25th International Meeting on Organic Geochemistry IMOG 2011

P-354
Sulfur isotope fractionation during thermochemical sulfate
reduction as reflected by individual organic compounds
Alon Amrani 1 , Andrei Deev 2 , Alex Sessions 3 , Yongchun Tang 2 , Jess Adkins 3
1 The Hebrew University of Jerusalem, Jerusalem, Israel, 2 PEER Institute, Covina, United States of America,
3 California Institute of technology, Pasadena, United States of America
(corresponding author: alon.amrani@mail.huji.ac.il)
Thermochemical reduction of sulfate (TSR) to
sulfide and oxidation of organic matter involves
multiple reactions and redox changes. The
mechanism controlling this process is still enigmatic.
Attempts to study it were mainly focused on analysis
of inorganic sulfur reactants and products. However,
organic sulfur compounds have been shown to from
rapidly and affect significantly the rate of TSR (Amrani
et al., 2008). Their structural and isotopic analysis
may shed light on important and unexplored aspects
of the TSR mechanism.
In the present study, we performed a set of laboratory
simulation experiments to study how individual
organic compounds effected by TSR. The gold tubes
hydrous pyrolysis experiments performed at 360°C
with mineral buffer (silica gel and talc), n-C16 as an
organic model compound, and several inorganic
sulfur oxidizers, CaSO4, Na2SO4, Na2SO3 and S° with
distinct δ 34 S values. We applied a new technique
capable of measuring precise δ 34 S values in individual
compounds by GC-MC-ICPMS (Amrani et al., 2009).
We focused mainly on benzothiophenes (BT) and
dibenzothiophenes (DBT) as TSR tracers.
We observed large δ 34 S fractionation between DBT
and sulfates (Na2SO4 and CaSO4) of up to -21.6‰.
This fractionation was reduced rapidly with increase
of TSR reaction time, but never approached the initial
sulfate δ 34 S value. Hydrogen sulfide exhibits similar
fractionation as DBT at the earlier reaction time but
rapidly approach sulfate value. BT started with similar
fractionation as DBT but as the reaction proceeds it
has intermediates values between the DBT and H2S.
Experiments with 10 folds excess of sulfate have little
effect on the fractionation and the trend of reducing
fractionation with increasing TSR reaction time was
similar. The results for sodium and calcium sulfate
were close, suggesting that the dissolution of solid
CaSO4 was not a limiting factor. These results
suggest that other processes rather than the
concentration of sulfate, dominate the fractionation
during TSR. These processes may include isotopic
exchange between sulfate and it reduced forms that
mask the original kinetic isotope effect.
Experiments with elemental sulfur show small and
consistent δ 34 S fractionation between BT and initial S°
of -1 to -2.4 at all reaction times. This shows that the
δ 34 S recorded by the organosulfur compounds
reflecting the δ 34 S value of the TSR reduced sulfur
rather than isotope effect during their formation.
Therefore we can use organosulfur compounds as
markers for the δ 34 S of H2S derived from TSR.
The reaction with Na2SO3 yielded DBT and BT with
maximum fractionation of up to -9.6‰. As observed in
the experiments with sulfates, this fractionation
decreased rapidly with reaction time and BT changed
more rapidly than DBT. In this case H2S exhibit no
fractionation relative to Na2SO3 at all reaction times.
The fractionation during SO3 -2 reduction is surprisingly
large as the common assumption is that the most
significant fractionation occurs during the reduction of
SO4 -2 to SO3 -2 . The fractionation during SO3 -2
reduction could not be detected by H2S, because the
reaction is too rapid and mask the initial fractionation.
DBT can preserve the initial fractionation signal
because it stability and slow rate of formation. BT is
less stable and reflecting the cumulative δ 34 S values
during the course of reaction. These TSR markers
can therefore allow us to follow the initial steps of
TSR. Better understanding of these fractionations and
their kinetics will help us follow and understand more
closely TSR reaction mechanism.
References
Amrani, A., Sessions A.L., Adkins J. 2009.
Compound-specific δ 34 S analysis of volatile organics
by coupled GC/ICPMS. Analytical Chemistry 81,
9027-9034
Amrani, A., Zhang, T., Ma,Q., Ellis, G. S. ,Tang, Y.
2008. The role of labile sulfur compounds in
thermochemical sulfate reduction. Geochimica et
Cosmochimica Acta 72, 2960-2972.
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