Reporting small Δ 17 O values: existing definitions and concepts

The three‐isotope tracer Δ 17 O is increasingly used in atmospheric chemistry and other research areas. Thanks to the development of isotope‐ratio mass spectrometry (IRMS), δ 17 O and δ 18 O can be determined with a precision of a few 0.01‰, and values for Δ 17 O may be calculated with similar precision. However, interpreting small and precisely determined Δ 17 O values as a deviation from an expected mass‐dependent fractionation process is not straightforward. Several aspects are of high importance. In the present paper we review existing definitions, formulas and some other aspects of Δ 17 O reporting. One of the most confusing aspects is a variance of definitions and corresponding formulas. While Δ 17 O is traditionally defined to characterise a data point, i.e. Δ 17 O is considered as a deviation from an expected mass‐fractionation line, the recently introduced definition (Miller MF. Geochim. Cosmochim. Acta 2002; 66: 188) characterises a fractionation line itself, in terms of its ordinate intercept. The formulas corresponding to this definition gives a characteristic for a specific process. When the ‘traditionally defined’ Δ 17 O is in use, an expected fractionation processes—the key point for Δ 17 O reporting—should be defined and parameterised with the same accuracy as intended for reporting Δ 17 O. When Δ 17 O is reported for a data point, not only a value for λ but an ordinate intercept of a reference fractionation line should be given with high accuracy. We note that defining a single fractionation process is hardly possible for many natural compounds. For such compounds we propose to use a phenomenological reference line, namely an isotope composition range of natural sources. Next, aspects of Δ 17 O comparison and mass‐balance calculations are considered. All the aspects considered for Δ 17 O may be relevant for others three‐isotope tracers, e.g. Δ 33 S. Copyright © 2005 John Wiley & Sons, Ltd.