HR: 16:05h
AN: H54E-01 INVITED    [Abstracts]
TI: Formation and Maintenance of Sand-Mud Transitions
AU: * Hill, P S
EM: paul.hill@dal.ca
AF: Department of Oceanography Dalhousie University, 1355 Oxford Street, Halifax, NS B3H 4J1, Canada
AU: Milligan, T G
EM: milligant@mar.dfo-mpo-gc.ca
AF: Ecosystem Research Division Habitat Ecology Section, Bedford Institute of Oceanography 1 Challenger Drive, Dartmouth, NS B2Y 4A2, Canada
AU: Law, B A
EM: lawb@mar.dfo-mpo.gc.ca
AF: Ecosystem Research Division Habitat Ecology Section, Bedford Institute of Oceanography 1 Challenger Drive, Dartmouth, NS B2Y 4A2, Canada
AU: Curran, K J
EM: kcurran@dal.ca
AF: Department of Oceanography Dalhousie University, 1355 Oxford Street, Halifax, NS B3H 4J1, Canada
AB: The sand-mud transition (SMT) is a common boundary on continental shelves where the mean diameter of seabed sediments drops abruptly from ~ 100 μm to ~ 10 μm. This transition is important because sands and muds host different benthic communities, have different acoustic properties, and carry different loads of heavy metals and biogenic particles. The transition from sand to mud is also a basic facies change that can be used to infer properties of ancient environments of deposition recorded in sedimentary rocks. The SMT in general occurs because of decreasing shear stress on the seabed. In short, sands erode and deposit at higher shear stresses than muds. This simple notion, however, does not explain the abruptness of the transition, which often occurs over a depth range of only a few meters. Recent observations of in situ particle size distributions show that the size, settling velocity, and abundance of suspended flocs increase rapidly with decreasing shear stress. Because individual mud particles sink very slowly, deposition in flocs is the primary way of transferring mud from suspension to the seabed. The increases in floc abundance and settling velocity with decreasing shear stress therefore lead to formation of an SMT by causing a rapid increase in the depositional flux of mud. Observations of size sorting during controlled erosion of mixed grain size beds demonstrate that SMTs are accentuated by the cohesive effects of mud. If the mud fraction is low in sediment composed of both sand and mud, then muds will be winnowed from the sands at low shear stresses. If the mud fraction is high, then many grain sizes are bound together. Winnowing of muds from sands is reduced or eliminated. By this mechanism, sediment on the sand side of an SMT will be cleaned by subsequent resuspension, while sediment on the mud side will not, thereby retaining its poor sorting and small mean grain diameter. The processes described here need to be incorporated into sediment transport models. Floc settling velocity and abundance need to be defined in terms of boundary shear stress. The critical erosion shear stress of muds needs to be defined in terms of the percentage of fine sediment in the seabed. These changes to current models should allow them to reproduce the abruptness of sand-mud transitions.
DE: 1815 Erosion
DE: 3022 Marine sediments: processes and transport
DE: 4211 Benthic boundary layers
DE: 4219 Continental shelf and slope processes (3002)
DE: 4558 Sediment transport (1862)
SC: Hydrology [H]
MN: 2007 Fall Meeting