From: russ@pmafire.inel.gov (Russ Brown)
Date: Thu, 15 Oct 92 13:47:28 GMT
Organization: WINCO
Subject: Re: --Missing Carbon Still at Large
Summary: Aaaaaaarrrrgh 2
Followup-To: 
Organization: WINCO
Keywords: 

In article <1992Oct15.021933.13425@grebyn.com> rmg3@grebyn.com (Robert Grumbine) writes:
>In article <1466601801@igc.apc.org> Tom Gray <tgray@igc.apc.org> writes:
>>
>>Carbon from human sources can be distinguished from that in the
>>atmosphere because it contains a different ratio of the radioactive
>>isotope carbon-13, the magazine said, and researchers measuring CO2
>>in the oceans have concluded they absorb 2.1 billion tons of carbon
>>annually.  However, that number remains in dispute, with other
>>scientists believing it may be too high.
>
>  ARRRRGH!!
>
>  Sorry.
>
>  Carbon-13 is NOT radioactive.  Carbon comes in three isotopes.  Most
>of it is Carbon-12, which is stable.  A little bit is Carbon-13, which
>is STABLE.  An even tinier fraction is Carbon-14, which truly is 
>radioactive, with a half life around 5700 years.  The Carbon-14 is 
>created in the atmosphere when cosmic rays strike a Nitrogen-14 atom
>and convert a proton to a neutron.  All carbon-14 comes from the
>atmosphere, so sources from the earth can be neglected.  (PS, strictly
>speaking, not _all_ C-14 comes from this.  Some is also created in
>nuclear bomb tests, particularly those conducted above ground.  This
>fact is also used by oceanographers.)
>
>  Carbon-14 and Carbon-13 can both be used to diagnose human influences
>on the carbon cycle.  When we burn fossil fuels, we burn carbon sources
>with distinctive signatures.  The fossil fuels contain no carbon-14,
>since they were created millions of years ago.  (That they don't is
>a direct observation.)  Also, the plant matter that went in to making
>the fossil fuels is depleted, relative to the global inventory, in 
>carbon-13.  This happens because your high school chemistry teacher
>fibbed.  Isotopes are _not_ chemically identical.  The less massive
>isotopes are _slightly_ more reactive.  So plants (and people and
>anything else using carbon chemistry) tend to absorb Carbon-12 in 
>preference to Carbon-13.  The difference is slight, but measureable.
>
>  So, as a tracer, both isotopes can be used for tracking the carbon
>cycle.  Carbon-13 is preferable for longer periods because it stays
>around.  One line of research (consult the last 2-3 months of Science
>and Nature) is to measure the Carbon-13 trapped in air bubbles in the
>polar ice caps.  Results of the ice cap work suggest that the rise in
>atmospheric CO2 levels that accompanied the end of the last ice age was
>an even more complex process than we'd been thinking.  

Carbon-14 can also be produced by neutron activation of carbon-13 and
oxygen-17.  The natural steady-state global inventory is about 310
megacuries, and of this, about 5.9 megacuries is in the atmosphere.  The
annual contribution to the global inventory from U.S. nuclear facilities
(both power and defense) is about 540 curies.

When it undergoes radioactive decay, it releases a 0.157 MeV beta.

All in all, there are 13 isotopes of carbon, C-8 through C-20.

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