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