Global meteoric water line

The Global Meteoric Water Line (GMWL) describes the global annual average relationship between hydrogen and oxygen isotope (oxygen-18 [¹⁸O] and deuterium [²H]) ratios in natural meteoric waters. The GMWL was first developed in 1961 by Harmon Craig, and has subsequently been widely used to track water masses in environmental geochemistry and hydrogeology.

When working on the global annual average isotopic composition of ¹⁸O and ²H in meteoric water, geochemist Harmon Craig observed a correlation between these two isotopes, and subsequently developed and defined the equation for GMWL:^[2]

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

Where δ¹⁸O and δ²H (aka 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 (^{18, 16}O) and hydrogen isotopes (^{2, 1}H) 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]

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 of 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 of groundwater are often used to study the origin of groundwater^[11] and groundwater recharge.^[12]

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]

• Isotope fractionation
• Meteoric water
• Water cycle