A new box model for glacial-interglacial changes in atmospheric CO2 produces lower levels of atmospheric CO2 without changes in biological production or nutrient chemistry. The model treats the boundary between middepth water and deep water as a chemical divide that separates low-CO2 water above from high-CO2 water below. Atmospheric CO2 is reduced 21 ppm by reduced ventilation of the deep water below the divide. A further reduction of 36 ppm is due to CaCO3 compensation in response to lower CO3= below the divide. Colder surface temperatures account for an additional 23 ppm of CO2 reduction. The new mechanism leaves the glacial atmosphere lighter in Δ13C than in preindustrial time, as seen in ice cores and fossil plant material. Bottom water below the divide becomes strongly depleted in Δ13C without a change in nutrient concentrations.