Re: [asa] radiometric question

I do know what many fossil mollusks would have been eating, because
they are similar enough to modern species to pin down (and some have
shell features that relate to their lifestyle). The example holds
only for 14C, and mollusk haven't changed much over the relevant time
interval (except that humans have exterminated a couple hundred
species). Also, in the case of ones with old-appearing soft parts
becuase they eat bacteria that eat methane, the 13C ratios will also
be strange. Finally, the fossil environment can be examined-things
that consistently turn up in hydrocarbon seeps have a good chance of
tying into a chemosynthetic food chain, for example.

The key factor is that the conventional dating looks for consistency.
Also, knowing the conditions necessary for a particular dating
technique to work, you can examine a specimen for any evidence that it
may have been negatively affected.

The very first radiometric dates were done in the absence of any
precise estimates of what the expected age would be and yielded ages
much higher than was expected from Kelvin's ideas on physics. No
circularity there. Many other examples can be given of dates that
weren't exactly what was expected, e.g., moving the
Precambrian-Cambrian boundary from 575 (interpolated) to 545 million
in the early 1990's.

As already pointed out, having a rough guess as to what sort of age is
expected helps you to know what techniques will be appropriate and
about what setting would be appropriate in an analysis. It's possible
to analyze a genuine unknown, but it takes more work and thus more
money than if you have a good idea of precisely what measurement you
want.

To take a non-dating example, I'm trying to help go through an old
museum shell collection. When there's a good locality label, I can
look up what species are found there and generally have a good idea of
what it is (if I can find a good reference on them). Others have no
label or a wrong label. This takes a lot more work, trying to check
all the possibilities from around the world.

-- 
Dr. David Campbell
425 Scientific Collections
University of Alabama
"I think of my happy condition, surrounded by acres of clams"
To unsubscribe, send a message to majordomo@calvin.edu with
"unsubscribe asa" (no quotes) as the body of the message.