1. Sulfur isotopes in the rock record
James Farquhar ESSIC and Department of Geology, University of Maryland
The goal of this talk to provide a sense of the sulfur isotope signal that is preserved within the geological record and to describe the connections that have been made between the sulfur isotope record and atmospheric chemistry and evolution.
Research presented here supported by ACSPRF, NASA, and NSF

2. 2 parts The Geological story told by sulfur isotopes
Questions about the sulfur isotope record and the processes it records
The structure of this talk is in two parts. The first part focuses on the record and its interpretation, and the second part describes some very specific aspects of the data and results of photochemical experiments that may be relevant for future work.

3. Rationale for S-isotopes in geochemistry
(32S, 33S, 34S, & 36S)
(δ34S, Δ33S, Δ36S)
One of the issues in studying the sulfur cycle in natural and experimental settings is in focusing on a part of the problem that is likely to provide new insights into the way that it operates. This picture is of the meromictic lake Cadagno in Switzerland. This location and others are places where we have been working for the past few years on studies of biological cycling of sulfur. This is also a subject of research that is the focus of many other sulfur isotope geochemistry groups.
As Tracers of biological activity

4. Diamond Rationale for S-isotopes in geochemistry
Another application of sulfur isotopes is as tracers of mass-transfer. This is an image of a sulfide mineral inclusion in a diamond from south Africa. This particular inclusion possesses a sulfur isotopic composition that implies a prior history of surface sulfur cycling, even with an atmospheric component. Age constraints suggest that the sulfide in the inclusion formed early in Earth history.
As Tracers of mass transfer

5. Oxygen Rationale for S-isotopes in geochemistry
Biogeochemistry of oceanic sulfate
Atmospheric oxygen
A third, and by no means final area is the area that interests us today, which is the use of sulfur isotopes as tracers of planetary evolution. This sequence of slides marches us through four different compositional aspects of sulfur isotope geochemistry. At this stage of the talk it is not essential that we know what the various compositional notation means other than to know that it is a representation of the variability of the relative abundance of the different isotopes in sulfur that has been analyzed.
The first of these is of the delta 34S, which is a way of representing variations in the 34S/32S. These are data from sedimentary sulfide and sulfate minerals plotted as a function of sample age. What we see is a general expansion of the range of variation for delta 34S observed in sediments as the samples young. This is thought to be real, and to reflect a change in the biological cycling of sulfur. We will come back to this later in the talk.
The second slide is a plot of Cap Delta 33S (a measure of anomalous mass-independent 33S enrichments and depletions) plotted as a function of sample age. What we see is a dramatic change from a large degree of variability for D33S to a much more homogenous D33S at about 2400 million years ago. This change is interpreted to reflect a change in atmospheric and surface oxidation state about mid way through Earth’s evolutionary history.
Age (Gyr)
Modified from Holland (2006)
As Tracers of planetary evolution
Credit:  NASA Johnson Space Center (NASA-JSC)

6. Linking sulfur isotope variations to chemical and physical process
NOTATION:
Express compositions as ratios of isotopic ratios using:
d34S = [(34S/ 32S)I/(34S/ 32S)ref – ((34S/ 32S)ref /(34S/ 32S)ref ]
D33S is measured –predicted 33S/ 32S in a sample
D36S is measured –predicted 36S/ 32S in a sample
D33S = [(33S/ 32S)I/(33S/ 32S)ref-((34S/ 32S)I/(34S/ 32S)ref )0.515]
D36S = [(36S/ 32S)I/(36S/ 32S)ref-((34S/ 32S)I/(34S/ 32S)ref )1.9]
You may ask how do we link sulfur isotope variations to chemical and physical processes? To do this, I will need to describe a little more about the underlying principles of isotope geochemistry and we will start with the notation we use – delta notation and cap delta notation.
The delta notation developed in the 1940’s and was used to describe variations (deviations) in the isotopic ratio of two isotopes relative to that in a reference.
The Cap delta notation describes the deviation of a measured quantity relative to a predicted value (at least the way that I have defined it). It is my understanding that the cap delta notation is defined to approximate single-step equilibrium isotope exchange. Capital delta is given as the deviation of a measured quantity (delta 33S or d3lta 36S) from what it would be predicted to be if single-step equilibrium isotope exchange prevailed. The exponents and 1.90 are defined to approximate this equilibrium prediction, but we will see that it is still an approximation on the next slide.

7. D – notation: classical isotope effect reference frame
1st POINT:
Mass-dependent effects produce only small
variations for Δ33S (Δ36S)
CIE arise because isotope mass plays a part determining
Vibrational partition functions and internal energy
D33S = [(33S/ 32S)I/(33S/ 32S)ref-((34S/ 32S)I/(34S/ 32S)ref )0.515]
Mass-dependent effects produce significant
variations for 34S
There is an aspect of the Cap Delta notation that we all need to be aware of. And that is the existence of several different definitions of Cap Delta. One of these (the one that I just described) is designed to minimize Cap Delta for single-step equilibrium chemical reactions. Plotted here are the curves for equilibrium between various species and hydrogen sulfide. Presented here are results of molecular orbital calculatoins published by Otake et al(2008). They used a Hartree Fock level theory, and a relatively small basis set for these calculations, but the results are consistent with results made using higher order techniques. The figure illustrates how the D33S is minimized (near zero) for all temperatures for both sets of calculations and how the variation in D33S is within about +/ permil. This is only true if one defines D33S using this definition.

8. Mass-independent isotopic effects
NMD
Mass-independent isotopic effects
2nd POINT:
Mass-independent effects produce larger
Variations for Δ33S (Δ36S)
With or without variations for 34S
Factors in addition to mass play
roles in other types of chemical
reactions
Another class of effects called mass-independent effects and that are produced by chemical reactions can produce large variations in D33S. These effects were discovered in the oxygen system by Mark Thiemens and John Heidenriech who did experiments with oxygen and ozone.....They are a different type of effect than the mass-conservation effects and although there seems to be a tendency to lump all types of processes that produce nonzero D33S with mass-independent chemical effects, I prefer to stick with the distinction that MIF or NMF describe chemical reactions that have terms in them contributing to the fractionations other than mass.
Thiemens and Heidenreich, 1983 Science

9. Mass-conservation effects
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
3rd POINT:
Small magnitude signals for Δ33S
(larger for Δ36S)
produced by biological cycling
A third type of effect is associated with mixing.
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
Musings: There is also an added element relating to equilibrium versus kinetic isotope effects.The relationship between the different isotopes in KIE will have components that are classical in nature (Depending on vibrational partition functions) as well as elements that depend on other factors (such as commitment to catalysis – which can introduce a element like the mixing element described here). There can also be translational components in some reactions (which will have a different mass-dependence) or possibly components related to tunneling (other relationships). For present, I am assuming that most kinetic reactions depend strongly on the classical vibrational components, but this is a cautious assumption.
Δ33S (Δ36S) scale with 34S
Desulfomaculum acetoxidans
Spring et al., 2009

10. Increase in fractionation with time
How are these different types of isotope effects expressed in the geologic record?
Increase in fractionation with time
Observations δ34S values increase with time
Point: The record of δ34S shows a progression from relatively small fractionations early in Earth history to larger fractionations later in Earth history.
Description of the Data: There are two large jumps in the spread of δ34S values and possibly a number of other more subtle ones. The two large jumps include one that occurs in the late Archean ( Ga) and a second that occurs in the late Proterozoic (near the start of the NeoProterozoic, but possibly extending well into it) where there is a dramatic increase in the spread of δ34S values to about 60 permil.
Interpretation: The spread of δ34S values has been interpreted to reflect role of (1) biology (principally microbial sulfate reduction, but also disproportionation and reduction of sulfur intermediates, and oxidation of reduced and intermediate compounds), (2) how much these organisms fractionate the sulfur (which depends on the concentration of dissolved sulfate and substrates for growth in the local setting where medium where these organisms live), and (3) the ecology of the organisms that carry out these processes on a global scale (whether the role they play in moderating the signal occurs in the water column, or in the sediments and whether the sediments are bioturbated (burrowed) or not).

11. Decrease in Δ33S variation
With time
The ∆33S also varies with time, becoming smaller for the past 2400 million years, and being larger for samples older than 2400 million years old.
So the question we will focus on here will be related to mass-independent signals in the geologic record. It is not so much whether the chemical processes are inherently mass-independent, but this is an operating assumption, but rather that there is something going on that produces an anomaly, and knowing something about what it is that is going on, will provide new insights into the conditions that operated in the past. This is linked to the issues associated with the production of these effects, but may also include links to other issues such as shielding. I would however like to make the point that the sulfur isotope variations that we see are striking and that they occur on a planetary scale.

12. The relationship between d34S and D33S to illustrate the different types of isotope effects interpreted to be responsible for the observed variations.
Mass-dependent and mixing/biology.

13. The record prior to 2400 million years is clearly different from the mass-dependent or mixing relationships and has been attributed to mass-independent effects. There is scatter in the data that indicates that other effects (either MI or MD operated).
This slide and the next shows the covariation between Cap Delta 33S and other measures of sulfur isotope composition. We see changes in the relationship between D33S and d34S and also between D33S and D36S. These coincide with sample age and can be divided into samples older than, and younger than approximately 2400 million years old. There are other subdivisions, but these are the big ones, and the ones that we try to interpret from the sulfur isotope record. One interpretation is this one, given by Holland, (2006) and others which attributes the changes to variations in atmospheric oxygen content and changes in the biogeochemistry of sulfur in the oceans. This is the basic geological issue that underpins much of the research in deep time applications of sulfur isotopes,

14. Other aspects of data
When these effects were first identified, a part of the interpretation, was linked to the question of whether the effect was just for 33S – being an isotope effect for isotopes with odd mass (magnetic moment or nuclear volume (?)) – or whether it was an effect that depended on factors other than mass, perhaps analogous to effects seen with ozone, photolysis of CO2 or CS2, or ???.
So the question is related to what produced the mass-independent signals in the geologic record.
Note: It is not so much whether the chemical processes are inherently mass-independent, but this is an operating assumption, but rather that there is something going on that produces an anomaly, and knowing something about what it is that is going on, will provide new insights into the conditions that operated in the past. This is linked to the issues associated with the production of these effects, but may also include links to other issues such as shielding. I would however like to make the point that the sulfur isotope variations that we see are striking and that they occur on a planetary scale.

15. Question about Origin of MIF
In 2000 (published in 2001) we conducted pilot experiments to explore the possibility that photochemistry involving sulfur dioxide
The question then arises: “What is the connection between the anomalous (mass-independent) sulfur isotope signal and the early atmosphere?”
There are several connections. The first is that sulfur gases (sulfur dioxide and hydrogen sulfide) are introduced continuously to the atmosphere from voclanic and biological sources. The second is that these gases undergo a host of different chemical transformations in the atmosphere as a result of reactions with other species in the atmosphere (principally as oxidation reactions) and as a result of interactions with electromagnetic radiation (UV radiation and light) in the atmosphere.
Farquhar et al., 2001

16. Link to Ozone and Oxygen Why geoscientists care?
There was also a connection between the chemistry of sulfur dioxide and deep ultraviolet radiation.
We had done some exploratory experiments and found that large non mass-dependent isotope effects could be produced by irradiating sulfur dioxide with deep ultraviolet radiation. This combined with the inferences about SO2 being a relevant gas in the past atmosphere led us to explore what the connection might be with atmospheric composition.
We already had one connection – the presence of the signal early in Earth history required the atmosphere to play a role in controlling the isotopic composition of sulfur and this does not occur in the present-day suflur cycle because of oxidative weathering. The combined fluxes of oxidative weathering of pyrite, and of the sulfate built up over geologi history are about 200 Tg/yr whereas the atmospheric fluxes are about 10 Tg/yr (from memory). This implies less oxygen for oxidation. What other connections might exist?
One of these is related to UV. In the present oxygen-rich atmosphere, ozone is produced and it is this ozone that absorbs UV and make better conditions for life at the surface. Ozone and oxygen and UV are connected, and we can illustrate this in the following slides.

17. Link to atmospheric oxygen levels: Sulfur chemistry and
Connection: Oxygen and ozone concentrations control available UV radiation
Link to atmospheric oxygen levels:
Sulfur chemistry and
atmospheric transparency
I show this plot and the next few to make a link between atmospheric composition and the transparency of the atmosphere to ultraviolet light.
These following is a series of plots that I have recalculated from work done on the column depth of ozone as a function of oxygen content by Joel Levine and Jim Kasting in the early 1980’s. They are calculated using an incident solar spectrum (the spectrum incident to the top of the atmosphere), for a given zenith angle (50º), and with the prescribed ozone and oxygen column depths.
On this plot the blue field is the intensity of radiation that makes it through the atmosphere. In this slide it is for the present atmosphere. In the next slide it is for 1/10 of Present levels.

18. Oxygen and ozone concentrations control available UV radiation
1/10 PAL

19. Oxygen and ozone concentrations control available UV radiation
1/100 PAL

20. Oxygen and ozone concentrations control available UV radiation
1/1000 PAL

21. Oxygen and ozone concentrations control available UV radiation
1/ PAL

22. Link to atmospheric oxygen levels: Sulfur chemistry and
atmospheric transparency
1/ PAL
and we see how the atmosphere becomes transparent for just the same radiation that produces the mass-independent fractionation for photolysis of sulfur dioxide. A back of the envelope calculation of the lifetime of sulfur dioxide against photolysis compared to its lifetime against rain out (4 days – Warneck) or physical processes (40 days – Warneck) and deposition suggests that a level of oxygen necessary to have a comparable sink is between 10-1 and 10-2 PAL. (quantum yield assumed 0.5)
Below this level one would expect these reactions to be switched on in the lower parts of the atmosphere.

23. Second link – cycling of sulfur insufficient to homogenize Δ33S
Limits oxidative weathering – consistent with geological evidence
P – pyrite
R – rutile
Z – zircon
C - chromite
Wacey et al. 2010

24. Third link to atmospheric oxygen levels: Formation of sulfur aerosols
creates a second pathway for transfer of
atmospheric signals to surface sulfur pools
A third link to atmospheric chemistry was provided by Jim Kasting and Alex Pavlov. They noted that the pathways for formation of aerosol S8 were blocked by oxygen and other oxygen species. They also noted that there was a threshold when the S2 would be abundant enough for the S2+ S2 reaction to occur. Once this threshold was passed, the sulfur would leave the atmosphere by two pathways, AND they argues that this was a much more efficient way to transfer an atmospheric signal to the rock record. In other words, the bottleneck produced by passing most sulfur through sulfate aerosols (as it is in the present atmosphere) would act as a strong agent for homogenizing the signature of atmospheric chemistry. While it need not completely remove the signal, it was suggested that the transfer of the signal would be much less efficient in high oxygen atmospheres.
The threshold they identified was 10e-5 PAL, and my understanding (although not explicitly stated by the authors) is that this is the level at which sulfur exceeds oxygen and it relates to the sulfur burden of the atmosphere.
Kasting JF, SCIENCE, 293: , 2001
Also Pavlov et al. 2002

25. Developments in the past 10 years
Offset in absorption features
conducive for shielding effects
Effects related to UV spectrum
Danielache et al., 2008

26. Developments in the past 10 years
Reduction of sulfate using amino acids
Hypothesis: either an MIE
or a new type of isotope effect
(may be relevant in geological systems)
Possible alternative chemical pathways for MIF
Watanabe et al., 2009

27. Developments in the past 10 years Geochemical Interpretations
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)
More detailed focus on the record
and
development of models for interpretation

28. Developments in the past 10 years Geochemical Interpretations
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).
Ono et al. 2003

29. Developments in the past 10 years Geochemical 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)

30. Developments in the past 10 years Geochemical Interpretations
A 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
Halevey et al. 2010

31. Significant issues remain
Sampling the sulfur isotope record
(representative sample or not?)

32. Bias toward samples with high Δ33S? Or Missing pool of
sulfur with negative Δ33S?
Sample density
too low
A second aspect of the data that shows a problem with the dataset is related to the means. In principle, a balanced record should preserve the same composition as the total sulfur, and the total sulfur in the system should preserve the composition of the source (juvenile sulfur with near zero D33S). The yellow dots show that the mass-balance is not satisfied. This is important because it can influence the information that is extracted by models about the MI and MD source reactions.
Illustration of mean for analyses done so far grouped into 100 million year bins: This illustration shows that the data collected to date imply a sample bias in the Neoarchean. Understanding the cause of this bias is critical for making interpretations. It may be that the samples analyzed have a lithological bias towards samples with high Δ33S. It may be that there is a missing pool. The former would imply a bias imposed by choices made during sample selection. The latter might imply something like a global deep water sink for sulfur with negative Δ33S (hydrothermal or biological sulfate reduction).

33. Significant issues remain
Sampling the sulfur isotope record
(representative sample or not?)
Characterizing variability in the early sulfur isotope record

34. What is the true nature of temporal variablity?
There also may be additional structure in the record. For instance (1) in the interval leading up to the disappearance of the mass-independent signal from the rock record may have two steps – one step at about 2.5 Ga when the maximum of the positive signal drops from ~10-12 ‰ to about 2 ‰ and when the negative signal all but disappears, and a second step when the evidence for an externally-imposed signal disappears altogether. (2) The spread in the data in the Archean (4.0 to 2.5 Ga) also appears to show variation, with an early Archean ( Ga) interval with a range of about 6‰, a middle Archean interval ( Ga) with a range of about 3 , and a late Archean interval with the largest (~15 ‰) range. The Early interval corresponds roughly to the Eoarchean and Paleoarchean, the middle interval to the Mesoarchean, and the late interval to the Neoarchean.
This structure also appears to be related to changes in the relationships between D33 and D36. This data is incomplete and it shows variability that appears to be real. It also has some problems. There is enough evidence, however to point to changes over time and to changes that occur on the formation level (for specific intervals). I suspect that identifying where these changes occur and what they are caused by, will be one of the important next steps for geochemical S-MIF research.
Interpretation: This variation has been a focus of studies seeking to understand changes in the reactions responsible for generating the signal, the way that these reactions are incorporated into the pathways that define the atmospheric and oceanic sulfur cycle, and the role of mass-dependent reactions that may contribute sulfur to, or between, global sulfur pools.

35. Martian meteorites Difference: lack of anomalous Δ36S Shergotites
Other Martians
I added this slide to make a point. Mass-independent signals are not restricted to Earth. There are signals in organic compounds (sulfonic acids) from primitive meteorites, and there are signals from Martian meteorites. The martian signals might reflect similar processes to those seen on Earth. They do not have a D36S anomaly. The sulfate appears to have 33S depletions. Some of the sulfides in basalts appear to have 33S enrichments. This is another area and chemistry to explore.
Work of Franz and Kim, unpubl.

36. 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
The final issue that I want to touch on has to do with the experimental studies that have been done to date.
There are issues about the spectroscopic measurements, such as whether the differences in the

37. Role of shielding and primary photochemical IE
Effects related to UV spectrum
Offset in absorption maxima, minima,
and width, carry implications
for isotope effects.
Danielache et al., 2008

38. Issues with experiments
Point: Relationships between
Isotope effects and UV spectrum

39. Issues with experiments
Masterson et al. 2011
Point: Systematic relationships between
Isotope effects and pressure

40. Issues with experiments
Point: Systematic relationships between
Isotope effects and identity of oxygen
Experiments with S18O2 and S16O2 (Heather Franz, unpub)

41. END