Sulfur Isotope Ratios and Constraints on
Climate
Alexander A. Pavlov
LPL, University of Arizona, Tucson
Main Message
• Anomalous fractionation (MIF) in sulfur
isotopes can be used as a constraint on the
total amount of atmospheric sulfur and on
the overall atmospheric oxidation state.
Sulfur Isotopes
1) Sulfur has 4 stable isotopes: 32S (95.02%),
33S(0.75%), 34S (4.21%), and 36S (0.02%)
2) δ = ( Rsample/Rstandard – 1) x 1000
R = 34S/32S for δ34S
3) Typical terrestrial values for δ34S are within ±
50 per mil
MIF in Sulfur
• Standard mass fractionation line (MFL):
δ33S ≅ 0.515 x δ34S; δ36S ≅ 1.91 x δ34S
• In Archean sediments and Martian meteorites, the
isotopes fall off this line (Farquhar et al., 2000)
• Δ33S = δ33S - 0.515 x δ34S
Δ33S = δ33S – 1000 x [(1 + δ34S/1000)0.515 – 1 ]
Sulfur MIF is measured Δ33S – deviation from
MFL.
Δ33S
Farquhar et al., JGR 2001
Analogy with Oxygen MIF
• Sulfur is not the only element which show
MIF (anomalous fractionation)
• 16O, 17O, 18O
• Δ17O = δ17O - 0.52 x δ18O
• Used to constrain stratosphere/troposphere
exchange
• Oxygen MIF is produced in the stratosphere
and oxygen bearing species which do not go
through the stratosphere would not acquire
MIF
The only(!) experiment so far….
Farquhar’s experiment
Sulfate
Δ33S < 0
UV
SO2 mixture
SO2 remaining
Δ33S < 0
Δ33S = 0
Δ33S > 0
Elemental S
Farquhar et al., JGR 2001
Why is it exciting for the planetary atmospheres?
Mass-dependent fractionation after deposition (biology, aqueous
alteration etc.) can change (δ33S, δ34S) but it cannot introduce
new produce Δ33S . The only way to lose Δ33S is to mix the
atmospheric sulfur with unfractionated sulfur after deposition.
Δ33S through time. Note that Δ33S essentially disappears when oxygen
became abundant in the Earth’s atmosphere.
Compiled from Farquhar et al. (2000), (2002), Mozjsis et al. (2003), Ono et al. (2003), Hu et al. (2003), Bekker et al. (2004), Johnston et al.
(2005a), Whitehouse et al. (2005), Papineau et al. (2005). All unpublished data was collected at UMd.
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Summary of the sulfur MIF data
• So far sulfur MIF signature was observed
in:
a) Martian meteorites (small but non-zero Δ
33S )
b) Archean rocks (large Δ33S )
c) Antarctic ice cores during deposition of the
volcanic sulfates from the large volcanic
eruptions (Δ33S depends on the magnitude
of eruption)
Physical mechanism for the
sulfur MIF
• We know that MIF should be produced in
the gas phase by some photochemical
reaction. But which one?
• Currently three reactions were suggested as
the primary sulfur MIF source:
A)SO2 + hν  SO + O (Farquhar et al.)
B)SO2* + SO2  SO + SO3 (Savarino et al.)
C)SO3 + hν  SO2 + O (Pavlov et al.)
How does MIF signature place a constraint on
the atmospheric oxidation state?
Input
Oxidizing Atmosphere
OH, O2, O
33
SO
+
hν
2
SO
:
Δ
S
<
0
Volcanic SO2 flux
2
Δ33S = 0
SO: Δ33S > 0
A lot!
Input
Reducing Atmosphere
Volcanic SO2 flux SO2 + hν SO2: Δ33S < 0 OH, O2, O
Δ33S = 0
SO: Δ33S > 0
Not a lot!
Output
H2SO4
Δ33S = 0
Output
H2SO4, SO2
Δ33S < 0
H2S, HS, S8
Δ33S > 0
Low-oxygen Sulfur Cycle
S8 and Sulfate fluxes respond dramatically
to the increase in oxygen/oxidants.
2
/sec)
1010
109
S8
SO2
108
H2SO4
107
Sulfur Deposition Fluxe
10-10
10-9
10-8
10-7
Oxygen abundance f(O2)
10-6
• What about sulfur MIF in the present
Earth’s atmosphere?
• How does sulfur MIF signal place a
constraint on the total abundance of the
atmospheric sulfur?
Δ33S > 0
hν
SO2
Δ33S < 0
SO
“Initial” fractionation during SO2 photolysis.
Fast conversion of SO back to SO2 cancels
MIF in the aerosol
Slow
Fast
Aerosol
Δ33S = 0
Δ33S > 0
SO3
hν
SO2
Δ33S < 0
Fast
Slow
Aerosol
Δ33S ≠ 0
“Initial” fractionation during SO3 photolysis.
Slow conversion of SO2 to SO3 allows MIF
to be incorporated into the aerosol
Pinatubo eruption
1) 30 Mt of SO2 into the stratosphere between 10°S and
10°N, at 16-30 km altitude in June.
2) MIF happens during SO3 photolysis only
SO3 (Δ-) + hν → SO2 (Δ+) + O
3) Volcanic sulfur have Δ33S = 0
Main Message
1) In the weakly reducing atmospheres Δ33S is
always present and depends on the total
sulfur input into the atmosphere
2) In the oxidizing atmospheres MIF is much
smaller but can be detectable if there is
significant amount of sulfate aerosols at the
high altitudes.
Implications for Venus
• There are several reservoirs of oxidized and
reduced sulfur on Venus. Therefore, it is
hard to avoid sulfur MIF in Venus aerosols.
• The magnitude of sulfur MIF would place a
constraint on the mixing between lower and
upper atmosphere of Venus and the sulfur
cycle in general.