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Sulfur isotopes 11/14/12 Lecture outline: 1)sulfur cycle 2)biological fractionation 3)S isotopes in the geologic record 4)mass-independent S isotope fractionation.

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Presentation on theme: "Sulfur isotopes 11/14/12 Lecture outline: 1)sulfur cycle 2)biological fractionation 3)S isotopes in the geologic record 4)mass-independent S isotope fractionation."— Presentation transcript:

1 Sulfur isotopes 11/14/12 Lecture outline: 1)sulfur cycle 2)biological fractionation 3)S isotopes in the geologic record 4)mass-independent S isotope fractionation Authigenic marine barite (BaSO 4 ) separated from deep-sea cores SEM Photo: Adina Paytan Hydrothermal barite separated from black smokers SEM Photo: Kim Cobb

2 The sulfur cycle SO 2

3 From Don Wuebbles, Univ. Illinois UC,

4 Sulfur stable isotopes: 32 S: 95.02% 33 S: 0.75% 34 S: 4.21% 36 S: 0.02% Sulfur isotope standard: Canyon Diablo Triolite 32 S= S= S= S= Five oxidation states: +6: e.g. BaSO 4 +4: SO 2 0: S (s) -1: FeS 2 -2: e.g. H 2 S Introduction to sulfur isotopes R t marine sulfur = 20Ma

5 Equilibrium fractionations relative to H 2 S S 6+ S 4+ S ln  H2S Biologically-mediated SO 4 reduction NOTE: the bacterial reduction of sulfate occurs via kinetic fractionation  larger  -naturally-occurring sulfides commonly depleted by 45 to 70‰! -bacterial sulfate reduction  takes place in anoxic environments, where SO 4 is reduced in place of O 2 Thermochemical sulfate reduction - occurs at temps >100ºC -usually goes to near-completion -little fractionation

6 SO 4 2- H 2 S(g) Raleigh fractionation during sulfate reduction Use equations from Raleigh  18 O lecture to calculate  34 S of sulfate, sulfide as a function of fraction remaining.  34 S of sulfate becomes heavier as light sulfide forms  34 S of sulfide becomes heavier as sulfate source becomes heavier What would be the  34 S of the total S at the end of the distillation? but  varies widely, depends on environmental conditions

7 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  34 S 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  34 S of sulfide and sulfate in a solution depends on the relative proportions of H 2 S, HS -, and S 2-, which depends on pH, O 2 fugacity, total [S] SO… understanding present-day sulfur isotope variability in a given system is complicated ….

8 Phanerozoic  34 S evolution  34 S and  13 C not anti-correlated, as observed for last 1 billion years Cenozoic  34 S evolution atmospheric O 2 did not change very much during the last 100Ma, so reduced S and C are not the only controls on atmospheric O 2 Why anti-correlated over last 1Ga? increase burial C(org), = higher  13 C =higher atmos. O 2 =oxidize sulfides (low  34 S) to SO 4 =lower oceanic  34 S Paytan et al., 1998

9 measured  34 S of marine barite (BaSO 4 ) Main factors that influence evolution of Cenozoic  34 S: 1.deposition/burial of pyrite 2.deposition/burial of sulfates 3.intensity of hydrothermal activity and volcanism What does it mean that variations occur on timescales shorter than 20Ma (R t of oceanic sulfur)? What happened at 55Ma? Why might this affect marine  34 S?

10 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  34 S excursions in sediments be measureable? Fact: Early work on Martian meteorites and Archean sediments revealed significant  34 S excursions

11 Mass-independent sulfur isotope fractionation Laboratory SO 2 photolysis from Farquhar and Wing, 2003

12 A new notation for deviation from the MDF line:  33 S = δ 33 S− 0.515×δ 34 S  36 S = δ 36 S− 1.90×δ 34 S For mass-dependent fractionation (MDF): δ 33 S = 0.515×δ 34 S δ 36 S = 1.90×δ 34 S Three-Isotope Plot MDF MIF  33 S

13 Evolution of the atmosphere: multiple isotopes and MIFs Ono, 2008

14 keep in mind uncertainties… Johnston, 2011

15 Archean mass-independent sulfur isotope fractionation Farquhar & Thiemens, 2000,2001  33 S = 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  33 S = 0 in all sediments Models suggest that atmospheric O 2 had to be less than Pa at 3Ga <1% of present-day

16 Archean mass-independent sulfur isotope fractionation from Lyons & Reinhard, 2011 the “Great Oxygenation Event (GOE)”

17 Early Earth sulfur cycle: uncertainties abound! from Farquhar and Wing, 2003

18 Snowball Earth and the Sulfur Cycle

19 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

20 Cap carbonate overlying diamictite; photo by Francis MacDonald

21 translates into progressive enrichment of oceans by continued burial of pyrite in ocean from Hurtgen et al., 2002

22 anomaly upon deglaciation should be recorded in cap carbonates

23 from Hurtgen et al., 2002 cap carbonates


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