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2/25/2009
1
Carbonate Diagenesis
Early Diagenesis
A Short Course VU March, 2009 Peter Swart University of Miami
Cements are indicative of diagenetic environments
2/25/2009
6
Allan And Matthews (1982)
• Five Distinctive Isotopic Zones– Calcrete, caliche, sub-aerial exposure surfaces– Penetrative calcretes (Rossinsky
• Surficial calcretes
– Vadose– Vadose– Freshwater- phreatic zone– Mixing-zone
• Marine-zone (Melim et al 1988)• Non-depositional surfaces (Swart and Melim,
2000)
Figure from Moore 2001
Caliche or Calcrete
• "A surface developed on carbonate rocks which are exposed to the atmosphere or developed associated with roots. The surface is frequently laminated."
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Caliche or Calcrete
• Associated with a pronounced negative shift in the carbon isotopic composition and a slight enrichment in the oxygen isotopic ratio
• The negative carbon is derived f th d iti ffrom the decomposition of organic material and the respiration plants
• The enrichment of oxygen arises from the evaporation near the surface.
CAICOSEXUMA
GUN KEYBPKKEY LARGO
ANASTASIASANSAL
-5
-4
-3
-2
-1
0
1
Oxy
gen
Iso
top
ic C
om
po
siti
on
Isotopic Calcretes
CAICOSEXUMA
GUNBPK
LARGOANASTASIA
SANSAL
Locality
-9
-8
-7
-6
-5
-4
-3
-2
-1
0
1
Car
bo
n I
soto
pic
Co
mp
osi
tio
n
100
105
-8 -6 -4 -2 0 2
Oxygen
Locality Locality
Data from Rossinsky and Swart (1993)
Data from Clino
Data from ClinoKievman and Swart (unpublished)
Carbon
110
115
120
-7 -6 -5 -4 -3 -2 -1
Carbon
Picture Rossinsky
Data from Rossinsky (1990)
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Additional Caliche Signatures
• Shifts towards more negative carbon isotopic signatures occur within an overall isotopic regime which is already depleted.
• Shifts in carbon appear to be independent pp pof climate
• Oxygen isotopic values are not always enriched and the overall depletion is related to climate, with heavier values in more arid climates.
Trace Elements
• Trace elements in isolated carbonate platforms are derived mainly from dust.
• Exposed surfaces tend to have higher concentrations of Fe and Mnconcentrations of Fe and Mn
• Surfaces which are related to roots have lower concentrations (penetrative calcretes Rossinsky et al., 1993)
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10
50
100
0 2 4 6 8
E 1E 2 S oilS o ilCavityCavityC avityC avityCavityH os tHos t
E 3?(P ipe)P ipeP ipeP ipeP ipe??
A boveS urfac eS urfac eHos t H os tHos tS urfac e
E x pos ure
Thous ands
F e (p p m)
150
200
250
E x pos ureS urfac e
E x pos ure??
E x pos ure
E x pos ure
E x pos ureE x pos ure
De
pth
(ft
)
Picture Rossinsky
Vadose Zone
• Constant oxygen isotopic composition
• Large source of homogenous oxygen
• Variable carbon isotopic
• 1 cc of seawater contains 1/16 moles of oxygen (0.0625 M), but only 0.002 M of C.
• Variable carbon isotopic composition
• Variable amounts of mixing of organically derived isotopically light carbon dioxide and dissolution of carbonate
Therefore it is easier to alter O than C
Figure from Moore 2001
Vadose Zone
Alteration
C
40
60
-8 -6 -4 -2 0 2
(ft)
Carbon
Variable C
Relatively constant O
Data from ClinoKievman and Swart (unpublished)
80
100
120
De
pth
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Freshwater Phreatic
Relatively constant, but negative C
The input of C into the groundwater is
"A zone located below the water table in which the pore space is filled mainly with water rather than air"
still isotopically depleted, but is better mixing than in the vadose zone
Relatively constant O
Large pool of homogenous oxygen
Phreatic Zone
50
100
-8 -6 -4 -2 0 2
)
Carbon
Vadose
150
200
De
pth
(ft
)
Relatively constant C
Relatively constant O
Phreatic
Data from ClinoKievman and Swart (unpublished)
McClain et al. (1992)
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-8.00 -4.00 0.00
0.00
4.00
Oxygen
Corals Florida
Algae Bahama Bank
Bottom Sediment Bahama Bank
-12.00
-8.00
-4.00
Carbon
Altered Pleistocene San Salvador
Inverted "J"
The inverted "J" is a pattern of C and O isotopes which is defines the
5
alteration of original material.
-4 -3 -2 -1 0 1-10
-5
0
Oxygen Isotopic Composition
Car
bon
Isot
opic
Co
mpo
sitio
n
MCL
Lohmann, 1985
Inverted "J" Part I
0
5The story starts with deposition of the original sediments
-5 -3 -1 1-10
-5
Oxygen
Ca
rbo
nsediments which may have a carbon value of +2 to +3 and an oxygen of -1 to -1.5 per mille.
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Perverted "J" Part II
0
5
n
As the original sediment becomes exposed to meteoric fluids it is recrystallized and starts to change its O
-5 -3 -1 1-10
-5
Oxygen
Ca
rbo
nchange its O isotopic composition while maintaining its C value. This is because there is a lot of oxygen circulating through the sediment, but relatively little C.
0
5
on
Inverted "J" Part IIIThe recrystallization of the sediment causes the O isotopic composition of the calcite to approach a
MCL
12
-5 -3 -1 1-10
-5
Oxygen
Ca
rbsteady value which is characteristic of the water which is doing the work and the temperature. This line is called the Meteoric Calcite Line (MCL).
3
Figure from Moore 2001
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Mixing-zone
Mixing-zone diagenesis produces covarying trends
"A zone located between the marine phreatic zone and the freshwater phreatic zone which is characterized by mixing betwen these two end members"
covarying trends between alteration by meteoric waters ( -ve C and O) and alteration by marine fluids (+ve C and O).
400
500
-5 -3 -1 1 3 5
Isotopic Composition
Mixing-ZoneCovarying trends
Carbon
4
-3
-2
-1
0
1
Oxy
ge
n
600
700
De
pth
(ft)
Oxygen-1 0 1 2 3 4
-4
Carbon
Data from ClinoKievman and Swart (unpublished)
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Vadose, phreatic, mixing-zone
200
300
400
-5 -3 -1 1 3 5
Isotopic Composition
Vadose
FreshwaterPhreatic
Surface
400
500
600
700
De
pth
(ft
)
Mixing-zone
Data from ClinoKievman and Melim and Swart
Marine-Phreatic
200
400
-5 -3 -1 1 3 5
Isotopic Composition
CarbonHeavy CarbonHea O gen
"A zone in which the porespace is filled with marine dominated fluids"
600
800
1000
De
pth
(ft
) Oxygen
Heavy Oxygen
100
200
300
-10.00 -5.00 0.00 5.00
th (
ft)
13C & 18O (o/oo)
Oxygen =red, carbon = blue
300
400
500
600
Dep
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Cementation is very fast!
Occurs within monthsRate decreases with depth
8 months
20 monthsoriginal
20 months
Example:Oolithic sand in 30 m water depth is a rock after 20 months
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