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Radiocarbon Dating in Environmental Archaeology
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Practice Calibrating 14C Samples

Let's say that you have considered all of the potential dating and sampling issues. You have sent your samples off to the lab and received the results back. What do you do know?

Because the date is only the conventional age, you need to transform it to calendar years by using a calibration program. Let's try it!

CALIB 4.4

Go to the CALIB internet site: http://radiocarbon.pa.qub.ac.uk/calib/ or open the program on your computer.

Click on Execute Version 4.4 html to use the program online.

Click on the Calibration Options Menu; select the 2-sigma and the AD/BC options. The box should now look like this:

14C Image 02

Now click on the Data Input Menu option to open the Data Entry Form. (You may need to scroll down to see it all.)

In the box labelled Unique Sample ID type in the lab number (WK-2191)

In the box labelled Radiocarbon age BP type in the conventional age (510)

In the box labelled Standard Deviation in age , type in the deviation (50)

If the sample is marine in origin or has a marine component, include the % in the box labelled % Marine carbon; if it is marine shell, you would put 100%. This sample is charcoal, so leave it blank.

Given that these samples are from New Zealand you must change the "Curve Selection" from Northern Hemisphere (NH) to Southern Hemisphere (SH). The remaining boxes do not need to be changed.

Your Data Entry Form should now look like this:

14C Image 04

Now click on the box labelled Enter Data (on the left-hand side) and the data will be entered into the lower screen.

Finally, click on the box labelled Calibrate (in the lower screen) to run the calibration program.

The results will be displayed on the upper screen. (You will need to scroll down again to see them). If everything was entered correctly, you should see something like this:

14C Image 06

These figures tell you that the most likely age of your sample is between AD 1392 and 1504 (a 96.3% chance). Round this figure to AD 1390-1500. It is also possible (though not very likely) that the sample dates to the period between AD1592 and 1616 (3.6%) or AD1325 and 1326 (0.1%). When presenting your results, be sure to round off to the nearest "10".

Now, calibrate the rest of the samples. Be sure to consider the following:

Graphs

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. Select the Probability Distribution option, and then click on Make Another Plot .

14C Image 08

This produces a graph like the one shown below. 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 1440.

14C Image 10

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.


OxCal 3.9v

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)

Click OK

Here's what your graphic should look like which includes the probability distributions:

14C Image 12

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.

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