Diffuse venting at the ASHES hydrothermal field: Heat flux and tidally modulated flow variability derived from in situ time‐series measurements

Abstract

Time-series measurements of diffuse exit-fluid temperature and velocity collected with a new, deep-sea camera, and temperature measurement system, the Diffuse Effluent Measurement System (DEMS), were examined from a fracture network within the ASHES hydrothermal field located in the caldera of Axial Seamount, Juan de Fuca Ridge. The DEMS was installed using the HOV Alvin above a fracture near the Phoenix vent. The system collected 20 s of 20 Hz video imagery and 24 s of 1 Hz temperature measurements each hour between 22 July and 2 August 2014. Fluid velocities were calculated using the Diffuse Fluid Velocimetry (DFV) technique. Over the 12 day deployment, median upwelling rates and mean fluid temperature anomalies ranged from 0.5 to 6 cm/s and 08C to 6.58C above ambient, yielding a heat flux of 0.29 6 0.22 MW m 22 and heat output of 3.16 2.5 kW. Using a photo mosaic to measure fracture dimensions, the total diffuse heat output from cracks across ASHES field is estimated to be 2.05 6 1.95 MW. Variability in temperatures and velocities are strongest at semidiurnal periods and show significant coherence with tidal height variations. These data indicate that periodic variability near Phoenix vent is modulated both by tidally controlled bottom currents and seafloor pressure, with seafloor pressures being the dominant influence. These results emphasize the importance of local permeability on diffuse hydrothermal venting at mid-ocean ridges and the need to better quantify heat flux associated with young oceanic crust.

Acoustic and In‐Situ Observations of Deep Seafloor Hydrothermal Discharge: An OOI Cabled Array ASHES Vent Field Case Study

“…Based on the in-situ temperature measurements made at the Lucky Strike Hydrothermal Field on the Mid-Atlantic Ridge, Barreyre et al, (2014) suggested that diffuse flows are mostly affected by bottom currents while focused flows are mostly affected by tidal loading. Mittelstaedt et al, (2016) observed tidal variations in the velocity and temperature of diffuse discharge from a fractured network at ASHES, which were attributed to the combined effects of tidal pressure and bottom currents.…”

“…Their measurements also indicate that the vent flow is overpressured and driven predominately by a significant subsurface pressure gradient as opposed to thermal buoyancy. Most recently, Mittelstaedt et al, (2016) measured heat flux of diffuse flow venting from a narrow fracture near the southern end of the depression region using the Diffuse Effluent Measurement System (DEMS), which is a camera system augmented with thermistors that can measure the flow rate and temperature of diffuse-flow effluents. They obtained an estimated heat flux of the crack: 0.07-0.51 MW/m 2 , which was subsequently extrapolated over all the cracks detected in a photomosaic survey to get a total heat flux from venting fractures of 0.1-4 MW.…”

Depth‐Dependent Permeability and Heat Output at Basalt‐Hosted Hydrothermal Systems Across Mid‐Ocean Ridge Spreading Rates

“…Note that the velocity perturbation at the seafloor ( ) is of particular interest since it can be measured by seafloor instruments such as a direct flow meter (e.g., Germanovich et al, ) or camera systems that use image analysis methods (e.g., Crone et al, ; Mittelstaedt et al, ). It is expressed as: …”

Rapid variations in fluid chemistry constrain hydrothermal phase separation at the Main Endeavour Field

“…For instance, observations of relatively short term variability in fluid chemistry in this region include tidally driven oscillations in temperature and chloride at high temperature vents attributed to subsurface mixing of brines with near‐critical fluids [ Larson et al ., ], as well as tidal variability in the temperature measured by the BARS sensor package deployed at the Grotto structure in the MEF [ Xu et al ., ]. Similar patterns are present in both high and low temperature flow in other systems, shedding light on the relative roles of tidal currents, tidal pressure and porosity structure on fluid characteristics [eg Barreyre et al ., ; Mittelstaedt et al ., ]. Modeling efforts can elucidate the effects of tidal loading on venting fluids [ Crone and Wilcock , ], demonstrating the interplay between high resolution sampling and building an applied theoretical framework.…”