Modeling the effects of tidal loading on mid-ocean ridge hydrothermal systems

[ 1] Tidal signals are observed in numerous time-series measurements obtained from mid-ocean ridge hydrothermal systems. In some instances these tidal signals are clearly the result of ocean currents, but in other instances it appears that the signals may originate in the subseafloor formation. In order to explore the effect of ocean tidal loading on mid-ocean ridge hydrothermal systems, we apply a one-dimensional analytical model of tidal loading on a poroelastic half-space and develop a two-dimensional numerical model of tidal loading on a poroelastic convection cell. The one-dimensional models show that for a reasonable range of fluid, elastic, and hydrological properties, the loading efficiency may vary from near zero to near unity and the diffusive penetration depth for tidal pressure signals may vary from tens of meters to kilometers. The two-dimensional models demonstrate that tides may generate significant vertical and horizontal pressure gradients in mid-ocean ridge hydrothermal systems as a result of spatial variations in fluid temperatures and the elastic and hydrological properties of the crust. These continuum models predict that outflow temperature perturbations will be very small (< 10(-4) degrees C), but in real systems where the continuum hypothesis does not always apply, the perturbations may be on the order of similar to 0.1 degrees C. The models predict relatively large perturbations to fluid velocity at the seafloor. For high-temperature vents the outflow perturbations normalized to the mean flow velocity increase as the permeability decreases. Flow reversals at the seafloor are predicted in some regions of net low-temperature outflow and net inflow during the tidal cycle. In the subseafloor, tidally induced flow perturbations are likely to significantly enhance mixing and fluid exchange below the seafloor in regions of slow flow and in regions where there are strong gradients in temperature or in the mechanical and hydrological properties of the crust. Tidally enhanced mixing and fluid exchange may significantly influence the extent and character of microbial production in the subseafloor.