Stable Isotope Ratio Analysis		  
 


As mentioned before, stable isotope ratios 18O and 13C are computed by

18O =
 [ (18O/16O)sample / (18O/16O)standard  -1] * 103 per mil
13C =
[ (13C/12C)sample / (13C/12C)standard  -1] * 103 per mil

Certain fossile carbonates such as "Vienna Pee Dee Belemnite" (VPDB) are used as standards.

  
 
Stable isotopes are measured by mass spectrometry. Most geological and paleobiological departments have mass spectrometers suitable for the analysis of oxygen and carbon in calcium carbonate.

In order to read the temporal record of stable isotope ratios in a calcareous skeleton, a sequence of small calcium carbonate samples has to be collected along the growth axis. Dentist drill bits fitted to small hobby drills have been found to be quite suitable for this tasks. With dentist drill bits a distance between subsequent samples of about 1.5mm can be achived. Well equipped geology departments may have micro driling devices which allow for better spatial resulution.

Usually one sample is about 0.050 to 0.100 mg calcium carbonate, depending on the mass spectrometer type and the taxon under investigation. Some pilot measurements can help to determine optimum sample mass.

  
 
Caveat : Many types of calcareous skeletons consist of several "layers", and not all of these layers are proper records of time. The whole surface of the innermost layer of bivalve shells, for instance, is altered permanently and hence does not represent a temporal record. Therefore, you have to understand structure and growth dynamics of a skeleton when using stable isotopes in growth studies.
 
 

Introduction
Chemistry
18O
 13C Methods