Re: When is a Year Not a Year?

[Jeff Hecht <jeff@jeffhecht.com>]

At 5:37 PM -0400 7/8/02, philidor11 wrote:
> (Radiocarbon years do not equal calendar years because the amount of 
> radiocarbon in the atmosphere through time has not been constant; it 
> varies as much as 15 percent, making radiocarbon time older or 
> younger than calendar time.  In the case of Clovis, a radiocarbon 
> age range of 11,000 to 11,600 years is actually about 13,000 to 
> 13,500 calendar years.  All dates in this article are in radiocarbon 
> years.)
>
> And I wonder if the same ratio of difference (11,300/13,250) applies 
> to all dates used.  Clarifies my mind.
> Anyway, when we get substance-based dates for dinos, can I assume 
> they are calendar years, or should I do an equivalent to my 
> meters-to-feet conversion calculation?
> Gee.
Other radioisotope years are "real" years. The problem with 
radiocarbon dates is that the radioactive carbon-14 is produced by 
high-energy particles hitting the upper atmosphere and causing 
nuclear reactions that produce the radioactive isotope. The charged 
particle flux varies over time. "Radiocarbon years" are calculated on 
the assumption that radiocarbon production is constant, so they're a 
first approximation. Conversion tables have been developed that can 
convert those radiocarbon dates to real calendar years, which are 
more meaningful, but there's no simple conversion formula because the 
radiocarbon production varies quite a bit, with no simple 
relationship available. (Archaeologists can calibrate with tree ring 
and ice core data, for example, because they record each year in a 
countable way.) -- Jeff Hecht
--
Jeff Hecht, science & technology writer
jeff@jeffhecht.com; http://www.jeffhecht.com
Boston Correspondent: New Scientist magazine
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