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Carbonate Diagenesis

Early Diagenesis

A Short Course VU March, 2009 Peter Swart University of Miami

Cements are indicative of diagenetic environments

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Figure from Moore 2001

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Figure from Moore 2001

Meteoric Diagenesis

Figure from Moore 2001

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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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Picture Rossinsky

Picture Rossinsky

Picture Rossinsky

Picture Rossinsky

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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Data from ClinoKievman and Swart (unpublished)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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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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McClain et al. (1992)

McClain et al. (1992)

McClain et al. (1992)

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18 O

13C

McClain et al. (1992)

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

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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).

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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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Cements are indicative of diagenetic environments

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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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Figure from Moore 2001

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Swart et al 1989

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Late Diagenesis