This is an interesting and real issue. We have coupled an IRMS to a 
thermal analysis system, so we can get on-line analysis of CO2 evolved 
as a complex material degrades on heating.  There is no doubt that 
organic matter typically persists to 500°C in an oxidising atmosphere, 
and in some soil samples 'black carbon' persists to higher 
temperatures.   The difference between d13C for CO2 derived from 
combustion of organics can be 20 per mille different from that derived 
from carbonate mineral breakdown, so a little organic matter can make a 
big difference.

It matters very much which carbonate minerals are present, as they 
decompose at different temperatures, as Steve has pointed out.

If any method trials are planned, the coupled TG-IRMS approach would be 
interesting (it tells you about both mineral and organic matter, 
quantifying both from measured weight losses).  But, our lab is being 
moved, so not much can happen before March next year!

References (all available on line) that describe this system include:

Lopez-Capel, E., de la Rosa-Arranz, J. M., González-Vila, F. J., 
González-Perez, J. A. and Manning, D. A. C.  Elucidation of different 
forms of organic carbon in marine sediments of the Atlantic Coast of 
Spain by thermal analysis coupled to isotope ratio and quadrupole mass 
spectrometry. Organic Geochemistry, in press.
Lopez-Capel, E, Abbott, GD, Thomas, KM, & Manning, D. A. C. Coupling of 
thermal analysis with quadrupole mass spectrometry and isotope ratio 
mass spectrometry for simultaneous determination of evolved gases and 
their carbon isotopic composition. J ANAL APPL PYROL 75 (2): 82-89, 
2006
Lopez-Capel, E, Bol, R, Manning, DAC Application of simultaneous 
thermal analysis mass spectrometry and stable carbon isotope analysis 
in a carbon sequestration study RAPID COMMUN MASS SP 19 (22): 3192-3198 
2005
Manning, DAC, Lopez-Capel, E, Barker, S. Seeing soil carbon: use of 
thermal analysis in the characterization of soil C reservoirs of 
differing stability MINERAL MAG 69, 425-435 AUG 2005

David Manning


Professor of Soil Science
School of Civil Engineering and Geosciences
Newcastle University
Newcastle upon Tyne
NE1 7RU

telephone (+44) 0191 222 7893