Stable Isotopes

• stable isotopes most commonly used in isotope hydrology (Tab)
• natural abundances of isotopes (Tab)
• encyclopedia entry for Harold C. Urey
• GMWL (Fig)
• temperature dependence of equilibrium  fractionation (Fig) (Tab)
• quantitative description of equilibrium and kinetic fractionation (Fig)
• a_k  is a function of 1-h (data for smooth ocean) (Tab)
• fractionation of oxygen isotopes during condensation of water from vapor at 25^oC according to the Rayleigh fractionation (Fig)
• network (Fig), operational network (Fig)
• GMWL (Fig)
• Isotopes in the water cycle (Fig)

• stable isotopes most commonly used in isotope hydrology (Tab)
• we will focus here on the stable isotopes of H and O
• relative abundance of H and O isotopes in terrestrial waters

• D, was first isolated by the Columbia chemist Harold C. Urey in 1931.
• stable isotope ratios are typically measured in the so-called d notation:

• so for example: d¹⁸O = -10 ‰ means that the sample is depleted relative to a standard in ¹⁸O by 1%

Standards

• the original standard was proposed by Harmon Craig in 1961
• SMOW ("Standard Mean Ocean Water") which never existed but was calibrated relative to a sample from the Potomac River (STD NBS-1)
• (¹⁸O/¹⁶O) _SMOW = 1.008 (¹⁸O/¹⁶O) _NBS-1 = (1993.4 ± 2.5) * 10^-6
• (²H/¹H) _SMOW = 1.008 (²H/¹H) _NBS-1 = (158 ± 2) * 10^-6
• subsequently the IAEA (International Atomic Energy Agency) prepared a standard water from distilled seawater that was modified to have an isotopic composition close to SMOW
• (¹⁸O/¹⁶O) _VSMOW =  (2005.2 ± 0.45) * 10^-6
• (²H/¹H) _VSMOW =  (155.76 ± 0.05) * 10^-6
• for isotopically very depleted samples another standard, SLAP ("Standard light Antarctic Precipitation")
• d¹⁸O_SLAP = -55.50 ‰ VSMOW
• d²H_SLAP = -428.0 ‰ VSMOW
• the VSMOW is the currently accepted since ~ 3 decades
• often measurements are reported in SMOW although they use VSMOW as reference (bad practice!)
• for isotopic measurements on carbonates, organic carbon and hydrocarbons the PDB standard is used
• H. Craig introduced the PDB standard in 1957 (PDB = sample taken from an internal calcite structure from a fossil Belemnitella americana  from the Cretaceous Pee Dee Formation in South Carolina
• after the standard was used up it was replaced by VPDB, which is supposed to be identical to PDB
• d¹⁸O_VSMOW = 1.03091 * d¹⁸O_VPDB + 30.91 ‰
• precision of measurements is of the order of 0.1 ‰ for d¹⁸O and 1 ‰ for d²H (=dD)

Fractionation

• Harmon Craig found in 1961 that d²H and d¹⁸O in precipiotation are related (Fig)
• he defined the Global Metoric Water Line (GMWL, in SMOW):

Notation

• isotope fractionation occurs any thermodynamic reaction due to the differences in the reaction rates of different molecular species
• it is expressed by the fractionation factor a, which is the ratio of the isotope ratios for the reactant and product:

• for example for a water/vapor system:

Equilibrium fractionation

• heavy isotopes are typically enriched in the more condensed phases
• in case of the water/vapor system the lighter molecule has an easier time to break up the hydrogen bonds and will be enriched in the vapor phase, in the case of oxygen isotopes by 1%
• temperature dependence of the fractionation (Fig)
• the  fractionation is often expressed as deviation from 1 in ‰:

• for example, at 20^oC the equilibrium fractionation factors are: e_e (²H) = 85 ‰, e_e (¹⁸O) = 9.8 ‰ ( d²H vs d¹⁸O Fig)

• quantitative description: consider the following situation (Fig)
• for the liquid phase we can write:
  

Non-equilibraium or 'kinetic' fractionation

• in case the relative humidity drops below 100% (h<1) (Fig) the water molecules have to penetrate the molecular boundary layer above the water which reults in a kinetic fractionation

• a_k is a function of 1-h (data for smooth ocean)

 

h	ek (2H) (‰)	ek (18O) (‰)
1	0	0
0.9	-0.5	-0.6
0.8	-1.1	-1.2
0.5	-2.6	-3.0

• kinetic fractionation does influence the fractionation of both isotopes in similar ways (different from equilibrium fractionation!)
• ( d²H vs d¹⁸O Fig)

Combined fractionation

• the total fractionation can then be described in the following way:

• when looking at the formation of precipitation the process is reversed, but is usually presumed to be an equilibrium fractionation
• fractionation of oxygen isotopes during condensation of water from vapor at 25^oC according to the Rayleigh fractionation (Fig)

Global network of Isotopes in Precipitation (GNIP)

• maintained by the IAEA/WMO
• measures ³H, ²H, and ¹⁸O
• complete network (Fig), operational network (Fig)
• GMWL is the compilation of all measurements in precipitation (Fig)
• the isotopes of the water molecule, tritium, deuterium and oxygen-18, label the global water cycle (Fig)

Resources

• IAEA/WMO Global Network of Isotopes in Precipitation (GNIP)
• GNIP booklet
• Environmental Isotopes in the Hydrological Cycle : Principles and Applications (IAEA website)
• very comprehensive on-line resource