The Shuram carbon isotope excursion represents a late Precambrian perturbation to Earth’s carbon cycle. Several mechanisms have been proposed, including global ocean oxygenation, methane hydrate release, and diagenesis. The plurality of hypotheses in part stems from a lack of chronostratigraphic constraints that are needed to provide the boundary conditions for geochemical models. In this study we use magnetostratigraphy and astrochronology to establish a chronostratigraphic framework for the excursion. Paleomagnetic and isotopic results from the Flinders Ranges, South Australia, and from Death Valley, California (USA), demonstrate that the nadir of the excursion is coincident with a correlative polarity transition at each locality, suggesting global synchroneity. Rock magnetic cyclostratigraphy yields an astrochronologic estimate of ∼8 m.y. for the excursion’s duration at both locations. Based on these observations, and the chronologic requirements of each proposed mechanism, we rule out diagenesis and methane hydrate release as sole causes for the excursion, and suggest reexamination of the ocean oxidation hypothesis.