Transcript d 13 C model

Dating Old Groundwaters
Stable Isotopes and Paleogroundwaters
The Environmental
Radioisotopes
Isotope
3H
14C
36Cl
39Ar
81Kr
85Kr
129I
Half-life
(years)
12.43
5,730
301,000
269
210,000
10.72
15,700,000
Dating range
<50
1,000 to 30,000
> 50,000
50 to 1000
>50,000
<50
>1 million
6 neutrons
+ 6 neutrons
12C
14C
12C

1
1,000,000,000,000
= 10–12
= 100 percent modern C (pMC)
100 pMC
14C
14N
Half-life = 5730 years
Radiocarbon decay
120
–lt
at = ao ∙ e
a t 14C pmC
100
æ a t 14 C ö
t = –8267 ∙ ln ç 14 ÷
è ao C ø
80
60
40
20
0
0
5,730
11,460
17,190
Time in years
22,920
28,650
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 14CO2 100 pMC
14C
dilution:
CaCO3 + 14CO2  2HCO3 ~ 50 pMC
14C
decay < 50 pMC
Measured
DI14C
<< 50 pMC
dilution vs decay?
Correcting
14
for C
dilution
(reaction)
Statistical correction (STAT model)
for recharge through different soil types (Vogel, 1970):
0.65 – 0.75
0.75 – 0.90
0.90 – 1.00
for karst systems
for fine-grained carbonate soils (loess)
for crystalline rocks (granites)
a14 C DIC
t = -8267 × ln
q STAT × ao 14 C
Dilution correction with 13C
Typical evolution of
d13C
initial=
?
14C
and
13C
in DIC
Soil 14CO2 100 pMC
14C
d13C
dilution:
CaCO3 + 14CO2  2HCO3 ~ 50 pMC
14C
decay < 50 pMC
d13Cfinal= DIC
Measured
DI14C
Correcting for 14C-free Carbonate
d 13C mixing ( d 13C model)
Pearson (1965) and Pearson and Hanshaw (1970)
d13 C DIC - d13 C carb
q = 13
d C rech - d13 C carb
a14 C DIC
t = -8267 × ln
14
13
13
where: d CDIC = measured C in groundwaterq STAT × ao C
d13Crech = d13C of DIC in recharge
(~ –15‰)
2
d13Ccarb = d13C of the calcite being dissolved (~0‰)
a14CDIC
t = –8267 · ln –————
q · ao 14Catm
Fossil groundwater
of Southern Oman
The aquifers of Southern Oman
2
H ‰ VSMOW
20
0
-20
Monsoon
Interior Plateau G/W
-40
Coastal Plain G/W
-60
-8
-6
-4
-2
0
d18O ‰ VSMOW
2
4
10
8
Monsoon
Tritium (TU)
6
4
2
0
Coastal Plain
-2
Interior Plateau
Dhofar
Mountains
-4
-6
-8
0
20
40
Sample
60
80
14
NAME
b
CDIC13
CDIC
pmc
Temp.
pH
Ca2+
Mg2+
meq/l
meq/l
Na+
K+
HCO3-
Cl-
SO42-
‰
°C
-6.3
28.0
7.26
2.85
2.25
1.59
0.05
5.94
2.38
0.20
meq/l meq/l meq/l
meq/l
meq/l
HS- c
meq/l
Oman
Dhofar Recharge Area
Average (2)
69.8
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
56
3.3
2.3
-2.0
-2.1
34.0
35.0
7.55
7.45
3.59
5.39
4.41
4.29
6.00
5.43
0.27
0.27
2.61
2.00
6.09
6.46
2.63
4.65
0.2
0.8
Radiocarbon ages corrected for
carbonate dissolution
d13CDIC - d13Ccarb
q = d13
Crech - d13Ccarb
Some Additional Complications to 14 C Dating
Matrix diffusion of 14 C
Sulphate reduction
Incorporation of geogenic CO 2
Methanogenesis: 2CH2O  CO2 + CH4
(14C loss)
(14C dilution)
(14C dilution)
(14C dilution)
DOC in groundwater
Humic acid
HC O
(Sugar)
(HC OH) 4
COOH
OH
R CH
O
HO
N
O
O
H
..
O
O
HC CH2
O
O
O
OH
O
O
CH
NH
OH
R CH
C
O
COOH
OH
N
OH
.. H
HC O
O
COOH
O
COOH
(Peptide)
O
NH
Humic acid CH2O
COOH
O2
H2O
15
NO3
N2
10
M nO2
M n2+
pe
Fe(OH) 3
2+
Fe
5
SO4
0
2–
–
HS
-5
H2O
H2
CO2
CH4
-10
3
5
pH
7
9
Oxidation of organic carbon
by sulphate reduction
Hydrocarbon:
CH4 + SO42-  HS- + HCO3- + H2O
Fixed organic carbon:
CH2O + ½SO42-  ½H2S + HCO3-
• dilutes 14C pool
• lost d13C and DIC as measures of dilution
• precludes classical age corrections
14
NAME
b
CDIC13
CDIC
pmc
Temp.
pH
Ca2+
Mg2+
meq/l
meq/l
Na+
K+
HCO3-
Cl-
SO42-
‰
°C
-6.3
28.0
7.26
2.85
2.25
1.59
0.05
5.94
2.38
0.20
meq/l meq/l meq/l
meq/l
meq/l
HS- c
meq/l
Oman
Dhofar Recharge Area
Average (2)
69.8
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
56
3.3
2.3
-2.0
-2.1
34.0
35.0
7.55
7.45
3.59
5.39
4.41
4.29
6.00
5.43
0.27
0.27
2.61
2.00
6.09
6.46
2.63
4.65
0.2
0.8
Sulphate reduction
d13C
Soil 14CO2
d13C
14C
d13C 
q=?
dilution - carbonate dissolution
and exchange
DOC + SO4  H2S + DIC
14C
decay
Measured
DI14C
Oman
35
Age (kaB .P .)
30
Uncorrected
25
20
carbonate dissolution
15
10
Pluvial
5
Sulphate reduction
0
64
57
63
66
Site
58
56
62
55