Radiocarbon dating and calibrating the obtained date into calendar years is a crucial step in understanding the timing and chronology of past events. Archaeologist is responsible in deciding which sample dates the event they want to date; they are also responsible in taking the sample and filling in the requested sampling reports for the collections and each laboratory preforming the radiocarbon dating service needs them to fill in a specific sample list (online or sent via email).  

Data Acquisition  

The radiocarbon lab gives you the following raw data: i) radiocarbon date in years BP (Before Present) and ii) uncertainty, that is the standard deviation or uncertainty associated with the radiocarbon date, iii) lab identifier, for organic materials also iv) d13C value will be provided (measured with AMS), and for bone you need to specifically ask for v) d13C and d15N values measured with EA-IRMS. 

To calibrate the radiocarbon dates there are several programmes available: 

1. OxCal developed by the Oxford Radiocarbon Accelerator Unit: https://c14.arch.ox.ac.uk/oxcal.html  

1. BCal, developed and hosted by the School of Mathematical and Physical Sciences at the University of Sheffield: https://bcal.shef.ac.uk/ 

1. ChronoModel is an open-source application developed at CNRS: https://chronomodel.com/ 

1. R-packages allowing calibration of radiocarbon dates like rcarbon: https://github.com/ahb108/rcarbon and  

OxCal Scripts and Data Analysis Tools  

Since OxCal is far the most used program, either online or via a standalone version, to calibrate the radiocarbon dates, we will briefly describe its workflow here. 

Input data: use the R_Date() function to input the radiocarbon date and its uncertainty 

-> Select calibration curve: choose the appropriate calibration curve (e.g., IntCal20 for Northern Hemisphere terrestrial samples) -> Run calibration: execute the calibration process to convert the radiocarbon date into a calendar date range (BCE/CE or AD) -> the program will generate a probability distribution of the calibrated date -> review results: Analyse the output, which includes the calibrated date range and probability distribution -> interpret the results in the context of your research question. 

Detailed descriptions about various scripts are available in the OxCal online manual: https://c14.arch.ox.ac.uk/oxcalhelp/hlp_contents.html  

Data reliability / limitation of method 

Major challenges that affect the reliability of radiocarbon dates and their calibration into calendar years cab ne summed as following: 

1. Fluctuations in atmospheric carbon-14. The concentration of carbon-14 in the atmosphere has varied over time due to factors like solar activity and geomagnetic field changes. These fluctuations require calibration using dendrochronology (tree rings) and other methods.
2. Calibration curves. Building accurate calibration curves is a complex and time-consuming process. It involves thousands of radiocarbon dates and specialized knowledge in dendrochronology. The curves can be “wiggly”, reflecting periods of higher or lower carbon-14 production rather than a steady state.
3. Measurement precision. The precision of radiocarbon dating instruments can affect the accuracy of the results. Instrument limitations and sample preservation issues can lead to errors.
4. Sample preservation. Poor preservation of samples can result in carbon loss, which affects the reliability of the radiocarbon date.
5. Reservoir effects. Samples from marine environments or certain freshwater systems can have apparent ages that are older than their actual age due to the reservoir effect, where carbon-14 levels differ from those in the atmosphere.
6. Contamination of samples with modern carbon can lead to inaccurate dates. Knowledge about the context of the sample together with careful handling and preparation of samples are essential to avoid contamination.

Fig. Quality assurance of the ORAU.

References

https://www.cambridge.org/core/journals/radiocarbon/article/radiocarbon-calibration-from-bane-to-blessing/8CDC01B87F2257B1A9DC709748BE07D5