The ratio of 13C to 12C 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 both labor-intensive, expensive and requires expert know-how. Here, we measure 13C/12C in natural sedimentary samples using Combustion Module - Cavity Ring Down Spectroscopy (CM-CRDS) with average precision and standard reproducibility of 0.05‰ and 0.2‰ (1 s.d., n = 17), respectively. The accuracy of the technique was determined from certified reference compounds to be <0.3‰. This is comparable to the performance using conventional mass spectrometry. We report data from a Cambrian succession of organic-rich shales straddling a positive d13Corg excursion of 2‰. We conclude that, optical determination of bulk organic d13C provides a high performance alternative to mass spectrometry and applicable for geochemical analyses.