# And today is -60...February 19, 2010 10:14 PM   Subscribe

What is today's date in the BP (carbon dating) calendar?
posted by eleanna to Science & Nature (12 answers total) 4 users marked this as a favorite

Doesn't the 'P' stand for 'present'?
posted by hattifattener at 10:41 PM on February 19, 2010

Ah, Wikipedia says that the "present" is defined as 1950 for radiocarbon dating purposes. (Which I find odd, since radiocarbon dating does actually date the object relative to the time you do the test.) So, yeah, today is year -60 BP.
posted by hattifattener at 10:45 PM on February 19, 2010

I'm not sure what you mean by "today's date," but if you're asking what the "present" is in Before Present, it's January 1, 1950. Here is some more information about how that decision was made.
posted by cirripede at 10:45 PM on February 19, 2010

Yup. I guess I should have put more information in here. The calendar sets 0 at 1950. So, for example, if carbon dating put a sample at 1930, the date would be 20 BP. I'm trying to figure out what a date after 1950 would look like.
posted by eleanna at 10:59 PM on February 19, 2010

Dates after 1950 wouldn't be represented on that scale.
posted by Chocolate Pickle at 11:00 PM on February 19, 2010

And...I should have previewed. So the scale doesn't represent dates after 1950?
posted by eleanna at 12:20 AM on February 20, 2010

Carbon-14 has a half-life of 5700 years, so if you measure its concentration with an absolute precision of 1%, you are probably wrong by 60 years anyway.
posted by fantabulous timewaster at 5:01 AM on February 20, 2010

Chocolate Pickle: "Dates after 1950 wouldn't be represented on that scale."

Why not? The simple logarithmic formulae for radiocarbon dating can produce positive or negative ages in years BP.
posted by turkeyphant at 5:04 AM on February 20, 2010

After 1950, atmospheric nuclear tests changed the amount of carbon-14 in the atmosphere. To get a meaningful date, not only would you have to measure the C-14 abundance very precisely (since only 1% of the half-life has elapsed since 1950), but you'd have to estimate a fold in a version of the way atmospheric C-14 changed over time at the location of your sample. Carbon dating just isn't useful on young samples. It also isn't useful on very old samples, where there may not be a measurable amount of carbon-14 left; there you use the decay of potassium or uranium.

Some people have done short-term radioisotope dating uses shorter-lived isotopes. The most notable is probably cesium-137, which has a half-life of 30 years. There was a paper a year or so ago on using cesium dating to determine the age of wines.
posted by fantabulous timewaster at 6:21 AM on February 20, 2010 [1 favorite]

A radiocarbon year is not the same as a calendar year. By convention, the intercept between the two time-measurement schemes is set at calendar date 1950 A.D., which is called "the present". Any other date derived from radiocarbon and expressed in calendar years has to be calibrated first.

That is, if you have a radiocarbon date of 1,000 B.P. you can't just subtract 1,000 from 1,950 and call it a Calendar date of 950 A.D. To be put onto the BC-AD scale the date MUST be calibrated. So, the year 2010 AD has no real meaning in the BP world.

As has been pointed out, bomb testing means any real dates after 1950 will not just be confused by surplus 14C in the atmosphere, but they will actually date into the future. It is conventional to report those dates as "modern", even though with accelerator dating the typical +/- (1 standard deviation) of the measurement error is to 4 significant digits which can translate into +/- of 15 or 20 years routinely.

But two other factors make recent dates problematic: Industrial hydrocarbon burning has diluted the background atmospheric levels of 14C for the last 300 years. And, for very recent dates, you have the opposite problem of very ancient ones: for the latter, too little 14C remains to measure accurately, for the former too little has decayed to accurately measure its absence. (remember the background starting ratio of 14C:12C+13C, from which all deviation is measured as a proxy for time passed, is about 1:1 trillion.)

So, what this all adds up to is: any actual date sample from 2010 (including your fingernails) is going to date from about 2500 AD in radiocarbon terms; AD/BC only has meaning via calibration of dates.

So, to answer your question, a date that had a 14C ratio of 1.0000 would be reported as "modern" by the radiocarbon dating lab and it would be discarded as meaningless. It would not be reported as -60 B.P.
posted by Rumple at 9:08 AM on February 20, 2010 [3 favorites]

Also, keep in mind that the B.P notation not only used 1950 AD as a standard present, it used the original half life of 14C as determined by Libby, of 5568 years. We now know that the halflife is closer to 5730 years (the Cambridge half life) and yet we report all dates from the lab in Libby years. These are known to be wrong, but they are all wrong in the same way. So a raw radiocarbon date, the only kind that you should see B.P. attached to, needs to be multipled by 1.03 just to put it onto accurate half life terms -- one more reason why 2010 doesn't = -60 BP

For those not familiar with radiocarbon dates as reported from the dating lab, I've uploaded an example here.
posted by Rumple at 9:19 AM on February 20, 2010 [2 favorites]

Thank you, Rumple. That's really clear and easy to understand.

And thank you, fantabulous timewaster, for the new information.
posted by eleanna at 9:14 PM on February 20, 2010

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