The geological cycling of carbon ties together the ocean-atmospherecarbon pool, Earth’s biosphere, and Earth’s sedimentary reservoirs. Perturbations tothis coupled system are recorded in the carbon-isotopic (13C) composition of marinecarbonates. Large amplitude 13C excursions are typically treated as individualevents and interpreted accordingly. However, a recent compilation of Phanerozoiccarbon isotopic data reveals that 13C excursions are a ubiquitous feature of thegeologic record, and thus should be considered in concert. Analysis indicates thatPhanerozoic carbon isotope excursions, as a whole, have characteristic durationsof 0.5 to 10 M.yr. and exhibit declining amplitude over time. These commonalitiessuggest a shared underlying control.Here we demonstrate that sinusoidal modulation of the sensitivity of organiccarbon and phosphate burial in a simple numerical model of the geologic carbon cycleresults in large, asymmetric 13C oscillations that exhibit their largest amplitudes in the0.5 to 10 M.yr. period range. As anoxia is known to strongly modulate the C:P burialratio of organic matter in sediments, we propose that sea-level oscillations were theprimary source of sinusoidal modulation for the geologic carbon cycle, and that theirdegree of influence on the carbon cycle was determined by the state of oxygenation ofbottom waters overlying the continental shelves. When oxygen minimum zones (OMZs)were large, shallow, and prone to expansion, sea-level changes would have had thecapacity to drive large changes in the areal extent of OMZs in contact with the sea-floor,resulting in strong leverage on the burial sensitivity of organic carbon and phosphate,and thus on 13C. Progressive oxygenation of the oceans, which was facilitated bybiological innovations, resulted in a decline in the amplitude of 13C excursions overthe Phanerozoic, and the biogeochemical stabilization of the Earth System.