Dating Old Groundwaters

Stable Isotopes and Paleogroundwaters

The Environmental Radioisotopes Isotope Half-life Dating range (years) 3 H <50 14 C 5,7301,000 to 30, Cl301,000 > 50, Ar to Kr210,000 >50, Kr < I 15,700,000 >1 million

6 neutrons + 6 neutrons 12 C

14 C 12 C 1 1,000,000,000,000 = 100 percent modern C (pMC) = 10 –12

100 pMC

14 C 14 N Half-life = 5730 years

Radiocarbon decay

DI 14 C

14 C dating groundwaters generally sample the DIC –CO 2 + H 2 CO 3 + HCO 3 – + CO 3 2– can also separate the DOC (CH 2 O) –humic acid –fulvic acid but DIC 5 to >10 ppm DOC <1 to 2 ppm

Typical evolution of 14 C in DIC Soil 14 CO pMC Measured Measured DI 14 C 14 C dilution: CaCO CO 2 2HCO 3 ~ 50 pMC CaCO CO 2 2HCO 3 ~ 50 pMC 14 C decay < 50 pMC << 50 pMC dilution vs decay?

Correcting for 14 C dilution (reaction)

Statistical correction (STAT model) for recharge through different soil types (Vogel, 1970): 0.65 – 0.75for karst systems 0.75 – 0.90for fine-grained carbonate soils (loess) 0.90 – 1.00for crystalline rocks (granites)

Dilution correction with 13 C

Typical evolution of 14 C and 13 C in DIC Soil 14 CO pMC Measured Measured DI 14 C 14 C dilution: CaCO CO 2 2HCO 3 ~ 50 pMC CaCO CO 2 2HCO 3 ~ 50 pMC 14 C decay < 50 pMC 13 C initial = ? 13 C 13 C final = DIC

13 C mixing ( 13 C model) Pearson (1965) and Pearson and Hanshaw (1970) q= CC C C 13 DIC 13 carb 13 rech 13 carb where: 13 C DIC = measured 13 C in groundwater 13 C rech = 13 C of DIC in recharge (~ –15) 2 13 C carb = 13 C of the calcite being dissolved (~0) Correcting for 14 C-free Carbonate t = –8267 · ln – a 14 C DIC q · a o 14 C atm

Fossil groundwater of Southern Oman

The aquifers of Southern Oman

O VSMOW 2 H VSMOW Monsoon Interior Plateau G/W Coastal Plain G/W

Sample Tritium (TU) Dhofar Mountains Interior Plateau Coastal Plain Monsoon

Radiocarbon ages corrected for carbonate dissolution q = C -C C - C 13 DIC 13 carb 13 rech 13 carb

Humic acid CH 2 O DOC in groundwater

Oxidation of organic carbon by sulphate reduction Hydrocarbon: CH 4 + SO 4 2- HS - + HCO H 2 O Fixed organic carbon: CH 2 O + ½SO 4 2- ½H 2 S + HCO 3 - Hydrocarbon: CH 4 + SO 4 2- HS - + HCO H 2 O Fixed organic carbon: CH 2 O + ½SO 4 2- ½H 2 S + HCO 3 - dilutes 14 C pool dilutes 14 C pool lost 13 C and DIC as measures of dilution lost 13 C and DIC as measures of dilution precludes classical age corrections precludes classical age corrections

Sulphate reduction 14 C decay DOC + SO 4 H 2 S + DIC Soil 14 CO 2 Measured Measured DI 14 C 14 C dilution - carbonate dissolution and exchange and exchange 13 C q = ?

Pluvial