Global Meteoric Water Line (GMWL)
Definition
The linear relationship δ2H = 8 × δ18O + 10 that describes the global co-variation of O and H isotopes in precipitation. Used as a reference to identify evaporative enrichment in samples that fall below the line.
- Equation
- delta2H = 8 x delta18O + 10
- Established by
- Craig, 1961, global precipitation dataset
- Forensic use
- Reference line for isotope provenance and evaporation checks
- Deviation signal
- Points below the line indicate evaporative enrichment
Common questions
How is the GMWL used to detect sample tampering or origin?+
A water or tissue sample's oxygen and hydrogen isotope ratios are plotted against the GMWL; a point close to the line is consistent with unaltered meteoric water, while a point falling below and to the right suggests evaporation, which can flag altered samples or identify arid-region origin.
Why can't the GMWL be applied directly everywhere without adjustment?+
Local precipitation follows a Local Meteoric Water Line that can differ in slope and intercept from the global average due to regional climate and evaporation effects, so forensic isoscape work typically references local calibration data alongside the GMWL.
What forensic applications rely on this isotope relationship?+
It underlies isoscape-based geographic provenancing of human remains, drugs, food and other organic materials, since the hydrogen and oxygen isotopes in body tissue and hair reflect the drinking water and food of the region where a person lived.
Related terms
- δ18O (Delta-18-O)
- The ratio of oxygen-18 to oxygen-16 in a sample expressed in per-mil deviation from the Vienna Standard Mean Ocean Water (VSMOW) standard....
- δ2H (Delta Deuterium)
- The ratio of deuterium to hydrogen in a sample expressed in per-mil deviation from VSMOW. Covaries with δ18O along the global meteoric...
- Isoscape
- A map of isotope ratio variation across a geographic area, derived from systematic sampling of soil, water, plant, or mineral material; used...
- OIPC (Online Isotopes in Precipitation Calculator)
- A web-based tool and database compiling the IAEA GNIP station network data and providing interpolated δ18O and δ2H predictions for any geographic...
- Tissue-Specific Incorporation
- The principle that different tissues record isotope signals over different time windows: enamel permanently records childhood; hair records month-by-month recent history; bone...