24. Horstad, I., Larter, S. R. & Mills, N. in The Geochemistry of Reservoirs (eds Cubitt, J. M. & England, W.
A.) 159–183 (Geological Society Special Publication No. 86, London, 1995).
Acknowledgements
We thank Saga Petroleum ASA, Norsk Hydro, PEMEX and the Mexican Petroleum
Institute for support and permission to publish.
Correspondence and requests for materials should be addressed to R. d. P.
(e-mail: rolando.di-primio@hydro.com).
letters to nature
176 NATURE | VOL 406 | 13 JULY 2000 | www.nature.com
.................................................................
Anomalous
17
O compositions
in massive sulphate deposits
on the Earth
Huiming Bao*, Mark H. Thiemens*, James Farquhar*,
Douglas A. Campbell*, Charles Chi-Woo Lee*, Klaus Heine†
& David B. Loope‡
* Department of Chemistry and Biochemistry, University of California San Diego,
Mail Code 0356, La Jolla, California 92093, USA
† Institute of Geography, University of Regensburg, 93053 Regensburg, Germany
‡ Department of Geosciences, University of Nebraska Lincoln, Lincoln,
Nebraska 68588-0340, USA
.................................. ......................... ......................... ......................... ......................... ........
The variation of
18
O that results from nearly all physical,
biological and chemical processes on the Earth is approximately
twice as large as the variation of
17
O. This so-called ‘mass-
dependent’ fractionation is well documented in terrestrial
minerals
1,2
. Evidence for ‘mass-independent’ fractionation
(D
17
O=
17
O - 0.52
18
O), where deviation from this tight rela-
tionship occurs, has so far been found only in meteoritic
material and a few terrestrial atmospheric substances
3
. In the
rock record it is thought that oxygen isotopes have followed a
mass-dependent relationship for at least the past 3.7 billion years
(ref. 4), and no exception to this has been encountered for
terrestrial solids
5
. Here, however, we report oxygen-isotope
values of two massive sulphate mineral deposits, which formed
in surface environments on the Earth but show large isotopic
anomalies (D
17
O up to 4.6‰). These massive sulphate deposits
are gypcretes from the central Namib Desert and the sulphate-
bearing Miocene volcanic ash-beds in North America. The
source of this isotope anomaly might be related to sulphur
oxidation reactions in the atmosphere and therefore enable
tracing of such oxidation. These findings also support the
possibility of a chemical origin of variable isotope anomalies
on other planets, such as Mars
6
.
We have analysed sulphates from laboratory experiments and
natural sources (Fig. 1). The
18
O and
17
O of seawater sulphate,
evaporites, sulphate from microbial sulphate reduction experi-
ments, and sulphates formed from mineral-sulphide oxidation in
air or soils, all fall on the mass-dependent terrestrial fractionation
line, given by the relationship
17
O = 0.52
18
O. The deviation from
this relationship, defined as D
17
O=
17
O - 0.52
18
O, is approxi-
mately zero ( -0.04 0.05‰, n = 36). The central Namib Desert
gypcretes and the Miocene volcanic ash-falls in the western United
States, however, possess sulphate
18
O and
17
O values that deviate
from the terrestrial fractionation line. The sulphate D
17
O of Namib
gypcretes ranges from 0.20‰ to 0.51‰ (Table 1), well outside the
experimental error of 0.05‰. Gypsum and other water-soluble
sulphate minerals from Miocene volcanic ash deposits in Nebraska
and South Dakota have strikingly large D
17
O values, up to 4.59‰,
in contrast to the zero or slightly positive D
17
O found in adjacent
soil and fluvial horizons (Table 2).
Sulphate minerals with D
17
O 0‰ are expected because
thermodynamic and kinetic (including biological) processes such
as evaporation, mineral-sulphide oxidation (by Fe
3+
or air O
2
), and
sulphate reduction generate mass-dependent compositions (Fig. 1).
Therefore, sulphate minerals with positive D
17
O values, such as
those found in the central Namib Desert gypcretes and the Miocene
volcanic ash-falls in the western United States require a different
process. The only documented terrestrial reservoirs with positive
D
17
O are from the atmosphere. Oxidants such as O
3
and H
2
O
2
are
known to have positive D
17
O values that range from 1 to more
than 25‰ (refs 7–10). Others, like OH and NO
x
, remain to be
measured.
Tropospheric O
3
and H
2
O
2
in rainwater may transfer their
positive D
17
O values to the product sulphate by in situ oxidation
of surface minerals (for example, sulphides). Our measurement of
the sulphate produced by marcasite (FeS
2
) oxidation in the air
yielded D
17
O 0‰, indicating that the in situ pathway is not the
major source of positive D
17
O. A more likely source is the wet and
dry atmospheric deposition of sulphate produced by atmospheric
oxidation of reduced gaseous sulphur compounds. The anomaly
can come from atmospheric oxidants such as O
3
,H
2
O
2
or OH
radicals as a result of aqueous or gas-phase S(IV) oxidation. A
similar explanation was invoked to interpret positive D
17
O values
(0.20‰ to 1.80‰) observed in aerosol and rainwater sulphate
11,12
.
Although the mechanisms of these atmospheric processes are still
subjects of intensive study, the connection between our D
17
O-
positive sulphate minerals and atmospheric oxidation processes is
unequivocal, as shown by the close association with a high flux of
atmospheric reduced sulphur compounds in our two reported
cases.
Gypcrete soils in the central Namib Desert occur extensively near
the coast and gradually thin off and disappear at about 50–70 km
from the coast, constituting one of the most extensive gypsum
accumulations in Africa. On the basis of
34
S, meteorological,
hydrological, and geological information, Eckardt and Spiro
13
suggest that sulphate in the Namib Desert originates mostly from
biologically produced marine sulphur (that is, not derived from sea
salt), particularly the oxidation of marine dimethyl sulphide
(DMS). This conclusion is also supported by the lack of correlation
between gypcrete accumulation and bedrock in this location and
the positive correlation between gypcrete accumulation and the
proximity to the ocean, a source of DMS. The Benguela Current,
Table 1 Isotopic compositions and occurrences of the Namib gypcretes
Sample
18
O
17
O D
17
O Depth below
surface (cm)
.............................................................................................................................................................................
GOR13-2 12.4 6.8 0.34 5
GOR13-3 8.3 4.6 0.33 16
GOR13-5 10.5 5.9 0.42 36
GOR13-6 11.6 6.4 0.35 52
GOR14-6 12.2 6.6 0.29 40
GBB16-6 11.8 6.4 0.26 45
GBB17-5 9.8 5.3 0.20 16
GBB17-6 9.5 5.2 0.26 46
GBB17-8 11.3 6.1 0.26 66
AUS6-5 9.9 5.6 0.40 10
SWA6-1 12.6 7.0 0.40 1
SWA6-2 12.0 6.6 0.31 9 (crack)
SWA6-3 11.4 6.3 0.34 5
SWA6-4 13.2 7.3 0.40 24
SWA6-6 13.0 7.1 0.37 50
SWA6GYP 11.1 6.2 0.38 52
SWA6-7 10.1 5.8 0.51 70
SWA9-10 11.3 6.1 0.23 46
.............................................................................................................................................................................
Samples from the same soil profile are grouped together. Soil profiles are listed with decreasing
distance from the ocean. Isotopic compositions are given in SMOW.
18
O = [(R
sample
/R
standard
) - 1] ×
1,000‰, where R is the number ratio
18
O/
16
O.
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