Dating Old Groundwaters

Stable Isotopes and Paleogroundwaters

The Environmental Radioisotopes

Isotope Half-life Dating range (years)

3H 12.43 <50 14C 5,730 1,000 to 30,000 36Cl 301,000 > 50,000 39Ar 269 50 to 1000 81Kr 210,000 >50,00085Kr 10.72 <50 129I 15,700,000 >1 million

6 neutrons+ 6 neutrons

12C

14C12C

1

1,000,000,000,000

= 100 percent modern C (pMC)

= 10–12

100 pMC

14C 14N

Half-life = 5730 years

0

20

40

60

80

100

120

0 5,730 11,460 17,190 22,920 28,650

Time in years

at14

C p

mC

Radiocarbon decay

at = ao ∙ e–t

t = –8267 ∙ lna

at

14

o14

C

C

DI14C

14C dating groundwaters

• generally sample the DIC– CO2 + H2CO3 + HCO3

– + CO32–

• can also separate the DOC (CH2O)

– humic acid– fulvic acid

• but DIC 5 to >10 ppm

DOC <1 to 2 ppm

Typical evolution of 14C in DIC

Soil Soil 1414COCO22 100 pMC100 pMC

MeasuredMeasuredDIDI1414CC

1414C dilutionC dilution:: CaCOCaCO33 + + 1414COCO2 2 2HCO 2HCO33 ~~ 50 pMC50 pMC

1414C decayC decay << 50 pMC50 pMC

<<<< 50 pMC50 pMC

dilution vs decay?dilution vs decay?

Correcting for 14Cdilution

(reaction)

Statistical correction (STAT model)

for recharge through different soil types (Vogel, 1970):

0.65 – 0.75 for karst systems

0.75 – 0.90 for fine-grained carbonate soils (loess)

0.90 – 1.00 for crystalline rocks (granites)

t 8267 lnC

q C

14DIC

STAT o14

a

a

Dilution correction with 13C

Typical evolution of 14C and 13C in DIC

Soil Soil 1414COCO22 100 pMC100 pMC

MeasuredMeasuredDIDI1414CC

1414C dilutionC dilution:: CaCOCaCO33 + + 1414COCO2 2 2HCO 2HCO33 ~~ 50 pMC50 pMC

1414C decayC decay << 50 pMC50 pMC

13Cinitial= ?

13C

13Cfinal= DIC

13 C mixing (13 C model)

Pearson (1965) and Pearson and Hanshaw (1970)

q = C C

C C

13DIC

13carb

13rech

13carb

where: 13CDIC = measured 13 C in groundwater13Crech = 13C of DIC in recharge (~ –15‰)2

13Ccarb = 13C of the calcite being dissolved (~0‰)

Correcting for 14C-free Carbonate

t 8267 lnC

q C

14DIC

STAT o14

a

a

t = –8267 · ln –———— a14CDIC

q · ao 14Catm

Fossil groundwaterof Southern Oman

The aquifers of Southern Oman

-60

-40

-20

0

20

-8 -6 -4 -2 0 2 4

18O ‰ VSMOW

2H

‰ V

SM

OW

Monsoon

Interior Plateau G/W

Coastal Plain G/W

-8

-6

-4

-2

0

2

4

6

8

10

0 20 40 60 80Sample

Tri

tium

(T

U)

DhofarMountains

Interior Plateau Coastal Plain

Monsoon

NAME 14CDIC

b 13CDIC Temp. pHCa2+ Mg2+ Na+ K+

HCO3- Cl-

SO42- HS- c

pmc ‰ °C meq/l meq/l meq/l meq/l meq/l meq/l meq/l meq/l

Oman

Dhofar Recharge Area

Average (2) 69.8 -6.3 28.0 7.26 2.85 2.25 1.59 0.05 5.94 2.38 0.20 nd

Najd Artesian Groundwaters

63 7.3 -1.7 32.0 8.85 2.80 2.86 7.35 0.31 2.23 6.31 1.94 0.5

66 7.9 -4.6 34.0 7.45 7.19 12.24 25.83 0.36 1.97 34.57 8.92 0.4

57 4.2 -1.8 37.5 7.10 5.24 3.54 7.22 0.37 4.04 7.00 3.02 0.7

62 3.0 0.0 38.0 7.55 3.65 6.29 10.83 0.22 1.87 13.80 3.81 0.6

64 2.7 -0.6 34.0 7.55 2.99 4.24 10.70 0.51 0.75 11.23 5.46 1.8

58 6.3 -4.8 37.0 7.76 3.99 5.04 6.96 0.32 3.12 7.23 5.83 1.6

55 3.3 -2.0 34.0 7.55 3.59 4.41 6.00 0.27 2.61 6.09 2.63 0.2

56 2.3 -2.1 35.0 7.45 5.39 4.29 5.43 0.27 2.00 6.46 4.65 0.8

Radiocarbon ages corrected forcarbonate dissolution

q = C - CC - C

13DIC

13carb

13rech

13carb

Some Additional Complications to 14C Dating

Matrix diffusion of 14C (14C loss)Sulphate reduction (

C dilution)

Incorporation of geogenic CO2 (14C dilution)Methanogenesis: 2CH2O CO2 + CH4 (14C dilution)

O H

O

O

O

O

(Sugar)

(Pep tide)

O

O

O

O

O

O

O

O

O

O

O

O

NN H

N H

H

H

N

C O O H

C O O H

C O O HC O O HC O O H

H O

O H

O H ) 4

O H

O HO H

C H

C H 2H C

H C

H C

C

(HC

C H

C H

R

R

.

.

.

.

H um ic a c id

Humic acid CH2O

DOC in groundwater

-10

-5

0

5

10

15

3 5 7 9pH

pe

O2

H2O

H2OH2

NO3

N2

MnO2

Mn2+Fe(OH)3

Fe2+

SO42–

HS–

CO2

CH4

Oxidation of organic carbonby sulphate reductionHydrocarbon:Hydrocarbon:CHCH44 + SO + SO44

2-2- HS HS-- + HCO + HCO33-- + H + H22OO

Fixed organic carbon:Fixed organic carbon:CHCH22O + ½SOO + ½SO44

2-2- ½H ½H22S + HCOS + HCO33--

Hydrocarbon:Hydrocarbon:CHCH44 + SO + SO44

2-2- HS HS-- + HCO + HCO33-- + H + H22OO

Fixed organic carbon:Fixed organic carbon:CHCH22O + ½SOO + ½SO44

2-2- ½H ½H22S + HCOS + HCO33--

• dilutes dilutes 1414C poolC pool• lost lost 1313C and DIC as measures of dilutionC and DIC as measures of dilution• precludes classical age correctionsprecludes classical age corrections

NAME 14CDIC

b 13CDIC Temp. pHCa2+ Mg2+ Na+ K+

HCO3- Cl-

SO42- HS- c

pmc ‰ °C meq/l meq/l meq/l meq/l meq/l meq/l meq/l meq/l

Oman

Dhofar Recharge Area

Average (2) 69.8 -6.3 28.0 7.26 2.85 2.25 1.59 0.05 5.94 2.38 0.20 nd

Najd Artesian Groundwaters

63 7.3 -1.7 32.0 8.85 2.80 2.86 7.35 0.31 2.23 6.31 1.94 0.5

66 7.9 -4.6 34.0 7.45 7.19 12.24 25.83 0.36 1.97 34.57 8.92 0.4

57 4.2 -1.8 37.5 7.10 5.24 3.54 7.22 0.37 4.04 7.00 3.02 0.7

62 3.0 0.0 38.0 7.55 3.65 6.29 10.83 0.22 1.87 13.80 3.81 0.6

64 2.7 -0.6 34.0 7.55 2.99 4.24 10.70 0.51 0.75 11.23 5.46 1.8

58 6.3 -4.8 37.0 7.76 3.99 5.04 6.96 0.32 3.12 7.23 5.83 1.6

55 3.3 -2.0 34.0 7.55 3.59 4.41 6.00 0.27 2.61 6.09 2.63 0.2

56 2.3 -2.1 35.0 7.45 5.39 4.29 5.43 0.27 2.00 6.46 4.65 0.8

Sulphate reduction

1414C decayC decay

DOC + SO4 H2S + DIC

Soil Soil 1414COCO22

MeasuredMeasuredDIDI1414CC

1414C dilution C dilution - carbonate dissolution- carbonate dissolution and exchangeand exchange

13C

13C

13C

q = ?

0

5

10

15

20

25

30

35

64 57 63 66 58 56 62 55Site

Age

(kaB

.P.)

Uncorrected

carbonate dissolution

Sulphate reduction

Oman

Pluvial