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A new version of the radiocarbon calibration curve for the Southern Hemisphere, Northern Hemisphere and marine environment will bring enhanced accuracy in establishing ages and reconstructing environmental information.


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Radiocarbon calibration

Data are from Reimer et al. Compiled atmospheric bomb radiocarbon curves for 4 different zones Northern Hemisphere zones and Southern Hemisphere zone for age calibration Hua and Barbetti, World map showing the areas covered by the 4 zones Hua and Barbetti, An example of bomb-pulse radiocarbon dating of a terrestrial sample from Northern Hemisphere zone 1.

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For a radiocarbon value measured in a sample S Fsbomb radiocarbon delivers two possible calendar dates T1 and T2indicated by the grey boxes Hua, Radiocarbon dating is one of the most reliable and well-established methods for dating the Holocene and Late Pleistocene. Natural radiocarbon or 14 C is produced in the atmosphere by the interaction of the secondary neutron flux from cosmic rays with atmospheric 14 N.

Following its production, 14 C is oxidised to produce 14 CO 2which is then transferred to other carbon reservoirs, such as the biosphere and oceans, via photosynthesis and air-sea exchange of CO 2respectively.

New radiocarbon calibration curves for a better dating method

Living organisms take up radiocarbon through the food chain and via metabolic processes. When an organism dies, the original 14 C concentration of the organism starts to decrease by radioactive decay. Radiocarbon age of that organism is determined by measuring its residual 14 C concentration and by assuming a constant level of atmospheric 14 C through time.

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However, not long after the establishment of the radiocarbon dating method in the late sit was recognised that the 14 C concentration of the atmosphere in the past had not been constant. Variations in atmospheric 14 C concentrations are mainly due to variations in the rate of radiocarbon production in the atmosphere, caused by changes in the Earth's magnetic field and variability in solar activity, and changes in the carbon cycle.

Radiocarbon calibration

The result is that radiocarbon and calendar ages are not identical, and the radiocarbon ages have to be converted to calendar ages using a calibration curve, which describes the atmospheric 14 C concentration in the past measured in precisely and independently dated materials. The current internationally-ratified calibration curve for terrestrial samples e.

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This curve is based on dendrochronologically-dated tree rings for the periodcal yr before present BP, with 0 BP being AD For the remaining period 12, cal yr BP, the curve is derived from independently dated marine C14 dating calibration curve such as foraminifera and corals. There is a small difference in the natural atmospheric 14 C concentration between the Northern and Southern Hemispheres. As a result, more 14 C in the southern troposphere is transported to the oceans through air-sea exchange of CO 2 and more 14 C-depleted CO 2 from the oceans see discussion later is transported to the southern troposphere.

Natural 14 C levels in the southern troposphere are therefore usually lower than those in the northern troposphere, and the radiocarbon ages of terrestrial materials in the Southern Hemisphere for a particular period of time are usually older than those in the Northern Hemisphere.

The current internationally-ratified radiocarbon calibration curve for terrestrial samples from the Southern Hemisphere is SHCal This curve covers the past 11, cal yr, which is based on the dendrochronologically-dated tree rings for the last millennium and on model ages for the remaining period. During this time the 14 C content of deep ocean waters is depleted by radioactive decay. The surface ocean exchanges with the atmosphere and the 14 C-depleted deep ocean and has a 14 C level intermediate between these two reservoirs.

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Marine samples living in the surface ocean e. To calibrate a radiocarbon date for a surface ocean sample, one can use IntCal04 curve with a known value of R. Alternatively, one can use the current internationally-ratified marine calibration curve Marine04 Fig. The latter method is generally preferred.

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However, recent studies have reported variations of these values of several hundreds to a couple of thousands of years for several regions during Late Glacial and the Holocene. These variations are due to changes in ocean circulation and the carbon cycles associated with climatic change.

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A large amount of 14 C was artificially produced when hundreds of nuclear test weapons were detonated in the atmosphere, mostly in the Northern Hemisphere, in the late s and early s. Nuclear bomb blasts produced intense fluxes of thermal neutrons, which in turn interacted with atmospheric 14 N to form 14 C.

As a result, the atmospheric 14 C level reached a maximum in the Northern Hemisphere inat almost double its pre-bomb level.

Why radiocarbon measurements are not true calendar ages

Since then, the atmospheric 14 C concentration has been decreasing due to rapid exchange between the atmosphere and other carbon reservoirs mainly the biosphere and oceans. The main feature of atmospheric bomb 14 C is that there are ificantly different atmospheric 14 C levels between consecutive years during the bomb period, offering the possibility of dating terrestrial samples formed after by 14 C with a resolution of one to a few years. This dating method is usually called bomb-pulse dating for the interval from onwards to differentiate from traditional radiocarbon dating for the period from backwards.

Four zonal data sets of tropospheric bomb 14 C data at mostly monthly resolution three in the Northern Hemisphere and one in the Southern Hemisphere are available for use in bomb-pulse 14 C dating Figs. Calibration of 14 C ages is usually undertaken using a computer program.

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Several calibration programs are available on-line. Examples of radiocarbon calibration for the traditional radiocarbon dating and the bomb-pulse dating are shown in Figs. Glossary The Holocene: the period covering the last 11, yr. Dendrochronology: is the scientific method of dating based on the analysis of patterns of tree rings.

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Damon, P. Solar and terrestrial components of the atmospheric 14 C variation spectrum. In Sonett, C. University of Arizona Press, Tucson, p. Hua, Q. Radiocarbon: A chronological tool for the recent past. Quaternary Geochronology4,doi Review of tropospheric bomb radiocarbon data for carbon cycle modelling and age calibration purposes.

Radiocarbon 46, Hughen, K. Marine04 Marine radiocarbon age calibration, cal kyr BP. Radiocarbon46, McCormac, F. SHCal04 Southern Hemisphere calibration 0— Reimer, P. IntCal04 Terrestrial radiocarbon age calibration, cal kyr BP. Radiocarbon46, —

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This figure is directly based on the proportion of radiocarbon found in the sample.


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Radiocarbon dating measurements produce ages in "radiocarbon years", which must be converted to calendar ages by a process called calibration.