> Dear Ethan,
>
> I was intrigued by the results of roasting on forams. I've never been asked
> to analyse forams (so I can't comment specifically), but it is clear that
> the effects you observed from roasting are generally different from my
> own. I suspect part of the problem may have something to do with the
> quantity of organic matter and (perhaps) types of organic compounds present.
> Despite physically removing larger fragments of organic material from
> brachiopods before roasting, both brachs and bryozoans contain observable,
> although unquantified amounts of organic tissue prior to treatment-
> significantly otoliths don't appear to contain much organic matter (and
> consequently don't appear to be affected by roasting). If forams contain
> relatively small amounts of organic matter then this might explain the lower
> shifts in isotopic composition observed relative untreated and plasma ashed
> samples.
>
> In addition crushing of the samples may also increase the "reactivity" of
> biogenic carbonate when exposed to roasting. All of my test materials were
> crushed to homogenize them. Do you crush your forams prior to roasting?
>
> It strikes me that there are so many subtly different ways of preparing and
> pretreating samples that it's difficult to identify a uniform approach to
> all forms of skeletal carbonate. Only in cases where it can be demonstrated
> that roasting and other pretreatments give the same result is it reasonable
> to assume that the isotopic data are "right". In cases where different
> pretreatments give conflicting results it seems almost impossible to justify
> one set of data against another. I tend to believe that plasma ashing gives
> the most reliable results because it removes organics at low temperatures
> and thus reduces the possibility of isotopic exchange between organic matter
> and carbonate which is more likely to occur at higher temperatures associated
> with roasting. Opponents of plasma ashing may argue that this pretreatment
> does not remove all organic matter- they are probably correct. My feeling
> is that plasma ashing is efficient enough to render organic matter
> sufficiently inert to prevent any contamination of CO2 generated by
> phosphoric acid decomposition of carbonates.
>
> If you have any thoughts on this conundrum I'd like to here them.
>
> Regards,
>
> Steve Crowley
>