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DECOMPOSING COHESIVE SEDIMENT ENTRAINMENT

DECOMPOSING COHESIVE SEDIMENT ENTRAINMENT. Timothy Keen Naval Research Laboratory, Stennis Space Center, Mississippi 39529. Factors in Cohesive Sediment Consolidation and Entrainment. Sedimentological: Grain characteristics

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DECOMPOSING COHESIVE SEDIMENT ENTRAINMENT

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  1. DECOMPOSING COHESIVE SEDIMENT ENTRAINMENT Timothy Keen Naval Research Laboratory, Stennis Space Center, Mississippi 39529

  2. Factors in Cohesive Sediment Consolidation and Entrainment • Sedimentological: Grain characteristics • Physical: Oceanographic forcing and seafloor consolidation (dewatering) • Geochemical: Fluxes of chemical species during shallow burial; Grain surface chemistry • Biological: Infauna effects on mixing and chemical fluxes

  3. Entrainment • Total entrainment: A0 = fundamental entrainment (kg/m2) , c = bottom shear stress and entrainment shear stress m = empirical coefficient AB = biological processes coefficient AC = consolidation (physical) processes coefficient

  4. Entrainment Data: No Infauna Entrainment Rate (mg/cm2/s) Concentration (mg/l) dynes/cm2 dynes/cm2 Laboratory data on entrainment of sterile sediments (67% shale as indicated by filled symbols) from Lake Erie with water content ranging from 61-75% (M. K. Fukuda and W. Lick, 1980. J. Geophys. Res., 85, 2813-2824).

  5. Evaluating A0 and m for Sterile Sediments Comparison of laboratory data and model for water content of 73%.

  6. Standard Entrainment Parameter Results Comparison for water content of 73%, representing 1 day old sediment. Assumed to have no consolidation.

  7. Variable Water Content and Shear Stress

  8. Estimating Values for AC A simple model for consolidation: Comparison of measurements with modeled concentration using consolidation model. Plot of AC as a function of water content

  9. Linear Model for AB • Bioturbated Sediment Data (D.G. Lintern, et al, 2002. Fine Sediment Dynamics in the Marine Environment. Elsevier, New York, pp. 343-357). • These data show increasing erodibility with time because of infauna. • The model uses A0 = 0.458 and m = 0.735.

  10. Entrainment Data: With Infauna

  11. Linear Model of Bioturbation AB = A + B t A = 9.53721 B = 0.33687 r2 = 0.2151

  12. Bioturbation Model Results

  13. Future Research Directions • Bioturbation models • Biogeochemical models • Bed consolidation models • Coupled hydrodynamic/sedimentation/ bioturbation/ geochemical models

  14. Biogeochemical Properties & Processes in Aquatic Sediments That can be Coupled by Modeling • Spatial distribution of geochemical species • Primary reactions • Microbially driven • Oxidation of organic matter • Reduction of terminal electron acceptors (e.g., O2, NO32-, Mn(IV), Fe(III), SO42-) • Secondary reactions • Reoxidation • Abiotic reduction • Acid-base • Sorption • Transport • Molecular diffusion • Biodiffusion • Advection (sedimentation) • Non-local (bioirrigation) Froelich et al. (1979) GCA43, 1075-1090. Courtesy of Y. Furukawa

  15. 1D (vertical) (Berner, 1980) 2D (vertical + radial) (Aller, 1980) Calculating AB using a Biogeochemical Model Steady state diagenetic equation Courtesy of Y. Furukawa

  16. Conclusions • Decomposed entrainment equation • Biological effects represented by AB • Basic entrainment coefficient A0 • Physical consolidation contained in AC • Derived preliminary models for AB and AC • Comparison with laboratory data is encouraging • Future work on developing coupled model systems incorporating biological, physical, and geochemical processes

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