The ratio of C-13 to C-12 in marine sedimentary rocks holds important clues to the evolution of the carbon cycle through Earth history. Isotopic analyses are traditionally carried out using isotope ratio mass spectrometry (IRMS), but this technique is labor-intensive, expensive and requires expert know-how. Here, we measure C-13/C-12 in natural sedimentary samples using Combustion Module-Cavity Ring Down Spectroscopy (CM-CRDS) with an average precision and a standard reproducibility of 0.05 parts per thousand and 0.2 parts per thousand (1 sd, n = 17), respectively. The accuracy of the technique was determined from certified reference compounds to be <0.3 parts per thousand. This is comparable to the performance using routine laboratory mass spectrometry <0.11 parts per thousand (1 sigma). We report data from a Cambrian succession of organic-rich shales straddling a positive delta C-13(org) excursion of 2 parts per thousand. We conclude that the optical determination of bulk organic delta C-13 provides a high performance alternative to routine laboratory mass spectrometry and is applicable for geochemical analyses.
Journal of Analytical Atomic Spectrometry, 2013, Vol 28, Issue 4, p. 516-523