When working on the global annual average isotopic composition of Oxygen-18 and Deuterium in meteoric water, the geochemist Harmon Craig observed a correlation between these two isotopes. The equation for GMWL was then developed and defined by Harmon Craig: Where δ18O and δ2H are the ratio of heavy to light isotopes. The relationship of δ18O and δ2H in meteoric water is caused by mass dependent fractionation of oxygen and hydrogen isotopes between evaporation from ocean seawater and condensation from vapor. As oxygen isotopes and hydrogen isotopes have different masses, they behave differently in the evaporation and condensation processes, and thus result in the fractionation between 18O and 16O as well as 2H and 1H. Equilibrium fractionation causes the isotope ratios of δ18O and δ2H to vary between localities within the area. The fractionation processes can be influenced by a number of factors including: temperature, latitude, continentality, and most importantly, humidity,.
Applications
observed that δ18O and δ2H isotopic composition of cold meteoric water from sea ice in the Arctic and Antarctica are much more negative than that in warm meteoric water from the tropic. A correlation between temperature and δ18O was proposed later in the 1970s. Such correlation is then applied to study surface temperaturechange over time. The δ18O composition in ancient meteoric water, preserved in ice cores, can also be collected and applied to reconstruct paleoclimate. A meteoric water line can be calculated for a given area, named as local meteoric water line, and used as a baseline within that area. Local meteoric water line can differ from the global meteoric water line in slope and intercept. Such deviated slope and intercept is a result largely from humidity. In 1964, the concept of deuterium excess d was proposed. Later, a parameter of deuterium excess as a function of humidity has been established, as such the isotopic composition in local meteoric water can be applied to trace local relative humidity, study local climate and used as a tracer of climate change. In hydrogeology, the δ18O and δ2H composition in groundwater are often used to study the origin of groundwater and groundwater recharge.