Previous studies of permeability in deep-sea sediments have been motivated by efforts to understand the physical processes of compaction and fluid transport in deep-sea sediments. The objectives of a previous study by Bryant and Rack (1990), for example, were to determine the consolidation characteristics of the material and the affect of diatoms on the porosity and permeability of marine sediments. Many studies of permeability in deep-sea sediments have focused on environments where hydrologic processes are known to be active, along convergent margins such as Cascadia, Nankai, and Barbados (e.g., Moran and Christian, 1990; Taylor et al., 1991; Moran et al., 1995; Hempel, 1995) and the Juan de Fuca Ridge (Fisher et al., 1994). Fewer studies have addressed permeability in distal turbidites (e.g., Bryant et al., 1986; Wetzel, 1990), carbonates (e.g., Taylor, 1991), and clays (e.g., Bryant and Bennett, 1988; Bryant et al., 1990).
Bryant and Rack (1990) and Marsters and Christian (1990) measured the hydraulic properties of diatomaceous sediments, which were found to have some rather peculiar properties caused by the very high intragranular porosity of diatom frustules: the porosities of these sediments are high and remain so to considerable depths of burial; shear strengths are anomalously high; and measured permeabilities range from semipervious (2 x 10–13 m2, comparable to sandstone) to virtually impervious (4 x 10–18 m2). Bryant and Rack (1990) demonstrated a high degree of correlation (R = ~0.8) between porosity and the abundance of diatom frustules in the sediments from the Weddell Sea. They also noted that the samples with the highest permeabilities are rich in diatoms, whereas sediments that contain a lower concentration of diatoms can be essentially impermeable, even though their porosities are >40%.
The diatomaceous sediments from Site 1179 have properties similar to the properties of the sediments from the Weddell Sea. Throughout Unit I, to a depth of nearly 225 meters below seafloor (mbsf), the porosity of the sediments exceeds 80%. Porosities actually increase with depth in the upper part of the unit, reaching a maximum of ~86% at 140 mbsf; there is a corresponding decrease of grain density to a minimum near 2200 kg/m3. Because siliceous microfossils are composed of opaline silica with a density <2100 kg/m3, we suspect that these trends are related to the abundance of diatoms and that permeability is similarly affected.
The discovery of deep microbial activity (the "deep biosphere") provides a new impetus to measure sediment permeabilities. As noted previously, one of the objectives of drilling at Site 1179 was to search for microbes and/or chemical evidence of biological activity in the sediments as part of the ongoing effort to discover the depth and lateral extent of the deep biosphere. Two lines of evidence that may prove to be indicative of ongoing microbial activity in the sediment column at Site 1179 are observed variations of ammonia and sulfate concentrations with depth. Transport properties of diffusion, advection, and permeability are important to the search for deep biological activity. Chemical profiles that may provide indirect evidence of microbial activity are affected by advection and diffusion. Diffusion, the movement of ions and molecules down concentration gradients, depends on the diffusion coefficient, which is a material property of porous media. Advection, the physical movement of pore water driven by pressure gradients, depends on permeability, which is also a material property. Diffusion coefficients are approximately proportional to permeability because both diffusion and fluid flow depend on the connectivity of the pore spaces; a permeability of 10–12 m2 (1 nd) is roughly equivalent to the diffusion coefficient for salts in aqueous solution (Brace et al., 1968). Whether diffusion or advection is the dominant process depends on the pressure gradient. Where rates of advection and diffusion are very low, pore water chemistry profiles may be "fossils" that change little, even over geologic time. Conversely, high rates of either flow or diffusion cause mixing of the pore water; under these conditions, significant chemical gradients must be maintained by dynamic chemical processes such as microbial activity. Another consideration is that some degree of permeability is required to support life in the deep biosphere. Living organisms cannot persist at depth unless there are sufficient fluxes of nutrients, waste products, and microbes themselves through the sediments. Permeability is thus a measure of the viability of the sediment as a habitat for bacteria; impervious sediments are unlikely habitats for microbes.