1) The highest sulfur isotope anomalies (Δ33S = +1.66‰ and Δ36S = +2‰) were observed in nonvolcanic sulfate aerosols extracted from a South Pole snow pit from 1998-1999, similar in magnitude to the largest volcanic eruptions of the 20th century. 2) These anomalies are linked to stratospheric chemistry and dynamics changes induced by the very strong 1997-1998 El Niño Southern Oscillation event, including higher altitude transport of sulfur dioxide and carbonyl sulfide via deep convection. 3) Recurring negative sulfur isotope anomalies (Δ36S = -0.6 ± 0.2‰) in nonvol

2. Large sulfur-isotope anomaly in nonvolcanic
sulfate aerosol and its implications for the
Archean atmosphere
Robina Shaheena
, Mariana M. Abaunzaa
, Teresa L. Jacksona
, Justin McCabea,b
, Joël Savarinoc,d
, and Mark H. Thiemensa,1
a
Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA 92093; b
Pacific Ridge School, Carlsbad, CA 92009; c
Laboratoire
de Glaciologie et de Géophysique de l’Environnement, Centre National de la Recherche Scientifique, Unité Mixte de Recherche 5183, F-38041 Grenoble, France;
and d
Laboratoire de Glaciologie et de Géophysique de l’Environnement, Université Grenoble Alpes, Unité Mixte de Recherche 5183, F-38041 Grenoble, France
Edited†
by Barbara J. Finlayson-Pitts, University of California, Irvine, CA, and approved July 18, 2014 (received for review April 8, 2014)
Sulfur-isotopic anomalies have been used to trace the evolution of
oxygen in the Precambrian atmosphere and to document past
volcanic eruptions. High-precision sulfur quadruple isotope
measurements of sulfate aerosols extracted from a snow pit at
the South Pole (1984–2001) showed the highest S-isotopic anoma-
lies (Δ33
S = +1.66‰ and Δ36
S = +2‰) in a nonvolcanic (1998–1999)
period, similar in magnitude to Pinatubo and Agung, the largest
volcanic eruptions of the 20th century. The highest isotopic anom-
aly may be produced from a combination of different stratospheric
sources (sulfur dioxide and carbonyl sulfide) via SOx photochemis-
try, including photoexcitation and photodissociation. The source of
anomaly is linked to super El Niño Southern Oscillation (ENSO) (1997–
1998)-induced changes in troposphere–stratosphere chemistry and
dynamics. The data possess recurring negative S-isotope anoma-
lies (Δ36
S = −0.6 ± 0.2‰) in nonvolcanic and non-ENSO years, thus
requiring a second source that may be tropospheric. The genera-
tion of nonvolcanic S-isotopic anomalies in an oxidizing atmo-
sphere has implications for interpreting Archean sulfur deposits
used to determine the redox state of the paleoatmosphere.
UV photolysis | sulfur isotopes
Sulfur is a ubiquitous element on Earth. Its multiple valence
states (S−2
to S+6
) permit it to participate in a range of
photochemical, geochemical, and biochemical processes, and its
four stable isotope (32
S, 33
S, 34
S, and 36
S) allow tracing of chemical
reactions at a molecular level. Multiple sulfur isotopes (δ33
S, δ34
S,
and δ36
S) and concomitant anomalies (Δ33
S and Δ36
S)‡
in paleo-
sediments [>2.5 giga-annum (Ga)] have been used to trace the
origin and evolution of life and rise of oxygen in the Earth’s
paleoclimatic history (1–3). In the present atmosphere, the
concentration of sulfate in ice cores and associated S-isotope
anomalies has served as a forensic tool to help understand the
dynamics of volcanic emissions, such as transport and trans-
formation of sulfur to the stratosphere and its impact on ozone
chemistry (4–7). The low concentration of sulfate (SO4
2-
) in ice
cores during volcanically quiescent periods and associated ana-
lytical challenges to analyze all four S-stable isotopes at high
precision have restricted studies of the temporal distribution of
sulfur mass-independent signatures. Here, we present a high-
resolution seasonal record (1984–2001) of quadruple S-stable
isotopes and concomitant isotope anomalies of sulfate aerosols
extracted from a snow pit (1 × 1 m) at the South Pole (89.5° S,
17.3° W; 2,850 m) (8) to gain further insight into sources, pho-
tochemistry, and associated sulfur transformations of strato-
spheric sulfate aerosols (SSAs). The time period encompasses
two major volcanic eruptions and three large El Niño Southern
Oscillation (ENSO) events. A recent study has attributed a global
warming hiatus (9) to a super ENSO event (1997–1998); therefore,
data from this period are timely for understanding changes in
stratospheric sulfate aerosol chemistry that play an important role in
mitigating global warming trends via scattering of incoming solar
radiation. Oxygen triple isotope measurements of sulfate aerosols
(1980–2002) have recently revealed how ENSO-driven changes
affect the global transport and transformation of sulfate aero-
sols from the troposphere to the stratosphere and across hemi-
spheres (10).
Results and Discussion
The highest SO4
2-
concentration in snow [154 parts per billion
(ppb)] is observed after volcanic activity (Pinatubo, June 1991;
Cerro Hudson, August 1991). The addition of volcanic sulfate to the
stratospheric baseline sulfate aerosol (or background sulfate aerosol
as defined in SI Appendix, Section 2) produced a significant de-
crease in heavy sulfur isotopes. The baseline sulfate aerosol value of
δ34
SBG= 12‰ dropped to ∼3‰ (Fig. 1A) following the Pinatubo
eruption, and δ33
S, δ36
S tracked this isotopic trend. The contribu-
tion of sea salt sulfate at the South Pole is <9%, indicating long-
range transported aerosol to be the main sulfate component (10). A
broad range in δ33
S (1.61–11.32‰), δ34
S (2–20‰), and δ36
S (2.8–
37‰) values for the sampling time period indicates the origin
of sulfate aerosols from various sulfur sources and chemical and
dynamical processes. A significant positive correlation of δ34
S
Significance
The highest S-isotope anomaly is observed in a nonvolcanic
period, and the magnitude of anomaly is similar to the largest
volcanic eruptions of the 20th century. S-quadruple isotope
data provided the first evidence of how super El Niño Southern
Oscillation (ENSO) events (1997–1998) have affected the trans-
port and transformation of aerosols to the stratosphere; thus,
record of paleo-ENSO events of this magnitude can be traced
with the S-isotopic anomaly. High-resolution and high-precision
S-isotopic fingerprinting also revealed that the tropospheric
sulfate produced during fossil-fuel and biomass burning con-
tributes to the stratospheric sulfate aerosol layer, a contribution
previously unrecognized. The distribution of sulfur anomalies
mimics the Archean isotope record, which is used to track the
origin and evolution of oxygen on earth.
Author contributions: R.S., J.M., J.S., and M.H.T. designed research; R.S., M.M.A., and T.L.J.
performed research; R.S., J.M., and M.H.T. contributed new reagents/analytic tools; R.S.,
J.S., and M.H.T. analyzed data; and R.S. wrote the paper.
The authors declare no conflict of interest.
†
This Direct Submission article had a prearranged editor.
1
To whom correspondence should be addressed. Email: mthiemens@ucsd.edu.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.
1073/pnas.1406315111/-/DCSupplemental.
‡
MIF. Here, delta denotes the ratio of the least abundant to the most abundant isotope
{e.g., δ33
S = [(33
S)/(32
S)sample/(33
S)/(32
S)std − 1) × 1,000]} relative to the same ratio in
standard, which is Canyon Diablo Troilite (CDT) and expressed in parts per thousand (‰).
Most natural processes fractionate S isotopes in proportion to mass differences and are
described by δ33
S ≈ 0.515*δ34
S, and δ36
S ≈ 1.91*δ34
S, except UV photolysis of SO2. The
deviation from mass-dependent fractionation (MDF) is called anomalous or mass-inde-
pendent fractionation (MIF) and quantified by Δ33
S and Δ36
S.
www.pnas.org/cgi/doi/10.1073/pnas.1406315111 PNAS Early Edition | 1 of 5
EARTH,ATMOSPHERIC,
ANDPLANETARYSCIENCES

3. with δ33
S and δ36
S (δ33
S = 0.508*δ34
S + 0.2, r = 0.97; δ36
S =
1.94*δ34
S – 0.876, r = 0.99) is observed (SI Appendix, Fig. S2).
The maximum S-isotopic anomalies of Δ33
S = +1.6‰ and
Δ36
S = +2.0 ‰ observed in 1998–1999 (Fig. 1B) occurs after the
strongest El Niño event (1997–1998) of the decade. The peak is
associated with a sharp increase in potassium (K) concentration
(up to 42 ppb). This anomaly (Δ33
S) is ∼2 times higher than the
Pinatubo signal (Δ33
S = +0.9‰) whereas the Δ36
S is similar in
magnitude to the Agung eruption (Δ36
S = +2.5‰) (5), the
largest volcanic eruption of the 20th century.
These S-isotopic anomalies are within the reported volcanic
sulfate isotopic ranges between Δ33
S = −1‰ to +0.9‰ and
Δ36
S= −5‰ to +3‰ (5, 11), suggesting similar photochemical
processes (Fig. 2A). Laboratory experiments have demonstrated
that the S-MIF originates during SO2 photolysis at short wave-
lengths (λ < 300 nm), producing sulfate with positive Δ33
S and
negative Δ36
S values, and a wide array of slopes (Δ36
S/Δ33
S),
ranging from −1.1 to − 4.3 depending on wavelength (12),
pressure, and composition of the gaseous mixture (13) . The
observations suggest that the S-isotope anomaly in sulfate
aerosol in 1998–1999 is a consequence of SOx (SO2, SO3)
photochemistry (5, 6, 12, 14) in the short UV (<200 nm) region
of the solar spectrum above the ozone layer (>25 km) where this
wavelength is available in the present-day atmosphere, as will be
discussed. The S-isotopic anomalies observed in volcanic sulfate
aerosols are accompanied by a significant increase in sulfate
concentration (4, 5, 11). The Pinatubo and Cerro Hudson
eruptions in 1991 produced a factor of 3.7 sulfate concentration
increase in the snow record. The observed unprecedented
S-isotope anomaly is not accompanied by a massive Pinatubo-size
increase in sulfate concentration, thus requiring new, or highly
perturbed, chemical, photochemical, and dynamical processes.
Our high-resolution, seasonally resolved sulfate aerosol data reveal
the presence of a negative S-isotope anomaly (Δ36
S(avg) ∼ −0.6‰)
during nonvolcanic and non-ENSO baseline periods (SI Appendix),
suggesting the presence of a second isotopically anomalous
sulfate source. The S-isotopic anomaly during these time periods
is within the range reported for tropospheric sulfate aerosols
(15) of Δ36
S= −0.3‰ to −2‰ (Fig. 2B). In the present atmo-
sphere, short UV is blocked by the O3 layer; thus, the negative
anomaly in tropospheric sulfate aerosol cannot be attributed to
short-wavelength SOx photolysis. Romero and Thiemens (15)
suggested possible transport of stratospheric S-isotope anomaly
to the troposphere at low and mid latitudes. Considering the
tropospheric S burden (16) (SO2 from fossil-fuel combustion ∼
78 Tg S·yr−1
, biomass burning ∼ 2 Tg S·yr−1
, and natural sources
∼ 25 Tg S·yr−1
), it is unlikely that even a 10% transport of SSA
(0.01 Tg S·yr−1
) can produce such a significant isotopic change in
tropospheric sulfate aerosols. Alternatively, a 0.01% transport of
anomalous sulfate from the troposphere to stratosphere can
cause a significant change (Δ36
S = −0.9‰) in the isotopic com-
position of SSA (SI Appendix), provided tropospheric sulfur is
anomalous. Laboratory studies demonstrate that a negative S-iso-
tope anomaly can be produced by nonphotochemical processes,
such as primary sulfate produced during fossil-fuel combustion
(Δ36
S = −0.8‰ to −1.7‰) and biomass burning (Δ36
S = −0.1‰
to −2‰) (SI Appendix, Table S3) (17). The mechanism that gen-
erates the negative anomaly in such processes is unknown (radical
driven or recombination reactions may be operative, for example),
but, clearly, high-temperature sulfur oxidation processes are a via-
ble source for the tropospheric negative anomaly. The S-isotopic
composition (δ34
S, Δ33
S, and Δ36
S) of baseline sulfate suggests
transport of SO2 and SO4 to the stratosphere despite its normal
short tropospheric life time (∼2–5 d).
The observed positive sulfur anomaly during 1998–1999
requires a stratospheric photochemical process involving SO2. It
is generally accepted that only explosive volcanic eruptions
[volcanic explosivity index (VEI) > 4] have sufficient energy to
transfer tropospheric boundary material into the stratosphere
that attain altitudes where short UV region at λ < 300 nm is
available. The Smithsonian database of global volcanic eruptions
(www.volcano.si.edu) and Stratospheric Aerosol and Gas Experi-
ment II (SAGE II) archives (18), however, do not list any significant
plinian volcanic activity in 1998–1999, ruling out volcanic SO2 input
to the stratosphere as a source for the observed positive S-isotopic
anomaly. Increased SO4 concentrations from local (Antarctic)
sources is ruled out because sea salt sulfate and sulfate produced
from DMS oxidation is isotopically normal (5, 6). A potential new
source of the increase in sulfate concentration and S-isotopic
anomaly could be the higher altitude transport of SO2 and po-
tentially from carbonyl sulfide (COS) by deep convection to the
tropical tropopause layer, followed by SO2 photochemistry upon
stratospheric COS oxidation (19). COS, the most abundant
tropospheric S compound [∼500 parts per trillion (ppt)], is
transported to the stratosphere (19) and removed by photolysis
(∼70%) to SO2 above 25 km. Increased COS (20–50%) in the
tropical tropopause layer (the main entry region to the strato-
sphere), along with a substantial increase in other tracers of
biomass burning (BB), including CO, HCN, CH3Cl, NOx, NOy in
1996 and 1999–2000, has been observed (20). Potassium (K),
a tracer of BB, can serve in certain circumstances as a tracer of
Fig. 1. (A) The concentration profile (1983–2000) of SO4 and δ33
S, δ34
S, δ36
S
sulfate aerosol extracted from snow-pit samples at the South Pole. Note
anticorrelation between SO4 concentration and S-stable isotopes of SO4 aerosol
after Pinatubo and Cerro Hudson eruptions. (B) Sulfur-isotope anomaly (Δ33
S
and Δ36
S) of sulfate aerosols extracted from the snow-pit samples at the South
Pole (1983–2000). For comparison, the ENSO-O3 Index is also shown. Note
that 1997–1998 biomass burning increased the O3 concentration in the upper
troposphere/lower stratosphere (42) followed by a sharp decline in O3 and
concomitant increase in S-MIF. Purple bars indicate strong El Niño Southern
Oscillation events, and M stands for moderate ENSO.
2 of 5 | www.pnas.org/cgi/doi/10.1073/pnas.1406315111 Shaheen et al.

4. forest fires (21, 22), and ice-core data have revealed increased K
concentration after intensive biomass burning events from 1750
to 1980 (23). There are no global measurements of trace gases
available from 1998 to 1999 BB events. Andreae and Merlet (24)
estimated ∼1.8 Tg of S (∼15% contribution from COS), K (1.9
Tg), NOx (20.7 Tg), and CH3Cl (∼0.65 Tg) emissions from global
BB events. El Niño Southern Oscillations are known to signifi-
cantly affect chemistry, temperature, and dynamics of the tro-
posphere and stratosphere (25) (SI Appendix). ENSO-induced
variations in the upper troposphere/lower stratosphere (UT/LS)
ozone levels (26) have been captured in the oxygen triple isotope
data of sulfate aerosols (1981–2004) retrieved from the South
Pole through its effect on the SO2 oxidation pathways (10). The
1998–1999 and 1984 peaks are the only deviants from the bulk
S-isotope anomalies (Fig. 1B), and special processes are required
for these two time periods. Assuming COS and SO2 from wildfires
as the source of sulfur in this period with higher altitude trans-
port to the stratosphere (18, 19) via pyrocumulus nimbus clouds
(27), subsequent photochemistry of COS produced SO2 above
25 km could provide an extra S-isotope anomaly source. The
altitude for both SO2 photo excitation and photolysis in this case
likely differs from volcanic SO2 due to its production above the
ozone layer. Additionally, S-MIF signatures recorded in ice cores
after major volcanic eruptions are actually a mixture of anoma-
lously produced sulfate via SOx photolysis and mass-dependently
produced sulfate via SO2 + OH reaction (∼90% for Pinatubo
sulfate), thus diluting the actual S-MIF signal (6, 7).
The origin of the S-MIF is a function of the actinic light
spectrum for both photoexcitation and photodissociation
processes of SO2 (12, 28). Laboratory experiments have shown
that COS photolysis at λ < 220 nm produces elemental sulfur
S0
with no S-isotope anomaly (29). In an oxidizing environ-
ment, however, COS photolysis produces SO2 (19), and sub-
sequent photochemical transformations at short wavelengths
(<280 nm) can produce S-MIF in sulfate (12, 14). A recent
model that considers SO2 photoexcitation rather than pho-
tolysis and volcanic plume chemistry (including heteroge-
neous stratospheric chemistry) suggests that UV photo
excitation of SO2 is another route to the observed S-MIF in
volcanic sulfate. This new mechanism may also provide in-
formation about the ozone-depletion chemistry in the plume
(30) and is relevant for the present data. Laboratory experiments
indicate that SO2 photodissociation is wavelength-dependent
(Δ36
S/Δ33
S slopes vary from −1 at λ = 193 nm to −4 at λ = 248 nm
as shown in Fig. 2), and the deviant sulfate circled points (1998–
1999) may result from photochemistry at shorter wavelength, likely
in the bands below 220 nm. There are no numeric simulations
(including stratospheric heterogeneous chemistry and photo-
chemical transformations) that are directly applicable to the
present case where the dynamics and chemistry are perturbed as
a result of changes in stratosphere–troposphere dynamics and in-
tensive global BB following the super ENSO event (1997–1998),
and it is difficult to quantify the excess sulfur reaching an altitude
for the required wavelength (<220 nm). Based on the SO2 pho-
tolysis experiments at short wavelength (193 nm) by Farquhar
et al. (12), if Δ36
S(SO4)= 20‰ is assumed as an upper limit
{isotopic mass balance; Im [(Δ36
S(SO4) = 20‰) = (2‰_ENSO +
2.6‰_BG + ENSO)*40 ppb_background sulfate/excess SOx
from biomass burning]}, isotope mass balance suggests that the
incremental SOx required above background level in the strato-
sphere (>25 km where λ < 200 nm light is available) to produce
Δ36
S = 2‰ is 5 ppb.
The potential importance of different sulfur sources (e.g.,
COS and differing SO2 photochemistry) and the second tropo-
spheric source may have further consequences in the Earth’s
early atmosphere (31). Mass-independent isotopic compositions
in S-bearing molecules have been observed in the Earth’s oldest
rocks, which are interpreted as reflecting lowered oxygen and
ozone concentrations in the atmosphere allowing tropospheric
SO2 photochemistry at short wavelengths (1). There is debate as
to the oxidation state of Earth’s atmosphere–hydrosphere before
Fig. 2. (A) Comparison of the sulfur-isotopic anomaly in aerosols (1983–
2001) extracted from the snow pit (1 × 1 m) at the South Pole station (green
squares) with the volcanic sulfate aerosol retrieved from ice cores (4–6, 11). (B)
Comparison with tropospheric aerosol collected at various sites in the United
States (15) [La Jolla, CA (blue circles); Shenandoah National Park, VA (red
circles); and Bakersfield, CA (yellow circles] (SI Appendix, Table S4) and fossil-
fuel and biomass burning signatures (magenta triangles) (SI Appendix, Table
S3). Δ36
S/Δ33
S slope of SO2 photolysis experiments using ArF excimer (193 nm)
and KrF (284 nm) Xe-lamp (continuum from 220 nm to longer wavelength)
are also shown for comparison(12). (C) S-isotope anomalies in sulfate and
sulfide deposits from Precambrian rocks record (32) to the present-day sedi-
ments and comparison with aerosols in the present-day atmosphere.
Shaheen et al. PNAS Early Edition | 3 of 5
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5. ∼2.4 Ga. Large mass-independent fractionation of S isotopes in
pre-2.4 Ga sedimentary rocks and their absence in post-2.4 Ga
counterparts support the hypothesis for a reducing Archean
atmosphere–hydrosphere (32, 33). Models assume that UV pho-
tolysis of volcanic gaseous SO2 in a low pO2 atmosphere (3, 14,
34) is the source of S-isotope anomaly. The rate of volcanic
supply of SO2, COS, H2S, photochemical transformation, and
further reactions to form sulfates (S+6
), sulfite (S+4
), elemental S
(S0
), and sulfide (S−2
) determines the overall signature preserved
in the rock (35, 36). Processes that can remove SO2 from the
atmosphere, such as homogeneous oxidation with OH radical
and heterogeneous chemistry with O3 and H2O2 in the cloud, can
reduce the S-MIF in the atmospheric S pools (35). However, the
presence of large S-MIF in the Archean rock record (Δ33
S = −2‰
to +12‰ and Δ36
S= −10‰ to +2 ‰) (36) is used to constrain
the upper limit of 10−5
present atmospheric level (PAL) of at-
mospheric oxygen levels in the Archean atmosphere (34). The
mixing of atmospherically derived S-MIF pool with the micro-
bially derived S-MDF pools in the marine sediment lowers the
overall magnitude of atmospherically derived S-MIF signal.
Comparison of the S-isotope anomaly of marine paleo-sediments
(sulfates and sulfide) with present-day sulfate aerosol, including
tropospheric aerosols (Fig. 2C), reveals that the S-isotope
anomaly resides in a similar S-quadruple isotope space although
the magnitude of S-isotope anomaly in ice-core data is smaller
than the Precambrian record. The result suggests that both may
be produced by the same SOx photochemical processes and that
factors such as photolysis wavelength and pressure may be impor-
tant in accounting for some of the differences (12, 13). The pres-
ence of S-MIF in the present-day atmosphere in nonvolcanic
aerosols (encircled points) after the super ENSO 1997–1998 sug-
gests that these two sources (SO2 and COS) could contribute to
sulfur-isotopic anomalies in the Archean. Thermodynamic gas
phase equilibrium shows COS to be a stable product of reactions
such as CO2 + H2S → COS + H2O and CO + H2S → COS+ H2 in
reducing and oxidizing environments and has been detected in
terrestrial geothermal fluids and present-day volcanoes (37), the
atmosphere of Venus (38), and dense molecular clouds and comets
(39, 40). Consequently, COS is another plausible S species in a re-
ducing early earth environment. Numeric simulations of the CO2-
rich (1%) early Archean atmosphere suggest that COS (5 ppm) may
have provided a greenhouse effect (41). If COS undergoes hydro-
lysis reactions, it could lead to SO2 in both oxidizing and reducing
environments, and the anomaly observed may be a consequence of
both SO2 and COS photochemical transformations in early Earth.
This observation suggests that it is imperative to consider such
reactions in the early Earth models to facilitate the optimal un-
derstanding of the Earth’s early atmosphere and its oxygen record.
The present work suggests that modeling efforts on the con-
sequences of COS emissions from past sources should be explored,
especially isotopically, and should include aerial and subaerial
volcanoes, fumaroles, and oceans. It is apparent from the S-isotope
anomaly plots (Fig. 2) that short UV-processed aerosols produced
by potentially amplified COS/SOx sources lie within the range
reported for the early Archean sediments (sulfide and sulfates) and
for the two largest volcanic eruptions of the century, Pinatubo
and Agung; therefore, new models of the Archean folding in
COS and additional sulfur isotopic chemistry are needed to fur-
ther resolve the early Earth environment. A long-term record of
S-quadruple isotopes along with other tracers of biomass burning,
such as soot and other tracers, is needed to quantify past ENSO
and BB events and to assess future impact on stratospheric O3
chemistry during time periods of increased biomass burning in the
present-day atmosphere and stratospheric inputs.
Materials and Methods
Sulfate aerosol were extracted from a 1 × 1 m snow pit at the South Pole
(2,850 m high; snow accumulation rate 84 kg·m−2
·a−1
; mean annual tem-
perature −49.5 °C), Antarctica (8). The sample preparation for O-isotope
analysis and SO2 collection for sulfur isotope analysis were described earlier
(10). SO2 gas was converted to SF6 for S-quartet isotope analysis following
the method developed earlier in our laboratory (3, 12, 43). The δ33
S, δ34
S,
δ36
S, Δ33
S, and Δ36
S showed standard deviations of 0.06‰, 0.1‰, 0.4‰,
0.05‰, and 0.2‰, respectively, over the course of one year (sample size = 1–
2 μmole; SI Appendix, Table S2).
The mass independent signatures of S are measured as (3):
Δ33
S = δ33
S − 1,000*[(1 + δ34
S/1,000)0.515
− 1]
Δ36
S = δ36
S − 1,000*[(1 + δ36
S/1,000)1.91
− 1].
ACKNOWLEDGMENTS. We thank anonymous reviewers for critical evalua-
tion that greatly improved our manuscript. The National Science Foundation
Atmospheric Chemistry Division and polar program are recognized for their
support through Awards ATM0960594 and OPP0125761. J.S. thanks the
Agence Nationale de la Recherche [ANR-NT09-431976-volcanic and solar
radiative forcing (VOLSOL)] and the Centre National de la Recherche
Scientifique/Projet International de Coopération Scientifique exchange pro-
gram for their financial support for maintaining the collaboration with the
University of California, San Diego.
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Shaheen et al. PNAS Early Edition | 5 of 5
EARTH,ATMOSPHERIC,
ANDPLANETARYSCIENCES

7. Supplement: Large Sulfur Isotope Anomaly in Non Volcanic Sulfate and its1
Implications for the Archean Atmosphere2
R. Shaheen1
, M. Abaunza1
, T. Jackson1
, J. McCabe1,2
, J. Savarino3,4
, M. H. Thiemens1*
3
1
University of California San Diego, Dept. of Chemistry and Biochemistry, La Jolla, CA-92093,4
USA.5
2
Pacific Ridge School, 6269 El-Fuerte Dr. Carlsbad, CA-92009, USA.6
3
CNRS, LGGE (UMR5183), F-38041 Grenoble, France7
4
Univ. Grenoble Alpes, LGGE (UMR5183), F-38041 Grenoble, France.8
*
Corresponding author: mthiemens@ucsd.edu9
10
Supplement Section S1:11
Materials and Methods:12
Sulfate aerosol were extracted from a 1x1 m snow pit at the South Pole (2850 m high, snow13
accumulation rate 84 kg.m-2
.a-1
, mean annual temperature -49.5o
C), Antractica (1). The sample14
preparation for O-isotope analysis and SO2 collection for sulfur isotope analysis is described15
earlier (2). SO2 gas was converted to SF6 for S-quartet isotope analysis following method16
developed earlier in our laboratory (3-5). The 33
S, 34
S, 36
S, 33
S, and 36
S showed standard17
deviations of 0.06‰, 0.1‰, 0.4‰, 0.05‰, and 0.2‰ respectively over a course of one year18
(sample size =1-2 mole Table SII).19
The mass independent signatures of S are measured as (4):20
33
S = 33
S -1000*[(1 + 34
S/1000)0.515
-1] (1)21
36
S = 36
S - 1000*[(1 + 36
S /1000)1.91
- 1] (2)22
23
Supplement Section2:24

8. 2
Regular variation in sodium concentration with depth was used for annual layer counting from25
1984-2001. Spikes in nss-SO4 (156ppb) concentration and low S- isotopic values (34
S= 1.4-2.6)26
served as independent corroborator of age estimates as explosive volcanic activity of Pinatubo27
(15.13 o
N, 120.35 o
E, 1,745m, June 1991); and Cerro Hudson (45.90 o
S, 72.97o
W, 1,905m; Aug28
1991) led to excess deposition of aerosol sulfates (1991-1993) at the South Pole. Higher29
concentrations of nss-sulfate in the ice cores from Antarctica (Dome C and South Pole) and30
Greenland has been used to spot paleo volcanic activities (6-8). The reported sample dates for31
are calculated from a high resolution (~1cm) cations (Na, K, Mg) and anions (Cl, SO4, NO3,32
MSA) concentration profiles (1). The actual date of the composite samples used for both S and O33
isotopes may be shifted by + 4 months.34
The background S- isotope signal (33, 34, 36
S = 6.5 + 0.7, 12 + 1.4 and 21 + 2.7‰ respectively)35
was estimated from samples with sulfate concentrations ~ 40ppb  2ppb using volcanic quiescent36
period from 1985 to 1988. This period reflects photochemically produced sulfate aerosols in37
steady state conditions and is considered to arise from all other contributors to nss-SO4 except38
volcanoes. The estimated 34
S for the background is close to the non sea salt S-isotope signal39
34
S =11 + 0.3 ‰ observed in aerosols collected directly on filter papers at the South Pole for the40
entire year in 1999 (9). The isotopic signature (34
S) of SO4 from marine biogenic sources and41
sea salt are well constrained at ~18 and 21‰ (10-12) (13). The isotopic signature of volcanic42
sulfate can be estimated as:43
34
Svolcanic = (34
Snss – (1-fbg) x 34
Sbg )/fbg (1)44
Here 1-fbg denotes fraction of sulfate from the volcanic activities. Substituting 34
Sbg =12‰ with45
constant background input (fbg =40 ppb) in eq. 1, the volcanic signal showed highest depletion in46

9. 3
S-stable isotopes 33
S,34
 36
SstageI(1991.6 -1992.1) =-4.4‰,-13‰, -30‰; 33
S,34
 36
SstageII (1992.8)47
= -12.6‰ -23‰,-45‰.48
In a steady state, background stratospheric sulfate aerosols (SSA) is produced by the photo49
oxidation of S species (COS photolysis ~43%, convective uplift of SO2 containing polluted air50
masses into the stratosphere ~26%, SO2 from aviation ~1% and remaining 30% is direct SO451
transport to the stratosphere from the tropical tropopause) to H2SO4, followed by52
nucleation/condensation, downward and polewards transport under the influence of global53
planetary waves and has a residence time ~ 3-4 yr(14, 15). There is an ongoing debate on the54
relative contribution of S from various sources (anthropogenic and natural) towards persistence55
increase in SSA (16-19) over the last two decades. Current understanding of the contribution of56
different sources to the SSA layer is restricted by the lack of quantitative distinction between57
different sources of SO4 aerosols to the stratosphere and details of in-situ chemical reaction58
mechanisms of S- species.59
The isotopic signature (33
S,34
 36
S) of various sources are well constrained, therefore a high60
resolution record of sulfate aerosol retrieved from the South Pole snow pit can help to understand61
the sources and photochemical transformations. A week correlation of 33
S and 36
S with 34
S is62
observed (Fig. S3) which is interpreted to reflect change in the sources of S over time (1983-63
2001) and it may reflect shift in wavelength of the light available for photochemical processing64
of S-carrying species in the stratosphere (SO2, OCS, H2SO4). Laboratory data have shown that65
photochemically produced sulfate and elemental sulfur (So
) leads to positive or negative66
correlation depending on the wavelength used for photolysis of SO2. 34
and 36
S of SSA falls67
in the range noted for biomass burning and fossil fuel combustion suggesting transport of68
anomalous sulfate to the stratosphere. Considering S-load in the atmosphere, tropospheric69

10. 4
sulfate = 105 Tg, stratospheric sulfate aerosol = 0.013Tg (20). Ratio of troposphere/stratospheric70
sulfate =1.24 x10-4
g. Assuming initial SSA with 36
S=0 and tropospheric aerosol with 36
Savg =71
−1.2‰ (21). A simple mass balance suggests 0.01% transport of tropospheric sulfate can change72
36
S of SSA by −0.9‰.73
74
Supplement Section3:75
K, a classic tracer of biomass burning(22) is released as KCl and K2SO4 from plant tissues at76
higher temperatures (800 o
C -2000o
C) and its emission spectra has been documented during wild77
fire season of 1998-99 (23, 24). ENSO driven meteorological conditions such has high78
temperatures, severe droughts and low precipitation might have triggered extensive wild fire and79
biomass burning from 1998-99 (25-28). Satellite based estimates of the fire activities and80
biochemical modeling (29) indicated excessive carbon emission to the atmosphere (2.1  0.881
petagram) due to the release of CH4, CO2 and CO from wild fires in Southeast Asia (60%),82
Central and South America (30%) and boreal regions of Eurasia and North America (10%) (28,83
30, 31). A significant correlation between CO mixing ratio in southern hemisphere (10-30o
S)84
and southern oscillation index was observed with highest concentration of CO (almost double ~85
200ppb) at 250 hPa due to severe fires in both hemispheres after strongest ENSO event of the86
decade in 1997-1998 (25, 30). Long term record (1940 to1998) of North American fires also87
revealed fire-ENSO teleconnections (correlation between time series of atmospheric pressure,88
temperature and rainfall) with 15 out of 17 biggest fires were reported after moderate to strong89
ENSO events (32-34).90
The increase in K, NO3, Cl and SO4 concentrations in the snow samples deposited after the91
biomass burning events in 1998-99 and 1984-85 (Fig. 1a, and S1) lend some additional support92

11. 5
to the transport of BB signal to the South Pole, however, NO3 and Cl concentration may also93
increase due to photochemical transformation of other anthropogenic compounds in the94
stratosphere (e.g. CFC and NOx), we consider K as a more reliable tracer of BB.95
96
Fig. S1. Higher resolution concentration profile of Na+
, K+
, NO3
-
, Cl-
from 1982-2001 of snow samples extracted97
from the 1x1 m snow pit at the South Pole.98
99
100
101

12. 6
102
Fig. S2. Sulfur four isotope plot of the sulfate aerosols (1983-2001) extracted from the 1x1 m snow pit at the South103
Pole and comparison with the previous volcanic sulfate aerosol acquired from the Dome C and the South Pole ice104
cores (7) and tropospheric aerosol collected at La Jolla CA, USA(21).105

13. 7
106
Fig. S3. Sulfur four isotope plot of the sulfate aerosols (1983-2001) extracted from the 1x1 m snow pit at the South107
Pole and comparison with the previous tropospheric sulfate aerosol. Open symbol = 33
S, closed symbol =36
S. La108
Jolla, CA (blue circles) Shenandoah National Park, VA (red circles), Bakersfield, CA (blue triangles), fossil fuel and109
biomass burning signatures (magenta triangles).110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132

14. 8
Table.S1. S-isotope composition and concomitant S-isotope anomalies of sulfate aerosol retrieved from a 1x1 m133
snow pit at the South Pole. Depth profile indicate initial and final ice layer of the composite samples used.134
Sample Depth
(cm)
Year 33
S
(‰
34
S
(‰
36
S
(‰
33
S
(‰
36
S
(‰
70-86 2000.6 -2000.3 6.74 13.31 25.24 -0.084 -0.334
87-94 2000.2 -1999.9 7.42 14.51 27.51 -0.024 -0.338
103-108 1999.6-1999.3 8.41 13.67 25.95 1.397 -0.324
108-113 1999.3-1999.0 8.92 14.29 26.70 1.582 -0.784
113-117 1998.3-1998.9 6.99 13.39 27.79 0.118 2.056
122-133 1998.6-1998.4 6.47 12.78 24.08 -0.086 -0.479
140-148 1997.5-1997.6 7.99 15.86 30.43 -0.147 -0.093
154-162 1997.2-1996.9 8.23 16.51 30.70 -0.239 -1.073
168-176 1996.4-1996.2 6.32 12.57 23.13 -0.129 -1.027
177-188 1995.9-1995.7 6.10 12.0 22.72 -0.059 -0.322
194-200 1995.6-1995.0 6.03 12.57 22.34 -0.429 -1.822
250-256 1993.5-1992.9 3.47 7.01 13.34 -0.129 -0.093
268-274 1992.8-1992.6 1.61 3.55 6.60 -0.212 -0.183
274-279 1992.5-1992.3 3.97 7.71 14.25 0.005 -0.536
279-284 1992.2 -1991.9 2.08 2.23 2.82 0.929 -1.451
289-295 1991.6-1991.3 2.38 2.86 3.65 0.905 -1.826
295-305 1991.2-1990.9 11.32 20.01 36.76 1.069 -1.781
305-316 1990.8-1990.4 8.06 14.77 27.55 0.477 -0.862
316-320 1990.3-1990.1 7.63 14.75 28.15 0.067 -0.215
320-328 1990.1-1989.9 6.44 12.61 23.90 -0.034 -0.332
328-338 1989.8-1990.6 7.09 13.68 25.32 0.067 -0.973
338-347 1989.5-1989.2 7.05 13.89 26.25 -0.073 -0.448
347-356 1989.1-1988.6 5.99 11.83 21.87 -0.081 -0.844
356-365 1988.6-1988.4 6.36 12.83 22.76 -0.227 -1.902
365-378 1988.3-1988.0 6.58 12.97 23.81 -0.078 -1.120
386-397 1987.7-1987.3 5.36 10.50 19.34 -0.028 -0.813
402-412 1987.0-1986.4 6.57 12.96 24.63 -0.088 -0.285
420-428 1985.9-1985.3 7.58 14.82 28.04 -0.026 -0.472
428-441 1985.3-1984.9 6.85 13.28 25.90 0.037 0.379
441-445 1984.8-1984.2 5.68 11.09 21.59 -0.021 0.286
456-465 1984.2-1983 5.42 10.90 20.28 -0.171 -0.640
Sea salt contribution is ~ 7-9% and no correction is applied to S-isotope values.135
136
137
138
139
140
141
142
143
144
145
146
147

15. 9
Table.S2. S-isotope composition and concomitant S-isotope anomalies of SF6 and Ag2S converted to SF6148
using method described earlier (4).149
Date of Analysis SF6
(µmole)
33
S
(‰
34
S
(‰
36
S
(‰
33
S
(‰
36
S
(‰
09/19/2011 1 -2.01 -3.89 -7.42 0.002 -0.038
10/12/2011 1 -2.01 -3.92 -7.39 0.008 0.082
10/18/2011 1 -1.97 -3.809 -7.40 -0.01 -0.14
01/05/2012 1 -2.06 -3.79 -7.35 -0.11 -0.17
06/11/2012 1 -1.99 -3.95 -7.43 0.03 0.10
07/16/2012 1 -1.86 -3.66 -6.59 0.02 0.39
Avg 1.98 -3.83 -7.26 -0.01 0.03
SD 0.06 0.106 0.33 0.05 0.20
04/22/2011 0.26 -2.14 -3.86 -6.74 -0.14 0.62
04/22/2011 0.50 -2.37 -3.24 -7.69 -0.08 0.79
04/22/2011 0.31 -1.93 -3.74 -6.55 -0.01 0.59
Avg -2.14 -3.61 -6.99 -0.07 0.66
SD 0.22 0.32 0.61 0.06 0.10
*T3z-Z758 2.85 2.63 5.13 9.54 -0.003 -0.28
*T3z-Z763 2.49 2.67 4.97 9.04 0.11 -0.48
*T4z-Z779 3.63 2.52 4.94 8.90 -0.018 -0.56
*T4z-Z793 1.04 2.27 4.44 8.30 -0.008 -0.200
*T4z-Z974 1.32 2.39 4.65 9.18 0.001 0.268
*T4z-Z749 2.18 2.10 4.14 7.60 -0.02 -0.321
Avg 2.43 4.71 8.76 0.01 -0.26
SD 0.21 0.37 0.69 0.04 0.29
Here * represent Ag2S converted to SF6 and analyzed for S- quadruple isotopes on Mat 252 isotope ratio mass150
spectrometer.151
152
153
Table.S3. S-isotope composition and concomitant S-isotope anomalies of primary sulfate produced during burning154
process (35).155
Materials Sulfate
(µmole)
33
S
(‰
34
S
(‰
36
S
(‰
33
S
(‰
36
S
(‰
Mix savanna grass flaming 4.9 5.9 11.84 22.2 -0.18 -0.536
savanna grass-flaming 6.3 7.6 14.75 27.6 0.031 -0.761
savanna grass-smoldering 12.5 8.07 15.99 28.7 -0.133 -2.063
Lamto grass flaming 6.5 7.99 15.70 30.1 -0.065 -0.101
Rice straw flaming 10.7 4.68 9.55 17.0 -0.227 -1.32
Hay france flaming 23.1 7.86 15.48 28.5 -0.083 -1.275
Diesel fuel-Idle 4.6 6.83 13.28 24.7 -0.013 -0.818
Diesel fuel-accelaration 16.5 8.42 16.42 29.9 -0.003 -1.696
156
157
158
159

16. 10
Table.S4. S-isotope composition and concomitant S-isotope anomalies of aerosol sulfate collected at Bakersfield and160
Shenondoah National Park, VA.161
162
Sample ID Sampling date 33
S
(‰
34
S
(‰
36
S
(‰
33
S
(‰
36
S
(‰
SS-1 2nd 30 Dec 1998-4Jan 1999 4.15 7.94 14.9 0.07 -0.3
SS-1 3rd 30 Dec 1998-4Jan 1999 3.89 7.35 13.8 0.11 -0.3
SS-1 4th 30 Dec 1998-4Jan 1999 2.32 4.14 7.7 0.19 -0.3
SS-1 BU 30 Dec 1998-4Jan 1999 2.68 4.63 8.6 0.3 -0.3
SS-3 1st 9-12Jan 1999 5.04 9.76 18.5 0.02 -0.2
SS-3 2nd 9-12Jan 1999 3.23 6.16 11.6 0.06 -0.2
SS-3 3rd 9-12Jan 1999 1.71 3.21 6 0.06 -0.1
SS-3 4th 9-12Jan 1999 1.65 3.05 5.6 0.08 -0.2
SS-3 BU 9-12Jan 1999 2.26 4.17 7.5 0.11 -0.4
SS-5 2nd 13-16Jan 1999 2.18 3.95 7.3 0.14 -0.3
SS-5 3rd 13-16Jan 1999 2.05 3.42 6.1 0.29 -0.5
SS-5 4th 13-16Jan 1999 2.25 3.57 6.2 0.41 -0.6
SS-5 BU 13-16Jan 1999 2.57 4.09 7.2 0.46 -0.6
SS-7 2nd 16-20 Jan 1999 2.84 5.22 9.9 0.15 -0.1
SS-7 3rd 16-20 Jan 1999 2.91 4.88 8.9 0.4 -0.4
SS-7 4th 16-20 Jan 1999 3.48 6.33 11.4 0.23 -0.7
SS-7 BU 16-20 Jan 1999 5.67 10.79 20.1 0.12 -0.6
SS-9-BU 21-27Jan 1999 2.4 3.75 6.6 0.47 -0.5
SS-9 2nd-4th 21-27Jan 1999 2.96 4.98 8.7 0.39 -0.8
Shenandoah National Park, VA, USA
SNP01-BU 13-20Aug2001 2.85 4.68 7.4 0.44 -1.5
SNP01-6th 13-20Aug2001 2.73 4.57 7.6 0.38 -1.2
SNP01-5th 13-20Aug2001 2.53 4.2 7.1 0.36 -0.9
SNP01-4th 13-20Aug2001 2.87 4.91 8.5 0.34 -0.9
SNP01-3rd 13-20Aug2001 3.54 6.37 11.3 0.26 -0.9
SNP01-1st 13-20Aug2001 4.32 8.11 14.5 0.15 -1
SNP01-2nd 13-20Aug2001 4.28 8.11 14.9 0.11 -0.7
SNP02-BU 20-27 Aug2001 2.6 4.22 7.9 0.43 -0.1
SNP02-6th 20-27 Aug2001 2.34 3.8 6.2 0.38 -1.1
SNP02-5th 20-27 Aug2001 2.55 4.27 7.4 0.35 -0.8
SNP02-4th 20-27 Aug2001 2.62 4.48 7.6 0.31 -1
SNP02-3rd 20-27 Aug2001 3.44 6.24 10.9 0.23 -1
SNP02-2nd 20-27 Aug2001 3.8 7.03 12.3 0.18 -1.2
SNP03-BU 27Aug-03 sep 2001 2.6 4.12 6 0.48 -1.9
SNP03-6th 27Aug-03 sep 2001 2.34 3.74 5.4 0.42 -1.7
SNP04-BU 03-10 sep 2001 2.75 4.31 6.8 0.53 -1.5
SNP04-6th 03-10 sep 2001 2.75 4.45 7.4 0.45 -1.1
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