Abundances and Their Relevance to 14C Dating of Groundwater

So far we have discussed the occurrence of 14C in hydrological systems. In a similar way one can follow 13C. Its abundance in rocks, organic material, and groundwater is expressed in permil deviation of the 13C:12C ratio in the sample from that in a standard (PDB—a belamnite carbonate from the Pee Dee formation of South Carolina). Most marine carbonate rocks have d13C = -2 to 0%, whereas frequent values for organic material and CO2 in soil are —28% to —20%. Most plants have values around —23 + 3%, but certain plants have more positive values, around —12 + 2% (Tables 11.311.5 and Figs. 11.6 and 11.7).

Table 11.3 d13C Values for Selected Samples from Saudi Arabia

Source

d13C (%„)

Tuwayq Mountain limestone

0.8

Upper Dhruma limestone

0.9

Upper Wasia sandstone

-1.3

Carbonaceous shale, from 241 m

-25.7

Lignite, from 1450 m

-23.4

Thorn plant, living

-24.5

Source: From Shampine et al. (1979).

Source: From Shampine et al. (1979).

The concentration of d13C in groundwater is determined by the input of recharged water and by reactions with rocks. For example, the reaction of water charged with CO2 of d13C = —25% is

In contrast, in reactions with silicates, the original (organic) d13C

Table 11.4 Carbon Isotopes of Various Samples from a Forest Near Heidelberg, Germany

Analyses no.

Material

Collection date

d13C(%o)

14C (pmc)

H-415

Air Co2

Apr 1958

-23.8

109

H-414

Soil Co2

Apr 1958

-23.4

104

H-447

Growing leaves

Jun 1958

-29.5

109

H-655

Air CO2

Apr 1959

-23.8

123

H-680

Air CO2

Jun 1959

-23

129

H-701

Air CO2

Jul 1959

-21.8

126

H-723

Soil CO2

Mar 1959

-23

106

H-703

Growing leaves

Apr 1959

-28.1

112

H-702

Growing leaves

Jul 1959

-31.4

120

H-679

Humus 0-2 cm

Jun 1959

-25.7

94

H-749

Humus 2-5 cm

Jun 1959

-26.0

90

Source: From Vogel (1970).

Source: From Vogel (1970).

Table 11.5 d C in Soil CO2 and Associated Plants in Northern Chile

Station

d13C (%), soil

d13C (%), plant

1

-14.7

-14.0

2

-22.3

-28.1

3

-18.3

-20.0 to -23.6

values will be retained:

H2O + 2CO2 + 2NaAlSi3 O8 ?2Na+ + Al2O3 + 6SiO2 + 2HCO3 d13C = -25% d13C = -25%

H2O + CO2 + 2KAlSi3O8?2K+ + Al2O3 + 3SiO2 + 2HCO-d13C = -25% d13C = -25%

At this point the d13C data of the Hawaii study (Table 11.2) warrant discussion. The tritium values indicated a pre-1952 age, the 14C values of 85-

Fig. 11.6 13C histogram in plants from Australia. Two major groups evolve, the total range being from —36% to —10%. (Following Rafter, 1974.)
Fig. 11.7 13C histogram in plants from Salar de Atacama, Chile. Two major populations emerge: d13C = —23 + 3% and d13C = -14±2%, attributed to different systems in plant metabolism. (After Fritz et al., 1979.)

97pmc indicated no, or little, lowering of the initial 14C value (100 pmc) by interaction with secondary calcite (section 11.4). The corresponding d13C values are —19 to —15%. These values are closer to common plant material (—23 ± 3%) and far from calcareous rocks (—2 to 0%). Thus the 13C changed only slightly due to water-rock interactions in the basaltic aquifers.

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