HR: 0800h
AN: T31A-0484    [Abstracts]
TI: A particle sedimentation model of buoyant jets: observations of hydrothermal plumes
AU: * Bemis, K G
EM: bemis@rci.rutgers.edu
AF: Institute of Marine and Coastal Sciences and the Department of Geological Sciences, Rutgers University 71 Dudley Rd, New Brunswick, NJ 08901 United States
AU: Silver, D
T31A-0484 AF: Center for Advanced Information Processing and Department of Electrical and Computer Engineering, Rutgers University, Piscataway, NJ 08854 United States
AU: Rona, P
T31A-0484 AF: Institute of Marine and Coastal Sciences and the Department of Geological Sciences, Rutgers University 71 Dudley Rd, New Brunswick, NJ 08901 United States
AU: Cowen, J
T31A-0484 AF: Department of Oceanography, University of Hawai'i at Manoa, Honolulu, HI 96822 United States
AB: Existing transport and sediment deposition models rarely use measured (rather than predicted) flow velocity and particle concentration data. We are developing a sediment deposition model using observed velocities and concentrations in order to model the deposition and transport of hydrothermal plume particles and biologic material. Our prior work developing acoustic imaging techniques to measure the static and dynamic properties of seafloor hydrothermal plumes provides the necessary input to the preliminary model. The general approach is to assume exponential settling of particles from the margins of the plume. Exponential settling states that the rate of decrease in the mass concentration C of sediment suspended in a layer of thickness h is given by dm/dt = -vCA/h, where v is the terminal velocity, C is the mass concentration of particles of given diameter, A is the basal area of the volume through which the settling occurs, and h is the thickness of the layer. Assuming the total thickness h is the height of the plume, the vertically integrated sedimented mass becomes: ∫_zdm/dz = ∫_z(vCA/w)/∫_z(1/w), where ∫_z(1/w) is the rise time over the region of interest and w is the local vertical velocity in a volume element. In this study, we calculate the terminal velocity (based on the estimated Reynolds number) and net force on the particles of a given size everywhere within the plume and its surroundings based on local velocity in gridded volume elements. In each volume element where the net force is negative (positive is upwards), the loss of sediment from the volume element is calculated. The loss of sediment is then summed vertically to determine the sediment mass deposited on the seafloor. The results for five particle sizes were then combined using three different assumed size distributions. This methodology is applied both to acoustic observations and to the output of an integral (time averaged) model of plume dynamics. Preliminary tests, using the predicted velocity and concentration from a integral model, produced the expected sedimentation maps, where larger grain sizes fall out near the vent, mean grain size decreases away from the vent and transport into the upper plume layers is greatest for the smallest particles. We also applied the sedimentation model to acoustic observations of hydrothermal plumes at Grotto Vent, on the Endeavour Segment of the Juan de Fuca Ridge. Acoustic backscatter data was used to infer particle concentration (as a function of assumed particle size); Doppler shifts in the same acoustic backscatter data were used to infer vertical velocity. The sedimentation model results showed decreasing mean particle size away from the vent. The mass sedimentation patterns are sensitive to the total mass available, which, as inferred from the volume backscattering strength, is also a function of particle size. Bending of the plume in local currents shifts the locus of sedimentation away from the vent, especially for smaller particle sizes. Future efforts will refine the basic settling model and incorporate predictions of the transport of biological material.
DE: 3017 Hydrothermal systems (0450, 1034, 3616, 4832, 8135, 8424)
DE: 3035 Midocean ridge processes
DE: 3050 Ocean observatories and experiments
DE: 4259 Ocean acoustics
DE: 4863 Sedimentation (1861)
SC: Tectonophysics [T]
MN: Fall Meeting 2005