Oxygen isotope ratio cycle likely driven by the Lomagundi carbon isotope excursion. Oxygen isotope ratio cycle Oxygen isotope ratio cycles are cyclical variations in the ratio of the abundance of oxygen with an atomic mass of 18 to the abundance of oxygen with an atomic mass of 16 present in some substances, such as polar ice or calcite in ocean core samples, measured with the isotope fractionation. The ratio is linked to water temperature of ancient oceans, which in turn reflects ancient climates. Cycles in the ratio mirror climate changes in geologic history. Oxygen (chemical symbol O) has three naturally occurring isotopes: O, O,
Atomic mass this scaling "relative" to carbon-12. The relative isotopic mass, then, is the mass of a given isotope (specifically, any single nuclide), when this value is scaled by the mass of carbon-12, when the latter is set equal to 12. Equivalently, the relative isotopic mass of an isotope or nuclide is the mass of the isotope relative to 1/12 of the mass of a carbon-12 atom. For example, the relative isotopic mass of a carbon-12 atom is exactly 12. For comparison, the atomic mass of a carbon-12 atom is exactly 12 daltons or 12 unified atomic mass units. Alternately, the atomic