OxCal calibrations

[cecil frost]

I am an ecologist, somewhat to new to OxCal and have been calibrating radiocarbon dates  for Bison bison from the Neotoma database. The dates are mostly in the last 1000 years. Can anyone tell me why most of them giver me a messages that say "Warning! Date may extend out of range"

Thanks for any insights.

Cecil

[Christopher Ramsey]

Yes - sure - that is a good question.

The reason is that the IntCal20 curve ends in 1950 when the radiocarbon date value is around 200.  If your date is anywhere near this the assumption (lacking any other information) is that there could be a match beyond the end of the curve.  Of course we know that after than the radiocarbon levels rose rapidly but that is not given explicitly in the curve and so the algorithm reports that the date could extend beyond the curve.

I attach an extreme example of this where the last range is unconstrained.

You can get around this by using one the of the bomb curves which have the end of IntCal20 included.  As a default these use a much higher resolution so you might want to set that to 1 which is appropriate in this pre-bomb period.  A comparison command file would look like:

Options()
 {
  Resolution=1;
 };
 Plot()
 {
  Curve("IntCal20","intcal20.14c");
  R_Date("A",200,20);
  Curve("Bomb21NH1","bomb21nh1.14c");
  R_Date("B",200,20);
 };

Which gives a table output like:

and a plot like:

where you can see the final range is now curtailed and there is no error message.

Best wishes

Christopher

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