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.

Sulfur isotopes 11/14/12

Lecture outline:1) sulfur cycle

2) biological fractionation

3) S isotopes in the geologic record

4) mass-independentS isotope fractionation

Authigenic marine barite (BaSO4)separated fromdeep-sea coresSEM Photo: Adina Paytan

Hydrothermal bariteseparated fromblack smokersSEM Photo: Kim Cobb

The sulfur cycle

SO2

From Don Wuebbles, Univ. Illinois UC, http://www.atmos.illinois.edu/courses/atms449-sp05/

Sulfur stable isotopes:32S: 95.02%33S: 0.75%34S: 4.21%36S: 0.02%

Sulfur isotope standard:Canyon Diablo Triolite32S=0.950395733S=0.007486534S=0.041971936S=0.0001459

Five oxidation states:+6: e.g. BaSO4

+4: SO2

0: S (s)-1: FeS2

-2: e.g. H2S

Introduction to sulfur isotopes

Rt marine sulfur = 20Ma

Equilibrium fractionations relative to H2S

S6+

S4+

S-11000

ln

H2S

Biologically-mediated SO4 reduction

NOTE: the bacterial reduction of sulfate occurs via kineticfractionation larger

-naturally-occurring sulfides commonly depleted by 45 to 70‰!

-bacterial sulfate reduction takes place in anoxic environments, where SO4 is reduced in place of O2

Thermochemical sulfate reduction

- occurs at temps >100ºC-usually goes to near-completion-little fractionation

SO42-

H2S(g)

Raleigh fractionation during sulfate reduction

4

21.025SO

H S

Use equations from Raleigh 18O lectureto calculate 34S of sulfate, sulfideas a function of fraction remaining.

34S of sulfate becomes heavieras light sulfide forms

34S of sulfide becomes heavieras sulfate source becomes heavier

What would be the 34S of the totalS at the end of the distillation?

but varies widely, dependson environmental conditions

Equilibrium fractionations

Bacterial Sulfate Reduction -15 to -70‰ depletionThermochemical Sulfate Reduction -20‰ (at 100ºC)

-15‰ (at 150ºC)-10‰ (at 200ºC)

But you must know the starting 34S of the sulfate…

AND… we can use mineral pairs to establish T of mineral formationex: pyrite and chalcopyrite coprecipitated from same fluid

but you must know the starting d34S of the sulfide….

BUT… the 34S of sulfide and sulfate in a solution depends on the relative proportionsof H2S, HS-, and S2-, which depends on pH, O2 fugacity, total [S]

SO… understanding present-day sulfur isotope variability in a given system is complicated ….

Phanerozoic 34S evolution

34S and 13C not anti-correlated,as observed for last 1 billion years

Cenozoic 34S evolution

atmospheric O2 did not change very much during the last 100Ma,so reduced S and C are not the onlycontrols on atmospheric O2

Why anti-correlated over last 1Ga?increase burial C(org),= higher 13C=higher atmos. O2

=oxidize sulfides (low 34S) to SO4

=lower oceanic 34S

Paytan et al., 1998

measured34S of marine barite (BaSO4)Main factors that influence

evolution of Cenozoic 34S:1. deposition/burial of pyrite2. deposition/burial of sulfates3. intensity of hydrothermal

activity and volcanism

What does it mean that variationsoccur on timescales shorter than20Ma (Rt of oceanic sulfur)?

What happened at 55Ma?Why might this affect marine 34S?

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 34S excursions in sediments be measureable?

Fact:Early work on Martian meteorites and Archean sediments revealed significant34S excursions

Mass-independent sulfur isotope fractionation

Laboratory SO2 photolysis

from Farquhar and Wing, 2003

A new notation for deviation from the MDF line:33S = δ33S− 0.515×δ34S36S = δ36S− 1.90×δ34S

For mass-dependent fractionation (MDF):

δ33S = 0.515×δ34Sδ36S = 1.90×δ34S

Three-Isotope Plot

MDFMIF

33S

Evolution of the atmosphere:multiple isotopes and MIFs

Ono, 2008

keep in mind uncertainties…

Johnston, 2011

Archean mass-independent sulfur isotope fractionation

Farquhar & Thiemens, 2000,2001

33S = departure from massfractionation line (MFL)= 0 present-day

but highly variable in Archean sediments

Today atmospheric mass-independentrxns occur, but isotopes are re-mixedin surface and biological redox chemistry, so 33S = 0 in all sediments

Models suggest that atmospheric O2 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

Snowball Earth and the Sulfur Cycle

planet cools considerably,incipient glaciation,ice grows near 30

runaway ice albedomakes snowball

rising CO2 increasestemp., melts ice,reverse ice albedo feedback

temporary hothouseEarth after snowball

Cap carbonate overlying diamictite; photo by Francis MacDonald

translates into progressive enrichment of oceans bycontinued burial of pyrite in ocean

from Hurtgen et al., 2002

from Hurtgen et al., 2002

anomaly upon deglaciationshould be recorded in capcarbonates

from Hurtgen et al., 2002

cap carbonates