Dear Steve, Bill and others interested,
Tricky thing!
But taking it positively, why not attempting to
compare methods as far as reasonably possible? I
understand the difficulty of selecting the proper
sample to test, but in the absence of an ideal
one, maybe your lake marls would make a good
starting point (fine-grained, abundant and
homogeneous enough???).
If any of you takes the trouble of organizing a
little test, I can contribute facilities for
plasma ashing and vacuum roasting, together with
different carbonate prep systems: off-line
extraction, with reaction temperatures from 25 to
150 deg C (+/-0.1 deg); ISOCARB (common acid bath
at 90 deg C) and Multiflow (single drop at 70
deg).
Any body else willing to have a go at it?
Cheers,
Clemente
>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
--
*******************************************
Dr. Clemente Recio
Laboratorio de Isótopos Estables
Fac. de Ciencias
Univ. de Salamanca
Plaza de la Merced, S/N
E-37008 SALAMANCA
SPAIN
Phone: (+34) 923 29 45 00, Ext. 1540 (Automatic Switchboard)
Fax: (+34) 923 29 45 14
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