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 δ¹³C_orgexcursion 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.