SMOW

SMOW stands for Standard Mean Ocean Water. It is a reference material used in stable isotope geochemistry to express the isotopic composition of hydrogen and oxygen in water samples. SMOW represents the average isotopic composition of the Earth’s oceans and serves as a defined standard for calibrating measurements, rather than a single physical specimen.

The concept provides a scale for reporting the ratios of heavy to light isotopes, specifically D/H for hydrogen and 18O/16O for oxygen. Isotopic measurements are typically expressed as delta values (δD and δ18O) in per mil relative to the reference standard. In modern practice, laboratories commonly calibrate against Vienna Standard Mean Ocean Water (VSMOW), a refined reference material with fixed isotope ratios; δD and δ18O values for VSMOW are defined as zero, making sample values appear relative to VSMOW (and thus connected back to SMOW).

SMOW originated in the mid-20th century as a practical ocean-water standard, providing a stable baseline for interlaboratory comparisons and the interpretation of hydrological and climatological data. VSMOW offers an internationally consistent basis for comparison and is widely adopted in current analyses.

Applications of SMOW and its derivatives include hydrology, climatology, paleoclimatology, and geochemistry. By comparing isotopic compositions in water, ice, minerals, and organic matter, researchers infer processes such as evaporation, condensation, and planetary hydrological cycling, aiding reconstructions of past climates and hydrological regimes.