Global meteoric water line

When working on the global annual average isotopic composition of Oxygen-18 and Deuterium (²H) 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:[2]

${\displaystyle \delta {\ce {D}}=8.0\cdot \delta ^{18}{\ce {O}}+10\ {}^{0\!}\!/\!_{00}}$

Where δ¹⁸O and δ²H (also known as δD) are the ratio of heavy to light isotopes (e.g. ¹⁸O/¹⁶O, ²H/¹H).

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.[3] As oxygen isotopes (¹⁸O and ¹⁶O) and hydrogen isotopes (²H and ¹H) 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,.[3][4]

Local meteoric water line of Changsha, China, 1990. Data are monthly oxygen-18 and deuterium values from precipitation monitored at the local station (n=12).[5]

Craig 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.[2] A correlation between temperature (T) and δ¹⁸O was proposed later[6] in the 1970s. Such correlation is then applied to study surface temperature change over time.[7] The δ¹⁸O composition in ancient meteoric water, preserved in ice cores, can also be collected and applied to reconstruct paleoclimate.[8][9]

A meteoric water line can be calculated for a given area, named as local meteoric water line (LMWL), 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 (d=δ²H - 8δ¹⁸O)[3] 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,[10] study local climate and used as a tracer of climate change.[6]

In hydrogeology, the δ¹⁸O and δ²H composition in groundwater are often used to study the origin of groundwater[11] and groundwater recharge.[12]

Recently it has been shown that, even taking into account the standard deviation related to instrumental errors and the natural variability of the amount-weighted precipitations, the LMWL calculated with the EIV (error in variable regression)[13] method has no differences on the slope compared to classic OLSR (ordinary least square regression) or other regression methods.[14] However, for certain purposes such as the evaluation of the shifts from the line of the geothermal waters, it would be more appropriate to calculate the so-called "prediction interval" or "error wings" related to LMWL.[13]

• Isotopic fractionation
• Meteoric water
• Water cycle