Sulfur isotopes
11/14/12
Authigenic marine
barite (BaSO4)
separated from
deep-sea cores
SEM Photo: Adina
Paytan
Hydrothermal barite
separated from
black smokers
SEM Photo: Kim Cobb
Lecture outline:
1) sulfur cycle
2)
biological
fractionation
3)
S isotopes
in the geologic
record
4)
mass-independent
S isotope fractionation
The sulfur cycle
SO
2
From Don Wuebbles, Univ. Illinois UC, http://www.atmos.illinois.edu/courses/atms449-sp05/
Introduction to
sulfur isotopes
Sulfur stable isotopes:
32S: 95.02%
33S: 0.75%
34S: 4.21%
36S: 0.02%
Sulfur isotope standard:
Canyon Diablo Triolite
32S=0.9503957
33S=0.0074865
34S=0.0419719
36S=0.0001459
Five oxidation states:
+6: e.g. BaSO4
+4: SO2
0: S (s)
-1: FeS2
-2: e.g. H2S
Rt marine sulfur = 20Ma
Equilibrium fractionations relative to H2S
Biologically-mediated SO4 reduction
S6+
-naturally-occurring sulfides commonly
depleted by 45 to 70‰!
-bacterial sulfate reduction
takes place in anoxic environments,
where SO4 is reduced in place of O2
Thermochemical sulfate reduction
- occurs at temps >100ºC
-usually goes to near-completion
-little fractionation
1000lnaH2S
NOTE: the bacterial reduction
of sulfate occurs via kinetic
fractionation  larger a
S4+
S-1
Raleigh fractionation during sulfate reduction
d34S of sulfate becomes heavier
as light sulfide forms
d34S of sulfide becomes heavier
as sulfate source becomes heavier
a HSOS  1.025
4
2
but a varies widely, depends
on environmental conditions
Use equations from Raleigh d18O lecture
to calculate d34S of sulfate, sulfide
as a function of fraction remaining.
What would be the d34S of the total
S at the end of the distillation?
SO42-
H2S(g)
Equilibrium fractionations
Bacterial Sulfate Reduction  -15 to -70‰ depletion
Thermochemical Sulfate Reduction  -20‰ (at 100ºC)
-15‰ (at 150ºC)
-10‰ (at 200ºC)
But you must know the starting d34S of the sulfate…
AND… we can use mineral pairs to establish T of mineral formation
ex: pyrite and chalcopyrite coprecipitated from same fluid
but you must know the starting d34S of the sulfide….
BUT… the d34S of sulfide and sulfate in a solution depends on the relative proportions
of H2S, HS-, and S2-, which depends on pH, O2 fugacity, total [S]
SO… understanding present-day sulfur isotope variability in a given system
is complicated ….
Phanerozoic d34S evolution
Cenozoic d34S evolution
d34S and d13C not anti-correlated,
as observed for last 1 billion years
Paytan et al., 1998
Why anti-correlated over last 1Ga?
increase burial C(org),
= higher d13C
=higher atmos. O2
=oxidize sulfides (low d34S) to SO4
=lower oceanic d34S
atmospheric O2 did not change
very much during the last 100Ma,
so reduced S and C are not the only
controls on atmospheric O2
Main factors that influence
evolution of Cenozoic d34S:
1. deposition/burial of pyrite
2. deposition/burial of sulfates
3. intensity of hydrothermal
activity and volcanism
What happened at 55Ma?
Why might this affect
marine d34S?
What does it mean that variations
occur on timescales shorter than
20Ma (Rt of oceanic sulfur)?
measured
d34S of
marine
barite (BaSO4)
Archean Sulfur isotopes and the hunt for early life
Idea:
If sulfur-reducing bacteria were around billions of years ago on Earth or Mars,
shouldn’t large d34S excursions in sediments be measureable?
Fact:
Early work on Martian meteorites and Archean sediments revealed significant
d34S excursions
Mass-independent sulfur isotope fractionation
Laboratory SO2 photolysis
from Farquhar and Wing, 2003
Three-Isotope Plot
MIF
For mass-dependent
fractionation (MDF):
δ33S = 0.515×δ34S
δ36S = 1.90×δ34S
A new notation for deviation from the MDF line:
33S = δ33S− 0.515×δ34S
36S = δ36S− 1.90×δ34S
MDF
33S
Evolution of the atmosphere:
multiple isotopes and MIFs
Ono, 2008
keep in mind uncertainties…
Johnston, 2011
Archean mass-independent sulfur isotope fractionation
33S = departure from mass
fractionation line (MFL)
= 0 present-day
but highly variable in Archean
sediments
Today atmospheric mass-independent
rxns occur, but isotopes are re-mixed
in surface and biological redox
chemistry, so 33S = 0 in all sediments
Models suggest that atmospheric O2
had to be less than 10-5 Pa at 3Ga
<1% of present-day
Farquhar & Thiemens, 2000,2001
Archean mass-independent sulfur isotope fractionation
the “Great Oxygenation Event (GOE)”
from Lyons & Reinhard, 2011
Early Earth sulfur cycle: uncertainties abound!
from Farquhar and Wing, 2003
Snowball Earth and the Sulfur Cycle
planet cools considerably,
incipient glaciation,
ice grows near 30
runaway ice albedo
makes snowball
rising CO2 increases
temp., melts ice,
reverse ice albedo
feedback
temporary hothouse
Earth after snowball
Cap carbonate overlying diamictite; photo by Francis MacDonald
translates into progressive enrichment of oceans by
continued burial of pyrite in ocean
from Hurtgen et al., 2002
anomaly upon deglaciation
should be recorded in cap
carbonates
from Hurtgen et al., 2002
cap carbonates
from Hurtgen et al., 2002