Oxygen isotope systematics is the study of the distribution and variation of oxygen's stable isotopes, primarily the ratio of heavy oxygen-18 (¹⁸O) to light oxygen-16 (¹⁶O), within Earth's systems. This variation is driven by isotopic fractionation, where physical, chemical, and biological processes—such as evaporation, condensation, and mineral formation—preferentially separate the isotopes based on their mass. Because the degree of this fractionation is highly sensitive to temperature, the resulting ¹⁸O/¹⁶O ratio preserved in natural archives like ice cores, marine sediments, and rocks serves as a powerful proxy, allowing scientists to reconstruct past climates (paleothermometry), trace the movement of water through the hydrologic cycle, and determine the temperatures at which geological materials formed.