DETERMINATION OF SULFUR ISOTOPE COMPOSITION IN SULFATE FROM TWO HIGH ELEVATION SNOWPITS BY MULTI-COLLECTOR THERMAL IONIZATION MASS SPECTROMETRY USING A DOUBLE SPIKE

Abstract

The variability of stable sulfur isotopes in nature provides a chemical tool for tracing the various sources of sulfur and a useful tool for understanding the sulfur cycle. It is also well established that snow and ice preserve a record of the sources, sinks, and processing of sulfur that reflect changes in this cycle through time. Our ability to sample this record is however limited by the total sample concentration and the analytical requirements for isotopic analysis. A high-resolution double spike technique using multi-collector thermal ionization mass spectrometry was developed for stable sulfur isotope composition measurements of small concentration sulfate samples (ppb level).

The capability of this new technique was demonstrated by measuring internationally recognized standards of known isotopic composition and by measuring snowpit samples with low sulfate concentrations collected from the Inilchek Glacier, Kyrgyzstan and Summit, Greenland. The elemental and high resolution sulfur isotope data for the snowpit samples were used to calculate the relative seasonal contributions of anthropogenic and natural sulfur sources to sulfate at these high-elevation Northern Hemisphere sites.

The isotope composition results for the standards demonstrate the double spike technique to be competitive in accuracy and precision with the traditional methods but the sample requirement is smaller. The average uncertainties on the individual isotope composition measurements for the Inilchek and Summit samples were approximately ± 0.10 (2s) and ± 1.5 (2s), respectively. The larger uncertainties for the Greenland samples resulted from increased blank and the smaller sample size used for analysis. Decreasing the blank concentrations by an order of magnitude show that a factor of two to three improvement in the uncertainties on small sample sizes is attainable with the double spike technique.

The sulfur isotope values in the Inilchek snowpit demonstrate no seasonality; while the values observed in the Greenland snowpit exhibit strong seasonality, where the values are 34S-depleted in the winter months and are 34S-enriched in the summer months. Mass balance calculations indicate that anthropogenic sources are the main contributor (75%) to sulfate during most of the year for both locations.