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Objectives: The
broad objectives are to understand the geochemical mechanisms
of sulfur incorporation into sedimentary organic matter,
to study the changes in the abundance and forms of sulfur
during early diagenesis in various organic-rich
sedimentary environments, and to examine the influence of
sulfur incorporation in the preservation of sedimentary
organic matter. Project Description: Sulfur is
believed to be involved in preserving organic matter in
sediments, in converting this organic matter to
petroleum, and in controlling the timing of petroleum generation
from a source rock. The fundamental geochemical issue in
this matter is the mechanism of sulfur incorporation into
sedimentary organic matter. Although there is compelling evidence
to indicate that reactions of reduced inorganic sulfur
species with organic molecules occur during the early stages
of diagenesis and under very mild conditions, the
molecular mechanisms are still not well understood.
Previously, hydrogen sulfide has been considered to be
the primary sulfur reactant for organic sulfur formation
in sediments. Recently, evidence is emerging that H2S oxidation
products, especially polysulfides, are also important.
The roles of elemental sulfur, sulfite, and thiosulfate
in the formation of organic sulfur are still not clear.
This project, which is aimed at understanding the formation
and transformation of sedimentary organic sulfur during
early diagenesis, has four major components: (1) studies
of sulfur speciation in sediments, (2) mechanistic
studies of organic sulfur formation, (3) mechanistic studies
of sulfide oxidation to understand the formation of
different oxidation intermediates, and (4) analytical
methods development. An important goal of this project is
to use synchrotron-radiation-based X-ray absorption near-
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edge-structure
(XANES) spectroscopy for characterization and
determination of sulfur species, in addition to other state-of-the-art
techniques. Results: In continuing efforts to a
fuller understanding of the role of various sulfur
nucleophiles in incorporating sulfur into sedimentary
organic matter, the reactions of thiosulfate with
different model organic compounds were studied.
Thiosulfate is a major partial-oxidation product
generated from the oxidation of H2S in marine sediments.
Although biological pathways of thiosulfate
transformation have been recognized, chemical pathways
are still not clear. Our results show that an organic
thiosulfate (R-S2O 3-)
is formed from the reaction of thiosulfate with most
activated unsaturated organic molecules other than
carbonyl compounds; however, with alpha,beta unsaturated
carbonyl compounds (both unsaturated aldehydes and
ketones), a sulfonate and not an organic thiosulfate is
generated as the end product. The formation of sulfonates
represents a novel, hitherto unrecognized reaction
pathway involving thiosulfate. A possible mechanism may involve
the addition of thiosulfate to the alpha-carbon through the
outer sulfur atom, forming a cyclic intermediate first,
which then dissociates with the attachment of the
sulfonate-sulfur to the ß-carbon and the release of the
outer sulfur as elemental sulfur. The reaction is highly pH
dependent as the type of products formed and the kinetics
of the reaction are changed conspicuously with varying pH.
The mechanism of this reaction is currently under study
using FT-IR and NMR spectroscopic methods in addition to
XANES spectroscopy. The thiosulfate carbonyls reaction
could be an important geochemical pathway for forming
sulfonates in marine sediments.
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