dating old groundwaters. stable isotopes and paleogroundwaters
TRANSCRIPT
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