Determination of the initial 14C activity of the total dissolved carbon: A review of the existing models and a new approach

摘要: Carbon 14 age calculations are based upon the assumption that initial activity of material to be dated was 100% modern CO2 (100 pmc). This is generally good for organic origin (wood and, some extent, shells). Dissolved carbon in groundwater has two main sources: active from soil zone and less (or even ‘dead’) inorganic origin. Furthermore, existence a three-phase system (gaseous aqueous solid carbon) allows significant isotopic exchange. The estimation total dissolved requires use model. Existing models determination 14C (A0) (1) Vogel's model (A0 = 85 ± 5% carbon), (2) Tamers' diluted by origin), (3) Pearson's mixing these same sources carbon, evidenced their stable isotope content), (4) Mook's (same as plus correcting term accounting an exchange closed with CO2). A new presented which assumes simple end-members carbonate). Their relative proportions given comparison molality (measured bicarbonates calculated H2CO3, assuming equilibrium at temperature aquifer) (DIC). DIC determined on basis alkaline-earth molal concentration corrected gypsum dissolution base treatment can more simply substituted measurement field alkalinity, whose value, expressed concentration, identical twice DIC. In addition mixing, also leads expression, calculated, part contribution either or carbonate equilibrium, open system, other reservoir. different applied results confined aquifer ‘calcaires carboniferes’ northern France Belgium. value −24‰ (equivalent about −22.8‰ gaseous CO2) adopted 13C content dioxide. Values fractionation occurs between carbon-bearing compounds temperatures values Mook et al. (1974) Deines (1974). It assumed ∈14C% ≃0.2∈13C‰. Radiometric ages obtained Vogel Tamers approaches undercorrected. appears very sensitive variations δ lead overcorrected values. Pearson rather agreement our range average A0. For extremely high low A0 provides larger corrections. discussed 18O waters, appear too attributed glacial epochs. Average flow velocities 1.70 m yr−1 0.73 yr−1.