terminus ante quem 14C dates in glacial retreat

[Alistair Monteath]

Dear Oxcal users,

I am developing a continuous multi-phase model to constrain a chronology of a glacial retreat. I'm using published OSL, 14C and 10Be dates within this model, all of which date different aspects of the ice-retreat. I've been having issues with the 14C dates which drag the model output towards an unrealistically young age because they date material that was deposited an unknown period of time after the ice-retreat. They therefore only provide a minimum (terminus ante quem) age for deglaciation - often several thousand years after retreat. To correct for this I've been using the 'Before' command when inserting 14C dates into each phase (see below). Is this the correct method for this? There aren't many published examples using Bayesian models to constrain ice-retreat so I wanted to make sure I haven't got this wrong. 

End 3 Boundary
ETH-28530 R_Date(8660,70)
Beta-194057 R_Date(8850,40)
ETH-28529 R_Date(8975,60)
ETH-29211 R_Date(9000,70)
Beta-194055 R_Date(9520,40)
ETH-30175 R_Date(9665,75)
 Before
UCLM-E-05 C_Date(-14027.5,1006)
UCLM-E-04 C_Date(-9274.5,712)
UCLM-E-03 C_Date(-9315.5,714)
Holocene deglaciation Phase
Transition 2/3 Boundary
Standstill Interval
CLM-Mr-13 C_Date(-9913.5,745)
CLM-Mr-05 C_Date(-11010.5,813)
CLM-Mr-04 C_Date(-12678.5,922)
CLM-Mr-03 C_Date(-10673.5,794)
CLM-Mr-01 C_Date(-10003.5,751)
UCLM-W-08 C_Date(-10066.5,757)
UCLM-W-07 C_Date(-9959.5,749)
UCLM-W-06 C_Date(-11636.5,873)
Standstill Phase
Transition 1/2 Boundary
WAT-408 R_Date(12650,320)
AA43652 R_Date(12567,49)
SFU-223 R_Date(11700,260)
AECV-1203C R_Date(11620,170)
OxA-14273 R_Date(11620,150)
 Before
CLM-S-06 C_Date(-11087.5,832)
CLM-S-05 C_Date(-12830.5,929)
CLM-S-04 C_Date(-11235.5,857)
CLM-S-03 C_Date(-9102.5,720)
CLM-S-02 C_Date(-14964.5,1056)
ML-BR-07 C_Date(-11073.5,837)
ML-BR-06 C_Date(-11272.5,840)
ML-BR-04 C_Date(-12096.5,883)
ML-BR-03 C_Date(-11486.5,845)
ML-BR-02 C_Date(-11528.5,879)
ML-BR-01 C_Date(-11986.5,900)
PL-SN-05 C_Date(-11351.5,831)
Late Glacial Deglaciation Phase
Start 1 Boundary
 Sequence
 U(0,4)
 T(5)
General Outlier_Model  

[Bayliss, Alex]

Dear Alistair,

 

Yes, you can use BEFORE for a terminus ante quem. However, this is only valid if the material you are dating is definitely not residual (a residual sample cannot provide a terminus ante quem). This is probably why you haven’t found very many examples in the literature.

 

Alex

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[Alistair Monteath]

Hi Alex,

Thank you for your help with this - much appreciated!

Just to be clear, by a residual date (in this context) you mean a date that is associated with the deposition of old material during the advance of the ice?

Also, during the model construction is it best to place all of the terminus ante quem 14C dates within one BEFORE command or apply BEFORE to each date individually?

Thanks again

Ali

[Bayliss, Alex]

Hi Alistair,

 

Reworked material in a deposit from derived from earlier deposit. In your application might you be able to infer that a sample was not reworked based on its character (eg if it is from a species that only grew after the ice)?

 

It will simplify your code if you put all the dates that are before a particular event inside one Before command, but I don’t think it matters if you do each individually (as long as they are in the correct positions in the sequence).

To view this discussion on the web visit https://groups.google.com/d/msgid/oxcal/823eb54f-83bc-4581-a67a-43c20f84ba58n%40googlegroups.com.

[Alistair Monteath]

Thanks Alex,

We should be able to use the relative stratigraphy to deal with this (e.g. above the till etc).

Last question (sorry), the BEFORE command should be placed within each phase as I did in the initial example?

Ali

[Bayliss, Alex]

No brackets below.

 

If ‘OxA-14273’ is a taq for ‘Transition 1/2 Boundary’ and part of the ‘Late Glacial Deglaciation Phase’ phase then yes.

To view this discussion on the web visit https://groups.google.com/d/msgid/oxcal/04965e7c-8abf-49d5-b89e-8fc89ef02a96n%40googlegroups.com.

[Alistair Monteath]

Ok, so the BEFORE command will constrain the boundary after the phase it's placed in, not the other dates within the same phase?

[Bayliss, Alex]

It will constrain the boundary after the phase it is placed in, not the boundary before the phase it is placed in. It will affect the other dates in the phase only in so far as they are affected by the boundary after the phase.

 

A

 

From: ox...@googlegroups.com [mailto:ox...@googlegroups.com] On Behalf Of Alistair Monteath
Sent: 19 January 2021 11:20
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Subject: Re: terminus ante quem 14C dates in glacial retreat

 

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[Alistair Monteath]

Ok, that makes sense.

Thanks again for your time

Ali