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 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 an average precision and a standard reproducibility of 0.05‰ and 0.2‰ (1 sd, 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 routine laboratory mass spectrometry <0.11‰ (1σ). We report data from a Cambrian succession of organic-rich shales straddling a positive δ13Corg excursion of 2‰. We conclude that the optical determination of bulk organic δ13C provides a high performance alternative to routine laboratory mass spectrometry and is applicable for geochemical analyses.