Dear Max,

Thanks for making the connection between C isotopes and Ba concentration. I
am not bold enough to jump so far as yet. The best correlation I have ever
seen was in Cretaceous limestones between d13C and Mn. IMHO these things
ought to be discussed more in the literature in a context other than simply
diagenesis.

>I cannot guarantee that we are not measuring detrital silicates or trace
>barite.  We measure Si and Al to look for relationships in a suite of
>samples  which would suggest a detrital origin for the elements of real
>interest.
>
>We have many limestone data with Ba at over 1000 ppm.  When nature plays
>fair we have seen nice negatively-correlated linear relationships between
>Ba and delta 13-C in a series of rocks; and although I cannot PROVE that it
>is not trace barite, it still indicates operation of a systematic process.

Trace barite will of course dissolve in a limited way if present. However,
I have observed no changes in Ba concentration or Sr/Ba ratio between HCl,
HNO3, acetic acid or buffered solution (pH=4.9) leachates of >98% pure
limestones, which indicates that Ba is really in the carbonate. The next
test is SEM and microprobe analysis.

>In some cases we can be fairly sure that the Ba is in the carbonate -
>measurements on samples from exceptionally-preserved Jurassic fauna - still
>aragonite and uncompacted - give values over 1000 ppm (and a negative
>correlation with delta 13-C)

Yes, this is one thing I have been thinking. Do high Ba concentrations
indicate, perhaps better than high Sr, that the original carbonate phase
was aragonite? In my samples, I have Ba between 50 to 2000 ppm, whereas Sr
varies between 700 and 4500 ppm (with only slight correlation between the
two). Sr and Ba contents (and very low Mg contents) indicate that the
original phase was aragonite.
Also negative correlation between d13C and Ba could be argued as you
mention. I will get in touch with Dr Gruszynski asap. Cheers,

Graham

Graham Shields
Centre de Géochimie de la surface (CNRS)
1 rue Blessig
F-67084 Strasbourg
France

Tel: 0033 3 88 35 86 27
Fax: 0033 3 88 36 72 35