Eighty per cent of all mid-ocean spreading centres are slow. Using a mixture of global bathymetry data and ship-board multibeam echosounder data, we explore the morphology of global mid-ocean ridges and compare two slow spreading analogues: the Carlsberg Ridge in the north-west Indian Ocean between 57°E and 60°E, and the Kane to Atlantis super-segment of the Mid-Atlantic Ridge between 21°N and 31°N. At a global scale, mid-ocean spreading centres show an inverse correlation between segment length and spreading rate with segmentation frequency. Within this context, both the Mid-Atlantic Ridge super-segment and Carlsberg Ridge are similar: spreading at 22 and 26 mm/yr full rates respectively, being devoid of major transform faults, and being segmented by dextral, non-transform, second-order discontinuities. For these and other slow spreading ridges, we show that segmentation frequency varies inversely with flank height and ridge axis depth. Segments on both the Mid-Atlantic Ridge super-segment and Carlsberg Ridge range in aspect ratio (ridge flank height/axis width), depth and symmetry. Segments with high aspect ratios and deeper axial floors often have asymmetric rift flanks and are associated with indicators of lower degrees of melt flux. Segments with low aspect ratios have shallower axial floors, symmetric rift flanks, and evidence of robust melt supply. The relationship between segmentation, spreading rate, ridge depth and morphology, at both a global and local scale, is evidence that rates of melting of the underlying mantle and melt delivery to the crust play a significant role in determining the structure and morphology of slow spreading mid-ocean ridges.

The overall heat transfer through an active hydrothermal sulfide edifice, in particular the North Tower of Grotto, has been estimated at 80MW or greater based on the following in situ measurements and assumptions: (1) The heat transfer by diffuse flow is estimated at 33–380MW based on extrapolating the acoustically mapped area to all sides of the North Tower (“visible” area=30m2; extrapolated area=100m2) and using the range of available spot measurements of temperature (6–23°C) and vertical velocity (0.07–0.28m/s). The lower number (33MW) is more likely, but there is insufficient knowledge of the temporal and spatial variability of diffuse flow to be certain. (2) The heat transfer by focused flow is estimated at 30–70MW based on summing the estimated individual rates of heat transfer for 4 out of 7 documented black smokers and flanges. (3) Conductive heat transfer out of the mound is unknown, but is likely to be much less than the advective heat transfer. Additionally, the plume transport of heat is estimated at 20–40MW based on the direct measurement of temperature within the plume (at 5–25m above the top of the edifice). Despite uncertainties, the lower estimate of plume versus smoker heat transfer suggests that heat transfer is dominantly by diffuse flow. Furthermore, not all plumes from individual smokers may merge even for so small an area as the North Tower of Grotto.

Hudson Canyon is the largest shelf-sourced canyon system off the east coast of the United States, and hosts a productive ecosystem that supports key fisheries. Here we report the results of a multi-year interdisciplinary study of the geological, geochemical, and physical oceanographic features and processes in the canyon that underpin that ecosystem. High-resolution multi-beam bathymetric and backscatter data show that the contrasting morphology of the two perpendicularly oriented branches at the head of the Hudson Canyon is indicative of different states of geomorphological activity and sediment transport. Tightly spaced ridges and gullies extend perpendicularly towards the canyon axis from the canyon walls. Numerous depressions are found at the base of the canyon walls or along the canyon axis at depths from 300m to 600m. Elevated concentrations of dissolved methane in the water column, where the highest density of depressions occur, suggests that methane is actively venting there. The topography and reflective floors of circular depressions in canyon walls and their association with methane maxima suggest that these represent active methane gas release-collapse pockmarks with carbonate floors. Patterns of irregular, low-relief, reflective depressions on the canyon floor may also represent methane release points, either as gas release or cold-seep features. The presence of methane maxima in a region of strong advective currents suggests continuous and substantial methane supply. Hydrographic observations in the canyon show that multiple layers of distinct inter-leaved shelf (cold, fresh) and slope (warm, salty) water masses occupy the head of the canyon during the summer. Their interactions with the canyon and with each other produce shifting fronts, internal waves, and strong currents that are influenced by canyon topography. Strong tidal currents with along-canyon-axis flow shear help to drive the advection, dispersion and mixing of dissolved materials in the water column that likely help support the rich canyon ecosystem.

Mapping of physical benthic habitats at the head of Hudson Canyon was performed by means of integrated analysis of acoustic data, video surveys and seafloor sampling. Acoustic mapping, performed using AUV-mounted multibeam sonar, provided ultra-high resolution bathymetric and backscatter imagery for the identification of geomorphological features and the characterization of surficial sediments. Habitat characterization in terms of seafloor texture and identification of benthic and demersal communities was accomplished by visual analysis of still photographs from underwater vehicles. Habitat classes were defined on the basis of the seafloor texture observed on photos and then compared with the geophysical data in order to associate habitats to acoustic classes and/or geomorphological features. This enabled us to infer habitat distribution on the basis of morpho-acoustic classes and extrapolate results over larger areas. Results from bottom trawling were used to determine the overall biodiversity within the identified habitats. Our analysis revealed a variety of topographic and sedimentological structures that provide a wide range of physical habitats. A variety of sandy and muddy substrates, gravel patches and mudstone outcrops host rich and varied faunal assemblages, including cold-water corals and sponge communities. Pockmark fields below 300m depth suggest that methane-based chemosynthetic carbonate deposition may contributes to creation of specific benthic habitats. Hummocky terrain has been delineated along the canyon rims and associated with extensive, long-term burrowing activity by golden tilefish (Lopholatilus chamaeleonticeps). These results show the relationships of physical features to benthic habitat variation, support the notion of the area as a biodiversity hotspot and define essential habitats for planning of sustainable regional fisheries.

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Continuous time-series observations are key to understanding the temporal evolution of a seafloor hydrothermal system and its interplay with thermal and chemical processes in the ocean and Earth interior. In this paper, we present a 26-month time series of the heat flux driving a hydrothermal plume on the Endeavour Segment of the Juan de Fuca Ridge obtained using the Cabled Observatory Vent Imaging Sonar (COVIS). Since 2010, COVIS has been connected to the North East Pacific Time-series Underwater Networked Experiment (NEPTUNE) observatory that provides power and real-time data transmission. The heat flux time series has a mean value of 18.10 MW and a standard deviation of 6.44 MW. The time series has no significant global trend, suggesting the hydrothermal heat source remained steady during the observation period. The steadiness of the hydrothermal heat source coincides with reduced seismic activity at Endeavour observed in the seismic data recorded by an ocean bottom seismometer from 2011 to 2013. Furthermore, first-order estimation of heat flux based on the temperature measurements made by the Benthic and Resistivity Sensors (BARS) at a neighboring vent also supports the steadiness of the hydrothermal heat source.

Aegirine is a Na-pyroxene well known for exhibiting bright green colour under the microscope, or else green to light brown pleochroism within the aegirine-augite solid-solution. We report the occurrence of aegirine from metamorphic rocks associated to the Bayan Obo REE-Fe-Nb ore deposits (Inner Mongolia), which displays a peculiar optical characteristic, being colourless in thin section. Detailed compositional data for the Bayan Obo aegirine and for aegirine and aegirine-augite from nepheline syenites from the Boavista Island (Cape Verde Archipelago), and from other geological settings, reveal that the Bayan Obo aegirine is characterised by significantly lower Ti contents ( 2 ) with respect to the aegirine from other occurrences. Besides the colour effects, normally ascribed to Fe 2+ -related crystal field transitions and to Fe 2+ –Fe 3+ charge transfer transitions, our chemical and spectrophotometric data suggest that Fe 2+ –Ti 4+ charge transfer transitions may be the main colour-enhancer mechanism for the typical deep colour of aegirine.

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 approximate to 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 approximate to 7 cm s(-1), speeds which can bend a plume more than 45 degrees 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 approximate to 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 ((EFF)) that varies from approximate to 0.11 to approximate to 0.025 with increasing height (2-76 m) above the source.

The Cabled Observatory Vent Imaging Sonar (COVIS) was deployed at the Main Endeavour Field node of the Canadian NEPTUNE cabled observatory in September 2010 and has acquired long time series on plume and diffuse hydrothermal flows. This talk will focus on recent efforts by the Rutgers-APL collaboration to invert sonar data to determine heat flux from the Grotto plume complex. Inversion employs plume theory to relate velocity as determined by Doppler shift to buoyancy flux, hence heat flux. The primary uncertainties have to do with plume bending due to ambient current and short sampling times relative to dynamic changes in plume shape. These uncertainties have been quantified by means of special high-statistics experiments using COVIS. Time series for heat flux will be compared with ground truth obtained by thermometry using an ROV. [Work supported by NSF Grants OCE-0824612 and OCE-0825088.]

Deep-sea hydrothermal vents are conduits of heat and chemicals from the lithosphere to the hydrosphere. Their plumes rise hundreds of meters from the seafloor into the water column; during their ascent, they entrain ambient water and are subject to cross flows. Source fluxes can vary in time, partly in response to seismic and magmatic events. Long-term measurements of physical properties of hydrothermal plumes provide windows to conditions within Earth's interior. This article provides examples of long-term measurements of acoustic scattering recorded along Endeavour Segment of the Juan de Fuca Ridge. Acoustic backscatter data from particles and temperature fluctuations provide information on width, shape, and vertical velocity in the plumes from which entrainment and upward transport are estimated. Acoustic forward scatter by turbulence within the plume gives time series of the plume's path-averaged vertical velocity and temperature fluctuations and exhibits variability that is dependent on the ambient horizontal flow. At several vents, direct measurements of heat flux using in situ devices and video imagery have yielded an integrated heat output for various sulfide structures. In conjunction with these measurements, plume models that incorporate ambient stratification and horizontal tidal flows are yielding insights into the vertical and azimuthal dependence of entrainment, rise-height variability, and plume bending.

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We employ the classical problem of an inclusion in an elastic half‐space to model effects of sub‐surface serpentinization on crustal deformation, change of stress state, and surface uplift. At the TAG hydrothermal field on the Mid‐Atlantic Ridge, the model suggests that an anomalous salient 3 km in diameter and 100 m high that projects 3.5 km westward from the east valley wall may have resulted from a relatively deep‐seated well‐serpentinized body exhibiting transformational strain. The associated large strains would likely result in sub‐surface fracturing or faulting, but surface uplift may be relatively insensitive to the exact depth and shape of the serpentinized region. Serpentinization of a region beneath the footwall of the TAG detachment fault will tend to promote slip along some overlying normal faults, which may then enhance fluid pathways to the deeper crust to continue the serpentinization process. Our solution for the Miyazaki Plain above the Kyushu‐Palau subduction zone in SW Japan explains the observed uplift of ≈120 m. The small transformational strains associated with serpentinization in this region may promote thrust‐type events in the aseismic slip zone near the upper boundary of the subducting Philippine Sea Plate as well as intraplate earthquakes associated with normal faulting. The rate of serpentinization needed to produce the observed uplift at the Miyazaki Plain is significantly greater than that needed at TAG, though significantly smaller on per unit volume basis. Thermal effects of serpentinization in both regions appear to be small, but uplift data provide an additional constraint on inferring serpentinization from geological and seismological observations.

A cabled observatory vent imaging sonar (COVIS) has been developed to provide plume and Doppler imaging of hydrothermal vents and surrounding diffuse flow. The system was designed to be compatible with the power and data interface standards of the Neptune Canada cabled observatory. COVIS is a 4 m tall, titanium tripod employing a Reson 7125 multibeam sonar. The sonar transducers are positioned by a motor-driven three degree of freedom rotation system (pitch, roll, and yaw). A 400 kHz, 1 × 128 deg fan-beam projector is used with a receiver array that forms 256 beams having horizontal width 0.5 deg and covering a 128 deg azimuthal sector. Volumetric imaging of plumes is generated as the transducer array is scanned in 1 deg pitch steps. Doppler measurements of flow velocity over a 3-D grid are also derived. A 200 kHz, 28 × 128° broad beam projector is used to image the diffuse areas near the base of the hydrothermal vent edifices. Software allows for the creation of complex, arbitrary, autonomously executed experiments that control all aspects of the sonar and rotation system. COVIS was successfully deployed in September 2010. The design of COVIS provides insights relevant to future cabled acoustic systems. [Work supported by NSF.]

Hydrothermal plumes venting from black smokers and diffuse flow discharging from the surrounding area of the seafloor are important as agents of transfer of heat, chemicals, and biological material from the crust into the ocean in quantitatively significant amounts [Elderfield and Schultz, 1996]. An unprecedented time series of three‐dimensional (3‐D) volume images of plumes rising tens of meters from black smoker vents and of concurrent 2‐D maps of diffuse flow discharging from surrounding areas of the seafloor illuminates the turbulent behavior of hydrothermal fluid transfer into the ocean (seeFigure 1).

The Cabled Observatory Vent Imaging Sonar has been deployed at the Main Endeavour Node of the Canadian Neptune cabled observatory and has acquired data on plume and diffuse hydrothermal flows. Based on the Reson 7125 multibeam sonar and operating at 200 and 400 kHz, two-dimensional and three-dimensional time series are produced using plume backscattering, Doppler shift, and acoustic scintillation. Hydrothermal plumes and diffuse flow are important as agents of transfer of heat, chemicals, and biological material from the mantle and crust into the ocean in quantitatively significant amounts. High-frequency sonar measurements offer the possibility of inversion to obtain fluxes of central importance in these processes. Long-term time series, obtainable in cabled systems, allow observations of hydrothermal response to tidal, tectonic, and volcanic forcing. Examples will be given of plume bending due to currents, determination of entrainment of ambient water, time variation of diffuse flows, and Doppler determination of volume flux. [Work supported by NSF Grants Nos. OCE-0824612 and OCE-0825088.]

The development of seafloor hydrothermal research has followed a classic scientific progression in which discoveries were initially interpreted as special cases until further exploration revealed their more general significance. The first high-temperature seafloor hydrothermal system was found at the Atlantis II Deep of the slow spreading Red Sea in 1963. At that time, the hydrothermal activity was largely discounted as an anomaly associated with continental rifting rather than as part of an early stage of opening of an ocean basin that could continue with the development of ocean ridges as in the Atlantic. When high-temperature black smoker hydrothermal venting was found on the East Pacific Rise in 1979, the scientific consensus then held that the relatively high rate of magma supply at intermediate to fast spreading rates was required for such activity. Accordingly, high-temperature hydrothermal activity could not occur on the slow spreading half of the global ocean ridge system. High-temperature black smokers like those on the East Pacific Rise were first discovered on a slow spreading ocean ridge at the TAG hydrothermal field on the Mid-Atlantic Ridge in 1985. The scientific consensus then ruled out the possibility for such activity on the ultraslow portion of the ocean ridge system. Plumes indicative of active high-temperature black smokers were found on the ultraslow spreading Gakkel Ridge in the Arctic in 2001, and active black smokers were found on the Southwest Indian Ridge in 2006. A diversity of high-temperature hydrothermal systems remains to be found on ocean ridges, particularly at slow spreading rates.