S= 95.02% S= 4.21% 35. S=radioactive 36 S=0.02% S= 0.75% 34 VI V IV III II I 0 -I -II SO 4 S 2 O 6 H 2 SO 3 HS 2 O 4- S 2 O 3

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1 SULFUR ISOTOPES 32 S= 95.02% 33 S= 0.75% 34 S= 4.21% 35 S=radioactive 36 S=0.02% S-H S-C S=C S-O S=O S-F S-Cl S-S VI V IV III II I 0 -I -II SO 4 2- S 2 O 6 2- H 2 SO 3 HS 2 O 4- S 2 O 3 2- S 2 F 2 S H 2 S 2 H 2 S SF 6 S 2 F 10 SO 3 2-

2 One of the ten most abundant elements in the solar system. S is essential for all living things (plants, and animals) present in levels of 1000 s of ppm in organic matter such as humic substances, kerogen, hydrocarbons, and proteins. Used in fertilizers, present in landfill leachate plums and sewage. It is a major element in seawater and marine sediments. Associated with fossil fuel, ore deposits, and mining drainage. It is an important constituent of atmospheric aerosols and trace gases (from natural and anthropogenic sources).

3 Analysis by isotope ratio gas mass spectrometry Standard is the Canyon Diablo Troilite (FeS) Use of internal lab standards and IAEA-S-1 (Ag 2 S) (-0.3 relative to VCDT) IAEA-S-2 and IAEA-S-3 NSB 127 (BaSO relative to VCDT)

4 Mass Spectrometry (± 0.2 ) Analyses as one of two gases (SO 2 or SF 6 ) Sulfides - combustion with an oxidizing agent (CuO, Cu 2 O, O 2 ) Sulfates ppt. to BaSO 4, reduction & conversion to CdS or Ag 2 S Organic matter - combustion with O 2 to covert S to SO 2 SO 2 ~1mg Correction for O isotopes (measure Mass 66/64 34 S 16 O 16 O/ 32 S 16 O 16 O BUT 32 S 18 O 16 O, 33 S 16 O 16 O also 66, not easily correctable - fractionation un-correlated) Sticky. Relatively fast SF ~0.4mg, 6 Dangerous, labor intensive. (146/148) All isotopes (F has only one isotope) Mass independent fractionation

5 Units are Tg S

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7 Equilibrium Isotope Effects H 34 2 S + 32 K SO 2 H 32 2 S + 34 SO 2 K = 800 K Enrichment of 34 S in the more oxidized species.

8 It must be noted that although an isotope equilibrium constant may be calculated for a pair of sulfur compounds, there is no guarantee that isotope equilibrium will in fact be achieved. For example, in the case of the SO 2 and H 2 S exchange system it is necessary to have water present and to go to high temperatures so that the reaction paths may be established.

9 Igneous Rocks/Ore bodies Equilibrium Fractionation and Geothermometry Equilibrium between oxidized and reduced S (sulfate and sulfides) only achieved at Hi-T Coexist S minerals in ore deposits are seldom in equil. Sulfide pairs tend to be in equilibrium e.g., sphalerite and galena Order of enrichment (more 34 S) reflects bond strength Decrease in bond strength

10 Kinetic Isotope Effects Fractionation results from differences in reaction rates of the different isotopic species. The rate constant k 32 /k 34 k 32 for the competing 32 SO 2-4 H 32 2 S reactions is ~1.022 at room temperature. 34 SO 2-4 k 34 H 34 2 S 32 SO 2-4 reacts1.022 times faster than 34 SO 2-4. The product H 2 S is depleted by 22 relative to the remaining SO 2-4. mainly in sediment (marine and freshwater) pore waters - absence of O 2

11 Major fractionation in nature due to dissimilatory sulfate reduction by bacteria (SRB). Sulfide oxidation by bacteria involves little fractionation During assimilation of S by phytoplankton isotopic fractionation is negligible, with little enrichment in 34 S per trophic level/diet. Possible to distinguish between terrestrial and marine and between open ocean and sediment or hydrothermal sources of S. Small isotopic fractionation during mineral formation

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13 Dissimilatory Sulfate Reduction (4 enzyme catalyzed steps) external = internal = adenosyl = sulfite = hydrogen sulfide sulfate sulfate phosphosulfate dissimilatory sulfite membrane transport reductase reducase

14 I SO 4 2- out SO 4 2- in the cell ~ 2 II SO 4 2- in + enzyme SO 4 2- complex negligible isotope effect III SO 4 2- complex SO 3 2- (S---O) bond breaking step 22 IV SO 3 2- H 2 S rapid reduction relative to I-III ~ 25 Either step I or step II+III may be rate controlling

15 Factors effecting fractionation in sulfate reduction Reduction Pathway (organism) Reduction rate Sulfate concentrations Temperature Substrate availability and type Fe availability Conditions in the sediment ph More...

16 At low sulfate concentrations (e.g <10 M) the fractionation is reduced (step I is rate controlling). In a closed system the fractionation is small.

17 Sedimentary S isotope record through time Early ocean before ~2.5 Gyr ago: Sulfate limited, final stage of Rayleigh distillation Essentially all sulfate is consumed by MSR, pyrite has same isotope composition as initial sulfate with minimal apparent fractionation. Modern ocean since ~0.8 Gyr ago: Virtually unlimited sulfate reservoir, early stage of Rayleigh distillation Only partial consumption of sulfate by MSF, large apparent fractionation in pyrite. Solid lines indicate isotope composition of sulfate. Symbols indicate isotope composition of pyrite.

18 The difference between sulfate and sulfide in natural samples is more than could be explained by the isotopic fractionations measured in the lab. Therefore, additional processes are contributing to the fractionation of sulfur isotopes in the environment. Isotope fractionation accompanying elemental sulfur disproportionation contributes to the 34 S depletion of sedimentary sulfides. S 2 O H 2 O SO HS - + H + 4S + 4H 2 O SO HS - + 5H +

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21 Applications Stratigraphy and correlation (Paytan 2012) Microbial ecology pathways and processes (sulfate reduction, sulfite oxidation) Food web structure and S sources for metabolism Origin and evolution of life Oxygen balance in the atmosphere Pollution source and movement (ground water, soil, rivers, lakes, atmosphere) Petroleum and ore deposit processes (source, age, diagenetic alteration and temperature) Volcanic events (tree rings, ice core, geological record) Atmospheric chemistry and mixing processes Climate (clouds, DMS)

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26 Ice Cores Tree Rings

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28 The Sulfur Isotopic Composition of Cenozoic Seawater Sulfate: Implications for Atmospheric Oxygen The S isotope record of oceanic sulfate can provide evidence for large scale transfers of S between different sedimentary reservoirs Paytan et al., 1998, 2004

29 A Representation of the Sulfur Cycle volcanic subduction 10, 0 uplift weathering 72 0 to +10 burial gypsum 44, +20 pyrite 28, -16 seawater +20, Tg 10, 0 hydrothermal fluxes in units of Tg (S) yr -1, isotope ratios in %o CDT

30 Global changes in climate and atmospheric chemistry are intimately related to the sulfur sedimentary cycle. Seawater sulfate represents the main reservoir for the sulfur exogenic cycle and knowledge of its chemical and isotopic composition is of crucial importance for the understanding of geochemical cycles on Earth.

31 Seawater Sulfate S isotope Curve Cenozoic Cretaceous

32 steady state mass and isotope balance model 34 S of input 34 S of seawater S input flux F py = pyrite burial flux in SO4 ( S S )* F 34 S sw py in seawater-pyrite isotopic difference Evaluate the change in each of the model parameters needed to explain the difference in 34 S SO4. Kump 1989

33 Background changes between the Holocene and Cretaceous Holocene (pre anthropogenic) 34 S SO4 = 21 F in = 2.0 * mol/year F py = 0.9 * mol/year 34 S in = S py-sw = Cretaceous 34 S SO4 = 19 (record) F in = 2.7 * mol/year F py = 0.66 * mol/year 34 S in = S py-sw = 40.0 (record) Arthur et al., 1990 F in = ~35%, F py = ~30%, 34 S in = 2.4 Large Changes and fast rates of change Doubling of the input flux to the ocean A 34 Sin value of -7.3 Pyrite burial flux less than 20% of pre excursion flux

34 The biogeochemicals cycles of S and C are intimately linked with the principal processes that control the level of atmospheric oxygen. CO 2 + H 2 O = CH 2 O + O 2 2Fe 2 O Ca HCO SO 4 2- = 4FeS 2 +16CaCO 3 + 8H 2 O + 15O 2

35 A general inverse relationship between marine 34 S sulfate and 13 C carbonate has been recognized using the evaporite S record. The correlation is based on averaging over geological periods. 13 C carbonate P K Pt 3 C T J T R Q D S O S sulfate

36 Scatter Diagram of Cenozoic 34 S sulfate vs. 13 C carbonate 34 S (% 0 CDT) for marine barite data is averaged over one million year increments 0-10 m.y m.y m.y m.y m.y m.y m.y C (%o PDB) for bulk deep sea carbonates (Shackleton, 1987)

37 S Isotope Stratigraphy

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39 MIF in Sulfur Normally, isotopes separate along a standard mass fractionation line (MFL): 33 S x 34 S Prior to 2.3 Gyr ago the isotopes fall off MFL line (Farquhar et al., 2000)! 33 S= 33 S x 34 S

40 MIFs are likely generated through photochemical reaction in upper atmosphere. Before Great Oxidation Event (in the absence of atmospheric oxygen), MIFs are passed on to the ocean and preserved in the rock record After the GOE (oxygenated atmosphere) S isotope are homogenized through oxidation reduction reactions in the atmosphere.