4
Organic Matter: The Driving Force for Early Diagenesis
150
with the total amount of organic matter arriving at the
sediment-water interface. As a consequence, the sedimentary pyrite sulfide content is positively correlated
with the non-metabolized (resistant or unused) organic
matter content (TOC). The trendline in Figure 4.14 is
considered representative of normal marine (oxic)
environments. Data from the Black Sea plot above the
trendline (higher S/C ratios) because consumption of
organic matter by sulfate-reducing bacteria leads to
excess hydrogen sulfide, available for pyrite formation,
in the water column. In contrast to this, freshwater
sediments have very low S/C (or high C/S) ratios
because of the low sulfate concentrations in most
freshwater bodies. Although the trendline is based on
pyrite sulfur, it is not important whether the reduced
sulfur is present as metal sulfide (mostly pyrite) or
bound to the organic matter. This is a question of iron
limitation rather than sulfate reduction.
As outlined in Section 4.3.1 (and discussed more
extensively in Chap. 8) there is a close connection
between organic matter remineralization during early
diagenesis and microbial sulfate reduction. If all of the
sulfate reduction products were precipitated as pyrite
or bound into (immobile) organic matter, measuring
the amount of sulfur in these species would provide
an easy way for determining the amount of organic
matter that has been remineralized and was not
preserved in the sediment. However, the main product
of sulfate reduction, hydrogen sulfide, is volatile and
can escape from the sediment, particularly when
bioturbation of the surface sediment supports this
transport.
Release of hydrogen sulfide from the sediment
plays a less important role under strictly anoxic
conditions where fine lamination indicates that the
environment is hostile to burrowing organisms and
bioturbation does not occur. It has been shown that in
these cases the initial amount of organic matter
deposited can be estimated by measuring concentrations of reduced sulfur in such sediments. Considering the amount of organic matter consumed by
sulfate reduction, Lallier-Vergès (1993) defined a sulfate
reduction index (SRI) as
SRI = % initial organic carbon /
% preserved organic carbon
(4.9)
The amount of initial organic carbon then is the
sum of the preserved organic carbon (measured as
TOC) and the metabolized organic carbon (determined
from the sulfur content with stoichiometric correction
for the sulfate reduction process). Furthermore, the
diffusive loss has to be taken into account. With a
correction factor of 0.75 and a term 1/(1-qH 2 S) for the
diffusive loss, Vetö et al. (1994) calculated the initial (or
original) organic carbon content of a sediment before
sulfate reduction as
TOC orig = TOC + 0.75S · 1/(1-qH 2 S)
(4.10)
where TOC and S are the measured values of total
organic carbon and total sulfur. The authors estimate
that the diffusive H 2 S loss in non-bioturbated
sediments usually is less than 45 % and that this value
can only be reached in cases of very high organic
matter supply, high reactivity of this organic matter
and iron limitation. Littke et al. (1991b) and Lückge et
al. (1996) calculated that sulfate reduction consumed
between 20 and 50 % of the initially sedimented organic
matter (or 1-3 % of primary productivity) both from
ancient rocks (Posidonia Shale) and recent sediments
(Oman Margin and Peru upwelling systems). In a study
of the Pakistan continental margin in the northern
Arabian Sea (Littke et al. 1997b), they clearly demonstrated that the carbon-sulfur relationship only holds
in the laminated sections of the sediment profile
whereas it fails (strongly underestimates sulfate reduction) in the intercalated homogeneous, i.e. bioturbated,
sediments for the reason explained before.
4.4.2
Marine Versus Terrigenous
Organic Matter
As pointed out in Section 4.2.5, even deep-sea
sediments deposited in areas remote from continents
may contain a mixture of marine and terrigenous organic
matter. For any investigation of autochthonous marine
organic matter preservation or marine paleoproductivity,
these two sources of organic matter have to be
distinguished. Furthermore, global or regional climate
fluctuations have changed the pattern of continental
run-off and ocean currents in the geological past (see
Sect. 4.4.3). Being able to recognize variations in marine
and terrigenous organic matter proportions may, thus,
be of great significance in paleoclimatic and paleoceanographic studies.
A variety of parameters are used to assess organic
matter sources. Bulk parameters have the advantage
that they are representative of total organic matter,
whereas molecular parameters address only part of the
extractable organic matter, which in turn is only a small
portion of total organic matter. Some successful applications of molecular parameters show that the small
bitumen fraction may be representative of the total,
but there are many other examples where this is not the
case. On the other hand, oxidation of marine organic
Organic Matter: The Driving Force for Early Diagenesis
150
with the total amount of organic matter arriving at the
sediment-water interface. As a consequence, the sedimentary pyrite sulfide content is positively correlated
with the non-metabolized (resistant or unused) organic
matter content (TOC). The trendline in Figure 4.14 is
considered representative of normal marine (oxic)
environments. Data from the Black Sea plot above the
trendline (higher S/C ratios) because consumption of
organic matter by sulfate-reducing bacteria leads to
excess hydrogen sulfide, available for pyrite formation,
in the water column. In contrast to this, freshwater
sediments have very low S/C (or high C/S) ratios
because of the low sulfate concentrations in most
freshwater bodies. Although the trendline is based on
pyrite sulfur, it is not important whether the reduced
sulfur is present as metal sulfide (mostly pyrite) or
bound to the organic matter. This is a question of iron
limitation rather than sulfate reduction.
As outlined in Section 4.3.1 (and discussed more
extensively in Chap. 8) there is a close connection
between organic matter remineralization during early
diagenesis and microbial sulfate reduction. If all of the
sulfate reduction products were precipitated as pyrite
or bound into (immobile) organic matter, measuring
the amount of sulfur in these species would provide
an easy way for determining the amount of organic
matter that has been remineralized and was not
preserved in the sediment. However, the main product
of sulfate reduction, hydrogen sulfide, is volatile and
can escape from the sediment, particularly when
bioturbation of the surface sediment supports this
transport.
Release of hydrogen sulfide from the sediment
plays a less important role under strictly anoxic
conditions where fine lamination indicates that the
environment is hostile to burrowing organisms and
bioturbation does not occur. It has been shown that in
these cases the initial amount of organic matter
deposited can be estimated by measuring concentrations of reduced sulfur in such sediments. Considering the amount of organic matter consumed by
sulfate reduction, Lallier-Vergès (1993) defined a sulfate
reduction index (SRI) as
SRI = % initial organic carbon /
% preserved organic carbon
(4.9)
The amount of initial organic carbon then is the
sum of the preserved organic carbon (measured as
TOC) and the metabolized organic carbon (determined
from the sulfur content with stoichiometric correction
for the sulfate reduction process). Furthermore, the
diffusive loss has to be taken into account. With a
correction factor of 0.75 and a term 1/(1-qH 2 S) for the
diffusive loss, Vetö et al. (1994) calculated the initial (or
original) organic carbon content of a sediment before
sulfate reduction as
TOC orig = TOC + 0.75S · 1/(1-qH 2 S)
(4.10)
where TOC and S are the measured values of total
organic carbon and total sulfur. The authors estimate
that the diffusive H 2 S loss in non-bioturbated
sediments usually is less than 45 % and that this value
can only be reached in cases of very high organic
matter supply, high reactivity of this organic matter
and iron limitation. Littke et al. (1991b) and Lückge et
al. (1996) calculated that sulfate reduction consumed
between 20 and 50 % of the initially sedimented organic
matter (or 1-3 % of primary productivity) both from
ancient rocks (Posidonia Shale) and recent sediments
(Oman Margin and Peru upwelling systems). In a study
of the Pakistan continental margin in the northern
Arabian Sea (Littke et al. 1997b), they clearly demonstrated that the carbon-sulfur relationship only holds
in the laminated sections of the sediment profile
whereas it fails (strongly underestimates sulfate reduction) in the intercalated homogeneous, i.e. bioturbated,
sediments for the reason explained before.
4.4.2
Marine Versus Terrigenous
Organic Matter
As pointed out in Section 4.2.5, even deep-sea
sediments deposited in areas remote from continents
may contain a mixture of marine and terrigenous organic
matter. For any investigation of autochthonous marine
organic matter preservation or marine paleoproductivity,
these two sources of organic matter have to be
distinguished. Furthermore, global or regional climate
fluctuations have changed the pattern of continental
run-off and ocean currents in the geological past (see
Sect. 4.4.3). Being able to recognize variations in marine
and terrigenous organic matter proportions may, thus,
be of great significance in paleoclimatic and paleoceanographic studies.
A variety of parameters are used to assess organic
matter sources. Bulk parameters have the advantage
that they are representative of total organic matter,
whereas molecular parameters address only part of the
extractable organic matter, which in turn is only a small
portion of total organic matter. Some successful applications of molecular parameters show that the small
bitumen fraction may be representative of the total,
but there are many other examples where this is not the
case. On the other hand, oxidation of marine organic