Erodibility of pelagic carbonate ooze in the Northeast Atlantic

Shipboard erosion experiments were conducted on retrieved carbonate ooze sediments using the suction-stirring flume instrument of Gust and Muller [Gust, G., Muller, V., 1997. Interfacial hydrodynamics and entertainment functions of currently used erosion devices. Burt, N.T., et al. (Eds.) Cohesive Sediments. Wiley, Chichester, pp. 149-176] at two differing localities (site A, 3 800 in water depth; site B, 1100 in water depth) in the eastern North Atlantic. Step-wise increases in bottom stress were used to sequentially erode the surface sediment layers. Surficial sediment layers at both sites are moderately resistant to fluid stress, with erosion rates during periods of subthreshold fluid stress of the order 10(-3) kg m(-2) s(-1). Tidal flows in excess of similar to 18 cm s(-1) at 1 in above the bottom would be necessary to induce any significant erosion. Seabed sediments of site A possess a higher erodibility in comparison to site B: significant erosion occurs at a lower applied friction velocity (u(*crit.) = 1.0-1.2 cm s(-1)), time-averaged erosion rates are higher by similar to 25% and ultimately nearly twice as much sediment is entrained into suspension at the end of the experiment. Site B sediment, for which u(*crit.) = 1.4 - 1.6 cm s(-1), is slightly finer texturally and slightly more compact. However, the presence of interlocking silica spicules from the benthic sponge species Pheronema and Hyalonema within the sediment matrix at site B is suggested as the principal reason for the differing credibility. Direct observation of the size spectra of particles in suspension during active erosion suggests that low interfacial stresses (u(*) similar to 0.2-0.4 cm s(-1)) winnow clay clusters, complete and broken coccolith tests and small Foraminifera species from the sediment matrix, whilst progressively higher stresses entrain sequentially larger particles, including larger benthic and planktonic Foraminifera bodies. The supposition of Miller and Komar [Sedimentology 24 (1977) 709-721] that sand-size Foraminifera should be entrained by lower fluid stresses is not home out, most probably due to cohesive forces associated with the sediment matrix. The implications of these experiments to natural resuspension events and bottom nepheloid layer formation are discussed. (C) 2002 Elsevier Science B.V. All rights reserved.