radiation levels over geologic time

[Andrew Karam <karam.1@osu.edu>]

Dear Harold,

I am completing some research on changes in Earth's background radiation
levels over geologic time due to geologic and internal emitters.  Internal
dose is, of course, due to K-40 and has decreased by a factor of 8 or so in
the 3.8 billion years life has existed.  

Dose from geologic materials is more difficult to determine because the
chemical composition of the earth's crust has changed over time in addition
to the decay of uranium.  I can't go into much detail here, but roughly
speaking, beta-gamma dose from geologic emitters started out at about the
same levels as today, increased as the continental crust formed (most of
the U, Th, and K in the earth have been partitioned into the crust over
time), and then decreased again to present levels.  The maximum levels
depend on whose model of continental crust formation you choose to select,
but were probably not less than a few hundred mr/yr at a time of about 2.5
billion years ago (plus or minus a few hundred million years) and were
probably not more than 500 mr/yr about 3 billion years ago.  Radon levels
should have reflected U levels in the rock since, before the colonization
of land, soil would not have existed.  But radon levels were probably not
important.

Keep in mind that, for nearly 3 billion years, life consisted primarily of
algal mats and single-celled organisms floating in the ocean.  Without
lungs or gills to pump water or air in and out, the alpha dose was probably
very small because they only would have been exposed to alphas within one
path length of the cells.  Also, until about 2 billion years ago, the
earth's atmosphere was anoxic, so U and Th were insoluble in water
(actually, Th is still pretty insoluble).

I won't go into more detail here, but would be happy to provide you with my
results if you would like further detail.  I'm currently finishing up the
paper for publication, so (fingers crossed) there may be something in print
sometime in the future.  Incidentally, my co-author and I have also looked
at the possible effects of this on the accuracy of molecular clocks for
dating organism divergence times - it helps to bring the linear molecular
clock models into closer agreement with the geologic record.  We're about
done writing this one, too.  Pretty neat stuff, actually.

Sorry to give such a long answer to such a short question, but it's
something that fascinates me greatly.

Andy



Andrew Karam, CHP
karam.1@osu.edu

Ohio State University Radiation Safety Section
1314 Kinnear Road
Columbus, OH  43221
(614) 292-1284    	(614) 202-7002 (fax)