Sulfur isotopes in the rock record

James FarquharESSIC and Department of Geology,

University of Maryland

Research presented here supported by ACSPRF, NASA, and NSF

• The Geological story told by sulfur isotopes

• Questions about the sulfur isotope record and the processes it records

2 parts

Rationale for S-isotopes in geochemistry

(32S, 33S, 34S, & 36S)

(δ34S, Δ33S, Δ36S)

As Tracers of biological activity

DiamondRationale for S-isotopes in geochemistry

As Tracers of mass transfer

Oxygen

Credit:  NASA Johnson Space Center (NASA-JSC)

3.0 2.0 1.0 0 Age (Gyr)

Modified from Holland (2006)

Rationale for S-isotopes in geochemistry

As Tracers of planetary evolution

Atmospheric oxygen

Biogeochemistry of oceanic sulfate

NOTATION:NOTATION:

Express compositions as ratios of isotopic ratios using:Express compositions as ratios of isotopic ratios using:

3434S S = = [([(3434S/ S/ 3232S)S)II/(/(3434S/ S/ 3232S)S)refref – (( – ((3434S/ S/ 3232S)S)ref ref /(/(3434S/ S/ 3232S)S)refref ] ]

3333S isS is measuredmeasured – –predicted predicted 3333S/ S/ 3232S in a sampleS in a sample

3636S isS is measuredmeasured – –predicted predicted 3636S/ S/ 3232S in a sampleS in a sample

3333S S = = [([(3333S/ S/ 3232S)S)II/(/(3333S/ S/ 3232S)S)refref-((-((3434S/ S/ 3232S)S)II/(/(3434S/ S/ 3232S)S)refref ) )0.5150.515]]

3636S S = = [([(3636S/ S/ 3232S)S)II/(/(3636S/ S/ 3232S)S)refref-((-((3434S/ S/ 3232S)S)II/(/(3434S/ S/ 3232S)S)refref ) )1.91.9]]

Linking sulfur isotope variations to chemical and physical process

– notation: classical isotope effect reference frame

CIE arise because isotope mass plays a part determiningVibrational partition functions and internal energy

1st POINT:Mass-dependent effects produce only small

variations for Δ33S (Δ36S)

3333S S = = [([(3333S/ S/ 3232S)S)II/(/(3333S/ S/ 3232S)S)refref--((((3434S/ S/ 3232S)S)II/(/(3434S/ S/ 3232S)S)refref ) )0.5150.515]]

Mass-dependent effects produce significantvariations for 34S

Mass-independent isotopic effects

Thiemens and Heidenreich, 1983 Science

NMD

2nd POINT:Mass-independent effects produce larger

Variations for Δ33S (Δ36S)

With or without variations for 34SFactors in addition to mass playroles in other types of chemical reactions

Variations in Δ3xS also occur because of a linear dependence of isotope ratios when material is added to pools (mixing) OR when material is removed from

pools (e.g., Rayleigh effects)

• (34S/ 32S)tot = 32Xa(34S/32S)a + 32Xb(34S/ 32S)b

Instead of an exponential dependence that is used to define the reference fractionation arrays

• (33S/ 32S) a / (33S/ 32S) b ~ [(34S/ 32S)a/ (34S/ 32S) b] 0.515

These Principles apply in Biosynthetic Networks and in Biogeochemical Networks

Most significant impact on Δ36S

Mass-conservation effects

Desulfomaculum acetoxidansSpring et al., 2009

3rd POINT:Small magnitude signals for Δ33S

(larger for Δ36S)produced by biological cycling

Δ33S (Δ36S) scale with 34S

Increase in fractionation with time

How are these different types of isotope effects expressed in the geologic record?

Decrease in Δ33S variation With time

Other aspects of data

Question about Origin of MIF

Farquhar et al., 2001

Link to Ozone and OxygenWhy geoscientists care?

Connection: Oxygen and ozone concentrations control available UV radiation

Link to atmospheric oxygen levels: Sulfur chemistry and

atmospheric transparency

Oxygen and ozone concentrations control available UV radiation

Oxygen and ozone concentrations control available UV radiation

Oxygen and ozone concentrations control available UV radiation

Oxygen and ozone concentrations control available UV radiation

Link to atmospheric oxygen levels: Sulfur chemistry and

atmospheric transparency

Wacey et al. 2010

P – pyriteR – rutileZ – zirconC - chromite

Second link – cycling of sulfur insufficient to homogenize Δ33SLimits oxidative weathering – consistent with geological evidence

Kasting JF, SCIENCE, 293: 819-820, 2001Also Pavlov et al. 2002

Third link to atmospheric oxygen levels: Formation of sulfur aerosols

creates a second pathway for transfer of atmospheric signals to surface sulfur pools

Effects related to UV spectrum

Developments in the past 10 years

Danielache et al., 2008

Offset in absorption featuresconducive for shielding effects

Possible alternative chemical pathways for MIF

Developments in the past 10 years

Watanabe et al., 2009

Hypothesis: either an MIE or a new type of isotope effect

(may be relevant in geological systems)

Reduction of sulfate using amino acids

More detailed focus on the recordand

development of models for interpretation

Possible variations in the signal during the Archean

Ono et al., 2006; Ohmoto et al., 2006; Domagol Goldman et al., 2009; Halevey et al., 2010)

Developments in the past 10 yearsGeochemical Interpretations

Ono et al. 2003

Ono et al (2003, 2009) argued that changes in the MIF-S signal reflect changes in

• Where the chemistry occurs.

• the amount of sulfur released to the atmosphere and

• the oxidation state of the atmosphere (controlled by CH4).

Developments in the past 10 yearsGeochemical Interpretations

Domagol Goldman et al. 2009

Domagal-Goldman argued that climate feedbacks and organic haze controlled the available radiation and the expression of MIF(other studies – Ueno et al., 2009 explored other shielding processes)

• Archean climate control feedback loop (Pavlov et al. 2001)

Developments in the past 10 yearsGeochemical Interpretations

Halevey et al. 2010A model that describes the

production of MIF in terms of

• shielding by CO2, the proportion of sulfur that is photolyzed with MIF (assumed SO2), and the proportion of sulfur that is lost by non MIF processes (oxidation and H2S photolysis).

And the geologic preservation of MIF by

• the homogenization of sulfur in a one box (well-mixed) ocean by metabolic activity

Developments in the past 10 yearsGeochemical Interpretations

Significant issues remain

• Sampling the sulfur isotope record– (representative sample or not?)

Bias toward samples with high Δ33S?OrMissing pool of sulfur with negative Δ33S?

Sample density too low

Significant issues remain

• Sampling the sulfur isotope record– (representative sample or not?)

• Characterizing variability in the early sulfur isotope record

What is the true nature of temporal variablity?

Martian meteorites

Difference: lack of anomalous Δ36S

Work of Franz and Kim, unpubl.

ShergotitesOther Martians

Significant issues remain

• Sampling the sulfur isotope record– (representative sample or not?)

• Characterizing variability in the early sulfur isotope record

• Characterization of the source of the effect

Effects related to UV spectrum

Danielache et al., 2008

Offset in absorption maxima, minima,and width, carry implications for isotope effects.

Role of shielding and primary photochemical IE

Issues with experiments

Point: Relationships between Isotope effects and UV spectrum

Issues with experiments

Masterson et al. 2011

Point: Systematic relationships between Isotope effects and pressure

Issues with experiments

-60

0

60

120

180

240

300

360

-60 0 60 120 180 240 300 360

34S (per mil, initial SO2)3

6S

(p

er

mil,

in

itia

l SO

2)

Residual SO2

Residual S18O2

Elemental sulfur from SO2

Elemental sulfur from S18O2

Residual SO2

Residual S18O2

Elemental sulfur from SO2

Elemental sulfur from S18O2

Mass-dependent fractionation line

-40

0

40

80

120

160

200

-40 0 40 80 120 160 200

34S (per mil, initial SO2)

33S

(p

er

mil,

in

itia

l SO

2)

Residual SO2

Residual S18O2

Elemental sulfur from SO2

Elemental sulfur from S18O2

Residual SO2

Residual S18O2

Elemental sulfur from SO2

Elemental sulfur from S18O2

Mass-dependent fractionation line

Experiments with S18O2 and S16O2 (Heather Franz, unpub)

Point: Systematic relationships between Isotope effects and identity of oxygen

END