This research illustrates application of a fluid-inclusion technique for quantifying water depth of ancient carbonate platforms. Jurassic limestones of Monte Kumeta, Italy, were cemented with submarine calcite during a transition to carbonate platform termination. The calcite cements contain fluid inclusions consisting of Jurassic seawater and immiscible gas bubbles trapped during the growth and penecontemporaneous recrystallization of the cements. Crushing analysis indicates that gas bubbles are under pressures indicative of entrapment in water depths of 23–112 m. Assuming simple deepening and acknowledging chronostratigraphic errors, rates of relative rise in sea level were initially less than 7 m/m.y. followed by a rate of at least 33 m/m.y. These slow rates are evidence that the platform's demise was caused by an environmental perturbation other than rapid sea-level rise. The facies transitions and regional studies indicate that the perturbation resulted from nutrient excess or eutrophication in shallow water followed by deepening into ephemeral dysoxic waters at depths perhaps as shallow as 23 m.