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Author Topic:   14C calculations
Coyote
Member (Idle past 799 days)
Posts: 6117
Joined: 01-12-2008


(1)
Message 3 of 10 (714581)
12-24-2013 12:13 AM


Can't help you with your specific question.

In dealing with >600 radiocarbon dates I've obtained, I've always used either the measured age and calibrated using Calib. 4.3, or used the conventional age and calibrated using Calib. 5, 6, or now 7. Calib. 7 includes the new IntCal13 and Marine13 calibrations curves.

When you calibrate using the recent curves, such as IntCal13 and Marine13, a lot of factors are taken into account, so you don't have to deal with them individually.

[By the way, one of your quotes is from Taylor (1987). I worked with Taylor on a proposal for a radiocarbon project a number of years ago, but it never came together.]


Religious belief does not constitute scientific evidence, nor does it convey scientific knowledge.

Belief gets in the way of learning--Robert A. Heinlein

How can I possibly put a new idea into your heads, if I do not first remove your delusions?--Robert A. Heinlein

It's not what we don't know that hurts, it's what we know that ain't so--Will Rogers

If I am entitled to something, someone else is obliged to pay--Jerry Pournelle

If a religion's teachings are true, then it should have nothing to fear from science...--dwise1


Replies to this message:
 Message 4 by herebedragons, posted 12-24-2013 8:53 AM Coyote has responded
 Message 7 by RAZD, posted 12-24-2013 2:24 PM Coyote has acknowledged this reply

  
Coyote
Member (Idle past 799 days)
Posts: 6117
Joined: 01-12-2008


Message 5 of 10 (714614)
12-24-2013 11:44 AM
Reply to: Message 4 by herebedragons
12-24-2013 8:53 AM


Re: Examples?
Sure!

Here is a radiocarbon date I received a few years ago. The measured age was 4980±40 and the conventional age was 5390±40.

The conventional age was calculated by the laboratory using a C13/C12 measurement of -0.1. This corrects for isotopic fractionation, as the heavier isotopes (C13 and C14) are taken up into the food chain at lower rates than C12. This sample was a piece of mussel shell. The C13/C12 figure for mussel shell, at -0.1, is very typical of what we see.

The laboratory gave a calibrated age of BC 3660-3490 (5610-5440 BP) at 2 sigma, with an intercept of 5560 BP. As this sample was shell, they used the marine reservoir correction (Delta-R) of 225±35. Given that this sample was submitted several years ago, the calibration curve they used was IntCal09/Marine09.

Recalibrating this using Calib. 7.0 with the newer IntCal13/Marine13 gives the following at 2 sigma:

BC 3696-3449 and BP 5645-5398, with the intercept remaining at 5560 BP.

Calibrating this using the old Calib. 4.3 (using Marine98) gives the following:

BC 3659-3474 and BP 5608-5423, with the intercept at 5548.

So, the intercept changed by 12 years between the 1998 marine dataset and the two newer marine datasets. This is insignificant as we're dealing with ±40 in the initial measurement anyway.


Religious belief does not constitute scientific evidence, nor does it convey scientific knowledge.

Belief gets in the way of learning--Robert A. Heinlein

How can I possibly put a new idea into your heads, if I do not first remove your delusions?--Robert A. Heinlein

It's not what we don't know that hurts, it's what we know that ain't so--Will Rogers

If I am entitled to something, someone else is obliged to pay--Jerry Pournelle

If a religion's teachings are true, then it should have nothing to fear from science...--dwise1


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 Message 4 by herebedragons, posted 12-24-2013 8:53 AM herebedragons has not yet responded

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Coyote
Member (Idle past 799 days)
Posts: 6117
Joined: 01-12-2008


Message 8 of 10 (714627)
12-24-2013 2:25 PM
Reply to: Message 6 by RAZD
12-24-2013 1:49 PM


Re: Examples?
From the bottom of Beta Analysic's report page:

Dates are reported as RCYBP (radiocarbon years before present, "present"=AD 1950). By international convention, the modern reference standard was 95% the 14C activity of the National Institute of Standards (NIST) Oxalic Acid (SRM 4990C) and calculated using the Libby 14C half-life (5568 years). Quoted errors represent 1 relative standard deviation statistics (68% probability) counting errors based on the combined measurements of the sample, background, and modern reference standards. Measured 13C/12C ratios (delta 13C) were calculated relative to the PDB-1 standard.

The Conventional Radiocarbon Age represents the Measured Radiocarbon Age corrected for isotopic fractionation, calculated using the delta 13C. On rare occasions where the Conventional Radiocarbon Age was calculated using an assumed delta 13C, the ratio and the Conventional Radiocarbon Age will be followed by "*". The Conventional Radiocarbon Age is not calendar calibrated. When available, the Calendar Calibrated result is calculated from the Conventional Radiocarbon Age and is listed as the "Two Sigma Calibrated Result" for each sample.

I have never worried about the difference between the original half-life of 5568 and the modern one of 5730. As noted above, most reporting uses the old figure, and often there is a note saying to multiply by 1.03 if you want to convert to the new one.

Here is a page describing Beta Analytic's report procedures and report form:

http://www.radiocarbon.com/carbon-dating-results.htm

Have fun!


Religious belief does not constitute scientific evidence, nor does it convey scientific knowledge.

Belief gets in the way of learning--Robert A. Heinlein

How can I possibly put a new idea into your heads, if I do not first remove your delusions?--Robert A. Heinlein

It's not what we don't know that hurts, it's what we know that ain't so--Will Rogers

If I am entitled to something, someone else is obliged to pay--Jerry Pournelle

If a religion's teachings are true, then it should have nothing to fear from science...--dwise1


This message is a reply to:
 Message 6 by RAZD, posted 12-24-2013 1:49 PM RAZD has responded

Replies to this message:
 Message 9 by RAZD, posted 12-24-2013 2:29 PM Coyote has responded

  
Coyote
Member (Idle past 799 days)
Posts: 6117
Joined: 01-12-2008


(2)
Message 10 of 10 (714633)
12-24-2013 2:58 PM
Reply to: Message 9 by RAZD
12-24-2013 2:29 PM


Re: Examples?
We have found there is far more error to be found in sample selection than in anything the laboratory can do.

Example: in our area, the oldest abalone shell has been dated at 5910 BP, while the oldest mussel shell is 9420 BP. The lndians started using mussels early, and abalones, which take a lot more work, much later. If you date only abalone shells you miss out on the earliest 3,500 years of our prehistory.

Another example: some archaeologists are still using multiple pieces of shell in their dates. In any site that has gophers or squirrels, that almost certainly guarantees an error. We dated one site, using single pieces of shell, and found that they were completely randomized with respect to depth. But those single pieces, even though they had been moved around, still gave good dates. Including a large number of shell fragments in your sample will completely homogenize your date: if you have a two component site, Early and Late, that's a good way to get Middle Period dates even though the site wasn't occupied at that time.

A really sneaky one: changing populations and patterns of exploitation over time resulted in early mussel shells generally being pretty thick and robust, but shells from later in time were thinner. This reflects over-exploitation of the mussel beds where the mussels aren't given a chance to grow up. For a while in trying to date one site we were selecting just the nice thick shells, and we kept getting Early Period dates. Only when we selected thin shells did we get more recent dates.

Given all of these sources of bias, and a few others, we don't worry so much about what the laboratory does. We worry far more about what we and other archaeologists are doing!


Religious belief does not constitute scientific evidence, nor does it convey scientific knowledge.

Belief gets in the way of learning--Robert A. Heinlein

How can I possibly put a new idea into your heads, if I do not first remove your delusions?--Robert A. Heinlein

It's not what we don't know that hurts, it's what we know that ain't so--Will Rogers

If I am entitled to something, someone else is obliged to pay--Jerry Pournelle

If a religion's teachings are true, then it should have nothing to fear from science...--dwise1


This message is a reply to:
 Message 9 by RAZD, posted 12-24-2013 2:29 PM RAZD has acknowledged this reply

  
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