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• SIMS δ13C carb from Ediacaran Doushantuo Fm in China and Mooifontein Mbr in Namibia. • 10‰ variation in SIMS δ13C carb over μm-mm scales in lower Doushantuo Formation. • 4‰ variation in SIMS δ13C carb over μm-mm scales in Mooifontein Member. • Bulk-sample δ13C variation at m-scale match SIMS δ13C variation at μm-mm scales. • Bulk-sample data reflect influence of authigenic carbonate in Doushantuo samples. Carbonate carbon isotope (δ13C carb) chemostratigraphy is a valuable tool in Precambrian stratigraphic correlation. The effectiveness of this tool rests on the assumption that δ13C carb data record global seawater signals. However, in some cases δ13C carb data exhibit rapid and noisy stratigraphic variations that may have been influenced by authigenic or diagenetic carbonate minerals. To assess the contribution of non-primary minerals towards bulk carbonate carbon isotope values, we acquired SIMS (secondary ion mass spectrometry) δ13C carb data, electron microscopic data, and CL (cathodoluminescence) microscopic data from two Ediacaran successions—the lower Member II of the Doushantuo Formation in South China and the Mooifontein Member of the Zaris Formation in southern Namibia. The Doushantuo samples came from a stratigraphic interval with noisy meter-scale δ13C carb variations of up to 10‰, whereas the Mooifontein Member is characterized by consistent δ13C carb values with limited meter-scale variations of less than ~4‰. Our data show that the meter-scale stratigraphic variations in δ13C carb are also mirrored in the SIMS data at μm-mm scales in both Doushantuo and Mooifontein samples. In the Doushantuo samples, SIMS δ13C carb values of authigenic calcite vary by up to 10‰ over μm-mm scales and can be either higher or lower than those of the co-existing dolomite matrix, which also appears to be affected by authigenic carbonate on the basis of petrographic observation. Bulk-sample δ13C carb values measured on powders microdrilled from the same SIMS specimens are within the range of SIMS δ13C carb values. Thus, we infer that bulk-sample δ13C carb values of Doushantuo samples represent mixtures of different carbonate components. The Doushantuo and Mooifontein SIMS δ13C carb data validate the traditional method of using chemostratigraphic consistency to evaluate authigenic/diagenetic alteration, but given that secondary phases may be 13C-enriched relative to matrix carbonate, caution is warranted when taking maximum values to approximate primary chemostratigraphic trends. [ABSTRACT FROM AUTHOR]