Sulfur isotopes 11/14/12
Lecture outline:1) sulfur cycle
2) biological fractionation
3) S isotopes in the geologic record
4) mass-independentS isotope fractionation
Authigenic marine barite (BaSO4)separated fromdeep-sea coresSEM Photo: Adina Paytan
Hydrothermal bariteseparated fromblack smokersSEM Photo: Kim Cobb
The sulfur cycle
SO2
From Don Wuebbles, Univ. Illinois UC, http://www.atmos.illinois.edu/courses/atms449-sp05/
Sulfur stable isotopes:32S: 95.02%33S: 0.75%34S: 4.21%36S: 0.02%
Sulfur isotope standard:Canyon Diablo Triolite32S=0.950395733S=0.007486534S=0.041971936S=0.0001459
Five oxidation states:+6: e.g. BaSO4+4: SO2
0: S (s)-1: FeS2
-2: e.g. H2S
Introduction to sulfur isotopes
Rt marine sulfur = 20Ma
Equilibrium fractionations relative to H2S
S6+
S4+
S-11000
ln
H2S
Biologically-mediated SO4 reduction
NOTE: the bacterial reduction of sulfate occurs via kineticfractionation larger
-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
SO42-
H2S(g)
Raleigh fractionation during sulfate reduction
4
21.025SO
H S
Use equations from Raleigh 18O lectureto calculate 34S of sulfate, sulfideas a function of fraction remaining.
34S of sulfate becomes heavieras light sulfide forms
34S of sulfide becomes heavieras sulfate source becomes heavier
What would be the 34S of the totalS at the end of the distillation?
but varies widely, dependson environmental conditions
Equilibrium fractionations
Bacterial Sulfate Reduction -15 to -70‰ depletionThermochemical Sulfate Reduction -20‰ (at 100ºC)
-15‰ (at 150ºC)-10‰ (at 200ºC)
But you must know the starting 34S of the sulfate…
AND… we can use mineral pairs to establish T of mineral formationex: pyrite and chalcopyrite coprecipitated from same fluid
but you must know the starting d34S of the sulfide….
BUT… the 34S of sulfide and sulfate in a solution depends on the relative proportionsof 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 34S evolution
34S and 13C not anti-correlated,as observed for last 1 billion years
Cenozoic 34S evolution
atmospheric O2 did not change very much during the last 100Ma,so reduced S and C are not the onlycontrols on atmospheric O2
Why anti-correlated over last 1Ga?increase burial C(org),= higher 13C=higher atmos. O2
=oxidize sulfides (low 34S) to SO4
=lower oceanic 34S
Paytan et al., 1998
measured34S of marine barite (BaSO4)Main factors that influence
evolution of Cenozoic 34S:1. deposition/burial of pyrite2. deposition/burial of sulfates3. intensity of hydrothermal
activity and volcanism
What does it mean that variationsoccur on timescales shorter than20Ma (Rt of oceanic sulfur)?
What happened at 55Ma?Why might this affect marine 34S?
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 34S excursions in sediments be measureable?
Fact:Early work on Martian meteorites and Archean sediments revealed significant34S excursions
Mass-independent sulfur isotope fractionation
Laboratory SO2 photolysis
from Farquhar and Wing, 2003
A new notation for deviation from the MDF line:33S = δ33S− 0.515×δ34S36S = δ36S− 1.90×δ34S
For mass-dependent fractionation (MDF):
δ33S = 0.515×δ34Sδ36S = 1.90×δ34S
Three-Isotope Plot
MDFMIF
33S
Evolution of the atmosphere:multiple isotopes and MIFs
Ono, 2008
keep in mind uncertainties…
Johnston, 2011
Archean mass-independent sulfur isotope fractionation
Farquhar & Thiemens, 2000,2001
33S = departure from massfractionation line (MFL)= 0 present-day
but highly variable in Archean sediments
Today atmospheric mass-independentrxns occur, but isotopes are re-mixedin 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
Archean mass-independent sulfur isotope fractionation
from Lyons & Reinhard, 2011
the “Great Oxygenation Event (GOE)”
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 albedomakes snowball
rising CO2 increasestemp., melts ice,reverse ice albedo feedback
temporary hothouseEarth after snowball
Cap carbonate overlying diamictite; photo by Francis MacDonald
translates into progressive enrichment of oceans bycontinued burial of pyrite in ocean
from Hurtgen et al., 2002
from Hurtgen et al., 2002
anomaly upon deglaciationshould be recorded in capcarbonates
from Hurtgen et al., 2002
cap carbonates