Rounded shapes of pelagic sediments on fault escarpments follow a diffusion-type model

What is it about?

Information from cores tells us that pelagic sediments are strongly bioturbated. In terrestrial soils, bioturbation under the effect of gravity produces a downslope movement of the soil, in places with fluxes proportional to the soil surface gradient. Combined with considerations of conservation of mass, the surface topography of soil can follow a diffusion equation (hence soil-covered hillslopes tend to be smooth and hills between streams are parabolic-like, a solution to the diffusion equation in which soil removal along the streams acts as a constant-flux boundary condition). The question therefore arises as to whether bioturbation in marine pelagic sediments can also cause a downslope movement and therefore an evolution of the sediment surface according to a diffusion equation. In this study, the shapes of sediment deposits accumulated over the crests of fault scarps on the Galapagos Spreading Centre were quantified from sediment profiler data originally collected on the Scripps Deep Tow. The deposits do have shapes consistent with the diffusion equation. Using the age of the underlying crust to constrain the timescales of their development, the effective diffusivity of the sediment is 0.007 m^2/yr, though is strongly varied (0.002-0.015 m^2/yr).

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Why is it important?

The shapes of sediment deposits over scarps could potentially arise from other processes, i.e., their geometries are only fortuitously compatible with the diffusion model (enhanced bottom currents about the escarpment edges, for example, may have reduced deposition rates there). Nevertheless, quantifying rates of lateral movement is extremely difficult in the deep ocean, in contrast with rates of vertical mixing which are readily quantified with radionuclides, so estimates based on the method presented here should contribute to this issue.