Dear Bill,

I raised this subject on ISOGEOCHEM many years ago. I still can't
understand the logic. If it's OK to heat a sample at 200oC to "remove"
organic matter then I don't see any reason to heat a sample to any
greater temperature because the objective has already been achieved. In
fact I would suggest that it's a pretty stupid thing to do, particularly
if you're working with poorly structured biogenic carbonate. Or are we
saying that organic matter in calcite is much 'tougher' than organic
matter in aragonite?

Conversely, if heating to 350oC (or whatever) is what's required then
"roasting" can't be suitable for aragonite. Then one is forced to
conclude that any aragonite only heated to 200oC may still contain
reactive organic matter and any resultant isotopic data may be
compromised.

However, I would take this organic question much further. How do you
know when a treatment has been successful? For instance, there has been
some ISOGEOCHEM discussion that suggests that organic matter in contact
with phosphoric acid does not cause any problems. If this were the case
then you shouldn't need to use any organic removal techniques. Of course
this might be OK at 25oC, but most labs now use autoprep systems with
hot acid. Not only can the hot acid hydrolyse the organics, but immature
organic matter must start to breakdown thermally regardless of any acid
hydolysis.

I have tried different procedures (chlorox, oxygen plasma ashing, vacuum
roasting), but I can't give a definitive answer for the best method. For
example, I've tried treating different types of biogenic carbonate with
each technique and have found no systematic affects. Sometimes chlorox
and plasma ashing would give similar results and roasting different
values, for another sample the outcomes would be entirely different.
Sometimes carbon would be affected and not oxygen, sometimes the reverse
would be the case. This lack of any consistent behaviour is difficult to
understand and means that it is impossible to decide which procedure is
best.

As far as I'm concerned, trying to test different methods using
artificial mixtures is problematic because you need to intimately mix
suitable samples of pure carbonate (poorly structured carbonate, not
nice highly crystalline inorganic carbonate) with the right sort of
organic molecules (pigments, proteins, lipids, carbohydrates etc).
Perhaps using otolith, echinoid carbonate or low temperature inorganic
carbonate precipitate as a source of "organic-free" calcite might be a
starting point - not Iceland spar. In addition, we've been running lake
marls at Liverpool for a number of years and it is evident that organic
contaminants exist on virtually a molecular scale, so it's difficult to
see how you could accurately mimic a natural sample by mechanical
mixing. One possibility would be to prepare a low temperature
precipitate in the presence of organic acid anions?

I'd also like to mention a problem that "roasters" don't seem to
mention. Biogenic carbonate contains variable amounts of
stucturally-bound water. So in addition to any possible exchange between
the thermal decomposition products of organic molecules (perhaps bound
on the crystallite scale), you also have to consider the effects of
exchange between hot "H2O" and carbonate where water is present on a
molecular scale. In fact it may turn out that water is a more serious
problem for "roasters" than organics, and that variable amounts of
structurally bound water (either as H2O or OH) may account for the
apparent varying affects seen in different types of biogenic carbonate.

To be honest it strikes me that not every one can be right every time
and that at least some published data, based on samples treated using
each of the common organic removal techniques, must be compromised. I
couldn't begin to say how one might attempt to decide which data are
acceptable. The fact that you can decide that a data set "makes more
sense if a correction is applied" seems risible. How is it possible to
arbitrate in these cases? The idea that some data might seem more
palatable than others must surely only be a personal opinion. Of course
some data will appear completely unacceptable (outside what is
scientifically possible), but then where do you draw the line between
what is acceptable and what is "wrong"? Perhaps we should build in an
uncertainty to account for this. The problem here is that many labs
claim such high levels of precision, and that carbonate data are used to
make extremely fine distinctions in interpretation, that many
researchers/reviewers won't accept "realistic" uncertainty estimates.

At is stage it strikes me that we may unwittingly risk creating a
pseudo-science. Unfortunately, a detailed investigation of this problem
is not the sort of thing that attracts research funding. We may have to
face the fact that we may never fully know either the consequences of
organic contamination on our isotopic data or the effects of techniques
that attempt to remove organics.

Finally, I'd like to apologise for the length of this message. Once I
started to reply to Bill's message I couldn't stop. I also realise that
much of the contents may be unacceptable to many List Members, so I
should also apologise for any offence I may have caused.

I look forward to the implications of my message being "shot down in
flames".

Regards,

Steve Crowley


Department of Earth & Ocean Sciences
University of Liverpool
4 Brownlow Street
Liverpool
L69 3GP
UK

0151 794 5163/5164