Reporting of stable hydrogen, carbon, and oxygen isotopic abundances (Technical Report)

To eliminate possible confusion in the reporting of isotopic abundances on noncorresponding scales, the Commission on Atomic Weights and Isoto ic Abundances recommended at the 37* General Assembly at Lisbon, Portugal that (i) 'H/ H relative ratios of all substances be expressed relative to VSMOW (Vienna Standard Mean Ocean Water) on a scale such that 2H/'H of SLAP (Standard Light Antarctic Precipitation) is 0.572 times that of VSMOW, (ii) 13C/'2C relative ratios of all substances be expressed relative to VPDB (Vienna Peedee belemnite) on a scale such that 13C/12C of NBS 19 carbonate is 1.00195 times that of VPDB, and (iii) l8O/l6O ratios of all substances be expressed relative to either VSMOW or VPDB on scales such that 180/160 of SLAP is 0.9445 times that of VSMOW. P COMMENT Abundances of stable hydrogen, carbon, and oxygen isotopes in geochemical and environmental studies are generally expressed in parts per thousand (k or per mil) difference from a standard. Thus, for the oxygen isotopic composition of a sample x , we have r 1 The standard may be an actual reference material or a hypothetical material whose isotopic abundance is set by assigning an isotopic composition to an existing reference material. Irregularities concerning the choice of the standard have arisen for hydrogen, carbon, and oxygen isotopes (refs. 1 & 2). Friedman and O'Neil (ref. 1) point out that some laboratories are "tied" to each other by acceptance of the 6 values of certain comparison materials. The situation for the SMOW (Standard Mean Ocean Water) standard has become increasingly aggravated. The SMOW standard was originally a hypothetical water sample with abundances of stable hydrogen and oxygen isotopes similar to those of average ocean water (ref. 3). Its abundances of stable hydrogen and oxygen isotopes were defined in terms of NBS 1 water distributed by the U.S. National Bureau of Standards (now National Institute of Standards and Technology): ( 2H/'H) SMOw 1.050( 2W1H) ms 1 and ( l80/l6O) SMOW = l.O08( 180/'60)ms Subsequently, H. Taylor and S. Epstein of the California Institute of Technology used a hypothetical standard that they also called SMOW and defined it by assigning a 6 l 8 0 value of +15.5% to their laboratory reference material, a sample of Potsdam Sandstone (ref. 1). Thus, their oxygen isotope scale is defined by 18 oPotsdam SandstonelSMOW = +15*5%* Ym = parts per thousand 274 Stable hydrogen, carbon, and oxygen isotopic abundances 275 Additionally, the International Atomic Energy Agency (IAEA) distributed a water sample they named SMOW. This sample was near (but not the same) in isotopic composition to the original SMOW defined in terms of NBS 1 water. Thus, there are three independent usages of SMOW that we observe today, leading to three differing 2W1H and l80/l6O abundances with the same name. The IAEA recognized the dilemma of naming a reference water as SMOW and subsequently changed the name to VSMOW (Vienna Standard Mean Ocean Water). Furthermore, they recommended that abundances of hydrogen and oxygen isotopes of all materials (except marine carbonates) be expressed using VSMOW rather than SMOW (refs. 2, 4 & 5). However, these recommendations have not received wide distribution and may be unknown to numerous producers of data on abundances of oxygen and stable hydrogen isotopes, particularly in the rapidly expanding fields of environmental and climate studies. A second standard is used for reporting abundances of oxygen isotopes of marine carbonates and is named PDB (Peedee belemnite). Because the su ply of this material is exhausted, some laboratories have "tied" themselves to each other by adopting 6l 0 values of various carbonate reference materials. The IAEA recognized the potentially serious problem that oxygen isotopic scales in different laboratories might not correspond. They recommended that abundances of oxy en isotopes of carbonates be expressed relative to VPDB (Vienna Peedee belemnite) by adopting a 6 0 consensus value of -2.2% for NBS 19 carbonate relative to VPDB at a meeting in 1983 in Vienna (ref. 2). Thus, P