Diffuse Venting Research Articles

Seafloor hydrothermal systems

The discovery of seafloor hydrothermal systems approximately two decades ago has led to a major reassessment of the Earth's thermal and geochemical budgets and has revolutionized our understanding of biological processes. This review traces the development of the study of seafloor hydrothermal systems from the indirect evidence provided by conductive heat flow anomalies to the discovery of ≈ 350°C black smoker vents on the East Pacific Rise at 21°N. Although the review focuses on physical characteristics and processes, it outlines some key characteristics of vent fluid chemistry that provide constraints on physical models. Ridge crest systems have thermal power outputs ranging from 10 to 104 MW. They are transient systems, driven by magmatic heat sources, but episodic events such as megaplumes, the interplay between focused and diffuse venting, and other aspects related to their thermal, chemical, and biological evolution remain poorly understood. Advances will be made by continuing exploration and discovery to determine the full range of possible phenomena both on and off axis and in different tectonic settings. In order to understand the complete, integrated ridge system, however, future studies must include long‐term monitoring of an active system, deep drilling into the reaction zone, and mathematical modeling that incorporates both physical and chemical constraints.

Effluent from diffuse hydrothermal venting: 2. Measurement of plumes from diffuse hydrothermal vents at the southern Juan de Fuca Ridge

An array of temperature and velocity sensors was deployed for two weeks in the bottom boundary layer at the Southern Juan de Fuca Ridge during the National Oceanic and Atmospheric Administration VENTS'90 field experiment. The instruments were placed close to the seafloor to detect effluent from diffuse hydrothermal sources located 1–2 km distant. Over the duration of the field deployment, we detected effluent from two vent sources. The plume from the larger of the two sources was sensed at all of the instrument locations, permitting estimates of dilution based on the field measurements. Our data also permitted estimation of energy flux from diffuse vents (approximately 125±75 MW from a source to the north of the instrument array, and 12±8 MW from a small source to the east). The largest source of error in the data was estimation of the plume width. We have described a sensor array that would reduce this error in future work. Such an array would be suitable for a long‐term vent activity monitor.

Effluent from diffuse hydrothermal venting: 1. A simple model of plumes from diffuse hydrothermal sources

This paper focuses on modeling the fate of effluent from diffuse seafloor hydrothermal activity after it has been vented into the water column. The model was formulated using a number of simplifying assumptions which permit direct application of this model to field measurements. I have limited the configurations to those where the hydrothermal outflow velocities are smaller than horizontal current. I assume that the entrainment of ambient seawater into the plume is constant over the length of the plume. This permits formulation of a first‐order relation for the rise height and dilution in a diffuse hydrothermal plume as a function of downstream distance. The analytic model is compared with a simple laboratory simulation of the hydrothermal flow. The results suggest that diffuse hydrothermal effluent will penetrate to a height in the water column that is proportional to the overall dimension of the diffuse vent patch, multiplied by a dimensionless plume intensity parameter. I also show relations for plume dilution which will be compared with field data in part 2 of this work.

Geochemistry of north Cleft segment vent fluids: Temporal changes in chlorinity and their possible relation to recent volcanism

Hydrothermal vent fluids from the North Cleft segment of the Juan de Fuca Ridge between 44°54′ and 45°00′N were sampled in 1988, 1990, 1991, and 1992. In 1988, chloride‐depleted (relative to seawater) diffuse fluids with low dissolved metal concentrations (relative to other mid‐ocean ridge (MOR) fluids) were sampled over a distance of ∼10 km along axis. In 1990 and 1991, both high‐temperature (>200°C) and diffuse vents were consistently metal‐ and chloride‐enriched. The end‐member compositions of high‐temperature vents and nearby diffuse vents were very similar. There were small but significant correlated changes in diffuse and high‐temperature composition from 1990 to 1991. The large‐scale but temporary venting of low‐chlorinity fluids over the entire North Cleft segment provides a resolution to the apparent mass imbalance implied by observations of continuous venting of chloride‐enriched fluids. It is possible that a volcanic event along the North Cleft segment prior to 1987, for which there is firm geologic evidence, initially caused a boiling event which resulted in the preferential venting of vapor‐enriched fluids through 1988, followed by a transition to brine‐enriched fluids by 1990. High iron, low sodium, and low Sr/Ca ratios in the high‐chlorinity fluids suggest that the brine phase has continued to react and approach reequilibration with an alteration mineral assemblage after the phase separation event. The absence of chloride‐depleted fluids from 1990 onward, and the systematics of lithium, boron, and manganese with chloride in the high‐temperature fluids from North Cleft suggest that the evolution toward lower chlorinity at Monolith vent from 1990 to 1992 is caused by progressive dilution of a brine with hydrothermal seawater.

A method for quantitatively estimating diffuse and discrete hydrothermal discharge

Submarine hydrothermal fluids discharge as undiluted, high-temperature jets and as diffuse, highly diluted, low-temperature percolation. Estimates of the relative contribution of each discharge type, which are important for the accurate determination of local and global hydrothermal budgets, are difficult to obtain directly. In this paper we describe a new method of using measurements of hydrothermal tracers such as Fe/Mn, Fe/heat, and Mn/heat in high-temperature fluids, low-temperature fluids, and the neutrally buoyant plume to deduce the relative contribution of each discharge type. We sampled vent fluids from the north Cleft vent field on the Juan de Fuca Ridge in 1988, 1989 and 1991, and plume samples every year from 1986 to 1991. The tracers were, on average, 3 to 90 times greater in high-temperature than in low-temperature fluids, with plume values intermediate. A mixing model calculates that high-temperature fluids contribute only ∼ 3% of the fluid mass flux but > 90% of the hydrothermal Fe and > 60% of the hydrothermal Mn to the overlying plume. Three years of extensive camera-CTD sled tows through the vent field show that diffuse venting is restricted to a narrow fissure zone extending for 18 km along the axial strike. Linear plume theory applied to the temperature plumes detected when the sled crossed this zone yields a maximum likelihood estimate for the diffuse heat flux of8.9 × 10 4 W/m, for a total flux of 534 MW, considering that diffuse venting is active along only one-third of the fissure system. For mean low- and high-temperature discharge of 25°C and 319°C, respectively, the discrete heat flux must be 266 MW to satisfy the mass flux partitioning. If the north Cleft vent field is globally representative, the assumption that high-temperature discharge dominates the mass flux in axial vent fields leads to an overestimation of the flux of many non-conservative hydrothermal species by about an order of magnitude.

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.

Temperature and sound-speed measurements from diffuse hydrothermal vent plumes at the Southern Juan de Fuca Ridge

An array of sensors was deployed for 2 weeks at the Southern Juan de Fuca Ridge during the NOAA VENTS’90 field experiment. The instrument measured temperature at six heights on each of four moorings, and temperature, velocity, sound speed, and Reynolds’ stress on each of two tripods. The instruments were placed close to the seafloor to detect effluent from diffuse hydrothermal flow that was advanced horizontally by ambient currents. Temperature and sound-speed measurements were compared during intervals when no plume was evident, and when effects of plumes were clearly detected. The data showed that energy flux from diffuse vents was approximately 125±75 MW from a source to the north of the instrument array, and 12±8 MW from a small source to the east. The largest error in the data was due to estimation of the plume width. A sensor array that would reduce this error in future work will be described. Such an array could be suitable for a long-term vent field monitor.