TECHNICAL COMMENTS
Questions Regarding Precambrian
Sulfur Isotope Fractionation
⌬33S ⫽ ␦33Smeas ⫺ ␦33 Sexp
⫽ ␦33Smeas ⫺ 0.518 ␦34Smeas
(1)
⌬36S ⫽ ␦36Smeas ⫺ 1.90 ␦34Smeas
(2)
and
when ⌬33S or ⌬36S ⱖ 0.1‰ mass-independent fractionation is indicated.
Farquhar et al. (1) determined the values
of ␦33Smeas, ␦34Smeas, and ␦36Smeas on SF6
gas generated from a sample. When the SF6 is
not completely purified by a gas chromatographic column, the presence of impurity
gases (C-F-S-O-H compounds) may cause
the ␦33Smeas and especially the ␦34Smeas and
␦36Smeas values to differ considerably from
the true values (2), and thereby to create an
apparent mass-independent fractionation.
Four observations suggest that such problems
exist in the data reported by Farquhar et al.
(1, 3).
1) Most of the ␦34S values determined by
Farquhar et al. (1) differed significantly from
those determined on the same samples by
other researchers using the conventional SO2
method, a more reliable approach for determining ␦34S values but not for determining
␦33S and ␦36S (2, 4). The difference between
the ␦34Smeas values obtained by Farquhar et
al. (1) and those obtained by the other investigators was typically between 1 and 10 per
mil (‰), rather than the acceptable difference
of less than ⫾ 0.5‰. As equations 1 and 2
suggest, if the ␦34Smeas value differs from the
true value by ⫹3‰ but the ␦33Smeas and
␦36Smeas values are accurate, the apparent
⌬33S and ⌬36S values will become –1.5‰
and –5.7‰, respectively—and will therefore
pass the threshold of significant mass-independent fractionation— even if there is no
natural mass-independent fractionation in the
original rock sample. If the ␦33Smeas or
␦36Smeas is also inaccurate, as may have been
the case in the Farquhar et al. study (as
discussed below), the apparent ⌬33S and
⌬36S values can differ greatly from the values above.
2) On some samples, Farquhar et al. (1)
generated two to three sets of SF6 gas through
successive acid treatments. Essentially, all of
the sulfur in these rocks (except for one
sample) was originally in the form of disseminated pyrite, but some may have been converted to sulfate by recent oxidation (5). The
SF6 gases from such a rock sample may have
different sets of ␦33S, ␦34S, and ␦36S values
because of kinetic isotope effects during the
recent oxidation or the acid treatments. They
should, however, have identical sets of ⌬33S
and ⌬36S values to within ⫾0.05‰, if there
were no analytical problem. This was not the
case among the samples analyzed by Farquhar et al. (Fig. 1).
3) Some natural processes produce massindependent fractionation of 33S with or
without mass-independent fractionation of
36
S, but no natural process is known to produce mass-independent fractionation of only
36
S (1). The samples dating from less than
2.0 Ga, however, showed mass-independent
fractionation of only 36S (Fig. 1), an indicaFig. 1. Observed mass-independent fractionation of
33
S and 36S on carbon-rich
samples, adapted from (1).
Tielines connect the ⌬33S
and ⌬36S values of different
SF6 gases from the same
rock samples.
www.sciencemag.org SCIENCE VOL 292 15 JUNE 2001
1959a
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The discovery by Farquhar et al. (1) of
mass-independent isotope fractionation of
33
S and 36S in rocks formed more than ⬃2
billion years ago (Ga), but not in younger
rocks, has boosted the theory postulating a
dramatic change from an anoxic to an oxygen-rich atmosphere about 2 Ga. That is because the only known natural process that
may cause mass-independent fractionation of
both 33S and 36S involves atmospheric photochemical reactions by ultraviolet light in
the absence of an ozone shield (1). Here, we
suggest the strong possibility that the fractionation observed by Farquhar et al. (1) may
have been created by analytical rather than
natural processes.
The magnitude of mass-independent fractionation of 33S or 36S, expressed as ⌬33S or
⌬36S, is defined as the deviation of a measured ␦33S or ␦36S value from the ␦33S or
␦36S value expected from the mass-dependent fractionation relationships. That is,
tion that serious analytical problems exist for
⌬36S, not only for these younger samples but
also for older samples. True mass-independent fractionation of 36S thus may not have
existed in any of the rock samples.
4) Mass-independent fractionation of 33S
or 36S is commonly found in carbon-rich
rocks, such as shales bearing disseminated
pyrite. Our own experience suggests that
such rocks are more likely to produce impurity gases (C-F-S-O-H compounds) during
the analytical process. None of the samples
younger than 500 Ma analyzed by Farquhar
et al. showed mass-independent fractionation
of either 33S or 36S (5); these samples were
all carbon-poor, simple mineral separates (5)
that were less likely to produce impurity
gases. Therefore, the conclusion by Farquhar
et al. (1) that the mass-independent fractionation was found only in rocks dating from
more than 2.0 Ga is analogous to comparing
apples (carbon-rich rocks) and oranges (carbon-poor rocks).
Farquhar et. al. (1) are to be commended for recognizing the possible presence of
mass-independent fractionation of sulfur
isotopes in geological samples. The observations reported above, however, lead us to
seriously question the validity of the major
conclusion of that study that photochemical
reactions were responsible for mass-independent fractionation in rocks dating from
earlier than 2.0 Ga. To establish the true
changes in mass-independent fractionation
of sulfur isotopes over geologic time, more
systematic investigations must be carried
out on a variety of samples of all ages—
especially the disseminated-pyrite– bearing
TECHNICAL COMMENTS
shales from ⬍2.0 Ga, which have not yet
been investigated.
Hiroshi Ohmoto
Kosei E. Yamaguchi
Shuhei Ono
Astrobiology Research Center
The Pennsylvania State University
University Park, PA 16802
USA
References and Notes
1959a
Table 1. Sulfur isotope analyses.
Sample
24 January 2001; accepted 13 April 2001
Response: Ohmoto et al. contend that the
mass-independent fractionations we reported
are an artifact of our analytical technique.
Here, we report repeat analyses using our
technique and reanalyses using an independent technique (laser fluorination) that attest
to the robustness and accuracy of isotopic
data previously reported in (1). These analyses, and further independent measurement of
the effect by secondary ion mass spectrometry (2), confirm our original measurements
and support our conclusions.
Ohmoto et al. argue that C-F-O-H-S contaminants derived from organic material generate mass interferences that jeopardize our
analyses. Rumble et al. (3) have demonstrated, however, that a single pass through a gas
chromatograph is adequate to purify SF6 for
⌬33S analysis. We have used gas chromatography to purify SF6 before mass-spectrometry
for more than 10 years (1, 4–11) and have
repurifed samples that we have previously
analyzed as secondary checks for ␦36S. Although we have not found it necessary to
repurify our samples for ␦33S, we did so for
the very first mass-independent sample we
analyzed, pprg 199 (1). We found our analyses reproduced within 0.04‰ for both ␦34S
and ⌬33S. We also note that the mass interference invoked by Ohmoto and colleagues
produces an effect that is in the wrong direction to explain the ⌬33S that we found in
carbon-rich shales.
Although Ohmoto et al. hold that the SF6
technique is less reliable than the SO2 technique, the SF6 technique is actually well established (3, 4, 6, 7, 9–15); indeed, it has been
validated by the decision of the International
Atomic Energy Agency (IAEA) to define the
isotopic composition for the international
the authors of this response (Hu and Rumble)
fall within the error of our previous ␦34S
measurements (Fig. 1A).
Ohmoto et al. maintain that our different
acid extractions “should . . . have identical
sets of ⌬33S and ⌬36S values to within ⫾
0.05‰.” Our reanalysis of sulfur extracted
from sample pprg 2777 (Table 1A and Fig. 1)
reproduce the mass-independent measurement well within the stated ⫾ 0.3‰ analytical uncertainties. Likewise, our previous
measurements of barite (Table 1B) illustrate
the reproducibility of our technique for determination of ⌬33S for sulfur from samples that
are not carbon-rich shales. Independent analyses of our Ag2S by laser fluorination fall
within error of our previous ⌬33S measurements (Fig. 1B).
According to Ohmoto et al., our Arche-
Fig. 1. (A) Plot of ␦34S data remeasured in 2001
against ␦34S data reported in (1). (B) Plot of
⌬33S data remeasured in 2001 against ⌬33S
data reported in (1). Data were determined
from the same Ag2S samples and illustrate that
the discrepancy between previously reported
␦34S values (19) and those reported in (1) is not
due to mass interference associated with the
SF6 technique but instead likely traces to sample heterogeneity, such as that reported in (20).
Data also illustrate that the reproducibility of
⌬33S is comparable to the uncertainties reported in (1). UCSD, repeat analyses undertaken at
The University of California, San Diego, using
techniques of (1); Geophysical Lab, reanalyses
of samples from (1) undertaken at Geophysical
Laboratory, Carnegie Institution; Strauss and
Moore, values reported in (19).
␦34S
⌬33S
⌬36S
(A) Carbon-rich shale PPRG–2777
pprg 2777*
– 0.41
1.18 –1.5
pprg 2777*
– 0.39
1.21 –1.5
pprg 2777*
– 0.03
1.21 –1.7
Average
– 0.28
1.20 –1.5
Standard deviation
0.21
0.02
0.1
pprg 2777†
–1.35
2.04 –2.2
pprg 2777†
–1.10
2.06 –2.6
pprg 2777†
–1.40
2.10 –2.0
Average
–1.28
2.07 –2.3
Standard deviation
0.17
0.03
0.3
pprg 480†
12.81
0.12 – 0.5
pprg 480†
12.94
0.12 –1.5
pprg 480†
12.75
0.12 –1.7
Average
12.83
0.12 –1.2
Standard deviation
0.10
0.01
0.6
(B) Hydrothermal barite samples
GSWA 169711 A
4.72 – 0.95
0.8
GSWA 169711 B
5.45 – 0.91
1.0
GSWA 169711 C
5.26 –1.04
1.3
GSWA 169711 D
5.39 – 0.95
0.6
Average
5.21 – 0.96
0.9
Standard deviation
0.33 – 0.06
0.3
GSWA 169712 E
5.04 –1.02
0.8
GSWA 169712 F
4.31 – 0.91
1.3
GSWA 169712 G
4.31 –1.00
1.2
GSWA 169712 H
4.94 – 0.96
0.8
GSWA 169712 I
4.71 –1.02
1.0
Average
4.67 – 0.98
1
Standard deviation
0.34
0.05
0.2
(C) Samples other than C-rich shales with MIF
pprg 011*
4.15 – 0.48
0.2
pprg 011†
1.07 – 0.43
0.3
pprg 010 barite
5.16 –1.18
1.1
pprg 1443 barite
3.45 –1.29
3.1
SAF 9-6 barite
4.33 – 0.43
0.1
SAF 16-22 barite
3.62 – 0.56
0.3
SAF 16-23 barite
3.61 – 0.57
0.5
SAF 206-9 barite
4.23 – 0.53 – 0.4
Hoering barite‡
4.26 – 0.47
2.1
Hoering pyrite‡
–5.55 – 0.32
0.3
pprg 2429 pyrite
– 8.97
0.27
0.0
pprg 1419 pyrite
2.98
0.70 – 0.6
an 29284 gneiss pyrite
0.83 – 0.67
0.9
*Fraction of sulfur extracted with reduction solution.
†Fraction of sulfur extracted with reduction solution after oxidation of sample by fuming nitric acid
(135 °C).
‡Analysis from (21).
15 JUNE 2001 VOL 292 SCIENCE www.sciencemag.org
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1. J. Farquhar, H. Bao, M. Thiemens, Science 289, 756
(2000).
2. D. Rumble, T. C. Hoering, J. M. Palin, Geochim. Cosmochim. Acta 57, 4499 (1993).
3. www.sciencemag.org/feature/data/1052160.shl
4. J. R. Hulston, H. G. Thode, J. Geophys. Res. 70, 3475
(1965).
5. J. Farquhar, personal communication.
6. We thank J. Farquhar for his openness and willingness
to discuss with us the details of his study, and P.
Deines, L. Kump, and Y. Watanabe for valuable comments on an early draft of this manuscript. Financial
support was provided by the NASA Astrobiology
Institute and the NASA Exobiology Program.
Canyon Diablo Troilite (CDT) standard as
identical to the value measured by the SF6
technique, and not the companion measurements using the SO2 technique (16, 17). The
isotopic composition for seawater sulfate is
also defined using the SF6 technique rather
than the SO2 technique (12, 13). To demonstrate the reproducibility of our fluorination
technique for the Archean samples, we refluorinated previously extracted sulfur (in the
form of pure Ag2S) from samples highlighted
by Ohmoto et al. (Table 1A and Fig. 1). The
␦34S of the samples reproduce within their
stated uncertainties of ⫾ 0.3‰ and are confirmed to differ from the previously reported
values cited by Ohmoto et al. Triplicate analysis of Ag2S from sample pprg 480, for example, yielded 12.8 ⫾ 0.1‰, which differs
from the previously reported value of 6.0‰.
Independent analyses of the Ag2S by two of
TECHNICAL COMMENTS
an ⌬ S data are problematic because
“samples dating from less than 2.0 Ga . . .
showed mass-independent fractionation of
only 36S.” In (1), we defined ⌬36S as equal
to ␦36S – 1000[(1⫹␦34S/1000)1.90–1], with
the exponent of 1.90 chosen on the basis
of statistical-thermodynamic theory (18).
When we regress all of our measurements
of present-day ␦36S against ␦34S, we obtain
an empirical exponent of 1.84. As far as we
are aware, such a value is not an allowed
theoretical mass-dependent slope, and we
are now working on understanding its origin. The source of the negative ⌬36S that
Ohmoto et al. note in samples younger than
2.0 Ga is an artifact of the calculation using
the exponent 1.90 instead of 1.84. When
36
James Farquhar*
Huiming Bao
Mark H. Thiemens
Department of Chemistry and Biochemistry
University of California at San Diego
La Jolla, CA 92093, USA
Guixing Hu
Douglas Rumble III
Geophysical Laboratory
Carnegie Institution of Washington
5251 Broad Branch Road N.W.
Washington, DC 20015, USA
*Also Department of Geology and ESSIC
University of Maryland, College Park
MD 20742, USA
References and Notes
1. J. Farquhar, H. M. Bao, M. Thiemens, Science 289, 756
(2000).
2. S. J. Mojzsis et al., Abstracts, 11th Goldschmidt Conference, Homestead, VA, 19 to 24 May 2001 (Lunar
and Planetary Inst., Houston, TX, 2001), p. 3185.
3. D. Rumble, T. C. Hoering, J. M. Palin, Geochim. Cosmochim. Acta 57, 4499 (1993).
4. A. Paytan et al., Science 282, 1459 (1998).
5. S. K. Bains-Sahota, M. H. Thiemens, J. Chem. Phys. 90,
6099 (1989).
6. G. Beaudoin et al., Geochim. Cosmochim. Acta 58,
4253 (1994).
7. J. Farquhar, T. L. Jackson, M. H. Thiemens, Geochim.
Cosmochim. Acta 64, 1819 (2000).
8. J. Farquhar et al., Nature 404, 50 (2000).
9. X. Gao, M. H. Thiemens, Geochim. Cosmochim. Acta
55, 2671 (1991).
, Geochim. Cosmochim. Acta 57, 3159 (1993).
10.
, Geochim. Cosmochim. Acta 57, 3171 (1993).
11.
12. C. E. Rees, W. J. Jenkins, J. Monster, Geochim. Cosmochim. Acta 42, 377 (1978).
13. C. E. Rees, Geochim. Cosmochim. Acta 42, 383 (1978).
14. H. Puchelt, B. R. Sabels, T. C. Hoering, Geochim.
Cosmochim. Acta 35, 625 (1971).
15. D. Rumble, T. C. Hoering, Accts. Chem. Res. 27, 237
(1994).
16. T. B. Coplen, H. R. Krouse, Nature 392, 32 (1998).
17. H. R. Krouse, T.B. Coplen, Pure Appl. Chem. 69, 293
(1997).
18. J. R. Hulston, H. G. Thode, J. Geophys. Res. 70, 3475
(1965).
19. H. Strauss, T. B. Moore, in The Proterozoic Biosphere: A
Multidisciplinary Study, J. W. Schopf, C. Klein, Eds. (Cambridge Univ. Press, New York, 1992), pp. 711–798.
20. H. Ohmoto, T. Kakegawa, D. R. Lowe, Science 262,
555 (1993).
21. T. C. Hoering, J. Geol. Soc. India 34, 461 (1989).
22. This research was supported by the NSF.
㛬㛬㛬㛬
㛬㛬㛬㛬
23 February 2001; accepted 13 April 2001
www.sciencemag.org SCIENCE VOL 292 15 JUNE 2001
1959a
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Fig. 2. Plot of ⌬36S against ⌬33S for selected
data from (1) and (21), using exponent of
1.84 instead of 1.90. Error bars are from (1)
and (21).
the data plotted by Ohmoto et al. are recalculated with an exponent of 1.84 instead of
1.90 (Fig. 2), the supposedly anomalous
younger samples plot near the origin (within experimental error), and the relationship
between ⌬33S and ⌬36S for samples older
than 2.4 Ga persists. Also, we point out that
in many cases the ␴ error bars overlap for
data from the same sample.
Ohmoto and colleagues imply that we observed anomalous ⌬33S only for sulfur from
carbon-rich shales. The barite data and selected
other data (Table 1, B and C), however, came
from pure mineral separates and are not from
analyses of organic-rich whole-rock shales. We
observed mass-independent fractionations in
carbon-poor rocks and in minerals, as well as in
carbon-rich rocks that are older than 2.0 Ga.
Over the past 10 years we have analyzed hundreds of samples for ␦34S, and we have yet to
observe mass-independent fractionations in any
rock type—carbon rich or carbon poor—
younger than 2.0 Ga. We will continue to test
this hypothesis with additional analyses, but at
present have no indication that the assertion of
Ohmoto et al. is valid.
We have considered the criticisms made
by Ohmoto and colleagues with the seriousness they deserve; however, our analytical
data and reanalysis using independent methods argue against those criticisms, and support our original interpretations. We invite
other laboratories to undertake further tests of
our hypotheses and methods and will continue to do so ourselves.
Questions Regarding Precambrian Sulfur Isotope Fractionation
Hiroshi Ohmoto, Kosei E. Yamaguchi and Shuhei Ono
Science 292 (5524), 1959.
DOI: 10.1126/science.292.5524.1959a
http://science.sciencemag.org/content/292/5524/1959
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