Dear Steve and friends,

I have the distinct impression that I am stating the obvious, (forgive me if you have already dismissed this idea), however, I would like to comment on the stable carbon and oxygen isotopic variation you observed in your experiments with brachiopods and bryozoa.  It appears that you do not consider inter-specimen variation in the isotopic ratio preserved in biogenic CaCO3 as a possible cause of the observed differences between samples. 

A research team at Oxford have observed carbon and oxygen isotopic fluctuations of up to 3 per mil for d18O and around 4 per mil for d13C preserved in a single fossil mollusc: 
        (Purton et al., 1997, Gastropod carbonate d18O and d13C values record strong  seasonal productivity and stratification shifts in upper Eocene strata, England:        Geology, in press).

Other workers have documented similar variation within Recent bivalves and corals.  (I can give you numerous references if you are interested).  It might seem, therefore, that despite 'crushing up your specimens to homogenise the sample', you are observing real environmental variation of the isotopic composition of the seawater in which your brachiopods and byozoa grew, which has been recorded in the biogenic carbontate, and/or temperature fractionation effects, (particularily in d18O), rather than effects resulting from the presence of differing amounts of organic matter.

With regard to heating, we have worked with the larger foraminifera Nummulites, which we split equatorially by quenching a red-hot specimen in iced water.  Unpublished tests to investigate isotopic variation between specimens that were heated and those that were not heated revealed no significant difference in observed isotopic range.  (Incidentaly, samples were crushed and treated with Hydrogen Peroxide to remove organic material).

Gill et al. 1995, in Geology, v. 23, p. 333-336, suggests that roasting causes isotopic exchange with atmospheric CO2 and H2O.  This might be the 'relatively recent paper in Geology' that Scott was referring to in his earlier contribution to this discussion. 

I hope this is relevant.

With kind regards,

Louise Purton,
Oxford University, 
OX1 3PR, U.K.

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