These instructions, as noted above, can be used to calibrate the Conventional Age for any wood charcoal sample. However, to calibrate shell dates, there are some additional steps.
If the shell samples are all marine in origin, you must use the Marine (marine98.14c) dataset for calibrating these samples.
In addition, are there any locally or regionally available marine reservoir corrections? Go to http://radiocarbon.pa.qub.ac.uk/marine/ to find out. Be sure to incorporate these adjustments into your calibrations, if necessary, and provide a list of the offset that you used.
Alternatively, you may choose to use another calibration program such as OxCal to calibrate and/or report your dates. A benefit of using OxCal is that the graphs are easier to interpret and to use in presentations, although as far as your instructor is concerned, the software itself is not as intuitive to use as CALIB.
Let's use OxCal v.3.9 to calibrate a sample from Trinidad (OS-49084) using the terrestrial (intcal98.14c) dataset option.
Click on the OxCal icon.
File: Analysis Options
Choose your "Reporting" option (e.g., BP or BC/AD)
Choose your sigma "Range"
Click "Browse" and select the appropriate Radiocarbon Calibration Curve (e.g., intcal98.14C for terrestrial samples or marine98.14C for marine samples).
File: Radiocarbon Date
Input information for the sample (R_date = Sample Number)
Here's what your graphic should look like which includes the probability distributions:
Notice the probability distributions in the top right hand corner of the graphic. The area under the curve shows the likelihood of the date on the x-axis. In this graph, the most likely date is about AD 150 or cal. 1850 BP.