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A numerical model of ocean flow, hydrography, and transport is used to extrapolate observations of currents and hydrography and infer patterns of material flux in the deep ocean around Axial Seamount, a destination node of NSF's Ocean Observatories Initiative's Cabled Array. Using an inverse method, the model is made to approximate measured deep ocean flow around this site during a 35-day time period in the year 2002. The model is then used to extract month-long mean patterns and examine smaller-scale spatial and temporal variability around Axial. Like prior observations, model month-long mean currents flow anti-cyclonically around the seamount's summit in toroidal form with maximum speeds at 1500m depth of 10-11 cm/s. As a time mean, the temperature (salinity) anomaly distribution takes the form of a cold (briny) dome above the summit. Passive tracer material continually released at the location of the ASHES vent field exits the caldera primarily through its southern open end before filling the caldera. Once outside the caldera, the tracer circles the summit in clockwise fashion, fractionally re-entering the caldera over lower walls at its north end, while gradually bleeding southwestward during the modeled time period into the ambient ocean. A second tracer release experiment using a source of only two-day duration inside and near the CASM vent field at the northern end of the caldera suggests a residence time of the fluid at that locale of 8-9 days.

[1] A turbulent convection model for a hydrothermal fluid discharging into a tidally modulated, stratified cross flow is used to investigate time-variable conditions in plumes, such as the one rising from Dante, a sulfide mound at ∼2175 m depth on the Endeavour segment of the Juan de Fuca Ridge. That plume is the consequence of the coalescence of 10 or more small, individual plumes from chimneys discharging hot, salt-diminished fluid into the near-bottom ocean. At Dante, the discharge encounters ambient horizontal currents with speeds oscillating from near zero to a maximum of ∼7 cm s−1, speeds which can bend a plume more than 45° from the vertical. Model results are compatible with field measurements of the plume footprint size and vertical velocity both 20 m above the source when earlier estimates for Dante's heat flux of ∼50 MW drive the convection. The small-scale short period variability of velocities and properties distributions observed in the field is mimicked in model results. Plumes pool above a source during periods of weak cross flows but stream away from the source, with more diluted concentrations and lower rise heights, at other times. Plume distributions, at identical cross-flow speeds, differ whether the flow is accelerating or decelerating. Small changes in background hydrographic profiles create differences in rise heights comparable to those caused by large changes in source buoyancy flux. If put into an entrainment context, results suggest an entrainment coefficient (αEFF) that varies from ∼0.11 to ∼0.025 with increasing height (2–76 m) above the source.

The vertical position of larvae of vent species above a mid-ocean ridge potentially has a strong effect on their dispersal. Larvae may be advected upward in the buoyant vent plume, or move as a consequence of their buoyancy or by active swimming. Alternatively, they may be retained near the bottom by the topography of the axial trough, or by downward swimming. At vents near 9°50′N on the axis of the East Pacific Rise, evidence for active larval positioning was detected in a comparison between field observations of larvae in the plankton in 2006 and 2007 and distributions of non-swimming larvae in a two-dimensional bio-physical model. In the field, few vent larvae were collected at the level of the neutrally buoyant plume (~75 m above the bottom); their relative abundances at that height were much lower than those of simulated larvae from a near-bottom release in the model. This discrepancy was observed for many vent species, particularly gastropods, suggesting that they may actively remain near the bottom by sinking or swimming downward. Near the seafloor, larval abundance decreased from the ridge axis to 1000 m off axis much more strongly in the observations than in the simulations, again pointing to behavior as a potential regulator of larval transport. We suspect that transport off axis was reduced by downward-moving behavior, which positioned larvae into locations where they were isolated from cross-ridge currents by seafloor topography, such as the walls of the axial valley—which are not resolved in the model. Cross-ridge gradients in larval abundance varied between gastropods and polychaetes, indicating that behavior may vary between taxonomic groups, and possibly between species. These results suggest that behaviorally mediated retention of vent larvae may be common, even for species that have a long planktonic larval duration and are capable of long-distance dispersal.

Topography has a strong effect on the physical oceanography over the flanks and crests of the global mid-ocean ridge system. Here, we present an analysis of the hydrography and circulation near the crest of the East Pacific Rise (EPR) between 9° and 10°N, which coincides with an integrated study site (ISS) of the RIDGE2000 program. The analysis is based primarily on survey and mooring data collected during the LADDER project, which aimed to investigate oceanographic and topographic influences on larval retention and dispersal in hydrothermal vent communities. Results indicate that the yearly averaged regional mean circulation is characterized by a westward drift of 0.5–1 cm s−1 across the EPR axis and by north- and southward flows along the western and eastern upper ridge flanks, respectively. The westward drift is part of a basin-scale zonal flow that extends across most of the Pacific ocean near 10°N, whereas the meridional currents near the ridge crest are a topographic effect. In spite of considerable mesoscale variability, which dominates the regional circulation and dispersal on weekly to monthly time scales, quasi-synoptic surveys carried out during the mooring deployment and recovery cruises indicate sub-inertial circulations that are qualitatively similar to the yearly averaged flow but associated with significantly stronger velocities. Weekly averaged mooring data indicate that the anticyclonically sheared along-flank flows are associated with core speeds as high as 10 cm s−1 and extend ≈ 10 km off axis and 200 m above the ridge-crest topography. Near the northern limit of the study region, the Lamont Seamount Chain rises from the western ridge flank and restricts along-EPR flow to five narrow passages, where peak velocities in excess of 20 cm s−1 were observed. Outside the region of the ridge-crest boundary currents the density field over the EPR near 10°N is characterized by isopycnals dipping into the ridge flanks. Directly above the EPR axis the ridge-crest boundary currents give rise to an isopycnal dome. During times of strong westward cross-EPR flow isopycnal uplift over the eastern flank causes the cross-ridge density field below the doming isopycnals to be asymmetric, with higher densities over the eastern than over the western flank. The data collected during the LADDER project indicate that dispersal of hydrothermal products from the EPR ISS on long time scales is predominantly to the west, whereas mesoscale variability dominates dispersal on weekly to monthly time scales, which are particularly important in the context of larval dispersal.

We investigated planktonic larval transport processes along an axially symmetric mid-ocean ridge with characteristics similar to that of the East Pacific Rise (EPR) segment at 9–10°N. The hydrodynamic basis for this study is a primitive equation model implemented in two dimensions (depth and across-ridge), forced at the open boundaries to provide suitably realistic simulation of currents observed on the EPR ridge crest from May to November 1999. Three-dimensional trajectories of numerical larvae are computed assuming homogeneity in currents in the along-ridge direction. Larval dispersal fluctuates significantly in time. Transport distance decreases systematically with height above the bottom where numerical larvae are less subject to strong currents along the flanks of the ridge. The probability that the simulated larvae will be located near the ridge crest at settlement depends strongly on their behavioral characteristics (vertical position in the water column during the larval stage) and the length of their precompetency period.

Spectra of currents measured along the 2100 m deep Juan de Fuca Ridge in the northeast Pacific have prominent tidal, inertial, and weather-band (3–7 day period) spectral peaks. The weather-band peak, in particular, has a number of interesting features. It is strongest in close lateral proximity to the ridge and strongest near ridge depth; intensification near the ridge is characteristic of trapped motion. Spectral peak intensities vary seasonally, with largest amplitudes occurring in autumn and winter; seasonal variation suggests that surface weather is forcing flow at depth. Together, trapping and seasonality indicate opportunistic amplification of oscillatory motion at the ridge. At all frequencies, forcing, topography, and stratification together shape current and hydrographic distributions near the ridge. Effects of those interactions for subinertial motion are quantified here using a primitive equation numerical model. Forcing period (1–10 days) and friction are the principal dependencies examined. Results show that flow over the Juan de Fuca Ridge can be amplified by factors of 3–4 for diurnal and up to a factor of 7 for weather-band frequencies. Amplification is ridge trapped, with effects extending many hundreds of meters upward and extending 5–10 km laterally for 5-day-period flow; the strength and area of amplification increase with increasing period over the 1- to 10-day-period band. Oscillatory weather-band flow leads to vertical velocities on the order of 0.3 cm s−1 on ridge flanks, which in turn cause periodic temperature (T) and salinity (S) variations with amplitudes of as much as 0.05°C and 0.01 practical salinity units, respectively. The vertical motion and consequent vertical displacement (>100 m) lead to isotherms that plunge below the crest alternately each side of the ridge, a distribution observed in conductivity-temperature-depth transect data. Near the ridge crest, cross-ridge baroclinic pressure gradients caused by cyclically plunging isopycnals are in near-geostrophic balance with the Coriolis force associated with along-ridge flow. Along-ridge current amplification is therefore closely tied to isopycnal movement up and down ridge flanks. Since weather-band oscillations lead to much larger cross-ridge baroclinic pressure gradients than do diurnal motions of the same amplitude, weather-band forcing causes greater along-ridge current amplification. Results vary strongly with forcing period and direction of forcing with respect to the ridge but depend only moderately on friction and turbulent mixing coefficients.

Tidal and inertial currents and profuse hydrothermal discharge at recently erupted Axial Volcano, Juan de Fuca Ridge, cause relatively large and rapid temperature (T) changes in the near-bottom water column. Measurements show short-term T variations of as much as 0.13°C at 30 m and 0.18°C at 150 m above bottom and currents that have strong tidal components and means of 3–5 cm s−1. Locations and magnitudes of the hydrothermal sources leading to the observed T variations have been inferred via an inverse calculation. Results imply noncongruent source regions around the mooring site for plumes from low- and high-buoyancy flux sources. Water column and seafloor observations in the volcano’s caldera region generally support the distribution of source types and sites inferred. A high-buoyancy flux, ephemeral venting site, unexpected on the eastern shoulder of the volcano, is also indicated by the inverse calculation and supported by water-column survey data. Over the O(10 km2) calculation region, heat ...

Plumes originating in high-temperature hydrothermal discharges entrain background water and often entrain diffuse-source fluids during convective ascent. The mixing of fluid from those three different sources determines the chemical composition of the ensuing neutrally buoyant plumes. Here convection models and aluminum data from the Trans-Atlantic Geotraverse (TAG) site are used to examine relative contributions of focused, diffuse, and background sources to anomalies in the plume. Our analysis shows that anomalously high Al values observed just above the seafloor at TAG, which we assume stem from diffuse flows, can contribute as much as 50% to the Al anomaly in the neutrally buoyant plume found >350 m above bottom. Background water contributes nearly an equal fraction of Al to the anomaly. More generally, entrained contributions depend on gradients in profiles external to the rising plume. Entrainment also depends on effective source area; distributed high-temperature vents entrain more fluid than a single, composite high-temperature vent. Larger effective source area is particularly significant for entrainment of diffuse-source or other tracers trapped within a few tens of meters of the seafloor. Downstream, cross-stream, and temporal variabilities exhibited in the three-dimensional model plume also underscore problems of sampling and interpreting actual plume data.

Total heat, salt, and other tracer masses released during a hydrothermal event are shown to be proportional to, but not necessarily equal to, volume integrals of resulting water column anomalies. Proportionality coefficients depend on anomaly definition, on background hydrographic and tracer profiles, on expansion coefficients of the equation of state at an appropriate pressure, and on tracer to heat anomaly ratios at the venting source. For Gorda Ridge event plumes, which are described in other papers of this issue, volume integrals of conventionally defined heat anomalies underestimate actual released heat by a factor of 2.4 if the discharge is not anomalously saline. Under certain combinations of hydrographic and source anomaly conditions, not unlike those found on the East Pacific Rise at 10°N, the apparent total heat released during an event can be deceivingly zero. This analysis also establishes a linear relationship between the ratio of tracer anomaly to heat anomaly at any point in the plume to the same ratio at the source. One consequence is that the ratio of anomalous 3 He to heat in Gorda Ridge event plumes is approximately 2.4 larger in the water column than it is at the source. Results are independent of the entrainment process involved in event plume formation, and they are shown to hold true even for background hydrographic profiles that do not vary linearly with depth.

Temperature and currents were measured ∼100 m south of the Pipe Organ vent site on the Juan de Fuca Ridge during a 5-day interval in September 1997 to examine the temporal variability of hydrothermal heat signals close to a source. Temperatures were sampled at three depths every 15 s while current speed and direction were sampled at a single depth hourly. Results show that rapid potential temperature (θ) changes of as much as 0.02 °C occurred at all three depths, partly caused by changes in current speed and direction. Spectra of θ showed no prominent spectral peaks, however. An analytic model of heat transport under variable currents was used to help put observed θ temporal variability into context and to point out problems with estimating hydrothermal heat fluxes from mooring data. Model θ distributions, when contoured on a horizontal plane and animated, demonstrate the pooling of effluent over vents at various times, the streaming of effluent as a very narrow plume away from vents at times of large current speed, and the spawning of boluses of heated fluid and their transport away from venting regions when a pooling period is followed by a streaming period. Model results also show that estimated heat flux based on mooring data is strongly dependent on analysis assumptions and suggest that vents are capable of causing measurable θ anomalies as far as several kilometers downstream.

A three-dimensional time-dependent convection model is used to describe circulation and property fields in rotating, stratified, and moving fluids near a point source of heat. The study context and, consequently, model scales are those of chronically discharging hydrothermal vent fields found at submarine ridge crests. Hydrothermal plumes having distinctive thermal and chemical anomalies have been observed to rise several hundreds of meters above the deep-sea floor before being advected away by background cross flows typically of magnitude 1–4 cm s−1. The model is used to study effects of rotation and indicate differences in plumes with respect to variation of subgrid-scale turbulence intensity and cross-flow strength. Counterrotating vorticity (ζ) couplets in all three coordinate directions develop in the lower plume stem at startup and follow the plume to the level of neutral buoyancy; for a nonrotational case (Ω=0), ζ patterns resemble those previously found for jets injected into homogenous cross flow. Ambient fluid entrainment into the convecting column is primarily from the upstream side, but deflection of background flow around both sides of the rising column is the root of the relative vorticity (ζz) couplet in the lower plume. Turbulence intensity within the buoyant region of the plume and/or globally controls smoothness and temporal variability of distal nonbuoyant plume distributions, allowing or preventing oscillations of potential temperature, θ, for example, at background buoyancy frequency, N. Over the range of turbulent mixing studied, rise height of plumes did not change appreciably, but breadth of plumes, counterintuitively, increased for decreasing turbulent mixing strength. Increasing cross-flow strength, U0, bends model plumes such that rise height ∝ U0−0.4. For the two largest values of cross flow, for which R, the ratio of maximum vertical velocity to U0, took values of 2.8 and 1.0, plumes showed evidence of bifurcation.

Effects of rotation on finite-length line plumes are studied with a three-dimensional nonhydrostatic numerical model. Geophysical convection with this source geometry occurs, for example, as the result of fissure releases of hot hydrothermal fluids at the seafloor from terrestrial release of hot gases and ash during volcanic activity along fissures and in the descent from the sea surface of brines formed during freezing of ice leads at high latitudes. Here the model treats the case of a starting plume of dense fluid descending into a rotating environment. Results are compared with laboratory experiments so that the validity of the model, particularly the nonlinear subgrid-scale mixing formulation, might first be established. Differences in plumes caused by varying rotation rate, &ohm, and buoyancy flux, B0, are the primary focus, with experiments in fluid of depth h spanning a convective Rossby number [B01/3/(2Ωh)] of 0.01−1.0. Rotation initiates spiraling of the descending plumes but it has litt...

The acoustic scintillation method is used to study the hydrothermal plume of Dante within the Main Endeavour vent field (MEF) at the Endeavour segment of the Juan de Fuca Ridge. Forty days of vertical velocity and temperature fluctuations were obtained from the rising plume above the Dante edifice in an environment where the flow is dominated by strong (5cm/s) semi-diurnal tidal currents and a northerly mean residual current (3cm/s). These acoustic measurements provide a window on deep-sea hydrothermal plume dynamics in strong oscillatory cross flows. A large eddy simulation, parameterized with anisotropic mixing coefficients, taking into account ambient stratification and time-dependent background flows and calibrated by the acoustic measurements, yields insight into turbulent processes, entrainment, plume bending, rise height, and, inferentially, mound heat flux. The turbulent dissipation rates for kinetic energy (e) and thermal variance (eθ) is approximated by computing the Reynolds averaged sub-grid s...

Recent measurements of large water column plumes in association with seismic activity and evidence of a fissure release of magma on the sea floor at the CoAxial Segment, northern Juan de Fuca Ridge, initiates questions about the response of the benthic ocean to large and sudden line discharges of heat. Here these issues are investigated using a three-dimensional non-hydrostatic hydrodynamical model. The model treats the case of a starting plume from a finite-length, narrow line source of heat ascending into a rotating environment stratified in temperature and salinity. The developing water column plume is studied over an initial time period of approximately f−1, where f is the Coriolis frequency. Midway into this interval the plume attains its maximum rise height of ∼900 m, for given buoyancy flux and stratification conditions. Thereafter, the plume spreads laterally and the effects of the earth's rotation come into evidence. Outward flow at 400–900 m above bottom is primarily transverse to the line source, quickly making the plume at those heights more circular than line-like.

[1] Time-mean abyssal current observations over the flanks of the East Pacific Rise (EPR) at 9–10°N and at the Juan de Fuca Ridge at 45°N document the occurrence of paired along-ridge current jets that are trapped to the ridge flanks and sheared across the ridge in an anticyclonic sense. A coincident feature, where local hydrothermal discharge effects are not in play, is the upward bowing of isopycnals over ridge crests and isopycnals plunging into ridge flanks. It would be tempting to explain the jets primarily as geostrophic responses to the doming/plunging isopycnal distribution, though that should lead to the question as to how the isopycnal perturbations originate. A numerical model of time-dependent flow on a cross-ridge (x–z) transect, forced in a way to be consistent with a yearlong, hourly sampled record of currents measured at the EPR ridge crest, is used to investigate some of the underlying physics. It will be shown that the jets can arise from oscillatory flows via eddy-momentum, eddy-heat, and eddy-salt fluxes that ultimately cause the isopycnals to dome over the ridge. As the probable offspring of velocity-velocity and velocity-density correlations that depend upon oscillatory motion, the jets are likely examples of stratified topographic flow rectification. An ancillary feature is a slight yearlong-averaged downward current O (0.1–0.5 mm/s) over the EPR ridge crest that crosses the time-mean, upward bowing isopycnals in a counter-intuitive vertical direction.

Laboratory analyses of sludges from four treatment plants were performed to determine solid content, size distributions, and settling spectra of sludge particulates in seawater. The settling experiments were conducted after partitioning samples by sieving into coarse and fine particulate fractions. For the fine fraction, experiments proceeded after controlled mixing of samples with filtered seawater to particulate concentrations low enough (approx. 10 mg/L) that flocculation was limited during subsequent measurement periods. Results show that 14.6-47.3% of the sludge particulates by weight had diameters greater than 63..mu.., with at least 5% exceeding 250 ..mu..m. Median settling velocities of this fraction ranged from 6 /times/ 10/sup /minus/2/ to 3 /times/ 10/sup /minus/1/ cm/s. The bulk of the particulates were smaller in diameter, and these flocculated in seawater. Resulting from aggregates had median settling velocities by volume ranging from 7 /times/ 10/sup /minus/4/ to 3 /times/ 10/sup /minus/3/ cm/s, with less than 10% of the particles settling more slowly than 10/sup /minus/4/ cm/s.

[1] We report the first 3-D numerical model study of abyssal ocean circulation and transport over the steep topography of the East Pacific Rise (EPR) and adjoining Lamont seamount chain in the eastern tropical Pacific. We begin by comparing results of hydrodynamical model calculations with observations of currents, hydrography, and SF6 tracer dispersion taken during Larval Dispersal on the Deep East Pacific Rise (LADDER) field expeditions in 2006–2007. Model results are then used to extend observations in time and space. Regional patterns are pronounced in their temporal variability at M2 tidal and subinertial periods. Mean velocities show ridge-trapped current jets flowing poleward west and equatorward east of the ridge, with time-varying magnitudes (weekly average maximum of ∼10 cm s−1) that make the jets important features with regard to ridge-originating particle/larval transport. Isotherms bow upward over the ridge and plunge downward into seamount flanks below ridge crest depth. The passage (P1) between the EPR and the first Lamont seamount to the west is a choke point for northward flux at ridge crest depths and below. Weekly averaged velocities show times of anticyclonic flow around the Lamont seamount chain as a whole and anticyclonic flow around individual seamounts. Results show that during the LADDER tracer experiment SF6 reached P1 from the south in the western flank jet. A short-lived change in regional flow direction, just at the time of SF6 arrival at P1, started the transport of SF6 to the west on a course south of the seamounts, as field observations suggest. Approximately 20 days later, a longer-lasting shift in regional flow from west to SSE returned a small fraction of the tracer to the EPR ridge crest.

[1] The extensive set of measurements previously taken at Fieberling Guyot provides a backdrop for model results on flow, hydrography, turbulent mixing, and derivative variables. A z-level primitive equation model driven at subinertial and superinertial tidal frequencies allows the nonlinear interaction of free and topographically trapped motions to jointly determine model outcomes. Model residual flows, mean hydrographic features, topographic amplification of oscillatory currents, turbulent dissipation (ɛ) and vertical diffusivity (KV) profiles are all examined in the light of the observations. Azimuthal residual flows are toroidal above the summit with maximum speed ∼20 cm s−1. Diurnal flow over the summit is intensified by a factor >20. Downward vertical flow as a time-mean (〈w〉t) occurs from ∼150 m above the summit apex to summit rim depth, with radially inward flow above the apex and radial outward flow below. Upward 〈w〉t rings the summit at larger r. The existence of the observed cold dome is shown to depend on 〈w〉t in the summit circulation cell. Vertical current shear is concentrated in a 150 m thick cap above the summit, accounting for a domed distribution of ɛ and KV with maxima of ∼10−7 W kg−1 and ∼30 cm2 s−1, respectively. Profiles most like the observations of KV are obtained with a subgrid-scale parameterization in which the turbulent Prandtl number (Pr) depends on the shear Richardson number (Ri).

[1] The relationship of tidal currents and turbulence over Irving Seamount is examined by model and measurement. Surface tides are partially scattered into internal tides and, particularly over the seamount summit, current strength and shear greatly increase. Current shear causes the model's turbulent mixing. At Irving, modeled turbulent mixing, as represented by vertical diffusivity, is largest O(10−3 m2 s−1) in the bottom boundary layer and in a turbulent column rising to the sea surface directly above the seamount. Pockets of less intense mixing occur within the turbulent column, but their locations are highly variable. Field measurements confirm a turbulent mixing column atop Irving of comparable intensity. Increased model diffusivity (average of 7 × 10−5 m2 s−1) also occurs to a distance of several hundred kilometers from the seamount flanks in a band vertically centered at ∼800 m depth.

[1] Convection in the Arctic Ocean mixed layer occurs under openings in sea ice where new sea ice is formed. The process of brine rejection during ice formation creates negatively buoyant fluid at the surface of ice leads. When the fluid convects downward the ocean mixed layer below the lead is stirred vertically and, through planetary rotational effects, along-lead flows at the lead edges are induced. These vertical and lateral motions have been previously studied using two-dimensional numerical ocean models and observed in field programs such as LeadEx. In this paper the generation of Arctic Ocean eddies through dynamic instabilities is investigated using a three-dimensional, nonhydrostatic ocean model. Results suggest that salt-enriched vortices can develop at the base of the mixed layer as a consequence of brine rejection, but the vortex formation requires a time period of several days, a time long enough that the progenitor lead would likely have closed. The scale of the eddies is found to depend on the magnitude of the buoyancy forcing, the width of the lead, and the duration of the buoyancy forcing. The number of vortices produced depends on lead width and duration of buoyancy forcing but is less sensitive to the magnitude of the buoyancy forcing. Vortex sizes, scaled appropriately, and vortex numbers are compared with those found in recent laboratory studies of convectively driven flows from line segment sources.

A nonhydrostatic primitive equation model is used to investigate motions and water column property distributions accompanying the rise of hot hydrothermal fluids into a rotating water column stratified in temperature and salinity. After preliminary model experiments used to fix the level of turbulent mixing, the model is applied to a megaplume event, the release of a large amount of hydrothermal heat into the benthic ocean over a period of several days. Events of this kind have produced water masses with anomalous temperature and salinity distributions ∼20 km in diameter, ∼800 m thick, centered vertically 700–1000 m off the seafloor. The model allows the study of the initial time development and maturation of the plume from such an event. Preliminary experiments demonstrate the sensitivity of plume rise to the level of turbulent mixing and to the ratio of turbulence viscosity to turbulence diffusivity, i.e., Prandtl number. Rise to maximum plume height occurs in 4–6 N−1, where N is buoyancy frequency. Consequences of a megaplume-sized release of heat are examined over an initial 30-day period. Vertical circulation accompanying a hydrothermal event leads to a slight, negative salinity anomaly below the plume's temperature anomaly core. The plume's heat (potential temperature) anomaly is shown to be a composite of hydrothermal heat and ambient heat that has been redistributed by the same vertical circulation cell. Horizontal circulation is dominated by an anticyclonic flow vortex centered just above the temperature anomaly maximum at ∼800 m above bottom and a cyclonic flow vortex at the seafloor. Azimuthal flow accounts for the largest fraction of kinetic energy within 4.5 hours (∼1.7 ƒ−1) after the start of heat release; azimuthal velocities reach maximum values of ∼70 cm s−1, 90% of which is attained within the initial 10 hours. Even as much as 4 weeks past cessation of heat release maximum azimuthal velocities are of the order of ∼10 cm s−1. The balance of vertical forces is hydrostatic by 12 hours, save for near-bottom frictional effects. During plume formation, the radial force balance is primarily cyclostrophic near axis and geostrophic at a distance.

A two-stage scavenging model is used to describe the transport of hydrothermal Mn to the sediments adjacent to ridge crests. Dissolved Mn is hypothesized to be scavenged by slowly settling metal-depositing capsuled bacteria which, in turn, are incorporated into rapidly settling macroaggregates. Upon reaching the seafloor, the Mn is subject to resuspension in particulate form and to remobilization within the sediment column and release back into the water column as dissolved Mn. Measured Mn distributions in the vicinity of the southern Juan de Fuca Ridge and estimated values of process rate constants are used to limit the range of possible model outcomes. The results present a picture of water column distributions and fluxes of dissolved, fine particulate, and large-particle associated Mn in a plume advecting off axis. The model and best available parameter values suggest that more than 80% of the hydrothermal Mn is deposited within several hundred kilometers of the ridge crest, though dissolved Mn concentrations beyond that distance exceed background levels by many times. The residence time of hydrothermal Mn in the water column is of the order of several years. An off-axis component of advection of the order of 0.1–0.3 cm/s is needed to make similar the model and measured distributions of Mn depositing in the sediments.

We have observed late fall and winter bedload sediment transport and the overlying current field in ridge and swale topography on the inner continental shelf south of Long Island, and can report movement of bed material at a water depth of 20 m to a distance of approximately 1500 m after several storm events. Movement over an 11-week observation period was longshore and oblique to the ridge crest at the experimental site. Currents were also predominately longshore, but long term averages demonstrate that a vertical shear existed in the fluid motion. Although the number of sediment transport "events" suggested by the current meter data is nearly balanced in eastward and westward directions, both estimates of transport from current speeds and sand tracer dispersion patterns show that several westward flowing events dominated the transport during a two and one-half month period. A quantitative upper bound of 31 cm/sec on the threshold velocity for sediment movement in this size range is also set by the data.

Current measurements made in the Hudson Shelf Channel during the summer of 1973 show essentially channel axial bottom current even though the channel aspect ratio is small in the area of measurement. Although the current record is of short duration, correlation of water movement with surface winds is suggested by the data. The sense of summertime nontidal bottom flow in the channel (up or down channel) would appear to be controlled by the surface wind direction (offshore or onshore). These results would suggest the likelihood of net down-channel flow during the summer months.

Suspended sediment distributions which take into account concentration-dependent settling velocities have been derived. The distributions have been compared to and have been found to be in good agreement with the high concentration data of Einstein and Chien [1]. The analysis shows that concentrations at the sediment bed in the Einstein-Chien data are close to those of saturated sediments, which leads to the suggestion that the reference level in the distributions always be taken at the bed where the reference concentration is flow independent. A suspended sediment distribution constructed in this way guarantees a reasonable value for concentration near the bed and depends on two diffusivity parameters which the Einstein-Chien concentration data indicate are related.

Rates, concentrations, and composition of mining discharge and the size and structure of the ensuing surface plumes were examined during North Pacific tests of scaled manganese nodule mining systems. Discharge was composed principally of bottom water and pelagic silts and clays, although nodule fragments with diameters less than 1 mm were also discharged at widely varying rates. Average flow rates of the discharge varied from 95 to 160 litres/s, with the solid fraction varying from 550 to approximately 2000g/s. The plume, as determined by particulate concentrations in excess of ambient oceanic conditions, extended approximately 5 km from the mining ship and had a width of about 1 km. Fe and Mn signatures allowed detection of the plume nearly 35 km from the source. The plume provided evidence of settling more rapidly than expected of silt and clay-size particles: a mean settling velocity of 6 × 10 −2 cm/s for the particulates in the plume and a mixed layer vertical turbulent eddy diffusivity of 1 × 10 −2 m 2 /s have been inferred from the data. Field and laboratory data together suggest that the rapid settling was due to flocculation of the discharge particulates.

The likelihood that manganese nodule mining discharge (essentially deep-seabed clays and some nodule fragments) would reside on the pycnocline for a long period of time was investigated by introducing mining waste particles into a two-layer laboratory settling column illuminated by laser. The experiments were repeated with polystyrene particles of uniform shape and size to further study the effect of a density interface on settling. The results indicate that mining particulates do not have sufficiently low density to accumulate on the pycnocline although a density interface can temporarily concentrate settling particles. A review of related studies of accumulation of inorganic particles on density interfaces suggests that the available evidence for pycnocline accumulation of inorganic particles is slight. A measurement of the wet density spectra of any oceanic discharge is necessary to accurately assess the possibility of particles rafting on a density surface.

Frequently the mixing length l in second-order closure models is assumed to have a constant value l0 = γL at large distances from the bottom with a magnitude proportional to the first moment L of turbulent intensity. Although it is often stated that turbulence closure model results are relatively insensitive to the value of mixing length parameter γ, we show that this is not the case for a second-order Level II model of the steady bottom boundary layer in an unstratified fluid. In particular, the eddy viscosity and diffusivity depend strongly on γ. Available oceanic data on geostrophic drag ratio lead to a value of γ of approximately 0.2–0.3. Atmospheric data for steady flow suggest a smaller value of 0.05–0.1 although the atmospheric observations are ambiguous about the choice of γ, possibly because it is difficult to find truly neutrally stratified and steady-state conditions in the bottom boundary layer. A value of γ between 0.18 and 0.20 is required for the model to match a similarity theory ...

A generalization of the one-pion exchange model for the reaction π−p → π+π−n in the resonant ππ region is developed by means of the five-point function of Bardakci and Ruegg. The model, using the slope of the pion trajectory as the only adjustable parameter, has appropriate spin content and proper Regge behavior. We have used aK-matrix unitarization scheme for mesons, generalized for the production process. The model is applied over a wide range of energies, and variations of all four remaining independent variables are also examined. The results are in general agreement with experiment, suggesting that the formulation is a desirable improvement over previous phenomenological models.

Calculations of cohesionless bottom-sediment movement within the New York Bight have been made by applying the transport formulae of Bagnold (1941, 1956, 1963), Einstein (1950), and Yalin (1963) to near-bottom current meter and surficial sand size observations. Current data were drawn from the records of eighteen long-term Savonius rotor current meter deployments at various locations within the Bight during Fall of 1973 and Spring of 1974. The assumptions underlying the calculations are that wave activity was minimal at recording sites, that a drag coefficient of 3 10-3 reflecting small scale roughness and large boundary layer Reynolds number is suitable to convert measured currents to friction velocities, and that laboratory threshold velocities apply in the marine environment. The calculations suggest that oceanic bottom sediment movement reaches maximum intensity during the fall and winter due to the added energy input from strong meterological events. Calculated transport quantities on the inner shelf tend to decrease as depth and distance from shore increase. However, during the Fall of 1973, the deep waters near the head of the Hudson Shelf Valley exhibited current flows directed to the north in excess of 40 cm/sec. These up-channel flow events appear to be in response to strong, sustained westerly winds. The maximum sediment transport rate caused by these current velocities is two orders of magnitude greater than that occurring at much shallower depths along the New Jersey coast during the same measurement period. The Spring 1974 current velocity field yields transport rates of lesser magnitudes relative to the previous fall, with a net down-channel sediment flux at the head of the Hudson Shelf Valley. The New York Harbor entrance appears to be the site of near-continual sediment transport generated by swift tidal currents.

Near-bottom current and particulate concentration data and a theoretical model of sediment resuspension together have led to inferences about the erosional response of a fine-grained sediment to bottom stress. Tidal boundary layer observations, made 5 m from the bottom in the main basin of Puget Sound, Washington, show that currents with speeds exceeding 40 cm/s cause an increase in concentration as much as sixfold from a background level of 1 mg/1. When the erosion rate, E = α|τ|η, is given a power law dependence on bottom stress τ, in accord with laboratory fine-sediment entrainment measurements, the field data and theoretical model together suggest that η = 4.0 and α = 1.7 × 10−6 g/cm2/s. Uncertainty in these values stemming from a variety of factors is discussed. The analysis also points to sediment resuspension of material with relatively large settling velocity (ws = 0.1 cm/s) taking place against a background of much finer particles and a depth of sediment reworking of approximately 0.3 mm under typical strong flow conditions. Comparison of this in situ erosion rate with previous laboratory rate estimates shows it to be comparable to rates measured for some freshwater sediments.

Prototype instrumentation designed to measure light scattering and transmission and the horizontal components of fluid velocity at a point 100 cm off the sea floor was deployed on the Long Island inner shelf for a 27-day period in October and November of 1976. Depth at the point of deployment was 10.5 m. Data were taken from sensors in hourly bursts consisting of 468 consecutive 1-s samples. Measurements taken with the electromagnetic current sensor were converted to absolute units by using a laboratory calibration, while absolute calibration of the turbidimeter in terms of suspended matter concentration was accomplished by using a combination of laboratory and field measurements. The results from the deployment show the effects of water motions at both wave and subtidal frequencies on sediment concentrations. Although burst mean fluid motions during the observation interval were primarily of tidal frequency, a single wind-forced event caused burst mean currents in excess of 38 cm/s. During that same event, suspended particulate concentrations rose to a burst mean of 79 mg/l from a prestorm level of 5 mg/l, an order of magnitude increase occurring over a period of approximately 12 hours. During the storm event, individual burst records show that fluid motions at surface wave frequencies and with amplitudes of over 100 cm/s occurred at the experimental site. The near-bottom suspended matter field at that time was found to have appreciable energy at surface wave frequencies as well. Changes in suspended matter concentration of the order of 130 mg/l were observed to occur in a period of 3–5 s. Increased burst mean concentration was seen to accompany increased wave activity, although increased mean flow with declining wave activity also led to increased suspended particulate concentration. When storm conditions (wave activity and mean flow much above deployment interval ambients) abated, concentrations returned to ambient levels in approximately 12 hours. Coherence between suspended matter concentration and horizontal fluid velocity was found to be statistically significant during four intervals of the deployment. In most cases the significance was centered around a broad frequency band near 0.1 Hz. Vertical near-bottom wave velocities were inferred from the current measurements. The analysis of coherence and phase between the vertical velocity and concentration supports the hypothesis that part of the measured concentration signal was due to the vertical oscillatory advection of the mean concentration field past the turbidimeter during the passage of surface waves.

The solution to the equations of two‐phase flow which describes hindered settling is unstable to small perturbations.

Kinematical effects which hinder settling are due to the upward return flow of the fluid and to the influence of the sediment on the hydrostatic pressure. Dynamical effects of increased drag must be accounted for with a nonlinear drag force law.

A semianalytic model of a time-dependent bottom boundary layer has been constructed in which flow and time-variable eddy viscosity are interdependent. Evaluated in the case of oscillatory forcing at tidal frequencies, the model shows that neglecting time variations in viscosity results in underestimates of maximum bottom stress and distortion of the flow profile near times of flow reversal. Acceleration and variable viscosity also add terms proportional to z In (z/z0) and z to the conventional steady state In (z/z0) profile near the bottom. Inferences based on the logarithmic profile for measurements made in time-dependent boundary layers may consequently yield inaccurate estimates of roughness length and friction velocity. Stress is seen to lag flow at points away from the bottom, as has been observed in measurements, though bottom stress always leads flow aloft. Bottom stress is found to depend linearly on the free-stream velocity in the limit of time-invariant viscosity. In the limit of strongly time-varying viscosity, bottom stress is more nearly quadratic. The friction coefficients are reasonably time independent only when the phase lead θ between bottom stress τB and free-stream velocity U is incorporated into the bottom friction expression. A generalized bottom drag law for oscillatory flow that encompasses all these features takes the form τB = gβ|U(t/T + θ)|β U(t/T + θ), where β has a value between 0 and 1, and θ is typically a few tens of degrees. In the examples evaluated, when β = 0, gβ (conventionally called r) ranges from 2–15×10−2 cm/s, and when β = 1, gβ (conventionally called CD) ranges from 1–5×10−3, the variation in both cases depending on surface Rossby number.

Eighteen near-shore current records within area 11 km/sup 2/ on the Long Island inner shelf have been examined for indication of small-scale topographic influence. In the frictionally influenced near-shore water column, currents over the 39-day records are generally oriented slightly clockwise of the longshore direction toward the principal axis of the local ridge-and-swale topography. This orientation angle is largest and closest to the local topographic axis when semidiurnal tidal currents dominate other flows, a result which may suggest that tidal currents play a role in molding the local topography. Viscous tidal current theory for a constant depth surface described the observed dependence of the semidiurnal (M/sub 2/) tidal current ellipticity and ellipse orientation on depth. The inferred vertical eddy viscosity coefficient ranges from 5 to 35 cm /sup 2//s.

Evidence of large-scale episodic venting of hydrothermal fluids was initially discovered in August 1986 in the form of a 130-km3 radially symmetric “megaplume” over the southern Juan de Fuca Ridge. We report here on the discovery in September 1987 of a second, smaller megaplume about 45 km north of the location of the first megaplume. The 3He/heat, 3He/dissolved Mn, and 3He/dissolved silica ratios in both megaplumes were typical of high-temperature vent fluids. Evidence from long-term records of current flow over the southern Juan de Fuca Ridge, and from the mineralogy and Mn chemistry of megaplume particles, makes it unlikely that the second megaplume was a reencounter of the first. A plume model that relates the heat flux to the observed plume rise height of ∼1000 m finds that the total heat content of the fluids that formed the megaplumes was 1016–1017 J, or equivalently a fluid volume of 3–8 × 107 m3 at 350°C. The geometry and suspended particle population of the first megaplume imply that such features are formed within a few days time. The extraordinary heat and volume fluxes associated with megaplumes (102–103 greater than ordinary vent fields), as well as their typical hydrothermal chemistry, suggest that they resulted from tectonic or hydraulic fracturing that suddenly increased the permeability of the hydrothermal fluid reservoir in the axial crust. The flux of hydrothermal heat from continuous venting and episodic megaplumes on the southern Juan de Fuca Ridge is presently 4 − 10 × 109 W, a factor of 5–10 greater than various geophysical model calculations for this ridge segment. This imbalance may be symptomatic of a recent surge in the local cycle of magmatic activity.