The Nemakit-Daldynian–Tommotian (ND-T) boundary marks the first appearance of metazoan reefs and calcite biomineralizers and is associated with the largest δ13C shift during the Phanerozoic Eon. Biological transitions in Earth history are often accompanied by excursions in the carbon isotopic composition (δ13C) of the ocean, where δ13C variability is interpreted to reflect changes in the global carbon cycle. The duration and thus rate of these δ13C anomalies are rarely known, making it difficult to constrain their possible causes and their relationship, if any, to biologic transitions. We report sedimentological and δ13C data from a new 2.5-km-thick section that spans the early Cambrian evolutionary “explosion” in the Moroccan Anti-Atlas Mountains. Three new zircon 206Pb-238U ages from tuffs within the stratigraphy constrain the timing of the ND-T boundary to 524.84 ± 0.09 Ma. Two of the tuffs exactly bracket the ND-T transition and constrain the duration of the −8‰ δ13C shift to 506 ± 126 k.y. With a simple box model, we explore a range of geochemical processes that could account for such a rapid ND-T δ13C shift, and conclude that metamorphic and/or volcanic fluxes of carbon may have been sustained at levels 4–16 times higher than today for millions of years.