Russell E. McDuff

Gradients in the composition of hydrothermal fluids from the Endeavour segment vent field: Phase separation and brine loss

Hydrothermal fluid samples collected in 1984, 1987, and 1988 from a large vent field near 47°57′N on the Endeavour segment of the Juan de Fuca Ridge (JFR) have been analyzed for major and minor elements and gases. There are of the order of 100 individual smoker vents on ∼10 large sulfide structures, which are localized along faults and fault intersections across the vent field. Each sulfide structure has a characteristic fluid composition, which varies very little from one vent orifice to the next, or from year to year, on a given structure. However, there are large gradients in fluid composition across the vent field, with endmember chlorinity increasing from ∼255 mmol/kg in the SW to 505 mmol/kg in the NE. End-member concentrations of major elements are well correlated with chlorinity, and endmember volatile concentrations in the lowest chlorinity fluids are approximately twice as high as in the highest chlorinity fluids. The gradients in composition across the vent field and measured vent fluid temperatures >400°C are consistent with supercritical phase separation and loss of brine phase below the seafloor. The factor-of-2 variation in CO2 (and H2S) is larger than expected for loss of a very high-chlorinity brine. Concentrations of iron and manganese are not positively correlated with chlorinity, suggesting that temperature and pH are more important in controlling metal solubility. Elevated ammonia and bromide/chloride ratios indicate that there has been subseafloor interaction between the hydrothermal fluids and organic matter, and high boron concentrations point to a sedimentary source.

Geology of a vigorous hydrothermal system on the Endeavour Segment, Juan de Fuca Ridge

A high-precision, high-resolution geologic map explicitly documents relationships between tectonic features and large steep-sided, sulfide-sulfate-silica deposits in the vigorously venting Endeavour hydrothermal field near the northern end of the Juan de Fuca Ridge. Water depth in the vent field varies from 2220 to 2200 m. Location of the most massive sulfide structures appears to be controlled by intersections of ridge-parallel normal faults and other fracture-fissure sets that trend oblique to, and perpendicular to the overall structural fabric of the axial valley. The fractured basaltic substrate is primarily composed of well-weathered pillow and lobate flows. As presently mapped, the field is about 200 by 400 m on a side and contains at least 15 large (> 1000 m3) sulfide edifices and many tens of smaller, commonly inactive, sulfide structures. The larger sulfide structures are also the most vigorously venting features in the field; they are commonly more than 30 m in diameter and up to 20 m in height. Actively venting sulfide structures in the northern portion of the field stand higher and are more massive than active structures in the southern portion of the field which tend to be slightly to distinctly smaller. Maximum venting temperatures of 375°C are associated with the smaller structures in the southeastern portion of the field; highest-temperature venting fluids from the more massive structures in the northern portion of the field are consistently 20°–30°C lower. Hydrothermal output from individual active sulfide features varies from no flow in the lower third of the edifice to vigorous output from fracture-controlled black smoker activity near the top of the structures. A different type of high temperature venting takes place from the upper sides of the structures in the form of “overflow” from fully exposed, quiescent pools of buoyant 350°C vent water trapped beneath overhanging sulfide-sulfate-silica ledges, or flanges. These flanges are attached to the upper, outer walls of the large sulfide edifices. Two types of diffuse venting in the Endeavour field include a lower temperature 8°–15°C output through colonies of large tubeworms and 25°–50°C vent fluid that seems to percolate through the tops of overhanging flanges. The large size and steep-walled nature of the these structures evidently results from sustained venting in a “mature” hydrothermal system, coupled with dual mineral depositional mechanisms involving vertical growth by accumulation of chimney sulfide debris and lateral growth by means of flange development.

Constrained circulation at Endeavour ridge facilitates colonization by vent larvae

Understanding how larvae from extant hydrothermal vent fields colonize neighbouring regions of the mid-ocean ridge system remains a major challenge in oceanic research. Among the factors considered important in the recruitment of deep-sea larvae are metabolic lifespan, the connectivity of the seafloor topography, and the characteristics of the currents. Here we use current velocity measurements from Endeavour ridge to examine the role of topographically constrained circulation on larval transport along-ridge. We show that the dominant tidal and wind-generated currents in the region are strongly attenuated within the rift valley that splits the ridge crest, and that hydrothermal plumes rising from vent fields in the valley drive a steady near-bottom inflow within the valley. Extrapolation of these findings suggests that the suppression of oscillatory currents within rift valleys of mid-ocean ridges shields larvae from cross-axis dispersal into the inhospitable deep ocean. This effect, augmented by plume-driven circulation within rift valleys having active hydrothermal venting, helps retain larvae near their source. Larvae are then exported preferentially down-ridge during regional flow events that intermittently over-ride the currents within the valley.

Physical characteristics of the Endeavour Ridge hydrothermal plume during July 1988

Abstract We conducted CTD-transmissometer tows from 8 to 26 July, 1988 within 15 km of the central hydrothermal vent site ( ≈ 47°57′N, 129°06′W) on the Endeavour segment of Juan de Fuca Ridge. Anomalies of temperature, salinity and light attenuation reveal possible new vent sites 4 and 8 km northeast and 6 km south of the central vent site. As a result of widespread plume dispersion, background values of potential temperature, salinity and light attenuation below the 1900 m depth exceeded those for “pristine” ambient waters by 0.05°C, 0.05 psu and 0.03 m −1 , respectively. Maximum plume anomalies relative to the background waters were of the order of 0.10°C, 0.010 psu and 0.10 m −1 at core depths of 2000–2050 m. Heat and salt anomalies were detectable more than 5 km from the central vent site whereas light attenuation (particle) anomalies were confined to within 2.5 km of the vent site. Based on the background water property anomalies and moored current meter records, the mean (time-averaged) heat fluxes for the survey region were+2.3(±1.5) × 10 8 W in the along-ridge direction (20°T) and−7.7(±4.7) × 10 8 W in the cross-ridge direction (110°T). Mean along- and cross-ridge salt fluxes were+7(±5)and−25(±15)kg s −1 ; mean particle fluxes were+0.09(±0.06)and−0.29(±0.18)kg s −1 . Estimates of the instantaneous fluxes derived from coincident current and plume measurements indicate that heat fluxes from the central vent field may have been as high as1.2(±0.6)×10 10 W and corresponding particulate fluxes as high as6(±3)kg s −1 .

Hydrothermal vents of Explorer Ridge, northeast Pacific

Abstract The first submersible exploration of Explorer Ridge found extensive hydrothermal fields in relatively old pillow basalts. Massive coalescing spires formed the basis for 25 m high mounds atop which chimneys emit grey ‘moke’ and water over 300°C. These sulfides are among the largest such ocean deposits found to date. Three types of vents were found: (1) abiotic iron- and zinc-rich vents; (2) low temperature biotic vents; and (3) high temperature H 2 S-rich vents. Coordinated suites of water, rock and animal samples indicated a basic similarity to vents on Juan de Fuca Ridge while demonstrating a wide range of variation.

Mapping the fluid flow of the Mariana Mounds ridge flank hydrothermal system: Pore water chemical tracers

We present a conceptual model of fluid circulation in a ridge flank hydrothermal system, the Mariana Mounds. The model is based on chemical data from pore waters extracted from piston cores and from push cores collected by deep-sea research vessel Alvin in small, meter-sized mounds situated on a local topographic high. These mounds are located within a region of heat flow exceeding that calculated from a conductive model and are zones of strong pore water upflow. We have interpreted the chemical data with time-dependent transport-reaction models to estimate pore water velocities. In the mounds themselves pore water velocities reach several meters per year to kilometers per year. Within about 100 m from these zones of focused upflow velocities decrease to several centimeters per year up to tens of centimeters per year. A larger area of low heat flow surrounds these heat flow and topographic highs, with upwelling pore water velocities less than 2 cm/yr. In some nearby cores, downwelling of bottom seawater is evident but at speeds less than 2 cm/yr. Downwelling through the sediments appears to be a minor source of seawater recharge to the basaltic basement. We conclude that the principal source of seawater recharge to basement is where basement outcrops exist, most likely a scarp about 2–4 km to the east and southeast of the study area.

Flow rate perturbations in a black smoker hydrothermal vent in response to a mid-ocean ridge earthquake swarm

Although there is indirect evidence for strong connections between tectonic processes and mid-ocean ridge hydrothermal flow, there are no direct observations of these links, primarily because measuring flow in these systems is difficult. Here we use an optical analysis technique to obtain a 44 day record of flow rate changes in a black smoker vent in the Main Endeavour field of the Juan de Fuca Ridge. We show that variations in the flow rate coincide with an earthquake swarm observed using an ocean bottom seismometer array. These observations indicate that connections between tectonics and flow are indeed strong, that hydraulic connections within this hydrothermal system are long ranging, and that enhanced tidal pumping of fluids may be initiated by earthquake activity. Because the effects of the swarm cross over an intervening vent field, we infer that the upflow zones feeding this field are narrow. Using the time lag between the swarm onset and the flow rate changes we estimate that the bulk permeability of the crust on the Endeavour segment ranges from 3.0 × 10−13 m2 to 6.0 × 10−12 m2.

Locating hydrothermal vents by detecting buoyant, advected plumes

An improved method of detecting buoyant hydrothermal plumes and locating their source vents is introduced. Plumes are detected by computing fluid stability from conductivity, temperature, and depth measurements acquired during navigated, towed, vertically oscillating casts over the Endeavour Segment of the Juan de Fuca Ridge. For each instability detected, the maximum range to its hydrothermal source is estimated by multiplying a theoretical plume equilibration time by a measured current velocity. Using an estimate of current direction, the method reliably locates plume sources where they are known to exist: in all of the focused vent fields mapped by submersible and in several isolated, diffuse flow sites. The method generates a distribution of hydrothermal sources that is more consistent with variations in surface permeability than with circulation cells spaced evenly along a uniformly permeable axis. Axial instabilities are nearly continuous along the heavily fissured and fractured western wall of the axial valley. Beyond the axial valley, instabilities evidence that hydrothermal upflow penetrates the outer slopes of the ridge crest, probably along a boundary between sheet and pillow flows.

Porosity of the upper edifice of Axial Seamount

Seamounts are not solid basalt structures, but have relatively high porosities in their upper crustal sections. At Axial Seamount, off the coast of Oregon, United States, we used on-bottom gravity measurements with a Bell gravity meter within deep-sea submersible Alvin to determine a porosity of 31% for the uppermost ∼100 m of the edifice. The southwestern caldera wall has a porosity of 22% and the caldera floor has a slightly higher porosity of 33%. Seafloor observations and models indicate that these high porosities result from large-scale structural features such as lava tubes and cracks, large lava drain backs, or regions of open pillow basalts in the near subsurface. These high-porosity zones can affect the subsurface permeability, and models of hydrothermal upflow zones explain observed localized gravity anomalies. The variety of hydrothermal alteration, hydrothermally active areas, and open porous features appears to be related to the high porosity that is inferred from geophysical measurements on this active seafloor volcano.

Scientific Rationale for Establishing Long-Term Ocean Bottom Observatory/Laboratory Systems

The oceanographic community is in a position scientifically and technologically to initiate programs leading to the installation of one or more permanently instrumented observatory/laboratory complexes on submarine spreading centers. The dynamic nature of these systems is well established. Yet, there has been no long term, inter-disciplinary effort focused on specific sites to document rates of change in system components, nor the interactions linking the physical, chemical, and biological processes involved. The ultimate goal of this natural laboratory approach would be to establish, then model, the temporal, and the spatial, co-variation among the active processes involved in generation and aging of 60 percent of the planetary surface. The technological and intellectual stimulation involved in successful implementation of natural seafloor laboratories will provide a new generation of dynamically-based, quantitatively testable models of ocean lithosphere genesis and of the biological and chemical consequences of its formation.