Doug Keenan’s long essay about the IPCC treatment of surface temperature trends that I discussed earlier had a digression into his work on the calibration of radiocarbon dates that he published in 2012 in the open access EGU journal Nonlinear Processes in Geophysics.

With the date 4530 ± 50 C BP, the peak of the radiocarbon date’s Gaussian distribution is close to the calibration curve between ~53 BP, so all these dates are assigned a high probability.

So although the radiocarbon date has a Gaussian distribution, the effect of this procedure is to weight parts of that distribution that are close to a plateau more heavily than parts of the distribution where the calibration curve is steep. Keenan wants the Gaussian distribution to be preserved.

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.

When presenting your results, be sure to round off to the nearest "10". Be sure to consider the following: The CALIB program can also plot these results on a graph.

To do this, you need to scroll down until you find the box shown below.

A benefit of using Ox Cal 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.

Ox Cal 3.9v Let's use Ox Cal v.3.9 to calibrate a sample from Trinidad (OS-49084) using the terrestrial (intcal98.14c) dataset option. 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).

But it makes the implicit assumption that the every radiocarbon date is equally likely.

It is easy to demonstrate that this is not true by taking the calibration curve and for each calendar date, finding the probability of each radiocarbon age. The probabilities assigned for each radiocarbon age are then summed. By inspecting the calibration curve, it is obvious that peaks in summed probability coincide with plateaux in the radiocarbon calibration curve.

This analysis gives the distribution of radiocarbon ages expected when dating objects with calendar ages 0BP, 5BP, 10BP, …, 50000BP. For example, the peak at 4480 C BP is the plateau in figure 1.