High-Resolution Sulfur Isotope Record from Two High-Elevation Snowpits: What is the Human Impact on the Remote Sulfur Cycle?
Jacqueline L. Mann, National Institute of Standards and Technology
5 Oct 2007
Sulfur has four naturally occurring isotopes; 32S, 33S, 34S and 36S. The relative abundances of these isotopes change in response to biogeochemical processing and consequently have become powerful tracers of the various sources of sulfur and for understanding the sulfur cycle. Snow and ice preserve a continuous chronological record of the sources, sinks, and geochemical processing of sulfur through time. The ability to decipher this record has been limited by the currently used analytical capability, which requires > 1µmole of S for precise isotopic measurements. In this work, δ34S was determined in snowpit samples using a new technique that is capable of 0.2‰ accuracy and requires < 5 µg natural S. The technique combines multiple-collector thermal ionization mass spectrometry (MC-TIMS) with a 33S-36S double spike and has been applied to snowpit samples collected from Kyrgyzstan and Greenland. The δ34S data from the Summit snowpit are the first continuous high-resolution (≈ 7 samples/year) data for this site. The δ34S values for the Inilchek ranged from 2.6 ± 0.4‰ (2σ) to 7.6 ± 0.4‰ (2σ) on sample sizes ranging from 0.3 to 1.8 µmol S. δ34S values for Greenland ranged from 3.6 ± 0.7‰ (2σ) to 13.3 ± 5‰ (2σ) for sample sizes ranging from 0.05 to 0.29 µmol S. The mass balance results show anthropogenic sulfate dominates (72%) throughout most of the year at both sites. Because of the reduction in sample size requirements of this technique, accessing the higher-resolution S isotope record of low concentration snow and ice is now possible.