Dear Max:
You must be reading the wrong journals... :-)
Gaffey S.J. and Bronnimann C.E. (1993) Effects of bleaching on organic
and mineral phases in biogenic carbonates. Jour. Sediment. Petrol. 63:
752-754.
This paper discusses the dissolution of various types of biogenic
carbonate...bottom line is that dilute sodium hypochlrite is the best
material to avoid dissolution of the carbonate phase - which is probably
what will induce an isotopic change.
There are citations of papers in this article:
Love and Woronow (1991) Chem. Geol. 93: 291-301.
Gaffey et al. (1991) Geochim.Cosmochim. Acta 55:1627-1640.
There was also a "recent" paper in Geology (within last two years) about
roasting coral aragonite and a subsequent discussion-reply.
I would start there. I vaguely remember sending a similar message about
a year ago...perhaps the isogeochem chatroom archives has that
discussion...?
Scott Carpenter
Max Coleman wrote:
> Reed
>
> It is a pity that there hasn't been a definitive comparison published
> in a
> well-read journal
>
> A former research student supervised by Chas Curtis and me, Iain
> Scotchman,
> did a thorough comparison of chlorox and plasma treatments as part of
> his
> PhD thesis (Scotchman IC 1984 Diagenesis of the Kimmeridge Clay
> Formation.
> Unpub PhD thesis, Geology Department, University of Sheffield) based
> on the
> method (I think)
> I think the plasma method was first described by
> Goreau TJ 1977 Coral skeletal chemistry: physiological and
> environmental
> regulation of stable isotopes and trace metals in Montastrea
> annularis.
> Proc. Roy. Soc. Lond. B196, 291-315 - note it might be Phil. Trans.
> Roy.
> Soc. Lond. instead of Proc. etc..
>
> (DOES ANYONE KNOW OF OTHER REFERENCES???)
>
> Iain's work was on organic-rich whole rock samples - often mud-rocks
> with
> lots of diagenetic carbonate. I don't know if Iain published his
> comparison elsewhere as part of one his carbonate diagenesis papers.
> In
> case he didn't, the end result is that plasma is perfect and bleach is
> bad
> for these (crushed) whole rock samples. The chlorox was NaOCl solution
> (1%
> w/v). Some of the detailed results follow
>
> 1. For our pure calcite lab standard
>
> Untreated
> d13C d18O
> -0.74 -9.10
> -0.72 -9.16
> -0.70 -9.15
> -0.71 -9.20
> --------------
> -0.72 -9.15 mean values
>
> plasma treated
>
> -0.74 -9.17
> -0.75 -9.25
> -0.69 -9.20
> -0.57 -9.06
> -0.71 -9.17
> --------------
> -0.69 -9.17 mean values
>
> So chlorox does not affect clean samples
>
> 2. The comparison for organic-rich samples is harder as there is no
> absolute standard but the following example of three pairs of
> duplicate
> analyses of samples plasma treated (none of which could not be
> measured
> untreated) show excellent reproducibility
>
> -0.09 -3.54
> -0.10 -3.58
> --------------
> +0.35 -3.56
> +0.35 -3.53
> --------------
> +0.66 -3.07
> +0.66 -3.05
>
> 3. Reproducibity of chloroxed samples was much worse
>
> For 18 pairs of duplicated organic rich samples the extreme samples
> were 1
> per mil apart for d18O
>
> 4. Comparison of chlorox and plasma treatments on 20 different
> org-rich samples
> gave chloroxed samples on average d13C -0.17 & d18O -0.19 relative to
> the
> plasma-ed samples. The difference was larger for the more organic
> rich
> samples but all gave good clean mass-spec runs
>
> 5. We think but (cannot prove) that one of the problems with
> chloroxing
> samples is that it is possible to lose some of the finer grain size
> particles while removing the chlorox solution from the sample - but
> won't
> apply to whole forams etc. This might have two effects
>
> a) it might affect the kinetic fractionation of carbonate-phosphoric
> acid
> reaction
>
> b) it might remove a part of the finer-grained fraction of the sample
> which
> is isotopically different from the bulk - a possible explanation for
> the
> greater effect on org-rich samples
>
> (Iain, if you read this, have you published these results any where?)
>
> I would be grateful to see if anyone else has done a more detailed
> comparison
>
> Max
>
> >There appears to be some difference of opinion in the stable isotope
> community
> >regarding the preferred method for removal of organic material from
> carbonates
> >(calcite in bulk sediment, ostracodes, forams, etc) prior to
> >extraction/analysis
> >for O and C isotopes. Soaking in bleach or hydrogen peroxide
> solutions
> >are time
> >consuming and appear to introduce small shifts in the isotopic
> composition of
> >the carbonate. Vacuum roasting, while more time efficient, also
> introduces
> >errors from what I have heard and read. Plasma ashers appear
> efficient, but I
> >do not know what, if any, changes they effect.
> >
> >I searched the ISOGEOCHEM archive and found that this issue was
> raised about a
> >year ago. Only one response to the initial question was tendered.
> At the risk
> >of being redundant I would like to open the topic for discussion
> again.
> >
> >We have dealt with H2O2 and bleach solutions and are currently
> looking at
> >alternatives to these methods. With this is mind, what are the
> >advantages/disadvantages of using a plasma asher versus vacuum
> roasting for the
> >removal of organic material? Are there any references addressing the
> issue?
> >
> >
> >
> >Reed McEwan
> >Stable Isotope Lab
> >Univeristy of Minnesota
>
> ________________________
> _______________________________________________
> Max Coleman, Professor of Sedimentology
> Postgraduate Research Institute for Sedimentology
> The University of Reading, Whiteknights, Reading RG6 6AB U.K.
> PRIS on the WWW: http://www.rdg.ac.uk/PRIS/home.html
> MAX on the web: http://www.rdg.ac.uk/PRIS/Sed/Staff/People/mlc1.html
>
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