Guidelines for Reporting Stable Isotope Data

The following note describes the IAEA and IUPAC-approved guidelines for reporting stable isotope data, quoted from Coplen (1996, Paleoceanography, 11:369-370) with the permission of the author. For alternate perspectives on this topic, check out the lively discussions from 1995 in the e-mail archive of the ISOGEOCHEM chat group.

Guidelines for the Reporting of Stable Hydrogen, Carbon, and Oxygen Isotope-Ratio Data

Tyler B. Coplen

U.S. Geological Survey, Reston, Virginia

Preface

Oxygen and carbon stable isotopic studies constitute one of the primary tools used in paleoceanographic reconstruction, yet the calibration to PDB (Peedee belemnite) is surprisingly poorly known. The supply of PDB has been exhausted for more than two decades, and the PDB d¹³C and d¹⁸O scales are not uniquely defined [Gonfiantini, 1984]. This came about because laboratories used different materials for calibration as the supply of PDB was exhausted. Some laboratories were tied to one material and other laboratories to other materials. In 1984, Gonfiantini [1984] reported that the d¹⁸O values (expressed relative to PDB) of the homogeneous, internationally distributed reference material NBS 19 calcite, as measured by 31 stable isotope laboratories from 19 countries, ranged from -2.42 to -1.53‰; d¹³C values ranged from +1.75 to +2.33‰. Fortunately, today, the ranges among stable isotope laboratories are smaller, and those among paleoceanographic isotope laboratories may be still smaller. Nevertheless, there is room for improvement.

Reporting d¹³C and d¹⁸O data from marine samples on corresponding scales is important because isotopic values are often needed and are routinely reported with precisions of about 0.03‰. There are numerous researchers who consistently report d¹³C and d¹⁸O values to hundredths of a per mill because the range in d¹³C of a marine core is often less than 1‰ and the range in d¹⁸O is only a few per mill.

The Commission on Atomic Weight and Isotopic Abundances of the International Union of Pure and Applied Chemistry (IUPAC) met in August 1995 and expanded their recommendations for the reporting of hydrogen, carbon, and oxygen isotopic data. These recommendations appear in the appendix. Recognizing that isotopic analyses of the same homogeneous material reported from different credible laboratories should yield the same isotopic composition within the uncertainty of the measurements, the International Atomic Energy Agency organized an Advisory Group Meeting on "Stable isotope reference and intercomparison materials for light elements," which was held in Vienna December 11--14, 1995. Twenty-nine participants from 14 countries attended this meeting. It was the unanimous opinion of the group that in light of increasing precision and accuracy available in isotopic determinations, IUPAC's uniform system of reporting isotopic data in the scientific literature should be adopted to the extent possible.

Perhaps the most important recommendation in the appendix is that isotopic data be expressed on scales defined in terms of existing internationally distributed reference materials. For the paleoceanographic community, isotopic data from carbonates would be reported relative to VPDB (Vienna PDB), which is defined in terms of NBS 19 calcite; isotopic data from water would be reported relative to the reference material VSMOW (see Table 1).

Recognizing that many paleoceanographic carbonate isotope laboratories do not have the capability to analyze water samples in order to normalize their d¹⁸O scales so that the d¹⁸O of SLAP reference water (see Table 1) is -55.5‰ relative to VSMOW, IUPAC recommendations suggest an alternative procedure. It is suggested that NBS 18 carbonatite, which differs in d¹⁸O value from that of NBS 19 by more than 20‰, be used for this normalization. For this purpose, two measurements of NBS 18 relative to VPDB on the normalized VSMOW-SLAP scale are available: Coplen, Kendall, and Hopple [1983] measured -23.01‰ and Stichler [1995] reports -22.97‰. Both are identical within analytical uncertainty.

One of IUPAC's recommendations deserves special mention to the paleoceanographic community. Specifically, authors can provide readers with more useful oxygen isotopic data if authors state the oxygen isotopic fractionation factors used for carbonate samples and NBS 19 calcite if they are not identical. By adopting these recommendations, individual paleoceanographic isotope laboratories can improve the quality of their data and can make their isotopic measurements more valuable for paleoceanographic reconstruction and to the scientific community at large.

Guidelines for the Reporting of Stable Hydrogen, Carbon, and Oxygen Isotope-Ratio Data

Introduction

Following recommendations [IUPAC, 1994; Coplen, 1994] and discussions of the Commission on Atomic Weights and Isotopic Abundances of the International Union of Pure and Applied Chemistry at the 38th General Assembly in 1995 in Guildford, United Kingdom, authors are requested to report stable hydrogen, carbon, and oxygen isotopic data as follows, stating the analytical method(s) employed.

Hydrogen

Water. Relative ²H/¹H values (d²H) of water should be expressed in per mill relative to VSMOW water (0‰) on a scale normalized such that the d²H of SLAP water is -428‰ exactly and so stated in the author's report.

Other substances. Relative ²H/¹H values (d²H) of other substances should be expressed in per mill relative to VSMOW water (0‰) on a scale normalized such that the d²H of SLAP water is -428‰ exactly and so stated in the author's report. It is recommended that the author's measured d²H of NBS 22 oil, NBS 30 biotite, or other internationally distributed reference material be reported, as appropriate to the analytical method.

Carbon

Carbonate. Relative ¹³C/¹²C values (d¹³C) of carbonate should be expressed in per mill relative to VPDB by assigning a value of +1.95‰ exactly to NBS 19 calcite.

Other substances. Relative ¹³C/¹²C values (d¹³C) of other substances should be expressed in per mill relative to VPDB by assigning a value of +1.95‰ exactly to NBS 19 calcite. It is recommended that the author's measured d¹³C of NBS 22 oil, USGS24 graphite, or other internationally distributed reference material be reported, as appropriate to the analytical method.

Oxygen

Water. Relative ¹⁸O/¹⁶O values (d¹⁸O) of water should be expressed in per mill relative to VSMOW water (0‰) on a scale normalized such that the d¹⁸O of SLAP water is -55.5‰ exactly and so stated in author's report.

Carbonate. Relative ¹⁸O/¹⁶O values (d¹⁸O) of carbonate should be expressed in per mill (on a scale normalized such that the d¹⁸O of SLAP water is -55.5‰ exactly relative to VSMOW water, and so stated in the author's report) either (1) relative to VPDB on a scale such that the d¹⁸O of NBS 19 calcite is -2.2‰ exactly, stating the value of the oxygen isotopic fractionation factor used to calculate the d¹⁸O of the carbonate sample and NBS 19 if they are not identical, or (2) relative to VSMOW water (0‰), stating the values of all isotopic fractionation factors upon which the d¹⁸O measurement depends. If d¹⁸O values cannot be reported on a normalized scale (perhaps because of the inability to measure water samples), the author's measured d¹⁸O of NBS 18 carbonatite or other internationally distributed reference material should be reported, as appropriate.

Other substances (oxygen gas, sulfate, silicate, phosphate, etc.). Relative ¹⁸O/¹⁶O values (d¹⁸O) of all other oxygen-bearing substances should be expressed in per mill relative to VSMOW water (0‰) on a scale normalized such that the d¹⁸O of SLAP water is -55.5‰ exactly and so stated in the author's report, stating either (1) the values of all isotopic fractionation factors upon which a d¹⁸O value depends, or (2) the author's measured d¹⁸O of NBS 28 quartz, NBS 30 biotite, NBS 127 barium sulfate, or other internationally distributed reference material. If d¹⁸O values cannot be reported on a normalized scale (perhaps because of the inability to measure water samples), the author's measured d¹⁸O of NBS 28 quartz, another internationally distributed reference material, or atmospheric oxygen should be reported, as appropriate to the analytical method.

Sources of Isotopic Reference Materials Isotopic reference materials may be obtained from the National Institute of Standards and Technology, Standard Reference Materials Program, Room 204, Building 202, Gaithersburg, MD 20899-0001 (Phone: 301-975-6776; Fax: 301-948-3730; E-mail: SRMINFO@enh.nist.gov) or the International Atomic Energy Agency, Section of Isotope Hydrology, Wagramerstr. 5, P.O. Box 100, A-1400 Vienna, Austria (Phone: 43-1- 206021735; Fax: 43-1-20607; E-mail: IAEO@iaea1.iaea.or.at).

Table 1.