Micha,

depending on the water content and the presence of metastable carbonate
phases in the core there may be transformation of the carbonate fraction into
more stable phases. Depending on a number of boundary conditions O isotope
exchange may occur. This will probably be not a serious problem for C
isotopes. During roasting some OM will decompose and create CO2.

Regards, Michael

Micha Horacek wrote:

> Dear isogeochemists,
>
> I think I have to formulate my request more precisely:
>
> The palmag cores are demagnetised by heating to temperatures above 350° C
> for at least two hours (but I do not think much longer) and "roasted" at
> maximum temperature of 500° for twenty minutes.
> The cores consist of massive rock, not powder. The cores are from impure
> carbonate rocks with low organic contents.
> Can the demagnetisation process possibly change the isotope ratios, for
> example if the carbonate content of the sample is rather small?
>
> Cheers,
> Micha