Radiocarbon Dating - Carbon Exchange Reservoir

Carbon Exchange Reservoir

Libby's original exchange reservoir hypothesis assumes that the exchange reservoir is constant all over the world. The calibration method also assumes that the temporal variation in 14C level is global, such that a small number of samples from a specific year are sufficient for calibration. However, since Libby's early work was published (1950 to 1958), latitudinal and continental variations in the carbon exchange reservoir have been observed by Hessel de Vries (1958; as reviewed by Lerman et al.). Subsequently, methods have been developed that allow the correction of these so-called reservoir effects, including:

• When CO₂ is transferred from the atmosphere to the oceans, it initially shares the 14C concentration of the atmosphere. However, turnaround times of CO₂ in the ocean are similar to the half-life of 14C (making 14C also a dating tool for ocean water). Marine organisms feed on this "old" carbon, and thus their radiocarbon age reflects the time of CO₂ uptake by the ocean rather than the time of death of the organism. This marine reservoir effect is partly handled by a special marine calibration curve, but local deviations of several hundred years exist.

• Erosion and immersion of carbonate rocks (which are generally older than 80,000 years and so shouldn't contain measurable 14C) causes an increase in 12C and 13C in the exchange reservoir, which depends on local weather conditions and can vary the ratio of carbon that living organisms incorporate. This is believed negligible for the atmosphere and atmosphere-derived carbon since most erosion will flow into the sea. The atmospheric 14C concentration may differ substantially from the concentration in local water reservoirs. Eroded from CaCO₃ or organic deposits, old carbon may be assimilated easily and provide diluted 14C carbon into trophic chains. So the method is less reliable for such materials as well as for samples derived from animals with such plants in their food chain.

• Volcanic eruptions eject large amount of carbon into the air, causing an increase in 12C and 13C in the exchange reservoir and can vary the exchange ratio locally. This explains the often irregular dating achieved in volcanic areas.

• The earth is not affected evenly by cosmic radiation, the magnitude of the radiation depends on land altitude and earth's magnetic field strength at any given location, causing minor variation in the local 14C production. This is accounted for by having calibration curves for different locations of the globe. However this could not always be performed, as tree rings for calibration were only recoverable from certain locations in 1958. The rebuttals by Münnich et al. and by Barker both maintain that while variations of carbon-14 exist, they are about an order of magnitude smaller than those implied by Crowe's calculations.

These effects were first confirmed when samples of wood from around the world, which all had the same age (based on tree ring analysis), showed deviations from the dendrochronological age. Calibration techniques based on tree-ring samples have contributed to increased accuracy since 1962, when they were accurate to 700 years at worst.