Sediment permeability plays a key role in controlling both the flux of solute and heat through marine sediments and patterns of hydrothermal circulation within the underlying oceanic crust. Interlayered silt-rich and clay-rich sediments almost completely bury basement on the eastern flank of the Juan de Fuca Ridge. Laboratory measurements indicate that silt-rich sediment permeability is 1 to 2 orders of magnitude greater than clay-rich sediment permeability at similar depths. Numerical simulations of coupled fluid flow and heat transfer using a simplified model of a sedimented ridge flank with smooth basement topography illustrate how differences in permeability between the two sediment types can influence patterns of hydrothermal circulation and the flux of heat and solute across the seafloor. The layered structure of the model domain is inferred from reflection seismic and seafloor heat flow data. Two distinct patterns of hydrothermal circulation are obtained, depending on whether silt-rich or clay-rich sediments compose the sediment layer above a permeable upper basement aquifer. A clay-rich sediment column nearly isolates circulation within the basement aquifer, resulting in closed convection. Computed seafloor heat flow profiles resemble heat flow measurements made on the eastern flank of the Juan de Fuca Ridge. Computed vertical fluid fluxes across the sediment column are small, in agreement with fluid fluxes estimated from sediment geochemical profiles. When the sediments are silt-rich, 17% of the flow within a convection cell crosses the sediment column. Open convection where basement topography is smooth is probably rare because sediment columns overlying oceanic crust are unlikely to be entirely silt. Each pattern of convection persists over a wide range of sediment thicknesses for a given sediment permeability and basal heat flow. A transition from open to closed convection occurs when the effective permeability of the sediment column is only slightly less than that of an entirely silt-rich column. Thus the addition of only a small amount of clay to an otherwise silt-rich sediment column may effectively isolate the crustal aquifer from the ocean. ¿ American Geophysical Union 1996