Poroelastic response of mid‐ocean ridge hydrothermal systems to ocean tidal loading: Implications for shallow permeability structure

Tidal oscillations of venting temperature and chlorinity have been observed in the long-term time series data recorded by the Benthic and Resistivity Sensors (BARS) at the Grotto mound on the Juan de Fuca Ridge. In this study, we use a one-dimensional two-layer poroelastic model to conduct a preliminary investigation of three hypothetical scenarios in which seafloor tidal loading can modulate the venting temperature and chlorinity at Grotto through the mechanisms of subsurface tidal mixing and/or subsurface tidal pumping. For the first scenario, our results demonstrate that it is unlikely for subsurface tidal mixing to cause coupled tidal oscillations in venting temperature and chlorinity of the observed amplitudes. For the second scenario, the model results suggest that it is plausible that the tidal oscillations in venting temperature and chlorinity are decoupled with the former caused by subsurface tidal pumping and the latter caused by subsurface tidal mixing, although the mixing depth is not well constrained. For the third scenario, our results suggest that it is plausible for subsurface tidal pumping to cause coupled tidal oscillations in venting temperature and chlorinity. In this case, the observed tidal phase lag between venting temperature and chlorinity is close to the poroelastic model prediction if brine storage occurs throughout the upflow zone under the premise that layers 2A and 2B have similar crustal permeabilities. However, the predicted phase lag is poorly constrained if brine storage is limited to layer 2B as would be expected when its crustal permeability is much smaller than that of layer 2A.

Section: Coupled Tidal Oscillations Of Temperature and Chlorinity Frocontrasting

confidence: 40%

Section: Coupled Tidal Oscillations Of Temperature and Chlorinity Fromentioning

confidence: 81%

Section: Poroelastic Modelmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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A preliminary 1‐D model investigation of tidal variations of temperature and chlorinity at the Grotto mound, Endeavour Segment, Juan de Fuca Ridge

Larson

Bemis

et al. 2017

“…However, recording time series of these quantities at the seafloor (i.e., in hydrothermal vent exit fluids) is technically challenging. By contrast, it is much simpler to measure temperature fluctuations in fluids discharging at the seafloor

(|, Δ T_{1} (|, 0, t))

(e.g., Barreyre & Sohn, ; Barreyre et al, ; Fornari et al, ; Larson et al, ; Lilley et al, ; Scheirer et al, ; Sohn, ; Tivey et al, ). We therefore complement our previous solutions with an analytical expression for temperature fluctuations due to tidal loading

(|, Δ T_{j} (|, z, t))

.…”

Section: A Model For Tidal Modulation Of Hydrothermal Systems With Dementioning

confidence: 99%

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

Barreyre

Olive

Crone

et al. 2018

Self Cite

The permeability of the oceanic crust exerts a primary influence on the vigor of hydrothermal circulation at mid‐ocean ridges, but it is a difficult to measure parameter that varies with time, space, and geological setting. Here we develop an analytical model for the poroelastic response of hydrothermal exit‐fluid velocities and temperatures to ocean tidal loading in a two‐layered medium to constrain the discharge zone permeability of each layer. The top layer, corresponding to extrusive lithologies (e.g., seismic layer 2A) overlies a lower permeability layer, corresponding to intrusive lithologies (e.g., layer 2B). We apply the model to three basalt‐hosted hydrothermal fields (i.e., Lucky Strike, Main Endeavour and 9°46′N L‐vent) for which the seismic stratigraphy is well‐established, and for which robust exit‐fluid temperature data are available. We find that the poroelastic response to tidal loading is primarily controlled by layer 2A permeability, which is about 3 orders of magnitude higher for the Lucky Strike site (∼10−10 m2) than the 9°46′N L‐vent site (∼10−13 m2). By contrast, layer 2B permeability does not exert a strong control on the poroelastic response to tidal loading, yet strongly modulates the heat output of hydrothermal discharge zones. Taking these constraints into account, we estimate a plausible range of layer 2B permeability between ∼10−15 m2 and an upper‐bound value of ∼10−14 (9°46′N L‐vent) to ∼10−12 m2 (Lucky Strike). These permeability structures reconcile the short‐term response and long‐term thermal output of hydrothermal sites, and provide new insights into the links between permeability and tectono‐magmatic processes along the global mid‐ocean ridge.

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

Mittelstaedt

Fornari

Crone

et al. 2016

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.