Stable Sulfur Isotopes

Source of atm oxygen

Burial of pyrite, Org C burial, and H escape, (photosynth)

Sink of atm oxygen

Pyrite oxidative weathering, Corg oxidative weathering, volcanic reduced gases, (resp)

Bacterial Sulfate Reduction

anaerobic microbial process that respires org m using SO4 and produces sulfide and creates a large fractionation btwn the species

occurs in euxinic environments and below Mn and Fe oxide reduction zones

When does fractionation occur in BSR

At each step where electrons are gained and results in negative DS and positive dS

What determines the extent of S fractionation

rate of sulfate reduction (lower rate results in higher fractionation which is opposite of Ca) and enough SO4 is present

Why does the concentration of SO4 impact BSR fractionation

when SO4 is too low, the microbes cant preferentially choose lighter isotopes (less picky w low conc)

Disproportionation

redox reaction that forms a reduced product and an oxidized product out of an intermediate compound w no external electron acceptor and donor

what does disproportionation lead to

Larger 34S depletions in sed sulfides (>-60permil)

Positive DS and dS

How can multiple S isotope ratios be used to determine different S reactions

small shifts in equilibrium value can generate non-zero DELTAB values from intracellular branching reactions (S exchanged btwn multiple reservoirs)

What mechanism forms low d13c and d34S

decrease organic carbon burial

What causes a lower S fractionation

decreased available reactants (low SO4 concentration)