Plants grown before fossil fuel depletion of atmospheric CO2 are enriched 1-2 ‰ relative to the above averages (Tieszen & Fagre 1993a).
1997), use C4 photosynthesis resulting in less discrimination against 13C and an average δ13C value of -12.5 ± 1.1 ‰ (n=455) (Cerling et al. Cacti and some members of the Agavacaea (yucca and agaves) use the CAM pathway, which alternates between C3 and C4 photosynthetic mechanisms and can produce isotope signatures as positive as C4 photosynthesis.
Samples more recent than 300 BP are indistinguishable from late 19 samples because of the Suess effect while at 50,000 BP; measurements are approaching the limit of detection.
A radiocarbon age is calculated from a re-arranged form of the first order decay equation as follows: where t = the time that has elapsed since removal of the carbonaceous material from the carbon cycle (C), again as measured in the radiocarbon laboratory.
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The current internationally-accepted value for living, terrestrial carbonaceous matter was determined from tree rings formed in the year 1890 and is quoted as 0.226 Bq g was observed due to the burning of fossil fuels (Suess, 1953; 1955), often termed the Suess effect.
Then, during the 1950s and early 1960s, the atmospheric testing of nuclear weapons increased the atmospheric C activity.
Subsequent consumption of plants by animals (and animals by other animals) transfers the C enrichment between the well mixed atmosphere and the terrestrial flora and fauna that atmospheric carbon supports, provided that due corrections are made for the degree of isotopic fractionation that takes place during the initial uptake and subsequent metabolic fixation of atmospheric carbon by the primary producers (plant life) and thereafter, any isotopic fractionation that occurs during subsequent transport through the food chain.
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Direct AMS results are not calibrated, nor corrected for the marine reservoir effect, as a number of site-specific calibration curves may be considered.
Calibrations performed by programs like CALIB and Ox Cal incorporate the compilations of INTCAL13.