OFF: what new century?!

[K Henderson]

Eric wondered...

>> Of
>> course, this doesn't get away from the problem that a radiocarbon year and a
>> 'real' calendar year aren't the same.  Like 10,000 BP 14C is really 11,700
>> BP 14Ccal (calibrated to calendar years).
>
>Now this is just plain f***ed.  What does an RC "year" correspond
>to, then?

Well, a year is a year.  But radiometric dating techniques are based on
various methodologies.  Some are based on the loss of parents, others on the
growth of daughters, and even some on the remains or traces of decay events
(like fission tracks and thermoluminescence).  And while the radiocarbon
method is based on the loss of parents (ie., 14C), the problem is that the
parents are continually regenerated in the atmosphere, producing a 'secular
equilibrium' of sorts.  But when an organism dies, no more carbon is
exchanged (usually, or at least hopefully) and so the 'clock' starts at that
point based on the 14C content of the atmosphere at that time.  But the
calculations of age assume that the production rate of 14C remains constant
over time, which it doesn't.  The strength of the magnetic field of the
Earth controls the amount of high energy cosmic radiation that invades and
produces 14C within the atmosphere.  And that isn't constant of course (and
in fact is known to reverse, N pole to S pole, over longer timescales).
Also, there is the problem of the distribution of 14C on the planet once it
forms.  Oceanic overturning (surface waters sinking to the deep ocean) is
not constant, and is believed to be responsible for major and rapid climate
changes in the recent history of the Earth, such as during the 'ice age.'
So the effective removal of 14C from the active hydrologic cycle during
periods of rapid overturning show up in the calibration against real-time
dating (e.g., trees, ice cores).  The "Younger-Dryas Plateau" is the best
manifestation of this.

And then of course, where there is strong upwelling of deep ocean water
(usually 1000 years 'old' or more) along coastlines, radiocarbon amounts (in
shallow marine seashells, for instance) will be less than they 'should' be,
because of decay during the period that the deep ocean water was not in
contact with the atmosphere.  (A global average of the relative age of
marine organisms on the radiocarbon time scale vs. terrestrial organisms is
420 years, i.e., marine organisms appear 420 years older because of the
influence of 'dead' carbon...in strong upwelling areas, this can be well
over 1,000 years.)

Oh, and then there was the strange event (that we're hoping to shed some
light on with these ice cores) that happened c. 37,000 years ago, where
there was a giant spike in the production rate of 14C and the other
cosmogenic radionuclides, like 10Be and 36Cl.  Some think it was a nearby
supernova, but I think that might not be a valid answer, since there's at
least one place that shows a dramatic, though short-lived, climate shift
that would suggest something more internal (to our solar system at least).
Others think maybe it was an aborted magnetic pole reversal, a very
intriguing idea.

Also, solar influences like the Maunder and Daulton minima (few to no
sunspots) of the last 500 years also influence the production
rate/calibration.  And as Jill suggested, humans have completely screwed up
radiocarbon dating for distant future generations, by 1) burning 14C-dead
fossil fuels (the Suess effect as it's called) and 2) nuclear testing, which
has generated far more 14C than anything else in nature.  So radiocarbon
dating over the last few hundred years is pretty worthless.  Instead, 210Pb
is often used for short timescales I think.

I guess that's more than you really cared to know.  :)

>ObPretendingToBeOnTopic:  In base 7237, one of my fave HW instrumentals would
>be "Spiral Galaxy 4".

How old does that make Simon House on 28.9.00?

Keith H. (FAA)