Karen G. Bemis

Production of small volcanoes in the Superswell region of the South Pacific

Abstract We use Sea Beam swath bathymetry and wide beam profiler data to investigate the production of small (50–700 m high) volcanoes in the region of the Superswell [1] in French Polynesia. Seamount population parameters are estimated for three study regions. We find that seamount abundances increase threefold going southward across the Marquesas fracture zone, the inferred northern boundary of the Superswell. In addition, we document that south of the Marquesas fracture zone, seamount abundances, on average, are equally as high on young crust (0–18 My) generated at the East Pacific Rise (9°–22°) as on the adjacent Superswell region ( ∼ 20–60 My old crust). The fact that seamount abundance estimates are the same in the two regions south of the Marquesas fracture zone leads to an uncertainty in whether the seamounts have been produced by ridge or off-axis processes, and suggests the following possibilities: (1) processes at the East Pacific Rise have produced more small seamounts south of the fracture zone than to the north for the last 60 My, (2) off-axis processes (possibly related to the Superswell) have led to excess seamount volcanism on both the Superswell region and the region adjacent to the East Pacific Rise, or (3) off-axis seamount production is anomalously high on the Superswell, and the production of seamounts has increased at the East Pacific Rise since 20 My.

Acoustic imaging, visualization, and quantification of buoyant hydrothermal plumes in the Ocean

We develop and apply visualization and quantification methods to reconstruct hydrothermal plumes in 3D from acoustic images and to make the first direct measurements from the reconstructions of scalar properties that describe the behavior of two buoyant plumes discharging from adjacent black smoker chimneys. The actual behavior is then compared to that predicted by a classic simple buoyant plume model. The images are reconstructed as isointensity surfaces of backscatter from particulate matter suspended in the plumes. The measurements pertinent to the role of the plumes as agents of dispersal of heat and mass into the ocean include change with height of diameter, particle distribution, dilution, centerline attitude, surface protrusions, and connectivity. The protrusions are the surface expression of eddies and appear to follow a bifurcating helical flow pattern that resemble simulation of the naturally forced flow of coherent vortex rings as the eddies rise with the buoyant plume. These direct measurements and the derived entrainment coefficient are generally consistent with behavior predicted by the simple buoyant plume model and support engulfment by vortex shedding as a primary mechanism for entrainment of surrounding seawater. Deviations from predicted buoyant plume behavior are diagnostic of particle dynamics.

A comparison of black smoker hydrothermal plume behavior at Monolith Vent and at Clam Acres Vent Field: Dependence on source configuration

Quantitative visualization of acoustic images is used to compare the properties and behavior of high temperature hydrothermal plumes at two sites with different source configurations, increasing our understanding of how plume behavior reflects source configuration. Acoustic imaging experiments were conducted at the Clam Acres area of the Southwest Vent Field, 21° N East Pacific Rise and at Monolith Vent, North Cleft segment, Juan de Fuca Ridge. At Clam Acres, black smokers discharge from two adjacent chimneys which act as point sources, whereas multiple vents at Monolith Vent define a distributed elliptical source. Both plumes exhibit consistent dilution patterns, reasonable fits to the expected power law increase in centerline dilution with height, and simple bending of plume centerlines in response to ambient currents. Our data suggest that point source vents are associated with ordered plume structure, normal entrainment rates, and initial expansion of isosurfaces while distributed source vents are associated with disorganized plume structure, variable entrainment rates, and initial contraction of isosurfaces.

Acoustic mapping of diffuse flow at a seafloor hydrothermal site : Monolith Vent, Juan de Fuca Ridge

Diffuse flow of hydrothermal solutions commonly occurs in patchy areas up to tens of meters in diameter in seafloor hydrothermal fields. It is recognized as a quantitatively significant component of thermal and chemical fluxes, yet is elusive to map. We report a new acoustic method to detect and map areas of diffuse flow using phase-coherent correlation techniques. The sonar system was modified to record phase information and mounted on DSV SEA CLIFF. The submersible occupied a stationary position on the seafloor and the transducer scanned the seafloor surrounding Monolith Vent, a sulfide edifice venting black smokers, at a nominal range of 17 m at a depth of 2249 m on the Juan de Fuca Ridge. Patchy areas of uncorrelated returns clearly stood out from a background of returns that exhibited ping-to-ping correlation. The areas of uncorrelated returns coincided with areas of diffuse flow as mapped by a video survey with the Navys Advanced Tethered Vehicle (ATV). Correlated returns were backscattered from invariant seafloor. Uncorrelated returns were distorted by index of refraction inhomogeneities as they passed through diffuse flow between the seafloor and the transducer. The acoustic method presented can synoptically map areas of diffuse flow. When combined with standard in situ measurement and sampling methods the acoustic mapping will facilitate accurate determination of diffuse thermal and chemical fluxes in seafloor hydrothermal fields.

Case study: a methodology for plume visualization with application to real-time acquisition and navigation

Applications of visualization techniques that facilitate comparison of simulation and field datasets of seafloor hydrothermal plumes are demonstrated in order to explore and confirm theories of plume behavior. In comparing these datasets, there is no one-to-one correspondence. We show the comparison by performing quantitative capturing of large scale observable features. The comparisons are needed not only to improve the relevance of the simulations to the field observations, but also to enable real time adjustment of shipboard data collection systems. Our approach for comparing simulation and field datasets is to use skeletonization and centerline representation. Features representing plumes are skeletonized. Skeleton points are used to construct a centerline and to quantify plume properties on planes normal to the centerline. These skeleton points are further used to construct an idealized cone representing a plume isosurface. The difference between the plume feature and the cone is identified as protrusions of turbulent eddies. Comparison of the simulation and field data sets through these abstractions illustrates how these abstractions characterize a plume.

Generating Realistic Images from Hydrothermal Plume Data

Most data used in the study of seafloor hydrothermal plumes consists of sonar (acoustic) scans and sensor readings. Visual data captures only a portion of the sonar data range due to the prohibitive cost and physical infeasibility of taking sufficient lighting and video equipment to such extreme depths. However, visual images are available from research dives and from the recent IMAX movie, volcanoes of the deep sea. In this application paper, we apply existing lighting models with forward scattering and light attenuation to the 3D sonar data in order to mimic the visual images available. These generated images are compared to existing visual images. This can help the geoscientists understand the relationship between these different data modalities and elucidate some of the mechanisms used to capture the data.

Activity Detection in Scientific Visualization

For large-scale simulations, the data sets are so massive that it is sometimes not feasible to view the data with basic visualization methods, let alone explore all time steps in detail. Automated tools are necessary for knowledge discovery, i.e., to help sift through the data and isolate specific time steps that can then be further explored. Scientists study patterns and interactions and want to know when and where interesting things happen. Activity detection, the detection of specific interactions of objects which span a limited duration of time, has been an active research area in the computer vision community. In this paper, we introduce activity detection to scientific simulations and show how it can be utilized in scientific visualization. We show how activity detection allows a scientist to model an activity and can then validate their hypothesis on the underlying processes. Three case studies are presented.

Acoustics advances study of sea floor hydrothermal flow

Sub-sea floor hydrothermal convection systems discharge as plumes from point sources and as seepage from the ocean bottom. The plumes originate as clear, 150–400°C solutions that vent from mineralized chimneys; precipitate dissolved metals as particles to form black or white smokers as they turbulently mix with ambient seawater; and buoyantly rise hundreds of meters to a level of neutral density where they spread laterally. The seepage discharges from networks of fractures at the rock-water interface as clear, diffuse flow, with lower temperatures, metal contents, and buoyancy than the smokers. The diffuse flow may be entrained upward into plumes, or laterally by prevailing currents in discrete layers within tens of meters of the sea floor. The role of these flow regimes in dispersing heat, chemicals, and biological material into the ocean from sub-sea floor hydrothermal convection systems is being studied on a global scale.

The relative effect of particles and turbulence on acoustic scattering from deep sea hydrothermal vent plumes revisited

The relative importance of suspended particles and turbulence as backscattering mechanisms within a hydrothermal plume located on the Endeavour Segment of the Juan de Fuca Ridge is determined by comparing acoustic backscatter measured by the Cabled Observatory Vent Imaging Sonar (COVIS) with model calculations based on in situ samples of particles suspended within the plume. Analysis of plume samples yields estimates of the mass concentration and size distribution of particles, which are used to quantify their contribution to acoustic backscatter. The result shows negligible effects of plume particles on acoustic backscatter within the initial 10-m rise of the plume. This suggests turbulence-induced temperature fluctuations are the dominant backscattering mechanism within lower levels of the plume. Furthermore, inversion of the observed acoustic backscatter for the standard deviation of temperature within the plume yields a reasonable match with the in situ temperature measurements made by a conductivity-temperature-depth instrument. This finding shows that turbulence-induced temperature fluctuations are the dominant backscattering mechanism and demonstrates the potential of using acoustic backscatter as a remote-sensing tool to measure the temperature variability within a hydrothermal plume.

Acoustic imaging and visualization of plumes discharging from black smoker vents on the deep seafloor

Visualization and quantification methods are being developed to analyze our acoustic images of thermal plumes containing metallic mineral particles that discharge from hot springs on the deep seafloor. The acoustic images record intensity of backscattering from the particulate matter suspended in the plumes. The visualization methods extract, classify, visualize, measure and track reconstructions of the plumes, depicted by isointensity surfaces as 3D volume objects and 2D slices. The parameters measured, including plume volume, cross sectional area, centerline location (trajectory), surface area and isosurfaces at percentages of maximum backscatter intensity, are being used to derive elements of plume behavior including expansion with height, dilution, and mechanisms of entrainment of surrounding seawater. Our aim is to compare the observational data with predictions of plume theory to test and advance models of the behavior of hydrothermal plumes through the use of multiple representations.