A study in the hydrodynamics of sediment transport

Abstract

Antarctica is a unique environment where sediments are influenced by the interaction of glaciers and marine currents. This results in little or no sorting of the sediment prior to its introduction into the marine environment. The existing data relating current velocity to grain size were not designed to apply to assess the ability of marine currents to winnow and sort texturally homogeneous glacial and glacial marine sediments. However, estimates from flume studies suggest currents with velocities greater than 4 cm/sec are required to erode consolidated sediments. A puzzling observation made in studying modern Antarctic sediments has been the degree to which compacted and cohesive glacial and glacial marine sediments are being reworked and re-sedimented by bottom currents which move at velocities less than 15 cm/sec. This study has focused on the role of biological mixing in the initiation of sediment erosion and entrainment at low current velocities. This more adequately reflects the conditions which occur today on high latitude seafloors. It was found that with simulated bioturbation of the bed material, particles in the silt and clay-size range were transported at current velocities less than 3 cm/sec, with sand-sized material removed from the bed by 15 cm/sec. Without simulated bioturbation, currents up to 2 cm/sec had a negligable effect on the bed. Important applications of the results from this study include understanding sediment dynamics. It does not appear likely that erosion and sorting of overcompacted and cohesive glacial and glacial marine sediments is possible without the action of bioturbation. The results from this flume study are useful in the indirect determination of the long term normalized current velocity using surface samples and physical oceanographic information. Results can also be applied in the estimation of paleocurrent velocity from downcore variations of textural data.