DECOMPOSING COHESIVE SEDIMENT
ENTRAINMENT
Timothy KeenNaval Research Laboratory, Stennis
Space Center, Mississippi 39529
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
Entrainment
• Total entrainment:m
cCBB AAAE
10
A0 = fundamental entrainment (kg/m2), c = bottom shear stress and entrainment
shear stressm = empirical coefficientAB = biological processes coefficientAC = consolidation (physical) processes
coefficient
Entrainment Data: No Infauna
Entrainment Rate(mg/cm2/s)
Concentration (mg/l)
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).
dynes/cm2 dynes/cm2
Evaluating A0 and m for Sterile Sediments
Comparison of laboratory data and model for water content of 73%.
(N/m2) T (s) A0 (kg/m2) mEB (mg/cm2/s) C (mg/l)
~0.06 ~5400 NA NA 0.00594 1028
0.06 5100 0.0079 3 0.01936 ~2600
0.06 5100 0.0031 3 0.00795 ~1040
~0.09 ~5400 NA NA 0.0804 11028
0.09 5100 0.0079 3 0.0808 ~11070
Standard Entrainment Parameter Results
Sample c (N/m2) (N/m2) T (s) A0
(kg/m2) m EB
(mg/cm2/s) C (mg/l)
Measured NA ~0.06 ~5400 NA NA 0.00594 1028
Measured NA ~0.09 ~5400 NA NA 0.0804 11028
Model 0.04 0.06 5400 2.32 2.51 0.00754 ~1095
Model 0.04 0.09 5400 2.32 2.51 0.0752 ~10930
Comparison for water content of 73%, representing 1 day old sediment. Assumed to have no consolidation.
Variable Water Content and Shear Stress
Sample c (N/m2) (N/m2) Water Content
(%)
A0
(kg/m2) m Meas.
CM
(mg/l)
FGS1D CF
(mg/l)
FACTOR CM/CF
14.4 0.04 0.0660 74.8 2.32 2.51 1134 2116 0.5359
10.3 0.04 0.0920 69-70 2.32 2.51 2589 12056 0.2147
14.5 0.04 0.0900 69.4 2.32 2.51 3301 10926 0.3021
10.6 0.04 0.0892 63-65 2.32 2.51 159 10487 0.0151
14.8 0.04 0.0900 62.9 2.32 2.51 111 10926 0.0102
10.8 0.04 0.1127 61-62 2.32 2.51 353 27963 0.0126
10.10 0.04 0.0615 61-62 2.32 2.51 23 1313 0.0175
Estimating Values for AC
A simple model for consolidation:)/(
101tX
C eAyA
Plot of AC as a function of water content
Comparison of measurements with modeled concentration using consolidation model.
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.
Entrainment Data: With InfaunaSample (N/m2) c (N/m2) TD (d)
EBO (kg/m2) EBM (kg/m2) EBO/EBM
ISIS1 0.2 0.177 7 0.201 0.0068 29.6
ISIS1 0.5 0.177 7 0.615 0.0048 128.1
ISIS3 0.1 0.01 19 0.107 0.0434 2.5
ISIS3 0.2 0.01 19 0.178 0.0075 23.7
ISIS3 0.5 0.01 19 0.517 0.1509 3.4
ISIS6 0.1 0.073 58 0.196 0.0042 46.7
ISIS6 0.2 0.073 58 0.272 0.0130 20.9
ISIS7 0.1 0.075 30 0.076 0.0038 20.0
ISIS7 0.2 0.075 30 0.187 0.0126 14.8
TAM5 0.1 0.065 43 0.107 0.0055 19.4
TAM5 0.2 0.065 43 0.357 0.0148 24.1
Linear Model of Bioturbation
AB = A + B t A = 9.53721 B = 0.33687
r2 = 0.2151
Bioturbation Model Results
Sample (N/m2) c (N/m2) TD (d)EBO (kg/m2) EBM (kg/m2) EBO/EBM
ISIS1 0.2 0.177 7 0.201 0.011 18.3
ISIS1 0.5 0.177 7 0.615 0.074 8.31
ISIS3 0.1 0.01 19 0.107 0.215 0.498
ISIS3 0.2 0.01 19 0.178 0.373 0.477
ISIS3 0.5 0.01 19 0.517 0.750 0.689
ISIS6 0.1 0.073 58 0.196 0.066 2.97
ISIS6 0.2 0.073 58 0.272 0.208 1.31
ISIS7 0.1 0.075 30 0.076 0.031 2.45
ISIS7 0.2 0.075 30 0.187 0.101 1.85
TAM5 0.1 0.065 43 0.107 0.064 1.67
TAM5 0.2 0.065 43 0.357 0.173 2.06
Future Research Directions
• Bioturbation models• Biogeochemical models• Bed consolidation models• Coupled
hydrodynamic/sedimentation/ bioturbation/ geochemical models
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), SO4
2-)
• Secondary reactions– Reoxidation– Abiotic reduction– Acid-base– Sorption
• Transport– Molecular diffusion– Biodiffusion– Advection (sedimentation)– Non-local (bioirrigation)
Froelich et al. (1979) GCA 43, 1075-1090.
Courtesy of Y. Furukawa
Calculating AB using a Biogeochemical Model
Steady state diagenetic equation
2D (vertical + radial) (Aller, 1980)
0),('),(
'2
2
R
r
rxCr
rr
D
x
rxCD
0))()(()(
' 02
2
RCxCxdx
xCdD
1D (vertical) (Berner, 1980)
Courtesy of Y. Furukawa
Conclusions
o Decomposed entrainment equation Biological effects represented by AB
Basic entrainment coefficient A0
Physical consolidation contained in AC
o Derived preliminary models for AB and AC
o Comparison with laboratory data is encouraging
o Future work on developing coupled model systems incorporating biological, physical, and geochemical processes