A poroelastic model for the tidal modulation of seafloor hydrothermal systems

Abstract

[1] Time series measurements of the temperature and exit velocity of hydrothermal effluent suggest that seafloor hydrothermal systems are modulated by tidal processes. Here we apply the theory of poroelasticity to predict the magnitude and phase of tidally induced changes in the temperature and flow rate of hydrothermal effluent at the seafloor. We construct a model in which the steady state upwelling of buoyant fluid in the crust is modulated by tidal loading of a one-dimensional seafloor by the overlying water column. The nature of the solution is controlled by the relative magnitudes of three length scales. These are (1) the depth H of the heat source below the seafloor, (2) the skin depth D over which pore pressure signals can diffuse during one tidal cycle, and (3) the advective length scale A over which the upwelling flow advects thermal signals during one tidal cycle. We consider the likely magnitude of the parameters in a real system as well as the limitations of a one-dimensional representation of that system. We then discuss how observational data on the magnitude and phase lag of temperature and flow rate could be used to constrain the subseafloor parameters that govern hydrothermal circulation within the seafloor.

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

“…Within layer 2B, the relative amplitude decreases exponentially toward a small but nonzero constant-the Skempton ratio, which is the proportion of the seafloor loading that is borne by the pore fluid in the absence of interstitial fluid flows [Jupp and Schultz, 2004b]. At 700 mbsf, the value of P r is 0.39.…”

“…Such response includes an instantaneous pore pressure change at all depths and a diffusive pressure change that propagates from the seafloor into the formation and across internal layer boundaries [Wang and Davis, 1996;Jupp and Schultz, 2004b]. Both instantaneous and diffusive pore pressure variations are dependent on the poroelastic properties of both the pore fluid and the crustal matrix framework.…”

“…Both instantaneous and diffusive pore pressure variations are dependent on the poroelastic properties of both the pore fluid and the crustal matrix framework. Pore pressure variations are governed by equations of poroelasticity, which have been used in many studies to investigate subseafloor pore pressure variations and their role in fluid flow response to tidal loading and geological events [Wang and Davis, 1996;Davis et al, 2000Davis et al, , 2001Jupp and Schultz, 2004b;Barreyre et al, 2014a;Barreyre and Sohn, 2016]. In this study, we use the one-dimensional multilayer poroelastic model developed by Wang and Davis [1996] to predict the tidally induced pore pressure variations beneath the MEF.…”

“…The matrix bulk modulus K m is the bulk modulus of the crustal matrix framework when its pore space is empty. In practice, we calculate K m using Gassmann's equation given in Jupp and Schultz [2004b].…”

“…According to Jupp and Schultz [2004b], under periodic tidal loading, the varying pore pressure gradient perturbs the flow rate at which hydrothermal fluid ascends along the subsurface discharge zone to oscillate at tidal frequencies. Furthermore, conductive and adiabatic cooling leads to a vertical temperature gradient throughout the discharge zone.…”