**Title: Measurement of sulfur isotope composition in two high-elevation
snowpits by Multiple-Collector Thermal Ionization Mass Spectrometry
(MC-TIMS) using a ^33^S-^36^S Double Spike**

The variability of sulfur isotope ratios, caused by mass dependent
fractionation during biogeochemical processing, is commonly used for
tracing the various sources of sulfur and for understanding the sulfur
cycle. A new analytical technique for the determination of δ^34^S and
sulfur concentration that is both highly accurate and precise has been
developed for the determination of sulfur isotope composition of low
concentration samples. The technique is based on the production of
singularly charged arsenic sulfide molecular ions (AsS^+^) by thermal
ionization using silica gel as an emitter and combines
multiple-collector thermal ionization mass spectrometry (MC-TIMS) with a
^33^S-^36^S double spike to correct instrumental fractionation. Because
the technique is based on thermal ionization of AsS^+^, and As is
mononuclidic, corrections for interferences or for scale
contraction/expansion are not required. The method has been applied to
three international sulfur standards (IAEA-S-1, IAEA-S-2, and IAEA-S-3)
and snow and firn samples collected from the Inilchek Glacier,
Kyrgyzstan (42.16°N, 80.25°E, 5100 m) and Summit, Greenland (72.58°N,
38.53°W, 3238 m).

The standards were measured to evaluate the precision and accuracy of
the new technique and to evaluate the consensus values for these
standards. Two different double spike preparations were used. The δ^34^S
values (reported relative to Vienna Canyon Diablo Troilite (VCDT),
(δ^34^S (‰) = \[(^34^S/^32^S)~sample~/(^34^S/^32^S)~VCDT~ -- 1) x
1000\]), ^34^S/^32^S~VCDT~ = 0.0441626) determined were --0.32 ± 0.04‰
(1σ, n=4) and --0.31 ± 0.13‰ (1σ, n=8) for IAEA-S-1, 22.65 ± 0.04‰ (1σ,
n=7) and 22.60 ± 0.06‰ (1σ, n=5) for IAEA-S-2, and --32.47 ± 0.07‰ (1σ,
n=8) for IAEA-S-3. The amount of natural sample used for these analyses
ranged from 0.40 to 2.35 μmol. Replicate determinations of each standard
showed less than 0.5‰ variability (IAEA-S-1 \< 0.4‰, IAEA-S-2 \< 0.2‰,
and IAEA-S-3 \< 0.2‰). The uncertainties reported are comparable to or
better then those obtained by gas source isotope ratio mass
spectrometers (IRMS). The δ^34^S measurements of the snow and firn
samples were used to estimate seasonal sulfate sources contributing to
precipitation in these regions. δ^34^S data from the Summit snowpit are
the first continuous high-resolution (≈ 7 samples/1 year) data for this
site. The δ^34^S values for the Inilchek ranged from 2.6 ± 0.05‰ (1σ) to
7.6 ± 0.06‰ (1σ) on sample sizes ranging from 0.3 to 1.8 μmol S. δ^34^S
values for Greenland ranged from 3.6 ± 0.19‰ (1σ) to 13.3 ± 2.51‰ (1σ)
for sample sizes ranging from 0.05 to 0.29 μmol S. For both the Inilchek
and Summit samples the uncertainties are dominated by blank corrections
and not by measurement uncertainty. The SO~4~^2-^ concentrations ranged
from 0.92 ± 0.009 μmol L-1 to 10.46 ± 0.08 μmol L-1 (95% C.I.) for the
Inilchek and from 0.16 ± 0.09 μmol L-1 to 0.92 ± 0.06 μmol L-1 (95%
C.I.)

for the Greenland snowpit. Although the Inilchek snowpit does not
demonstrate the seasonality seen in the Greenland snowpit, the mass
balance results show anthropogenic sulfate dominates (≈ 75%) throughout
most of the year at both sites. Because of the reduction in sample size
requirements of this technique, by as much as a factor of 10, accessing
the higher-resolution sulfur isotope record of low concentration snow
and ice is now possible.