Flow-induced uptake of particulate matter in permeable sediments

Abstract

We demonstrate the fast transfer of suspended particles from the boundary layer into the upper strata (z < 4 cm) of permeable sediments with topography-related interfacial water flows. The transport is driven by pressure gradients (Δ p ≤ 3 Pa) generated when bottom flows (u ≤ 10 cm s-1) are deflected by small surface structures (z < 3 cm) of hydrodynamical or biological origin. Acrylic pigment grains of 1- and 10- μm diameter traced the intrusion of particulate matter into sandy sediment (k > 2 x 10-11 m-2) incubated in a laboratory flume. Increased pressure up- and downstream of small mounds (z = 2.5 cm) drove water 5.5 cm into the core, carrying suspended particles (1 μm) to 2.2-cm sediment depth within 10 h. Simultaneously, decreased pressure at the downstream slope of the protrusions drew pore fluid from deeper layers (z ≤ 10 cm) to the surface. In the sediment, friction reduced the velocity of the particulate tracers, resulting in size fractionation and layers of increased particle concentration. Ripple topography (0.8-2.8 cm high) enhanced interfacial particle (1 μm) flux by a factor 2.3 when compared to a level control core. The pathways of the particle and solute tracers below a sediment ripple are explained with a source-sink model that describes the pore flow velocity field. Our results suggest that bedform-induced interfacial flows are important for the uptake of particulate organic matter into permeable shelf sediments.