A theoretical comparison of buoyancy‐driven and compaction‐driven fluid flow in oceanic sedimentary basins

Abstract

[1] Compaction-driven and buoyancy-driven fluid flow are the major driving forces in submarine basins; however, the impact of coupling these driving forces has yet to be fully evaluated. This is the first analysis to couple the transient interactions of compactiondriven and buoyancy-driven flow in oceanic sedimentary basins. We use finite element modeling to explore the effects of sediment compaction overlying a permeable oceanic crust layer where free convection occurs. Four numerical experiments examine the parameter space where each driving force dominates the flow field. In higher-permeability crust (k > 10 À13 m 2 ) buoyancy-driven flow dominates the fluid flow patterns in the basin, though compaction does cause minor alterations to fluid velocities and convection cell evolution. Compaction dominates the system when low permeability crust inhibits free convection (k < 10 À15 m 2 ) and can stimulate fluid flow in the oceanic crust by directing fluid from sediment pore spaces downward into the crust, possibly rejuvenating flow in older, lower-permeability submarine environments. In mixed-flow systems (k $ 10 À15 to 10 À13 m 2 ) both forces are approximately equal in importance and both contribute to the development of flow patterns. Fluid from the sediment layer is expelled downward into the permeable crust, which modifies the fluid flowlines. Instead of convection cells, a flow-through pattern forms in which all fluid is transported laterally across the basin. This increase in lateral transport could have significant implications for geochemical mass transport in submarine environments. Though flow rates are relatively small ($mm/yr), low-velocity fluid flow can be important in geochemical, mechanical, biological, and hydrogeologic systems.

Transient hydrogeologic models for submarine flow in volcanic seamounts: 1. The Hawaiian Islands

“…The two‐dimensional, finite element code OCRUST2D [ Christiansen and Garven , 2003] was developed to solve the continuity equation for fluid flow in deforming porous media, assuming one‐dimensional strain in the vertical direction and negligible thermal expansion effects. For variable‐density fluids, the mass conservation equation in compacting media is [ Person and Garven , 1994] and the energy conservation equation for heat transport is where ρ f is density of the fluid, ρ e is bulk density of the solid fluid mixture, is the Darcy velocity, ϕ is porosity, t is time, λ e is bulk thermal conductivity of the solid fluid mixture, T is temperature, c vf is specific heat of the fluid, and c e is the bulk specific heat of the solid fluid mixture.…”

“…For example, Holcomb and Searle [1991] estimate that landslide deposits associated with archipelagic aprons cover as much as 10% of the ocean floor. High sedimentation rates caused by the large‐scale slumps and debris flows drive compaction‐driven flow, which may enhance or retard fluid circulation caused by buoyancy‐driven flow in seamount environments and may stimulate flow in lower‐permeability environments [ Christiansen and Garven , 2003]. In addition to apron sediment, pelagic sediment that predates and underlies the volcanic edifice has been identified in seismic studies [ Morgan et al , 2000; Leslie et al , 2002] and may compact under the seamount load.…”

Transient hydrogeologic models for submarine flow in volcanic seamounts: 2. Comparison of the Hawaiian, Canary, and Marquesas Islands

“…We use the numerical code OCRUST2D [ Christiansen and Garven , 2003] in this analysis to solve the continuity equation for fluid flow in deforming porous media, assuming one dimensional strain in the vertical direction and neglecting thermal expansion effects [ Person and Garven , 1994], and the energy conservation equation for heat transport, where ρ f is density of the fluid, ρ e is bulk density of the fluid‐solid mixture, is Darcy flow rate (specific discharge), ϕ is porosity, t is time, λ e is bulk thermal conductivity of the porous medium, T is temperature, c vf is specific heat capacity of the fluid, and c e is the bulk specific heat capacity of the porous medium. The compaction algorithm of the code is based on the Athy equation, which scales porosity to depth and the Walder and Nur [1984] power law equation, which relates porosity to permeability.…”

“…Modifications to the values within the possible ranges are made so that the final, flexed cross section emulates seismic profiles of the islands. Further details of this code are discussed elsewhere [ Christiansen and Garven , 2003, 2004]. …”

“…Thick sedimentary aprons can host significant amounts of compaction‐driven fluid flow. Particularly in large volcanic island chains, this compaction‐driven flow can interact and alter buoyancy‐driven flow patterns in the upper oceanic crust [ Christiansen and Garven , 2003]. The combination of fluid flow driven by density gradients and compaction of mass wasting and pelagic sediments can create complex fluid flow patterns in seamount environments; these are the hydrologic issues explored in this paper.…”