Upper Cretaceous organic-rich carbonates in Israel contain benthonic foraminifera and varying amounts of early diagenetic infilling of sparry calcite. The sparry calcite has oxygen-isotope values (down to -9.5‰ PDB) that are significantly lower than those of the coexisting skeletal calcite (averaging -2‰ PDB). Low δ18O values are very common in carbonates that are associated with organic matter. These occurrences are conventionally explained as being due to carbonate formation under conditions of low salinity and/or elevated temperature. On the basis of petrographic data and local geologic evidence, however, none of the conventional explanations can be applied in this case. We advance an alternative model that relates the light oxygen compositions of the diagenetic calcite to depletion of 18O in pore water of normal salinity. The proposed operating mechanism is a direct consequence of organic matter decomposition in the sulfate-reducing zone. Being inherent to reducing marine environments, this model can be applied to many organic-rich rock associations.