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:
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
From Don Wuebbles, Univ. Illinois UC, http://www.atmos.illinois.edu/courses/atms449-sp05/
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=0.9503957 33 S=0.0074865 34 S=0.0419719 36 S=0.0001459 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
Equilibrium fractionations relative to H 2 S S 6+ S 4+ S -1 1000ln 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
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
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 ….
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
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?
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
Mass-independent sulfur isotope fractionation Laboratory SO 2 photolysis from Farquhar and Wing, 2003
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
Evolution of the atmosphere: multiple isotopes and MIFs Ono, 2008
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 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