana, to define both event-scale and longer-term patterns of mud mobilisation induced by waves. The ratio H₀³ / T², combining wave height H and period T, and the angle of wave incidence α, were singled out as the most relevant parameters for describing wave forcing. Typical ‘bank’ and ‘inter-bank’ profiles and corresponding mud densities, and a 3-month record of changes in the thickness of the fluid mud layer in an estuarine navigation channel were monitored by echo-sounding from October 2002 to January 2003. An 80-day record of bed-level changes in the intertidal zone was obtained from August to November 2004 using a pressure transducer. The results on the wave regime of French Guiana confirm a distinctly seasonal pattern, and highlight an increase in H₀³ / T² over the 44-yr period related to an increase in trade-wind velocities determined from corresponding trends in Atlantic wind pseudo-stress off the South American coast. Wave forcing over bank areas leads to the liquefaction of a 1–3 m-thick layer of mud that is transported onshore (and alongshore by the longshore component of wave energy). The episodic nature of high wave energy events generally results in the formation of mud bar features from the shoreward mobilisation of gel-like fluid mud. The effect of waves on mud is particularly marked following long periods of low energy, and especially at the onset of the high wave energy season (October to May), when even moderate wave energy events can lead to significant mobilisation of mud.Significant phases of increased wave energy are attended by higher long-term (annual) rates of longshore mud bank migration but the correlation is rather poor between the wave forcing parameter H₀³ / T² and migration rates because stronger wave forcing is generally associated with low angles of wave incidence. This suggests a complementary role of other hydrodynamic mechanisms, such as geostrophic and tidal currents, in longshore mud bank migration.     

Kinetic study reveals weak Fe-binding ligand,which affects the solubility of Fe in the Scheldt estuary

The chemistry of dissolved Fe(III) was studied in the Scheldt estuary (The Netherlands). Two discrete size fractions of the dissolved bulk (< 0.2 μm and < 1 kDa) were considered at three salinities (S = 26, 10 and 0.3).Within the upper estuary, where fresh river water meets seawater, the dissolved Fe concentration decreases steeply with increasing salinity, for the fraction < 0.2 μm from 536 nM at S = 0.3 to 104 nM at S = 10 and for the fraction < 1 kDa from 102 nM to 36 nM Fe. Further downstream, in the middle and lower estuary, this decrease in the Fe concentration continues, but is far less pronounced. For all samples, the traditionally recognised dissolved strong organic Fe-binding ligand concentrations are lower than the dissolved Fe concentrations.Characteristics of dissolved Fe-binding ligands were determined by observing kinetic interactions with adsorptive cathodic stripping voltammetry. From these kinetic experiments we concluded that apart from the well-known strong Fe-binding organic ligands (L, logK′ = 19–22) also weak Fe-binding ligands (P) existed with an α value (binding potential = K′ · [P]) varying between 10^11.1 and 10^11.9. The presence of this relatively weak ligand explained the high concentrations of labile Fe present in both size fractions in the estuary. This weak ligand can retard or prevent a direct precipitation after an extra input of Fe.The dissociation rate constants of the weak ligand varied between 0.5 × 10^− 4 and 4.3 × 10^− 4 s^− 1. The rate constants of the strong organic ligand varied between k_d = 1.5 × 10^− 3–17 × 10^− 2 s^− 1 and k_f = 2.2 × 10⁸–2.7 × 10⁹ M^− 1 s^− 1. The dissociation rate constant of freshly amorphous Fe-hydroxide was found to be between 4.3 × 10^− 4 and 3.7 × 10^− 3 s^− 1, more labile or equal to the values found by Rose and Waite [Rose, A.L., Waite, T.D., 2003a. Kinetics of hydrolysis and precipitation of ferric iron in seawater. Environ. Sci. Technol., 37, 3897–3903.] for freshly precipitated Fe in seawater.Kinetic rate constants of Fe with the ligand TAC (2-(2-Thiazolylazo)-p-cresol) were also determined. The formation rate constant of Fe(TAC)₂ varied between 0.1 × 10⁸ and 3.6 × 10⁸ M^− 1 s^− 1, the dissociation rate constant between 0.2 × 10^− 5 and 17 × 10^− 5 s^− 1 for both S = 26 and S = 10. The conditional stability constant of Fe(TAC)₂ (β_Fe(TAC)2′) varied between 22 and 23.4 for S = 10 and S = 26 more or less equal to that known from the literature (logβ_Fe(TAC)2′ = 22.4; [Croot, P.L., Johansson, M., 2000. Determination of iron speciation by cathodic stripping voltammetry in seawater using the competing ligand 2-(2-Thiazolylazo)-p-cresol (TAC). Electroanalysis, 12, 565–576.]). However, at S = 0.3 the logβ_Fe(TAC)2′ was 25.3, three orders of magnitude higher. Apparently the application of TAC to samples of low salinity can only be done when the correct β_Fe(TAC)2′ is known.     

The swarm dynamics of northern krill (Meganyctiphanes norvegica) and pteropods (Cavolinia inflexa) during vertical migration in the Ligurian Sea observed by an acoustic Doppler current profiler

A ship-mounted 153 kHz narrow-band ADCP and 1 m² MOCNESS were deployed between 16 and 24 Sept. 1997 in the Ligurian central zone (∼43°20′N 7°48′E). Results from both instruments showed that the zooplankton community performed vertical migrations that conformed to the classical pattern of ascent at dusk (∼18:30 h) and descent at dawn (∼06:30 h). Depth-discrete net samples between 0 and 500 m showed that the community was dominated by two species, the euphausiid Meganyctiphanes norvegica (Northern krill) and the pteropod Cavolinia inflexa, which migrated in separate discrete bands that were detectable by the ADCP. Information from the ADCP was used to estimate vertical migration speed in two ways: (i) from the trajectory of the back-scattering bands over time and (ii) from the Doppler-shift vertical velocity measured within depth zones at the corresponding time and depth of these bands. Estimates of the migration speed of C. inflexa were between 2 and 7 cm s⁻¹ upwards and between 4 and 7 cm s⁻¹ downwards. M. norvegica was estimated to migrate at speeds between 7 and 8 cm s⁻¹ upwards and over 11 cm s⁻¹ downwards. The consistently lower migration speeds estimated from Doppler measurements as compared with estimates obtained from measuring trajectories of back-scattering bands over time was believed to result from a methodological artefact. The Doppler measurements were nevertheless useful in a relative sense in revealing the relative speed of individuals within swarms. It was shown that individuals at the front of the upwardly migrating band of M. norvegica moved more slowly than those at the rear. These results illustrate the extra biological information that can be obtained by ADCPs compared with conventional echo-sounders.     

Acoustic discrimination of two zooplankton species (mysid) at 38 and 120 kHz

Using bifrequency sonar (38 kHz, 120 kHz) directed towards the open sea from the inside of a submarine cave, two species of Mysidacea (Crustacea), similar in shape but not in size, and with different patterns of behaviour, were observed simultaneously and discriminated, and their biomass was estimated during their migration. The first species, Hemimysis speluncola (<1 cm length), lives in the submarine cave, forming a distinct community that migrates horizontally. The second species, Siriella jaltensis (>1 cm length), lives in the open sea and migrates vertically. These species are both detected clearly at 120 kHz, but only the larger one, Siriella jaltensis, is detected at 38 kHz. This distinction allows one to determine the size limits of planktonic objects detected at these frequencies, i.e. 3 mm at 120 kHz and 10 mm at 38 kHz, corresponding to one quarter of the wavelength. The target strength (TS) of each species was calculated from the field data and compared to results calculated from three models. As the volume of the cave occupied by the Hemimysis population is about 300 m³, and the density of the swarm is 36,000 individuals/m³, the number of individuals in the cave is estimated to be around 10 million (or 24 kg). The density of the open-water population of Siriella is estimated to be 13 individuals/m³.     

Diel variations of the bathymetric distribution of zooplankton groups and biomass in Cap-Ferret Canyon,France

The bathymetric distribution, abundance and diel vertical migrations (DVM) of zooplankton were investigated along the axis of the Cap-Ferret Canyon (Bay of Biscay, French Atlantic coast) by a consecutive series of synchronous net hauls that sampled the whole water column (0–2000 m in depth) during a diel cycle. The distribution of appendicularians (maximum 189 individuals m⁻³), cladocerans (maximum 287 individuals m⁻³), copepods (copepods<4 mm, maximum 773 individuals m⁻³, copepods>4 mm, maximum 13 individuals m⁻³), ostracods (maximum 8 individuals m⁻³), siphonophores (maximum >2 individuals m⁻³) and peracarids (maximum >600 individuals 1000 m⁻³) were analysed and represented by isoline diagrams. The biomass of total zooplankton (maximum 18419 μg C m⁻³, 3780 μg N m⁻³) and large copepods (>4 mm maximum 2256 μg C m⁻³, 425 μg N m⁻³) also were determined. Vertical migration was absent or affected only the epipelagic zone for appendicularians, cladocerans, small copepods and siphonophores. Average amplitude of vertical migration was about 400–500 m for ostracods, some hyperiids and mysids, and large copepods, which were often present in the epipelagic, mesopelagic, and bathypelagic zones. Large copepods can constitute more than 80% of the biomass corresponding to total zooplankton. They may play an important role in the active vertical transfer of carbon and nitrogen.     

Distribution of deep-water corals along the North American continental margins: Relationships with environmental factors

Despite the increasing attention to assemblages of deep-water corals in the past decade, much of this research has been focused on documenting and enumerating associated fauna. However, an understanding of the distribution of most species of coral and the ecological processes associated with these assemblages is still lacking. In this study, we qualitatively and quantitatively described the habitats of two families of deep-water corals in relation to six oceanographic factors (depth, slope, temperature, current, chlorophyll a concentration and substrate) on the Pacific and Atlantic Continental Margins of North America (PCM and ACM study areas, respectively). This study focused primarily on the distributions of Primnoidae and Paragorgiidae because of the large number of documented occurrences. For each environmental factor, deep-water coral locations were compared to the surrounding environment using χ² tests. On both continental margins, coral locations were found to be not randomly distributed within the study areas, but were within specific ranges for most environmental factors. In the PCM study area, Paragorgiidae and Primnoidae locations were found in areas with slopes ranging from 0° to 10.0°, temperature from −2.0 to 11.0 °C and currents from 0 to 143 cm s⁻¹. In the ACM study area, Paragorgiidae and Primnoidae locations were found in areas with slopes ranging from 0° to 1.4°, temperature ranging from 0 to 11.0 °C and currents ranging from 0 to 207 cm s⁻¹. Although the patterns in habitat characteristics were similar, differences existed between families with respect to particular environmental factors. In both study areas, most environmental parameters in locations where corals occurred were significantly different from the average values of these parameters as determined with χ² tests ($p<0.05$) except for substrate in Paragorgiidae locations and depth in Primnoidae locations on the PCM. This is the first study to show coral distributional patterns at the continental shelf/slope scale.     

Volume transport and distribution of deep circulation at 165°W in the North Pacific

We conducted full-depth hydrographic observations between 8°50′ and 44°30′N at 165°W in 2003 and analyzed the data together with those from the World Ocean Circulation Experiment and the World Ocean Database, clarifying the water characteristics and deep circulation in the Central and Northeast Pacific Basins. The deep-water characteristics at depths greater than approximately 2000 dbar at 165°W differ among three regions demarcated by the Hawaiian Ridge at around 24°N and the Mendocino Fracture Zone at 37°N: the southern region (10–24°N), central region (24–37°N), and northern region (north of 37°N). Deep water at temperatures below 1.15 °C and depths greater than 4000 dbar is highly stratified in the southern region, weakly stratified in the central region, and largely uniform in the northern region. Among the three regions, near-bottom water immediately east of Clarion Passage in the southern region is coldest (θ<0.90 °C), most saline (S>34.70), highest in dissolved oxygen (O₂>4.2 ml l^?1), and lowest in silica (Si<135 μmol kg^?1). These characteristics of the deep water reflect transport of Lower Circumpolar Deep Water (LCDW) due to a branch current south of the Wake–Necker Ridge that is separated from the eastern branch current of the deep circulation immediately north of 10°N in the Central Pacific Basin. The branch current south of the Wake–Necker Ridge carries LCDW of θ<1.05 °C with a volume transport of 3.7 Sv (1 Sv=10⁶ m³ s^?1) into the Northeast Pacific Basin through Horizon and Clarion Passages, mainly through the latter (～3.1 Sv). A small amount of the LCDW flows northward at the western boundary of the Northeast Pacific Basin, joins the branch of deep circulation from the Main Gap of the Emperor Seamounts Chain, and forms an eastward current along the Mendocino Fracture Zone with volume transport of nearly 1 Sv. If this volume transport is typical, a major portion of the LCDW (～3 Sv) carried by the branch current south of the Wake–Necker and Hawaiian Ridges may spread in the southern part of the Northeast Pacific Basin. In the northern region at 165°W, silica maxima are found near the bottom and at 2200 dbar; the minimum between the double maxima occurs at a depth of approximately 4000 dbar (θ～1.15 °C). The geostrophic current north of 39°N in the upper deep layer between 1.15 and 2.2 °C, with reference to the 1.15 °C isotherm, has a westward volume transport of 1.6 Sv at 39–44°30′N, carrying silica-rich North Pacific Deep Water from the northeastern region of the Northeast Pacific Basin to the Northwest Pacific Basin.     

Impact of lugworms (Arenicola marina) on mobilization and transport of fine particles and organic matter in marine sediments

We investigated the impact of sediment reworking fauna and hydrodynamics on mobilization and transport of organic matter and fine particles in marine sediments. Experiments were conducted in an annular flume using lugworms (Arenicola marina) as model organisms. The impact of lugworms on sediment characteristics and particle transport was followed through time in sediments experimentally enriched with fine particles (< 63 μm) and organic matter. Parallel experiments were run at low and high water current velocity (11 and 25 cm s^− 1) to evaluate the importance of sediment erosion at the sediment–water interface. There was no impact of fauna on sediment composition and particle transport at current velocity below the sediment erosion threshold. At current velocity above the erosion threshold, sediment reworking by lugworms resulted in dramatic particle transport (12 kg dry matter m^− 2) to an adjacent particle trap within 56 days. The transported matter was enriched 6–8 times in fine particles and organic matter when compared to the initial sediment. This study suggests that sediment reworking fauna is an important controlling factor for the particle composition of marine sediments. A. marina mediated sediment reworking greatly increases the sediment volume exposed to hydrodynamic forcing at the sediment–water interface, and through sediment resuspension control the content of fine particles and organic matter in the entire reworked sediment layer (> 20 cm depth).     

Basin-scale variability of phytoplankton biomass,production and growth in the Atlantic Ocean

The latitudinal distributions of phytoplankton biomass, composition and production in the Atlantic Ocean were determined along a 10,000-km transect from 50°N to 50°S in October 1995, May 1996 and October 1996. Highest levels of euphotic layer-integrated chlorophyll a (Chl a) concentration (75–125 mg Chl m⁻²) were found in North Atlantic temperate waters and in the upwelling region off NW Africa, whereas typical Chl a concentrations in oligotrophic waters ranged from 20 to 40 mg Chl m⁻². The estimated concentration of surface phytoplankton carbon (C) biomass was 5–15 mg C m⁻² in the oligotrophic regions and increased over 40 mg C m⁻² in richer areas. The deep chlorophyll maximum did not seem to constitute a biomass or productivity maximum, but resulted mainly from an increase in the Chl a to C ratio and represented a relatively small contribution to total integrated productivity. Primary production rates varied from 50 mg C m⁻² d⁻¹ at the central gyres to 500–1000 mg C m⁻² d⁻¹ in upwelling and higher latitude regions, where faster growth rates (μ) of phytoplankton (>0.5 d⁻¹) were also measured. In oligotrophic waters, microalgal growth was consistently slow [surface μ averaged 0.21±0.02 d⁻¹ (mean±SE)], representing <20% of maximum expected growth. These results argue against the view that the subtropical gyres are characterized by high phytoplankton turnover rates. The latitudinal variations in μ were inversely correlated to the changes in the depth of the nitracline and positively correlated to those of the integrated nitrate concentration, supporting the case for the role of nutrients in controlling the large-scale distribution of phytoplankton growth rates. We observed a large degree of temporal variability in the phytoplankton dynamics in the oligotrophic regions: productivity and growth rates varied in excess of 8-fold, whereas microalgal biomass remained relatively constant. The observed spatial and temporal variability in the biomass specific rate of photosynthesis is at least three times larger than currently assumed in most satellite-based models of global productivity.