dealing with a calibration curve plateau

[Erik Marsh]

Hey all,

I have a set of dates from a few adjacent tombs. The dates are similar but not identical, and unfortunately, they fall on a calibration curve plateau, about AD 440–520. Does anyone have any suggestions for dealing with this? I want a more precise date for this burial event but the plateau is thwarting my efforts.

I noticed that the precise AMS dates (±15) are younger than the conventional ones – I don't know why this would be, or if I should trust the AMS dates more since they were processed with more modern methods. There is no apparent spatial pattern in the dates, for example, a single tomb has an earlier and a later date from this set. All of them should be from around the time of internment. However, the agreement index is low.

Most of the time I would expect nine dates from a single context to be enough to get a precise temporal estimate. But in this case, it seems that the best approach is to just use a single calibrated date from the most secure sample. This avoids any assumptions about the depositional sequence.

Any suggestions are welcome!

Erik

 Plot()
 {
  Curve("SHCal20","shcal20.14c");
   Combine("Tombs 1–3")
   {

    R_Date("Beta-208632",1580,40);

    R_Date("Beta-230124",1580,40);
    R_Date("CAMS-55919",1570,50);
    R_Date("Beta-212819",1550,40);

    R_Date("CAMS-55921",1550,40);

    R_Date("UCIAMS-102536",1645,15);

    R_Date("UCIAMS-102537",1615,15);
    R_Date("UCIAMS-102538",1625,15);
    R_Date("UCIAMS-102539",1605,15);
   };
 };

[Lori Barkwill Love]

Hi Erik,

I am familiar with these dates, as I was asked to do Bayesian chronological modeling for the site. I ran into the same problem. Not sure if you are just modeling the tombs or the site as a whole, but I did find that there were some issues with a few of the dates for the site. In the end, I was unable to produce an acceptable model (low agreement and/or low convergence) for the site. Feel free to contact me off list.

Best,

Lori

________________________

Lori Barkwill Love, Ph.D.

Department of Anthropology

University of Texas at San Antonio

lori.bar...@utsa.edu

lo...@tennesseespack.com

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[MILLARD, ANDREW R.]

Hi Erik,

 

The only way to deal with a plateau is to have a sequence of dates to exploit the structure of the calibration curve. On a plateau combining dates to get more and more precise radiocarbon ages doesn’t change the calibrated range much. Even 1605±1 calibrates to a 100-year range.

 

Best wishes

Andrew

--

Dr. Andrew Millard

Associate Professor of Archaeology,

Durham University, UK

Email: A.R.M...@durham.ac.uk 

Personal page: https://www.dur.ac.uk/directory/profile/?id=160

Scottish Soldiers Project: https://www.dur.ac.uk/scottishsoldiers

Dunbar 1650 MOOC: https://www.futurelearn.com/courses/battle-of-dunbar-1650

 

From: ox...@googlegroups.com <ox...@googlegroups.com> On Behalf Of Erik Marsh
Sent: 27 September 2022 15:05
To: OxCal <ox...@googlegroups.com>
Subject: dealing with a calibration curve plateau

 

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