Peter A. Rona

Two New Caridean Shrimps (Bresiliidae) From a Hydrothermal Field on the Mid-atlantic Ridge

ABSTRACT Rimicaris, a new genus, and two new species of caridean shrimps, R. exoculata and R. chacei, are recorded in swarms from a thermally influenced site in the rift valley of the Mid-Atlantic Ridge at 26°08.3N, 44°49.6W, 3,620-3,650 m depth. The shrimps appear to be the predominant macroinvertebrates in heated waters (at least 1.0°C above ambient, 2.37°C away from vents) on black sediments and layered rocks with the appearance of polymetallic sulfides precipitated from black smoker-type vents. Characters of these species indicate placement in the family Bresiliidae which is distributed in surface to mid-ocean depths. A key to the 7 bresiliid genera is given.

Morphology, mineralogy and chemistry of hydrothermal deposits from the TAG area, 26°N Mid-Atlantic Ridge☆

Abstract Metalliferous hydrothermal deposits have been observed and sampled by submersible from the TAG Hydrothermal Field on the Mid-Atlantic Ridge at 26°N. The deposits include surficial metal-rich staining on the surface of carbonate ooze deposits over considerable areas of the sea floor as well as discrete massive layered deposits. The latter include hydrothermal precipitates of Fe-silicate (nontronite), Mn-oxide (birnessite) and Fe-oxide (amorphous). The deposits range in size from less than 1 m to ∼ 15 m × 20 m. The composition and morphology of individual deposits varies from thick laminated crystalline birnessite precipitates, through Fe-rich tubular vents, to loose earthy birnessite, nontronite or amorphous Fe-oxide interlayered deposits. Fractionation and crystallinity are presumably controlled by local Eh conditions and the hydrodynamics of circulation in the upper crust. They are presumed to represent deposition from low-temperature hydrothermal solutions which had originally reacted with basaltic oceanic crust at depth at high temperature and had precipitated metal-rich sulfides within the crust. Occasional short-duration high-temperature black-smoker-type venting is inferred to have occurred based upon Cuue5f8Feue5f8Zn-rich layers in the sediments and observations of clam shells in bottom photographs.

Evidence for high-temperature hydrothermal venting on the Gorda Ridge, northeast Pacific Ocean

Abstract The first water-column survey of the axial valley of the Gorda Ridge, a slow- to medium-rate spreading center within 300 km of the coast of Oregon and California, found strong evidence for ongoing hydrothermal venting. At the northern end of the ridge, anomalously high concentrations of helium-3, dissolved manganese, particulate iron, and methane confirmed the hydrothermal origin of the above-bottom plumes identifiable as maxima in light-attenuation profiles. The presence of excess radon-222 and the highly soluble hydrothermal precipitate anhydrite in the plumes require a local vent source; the precipitation of anhydrite requires fluid temperatures of at least 130°C. Indications of hydrothermal activity elsewhere in the axial valley were inconclusive.

Geologic controls of hydrothermal activity in the Mid-Atlantic Ridge rift valley: Tectonics and volcanics

The rift valley at three widely separated sites along the Mid-Atlantic Ridge is characterized using geological and geophysical data. An analysis of bottom photographs and fine-scale bathymetry indicates that each study area has a unique detailed geology and structure. Spreading rates are apparently asymmetric at each site. Relationships between tectonic and volcanic structure and hydrothermal activity show that various stages in the evolution of the rift valley are most favorable for seafloor expression of hydrothermal activity. In a stage found at 26°08′ N, site 1 (TAG), the rift valley is narrow, consisting of both a narrow volcanically active valley floor and inner walls with small overall slopes. High-temperature hydrothermal venting occurs along the faster spreading eastern inner wall of this U-shaped rift valley. Site 2 (16°46′ N) has a narrow valley floor and wide block faulted walls and is at a stage where the rift valley is characterized by a V-shape. No neovolcanic zone is observed within the marginally faulted, predominantly sedimented floor and hydrothermal activity is not observed. The rift valley at site 3 (14°54′ N), with postulated extrusive volcanic activity and a stage in valley evolution tending toward a U-shape, shows evidence of hydrothermal activity within the slightly faster spreading eastern inner wall. Evidence for tectonic activity (inward- and outward-facing faults and pervasive fissuring) exists throughout the wide inner wall. Hydrothermal activity appears to be favored within a U-shaped rift valley characterized by a narrow neovolcanic zone and secondarily faulted inner walls.

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°u2009N 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.

Acoustic imaging of high-temperature hydrothermal plumes at seafloor spreading centers

We explore the possibility of using active sonar techniques to acoustically image high‐temperature ‘‘black smoker’’ hydrothermal plumes. We examine recent sonar images of a hydrothermal vent field at 11°N on the East Pacific Rise obtained from DSRV Alvin which may show the presence of plumes. For vent fields on the East Pacific Rise estimates are obtained of the minimum detectable concentration of precipitates as a function of the range between the sonar and the plume boundary. These estimates are compared with measured concentrations. Some of the anticipated advantages of acoustic imaging are discussed. We conclude that acoustic imaging techniques have the potential of providing a coherent framework for point sampling of physical and chemical properties of hydrothermal plumes and for determining the dynamics of their injection into the surrounding water mass.

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.

NOAA's VENTS program targets oceanic hydrothermal effects

The VENTS Program was established in 1984 to focus interdisciplinary research of the National Oceanographic and Atmospheric Administration on the oceanic effects of hydrothermal activity along seafloor spreading centers. Since then, the accelerating rate of discoveries in this area of research has resulted in increased recognition of the importance of seafloor hydrothermal venting as a fundamental process for transfer of mass and heat from the Earths interior to its surface—a process that is active intermittently, but not uncommonly, along the entire 60,000 km-long global spreading-center system. As the nations civilian ocean agency, NOAA undertakes research to understand processes that affect the ocean environment. The VENTS programs highest priority is studies that address large-scale hydrothermal chemical and thermal effects on the ocean, or the potential for such effects, that manifest themselves over relatively short time periods—from years to centuries.

Acoustic observation of hydrothermal flows

Acoustic backscatter has been used to observe hydrothermal flows at several sites on the Juan de Fuca Ridge at frequencies of 200 and 330 kHz. In the case of plume discharges (‘‘smokers’’), backscatter from particulates is used to obtain three‐dimensional images and to measure flow via Doppler shift. For diffuse flows, acoustic scintillation of the signal backscattered from the seafloor is detected using ping‐to‐ping correlation. Mapping of diffuse flows using both fixed and moving platforms will be discussed. The magnitude and time lag dependence of ping‐to‐ping correlation have been linked to flow parameters using the Rytov scattering approximation. [Work supported by NSF Grant OCE 98‐18841.]