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Carbonate GeochemistryCarbonate Geochemistry

Everything useful you need to know about carbonate geochemistry

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

Seawater

• Salinity– Concentration of all salts

– 1000 gms of seawater evaporated to dryness gives 35 gms of saltsgives 35 gms of salts

– 35 ppt, gm/kg, psu (practical salinity units)

– Also defined as conductivity, but does not include non-polar material.

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Seawater:cations

• Most Common cations in seawater

• Ca 2+

• Mg 2+

• Na +

• K +

• Mainly present as uncomplexed form

Anions

• Chloride, Cl-

• Bicarbonate, carbonate,HCO3-, CO3

2-

• Sulfate, SO42-

Molality of cations in seawater

• Sodium– Concentration = 10,773 ppm= 10773/23= 467 mM

= 0.467 M

• Magnesiumg– 1294 ppm = 1294/24 = 55 mM = 0.055 M

• Potassium– 399 ppm = 399/39 = 10 mM = 0.010 M

• Calcium– 420 ppm = 420/40 = 10 mM = 0.010 M

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Anions

• Chloride– 19344 ppm = 19344/35.5 = 546 mM = .546 M

• Sulfate2717 2717/94 28 M 0 028 M– 2717 ppm = 2717/94 = 28 mM = 0.028 M

• Bicarbonate– 142 ppm = 142/61 = 2.2 mM = 0.0022

Other Important Ions

• Strontium– 8 ppm, 8/87.62 = 93 M = 0.000093 M

• Bromine67 67/80 835 M 0 000835 M– 67 ppm, 67/80 = 835 M = 0.000835 M

• Na .31

• Cl .23

• Mg .15

• SO4 12

• 467

• 546

• 55

• 28

Riverwater (mM) Seawater (mM)

• SO4 .12

• Ca 0.37

• K 0.036

• HCO3 .87

• 28

• 10

• 10

• 2

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Conservative and non-Conservative

• Elements which generally do not participate in chemical reactions are called conservative– Na– Cl– K– Br

• Chemical reactions can include– Calcification, photosynthesis, respiration

Non-conservative

• Non-conservative elements include– C

– Ca

SO 2-– SO42-

– Mg

– Sr

– Fe, NO3-, NH4

-

Ca 2+ + CO32- = CaCO3

At equilibrium Ksp = CaCO3/ [Ca 2+ ] [CO32-]

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Concentrations of elements are in moles per kilogram (molal)

Ion molal product (IMP) of two elements = the concentration of one times the concentration of the

otherother

For CaCO3 the IMP = 0.011 x 0.0002

The effective molality is called the activity

The activity is only equal to the molality in an infinitely dilute solution

In other solutions the activity is related to the molality through the activity coefficient

So that:

= m

= activity coefficient

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Definitions

• Ksp = solubility product

• IMP= Ion Molar Product

• IAP = Ion Activity Product

• Activity= molality * activity coefficient

• Activity = effective concentration

• Ionic strength= I= ½ (m1c12+m2c2

2.mncn2)

Ionic Strength of Seawater

• I = ½(.546*1+ .467*1 + 0.028*4 + 0.01*4 + 0.01*1+ 0.055*4 + 0.002*1)

• Seawater or any solution must remain electrically neutralelectrically neutral

• mNa*c^2+ mK*c^2+mCa*c^2+mMg*c^2= mCl-*c^2+mSO4*c^2+mHCO3-*c^2

Thermodynamic Saturation Thermodynamic Saturation State of CaCOState of CaCO33

= [= [CaCa2+2+] [CO] [CO3322--]/Ksp(t,S,P)]/Ksp(t,S,P)

> 1> 1 SupersaturationSupersaturation

= 1= 1 EquilibriumEquilibrium

< 1< 1 Under SaturationUnder Saturation

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0 1 2 3 4 5

(m)

0

1000

2000

AragoniteD

epth

3000

4000

5000

6000

N. AtlanticN. Pacific

C a lc ite

0 2 4 6

(m)

0

1 0 0 0

2 0 0 0

Dep

th (

3 0 0 0

4 0 0 0

5 0 0 0

6 0 0 0

N . A tla n ticN . P a c if ic

Water

• H2O = H+ + OH-

• KH2O = 10 -14 = [H+][OH-]/ H2O

• pK = negative log = 14

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pH

7.4 7.6 7.8 8.0 8.2H

(m

)

0

1000

2000

DE

PT

H

3000

4000

5000

N. AtlanticN. Pacific

pCO2 (atm)

200 400 600 800 1000 1200 1400 1600

(m)

0

1000

2000

DE

PT

H

3000

4000

5000

N. AtlanticN. Pacific

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L A T IT U D E

-50 -40 -30 -20 -10 0 10 20 30 40 50 60 70

pH

in

sit

u

8.0

8.0

8 .1

8 .1

8 .2

8 .2

A tlan tic

P acific

L A T IT U D E

-70 -60 -50 -40 -30 -20 -10 0 10 20 30

pH

in

sit

u

7.6

7.7

7 .8

7 .9

8 .0

8 .1

8 .2

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1

2

1.462 3

2

{ }10

o

H

H COK

pCO

6.3531

2 3

{ }{ }10

{ }O

HCO HK

H CO

3

2 3{ }H CO

10.3332 2

3

{ }{ }10

{ }

CO HK

HCO

2 2 32 oH O pCO H CO

1.462 3

2

{ }10

o

H

H COK

pCO

2 3 3oH CO HCO H

6.3531

2 3

{ }{ }10

{ }O

HCO HK

H CO

21log * 356.3094 .06091964 21834.37 / 126.8339log 1684915 /K T T T T

10 33{ }{ }CO H

23 3HCO CO H

10.3332 2

3

{ }{ }10

{ }

CO HK

HCO

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1.462 3

2

{ }10

o

H

H COK

pCO

Take logs

2 3 2log{ } 1.46 logoH CO pCO

g

1.462 3

2

{ }10

o

H

H COK

pCO

Combine

2 3 2{ }OHH CO K pCO

6.3531

2 3

{ }{ }10

{ }O

HCO HK

H CO

1 23{ }HK K pCO

HCOH

2 3 2{ } HH CO K pCO

3 2log{ } 7.81 logHCO pCO pH

1 23{ }HK K pCO

HCOH

Combine

10.3332 2

3

{ }{ }10

{ }

CO HK

HCO

1 2 23{ }

2HK K K pCO

COH

Take logs

3 2log{ } 18.34 log 2CO pCO pH

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5

10

3 2log{ } 18.34 log 2CO pCO pH

3 2log{ } 7.81 logHCO pCO pH

2 3 2log{ } 1.46 logoH CO pCO

-25

-20

-15

-10

-5

0

5

0 5 10 15

pH

Co

nc

en

tra

tio

n

HCO3

CO3

H2CO3

120%

-25

-20

-15

-10

-5

0

5

10

0 5 10 15

pH

Co

ncen

tratio

n

HCO3

CO3

H2CO3

0%

20%

40%

60%

80%

100%

0 5 10 15

pH

Concentration

HCO3

CO3

H2CO3

22 3 3 3

oDIC mH CO mCO mHCO

70%80%90%

100%

ion

Bjerrum Diagram

0%10%20%30%40%50%60%

0 5 10 15

pH

Concentrati

HCO3

CO3

H2CO3

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Oxygen IsotopesOxygen Isotopes