Science News

Carbon-dating ivory

Thu, 4th Jul 2013

Mark Peplow

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Nuclear fallout from Cold War nuclear tests could help to identify poached ivory.

Poaching of elephant tusks and rhino horns is at an all time high, according to (c) Frank G. and Frances Carpenter" alt="Ivory tusks" />a CITES report released last year, with about 30,000 elephants killed each year. But taking tusks for ivory hasn’t always been illegal – a global trade ban only came into effect in 1989 - so when those policing the ban seize an ivory artifact, they need to know when it was actually taken from the animal.

That's where the fallout comes in. It helps with a particular type of radiocarbon dating, which relies on measuring the concentration of carbon-14, a radioactive isotope with a half life of about 5,700 years. Carbon-14 is produced when energetic neutrons hit nitrogen atoms; normally, these neutrons are produced when cosmic rays smash into our atmosphere.

Plants absorb that radioisotope along with the carbon dioxide they take in. Once they die, the radioactivity slowly dies away as that stock of carbon-14 decays. Measuring where a piece of plant matter is on the decay curve allows you to calculate its age.

There's generally an uncertainty of a few decades on samples younger than 10,000 years, and you need various fiddle factors in there to get a reliable answer, not least because of human activity skewing carbon-14 concentrations. The nuclear bomb tests of the 1950s and 1960s, for example, threw lots of hot neutrons into the atmosphere that almost doubled carbon-14 levels.

Now scientists led by Kevin Uno at Columbia University have proved that you can get accurate dates on relatively recent material by doing radiocarbon dating from the bomb curve itself. Publishing in PNAS, They took 29 samples of hippos teeth, elephant tusks, and hair, that were collected on known dates between 1905 and 2008 in East Africa, and measured the levels of carbon-14 using accelerator mass spectrometry.

They calibrated the results against the detailed measurements of carbon-14 made around the world since those bomb tests. This allowed them date any tissues formed after 1955 to within a year or so – in some cases, within 4 months.

This could form part of a suite of tools in ‘wildlife forensics’ – such as DNA analysis – which is helping to pinpoint exactly where and when a piece of ivory was actually taken. And it also provides valuable evidence to shore up prosecutions.

Those involved in stamping out ivory poaching need all the help they can get. That illegal trade is estimated to be worth $5-10 billion per year, and it has grown markedly since 2006.

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Would the age of the animal also be a factor?  So, for example, a 70 year old animal would have formed at least part of the tusks before the end of WWII.  CliffordK, Fri, 5th Jul 2013

Interesting point! I've written to Mark to alert him to this; let's see what he says... chris, Sat, 6th Jul 2013

Intriguing story. You could presumably get a second handle on the date from strontium 90, which did not appear in the atmosphere at all before 1945, and has decreased since the end of atmospheric nuclear tests in the 1960's. alancalverd, Wed, 10th Jul 2013

Mark, the author of the news story, ran into difficulties with the forum anti-spam system, so he sent me this in reply to Clifford's comment:

As per the story >>This allowed them date any tissues formed after 1955 to within a year or so ...

So they only got accurate ages on tissue formed after that year. Here's how they do that (from the paper) >> “Maximum accuracy with respect to the known
age is achieved by using tissues that undergo little or no turnover. Samples collected from the proximal, or most recently formed, portion of the tissue can
be used to determine date of collection, which is often, but not always, death.”

cheers
mark chris, Thu, 11th Jul 2013

Thanks Chris,

So, if one recovered a whole tusk, one could date the end closest to the skull, and get a good estimate of when the animal died.

But, for example, I have some ivory beads, and Mom has an ivory chess set that was given to her as a gift. 

So, if I tried to sell the chess set, or necklace, one would want to determine that they were manufactured before the international ban on importing ivory (of course, non destructive methods of analysis would be good).  But, already in manufactured form, I would think it would be difficult to determine where in the tusk they were located.  CliffordK, Sat, 13th Jul 2013

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