A cross-calibration of chlorine isotopic measurements and suitability of seawater as the international reference material

Abstract

A collection of 24 seawaters from various worldwide locations and differing depth was culled to measure their chlorine isotopic composition ( δ 37Cl). These samples cover all the oceans and large seas: Atlantic, Pacific, Indian and Antarctic oceans, Mediterranean and Red seas. This collection includes nine seawaters from three depth profiles down to 4560 mbsl. The standard deviation (2 σ) of the δ 37Cl of this collection is ±0.08‰, which is in fact as large as our precision of measurement (±0.10‰). Thus, within error, oceanic waters seem to be an homogeneous reservoir. According to our results, any seawater could be representative of Standard Mean Ocean Chloride (SMOC) and could be used as a reference standard. An extended international cross-calibration over a large range of δ 37Cl has been completed. For this purpose, 13 geological fluid samples of various chemical compositions and a manufactured CH 3Cl gas sample, with δ 37Cl from about −6‰ to +6‰ have been compared. Data were collected by gas source isotope ratio mass spectrometry (IRMS) at the Paris, Reading and Utrecht laboratories and by thermal ionization mass spectrometry (TIMS) at the Leeds laboratory. Comparison of IRMS values over the range −5.3‰ to +1.4‰ plots on the Y= X line, showing a very good agreement between the three laboratories. On 11 samples, the trend line between Paris and Reading Universities is: δ 37Cl Reading=(1.007±0.009) δ 37Cl Paris−(0.040±0.025), with a correlation coefficient: R 2=0.999. TIMS values from Leeds University have been compared to IRMS values from Paris University over the range −3.0‰ to+6.0‰. On six samples, the agreement between these two laboratories, using different techniques is good: δ 37Cl Leeds=(1.052±0.038) δ 37Cl Paris+(0.058±0.099), with a correlation coefficient: R 2=0.995. The present study completes a previous cross-calibration between the Leeds and Reading laboratories to compare TIMS and IRMS results (Anal. Chem. 72 (2000) 2261). Both studies allow a comparison of IRMS and TIMS techniques between δ 37Cl values from −4.4‰ to +6.0‰ and show a good agreement: δ 37Cl TIMS=(1.039±0.023) δ 37Cl IRMS+(0.059±0.056), with a correlation coefficient: R 2=0.996. Our study shows that, for fluid samples, if chlorine isotopic compositions are near 0‰, their measurements either by IRMS or TIMS will give comparable results within less than ±0.10‰, while for δ 37Cl values as far as 10‰ (either positive or negative) from SMOC, both techniques will agree within less than ±0.30‰.