Five new δ13C excursions are reported from the Upper Ordovician Galena Group of Iowa. The δ13Ccarb and δ13Corg trends are compared to sea-level trends based on conventional sedimentological and novel chemostratigraphic (εNd) methods. It is found that rising δ13C values occur during rising sea level, peak-magnitude δ13C values are coincident with inferred peak flooding, and declining δ13C values occur with declining sea levels. The relationship to sea level is explained by a simple model of episodic flooding of the Galena platform by cool, oxygen-depleted, nutrient-rich, bottom waters originating from the Sebree Trough, which was itself connected to the nearby Iapetus Ocean. Flooding of the inner Galena shelf during sea-level transgressions caused local changes in the carbon isotope balance of inner shelf waters due to (1) locally increased primary productivity and organic carbon burial, (2) locally increased exchange of 13C-enriched dissolved inorganic carbon (DIC) with surrounding epeiric sea water masses and the surface ocean, and (3) locally decreased 12C-weathering fluxes from the Transcontinental Arch. A sea-level driver of local C-cycle perturbations complicates the use of carbon isotopes as a chemostratigraphic tool because local and eustatic controls on sea level may combine to create different secular records of relative sea-level change in stratigraphically equivalent deposits of epeiric seas.