13.3 Chemical Diagenesis
a
AQUEOUS
DIFFUSION
(IN WATER
ONlY)
"
C1 = 29
in 1 m 3
WATER
,L __ _
675
SOlUTE FLUX SY DIFFUSION
a
A
J = -\- = D (C 2 - C 1 ) d
t = 1 a (year)
DIFFUSION COEFFICIENT
D- J (g/m~a)
(e.g., with D = 7.9'10 '6 cm 2 / s,.0.025 rri 2 / a
J = 0.025 g/m2.a)
b
CONVECTION-CONTROLLED
z
I
I
I
I
SED .SURFACE
C
I
I I-.. DISCHARGE
I
I I
T
I
I
I
I
\
,
' . . . . -4 CONVECTION
I CEll
SEVERAl
100
CONCENTRATION
GRADIENT= O.
FOR VARIOUS
CHEM. SPECIES
"
YOUNG
OCEANIC
CRUST
d
HIGH HEAT
FLQW
DIFFUSION-DOMINATED
m
I
I
!
Z
Fig. 13.17. a Meaning of the diffusion eoeffieient in
terms of diffusion transport under simplified eonditions. b-d Various types ofvertieal eoneentration
gradients in marine sediments: b in sediments of limited thiekness on top of young oeeanie erust; differenees in heat flow eause eonveetlve flow, whieh prevents ehemieal gradients. c Diffusion-dominated
Z
C
DIFFUSION-DOMINATED
CONCENTRAT ION
IN SEA WATER
e
C
CONCENTRATION
GRADIENT DUE TO
DIFFUSION ONLY
DIFF . COEFFICIENT
DECREASING WITH Z
++
SINK FOR Mg
DIFFUSION FROM EVAPORITES
CONCENTRATION C
INCREASING.f.
.v . . . .. . . . . . . .. . .
DIFFUSION --1
....... ALTERNATIVE :
COEFFICIENT I ....... L- DIFF. FROM
/ ., '4'
OVERlYING
......
EVAPORITES
i
++
++
Ca , Mg , CI
/
/
EVAPORITES (NaCI, CaS04 ' DOLOMITE)
Mi+ gradient, somewhat modified by depth-dependent diffusion coefficient or adveetion, in sediments
on top of oider oeeanie erust. d Same as c but for
Ca 2 +. e Diffusion-eontrolled, strong vertieal eoneentration gradients of ehemieal speeies assoeiated with
underlying (or overlying) evaporites. (Largely based
on Hesse 1986)