In carbonate provinces ferrous iron is stable only under reducing conditions; it will, therefore, not enter cements precipitated in the oxidizing zone above the water table. Once in the calcite lattice, however, ferrous iron can normally survive subsequent passages through oxidizing media. The ferroan character of a cement is thus good and lasting evidence for its precipitation under reducing conditions. In this latter environment, however, reducing conditions are only established in the presence of organic matter, the absence of which could result in local oxidizing conditions and thus in the formation of ferrous-iron-free cements. The latter are therefore not necessarily indicative of precipitation above the water table. Certain carbonate cements exhibit variations in the ferrous iron content, and represent diagenetic time lines.