Terrestrial Organic Matter in Marine Sediments
555
Microscopic Approaches
There has been much discussion on the relative
significance of the various geochemical and optical approaches of OM and kerogen typing (Powell
et al. 1982; Tissot and Welte 1984; Scnftle et al.
1993). Optical studies comprise organic petrography (maceral composition determined under incident white and fluorescent light) and palynology
(palynomorph association studied with translucent
light). Both methods register the composition and
diagenetic stage of individual organic particles either as macerals (Stach et al. 1982; Taylor et al.
1998) or as palynomorphs (Tyson 1995). Despite
the general concerns against quantitative optical
approaches, there is a significant number of studies covering the whole range of old, mature rocks
to modern, immature sediments showing that quantitative optical data convincingly correlate to bulk
geochemical proxies (e.g. Stein et al. 1986; Jones
1987; Wiesneret al. 1990; Littke 1993; Wagner and
Hi:ilemann 1995; Wagner subm.; for reviews see
Senftle et al. 1993, and Tyson 1995). Outlines and
interpretation of marine pollen data are discussed
by Dupont (this volume).
With regard to the scope of this study the following discussion concentrates on the application
of organic petrologic studies to young, immature
marine deposits. Particulate OM in marine
sediments is broadly dispersed and generally not
abundant. Identification and quantification of
small-sized organic particles (macerals) in marine
sediments is therefore restricted by various factors,
such as size, abundance and nature of macerals.
The recognition of individual macerals is limited by
the maximum resolution oflight microscopy (111m
particle diameter). Optical quantification of submicroscopic OM is hardly possible, although fluorescent submicroscopic OM is still recognizable as
"matrix fluorescence". Few studies of modern and
past marine settings enable a critical evaluation of
quantitative microscopic data (Wiesner et al. 1990;
Littke 1993; Wagner 1993; Littke and Sachsenhofer 1994, Littke et al. 1997a; Wagner 1998,
1999). Littke (1993) argues thatthe amount oftotal OM which is invisible, i.e. submicroscopical, can
be quantitatively assessed by comparison of total
counted macerals and TOC content. Due to the
average densities of organic and mineral matter an
approximate relation of2: 1 is expected ifTOC is
completely represented by visible POM (Scheidt
and Littke 1989). Similar relations are reported from
Triassic and Cretaceous deep-sea sediments off
NW Australia (Rullki:itter et al. 1992) and late
Quaternary sediments of the Norwegian-Greenland
Sea (Wagner 1993). Based on a comprehensive set
of microscopic data Littke (1993) concluded that
more than 80% of the total OM occurs in form of
discrete, optically resolvable particles in most marine sediments. On the other hand, this percentage
may be considerably smaller if marine OM dominates, i.e. <10% off Oman and Peru (ten Haven
et al. 1990; Uickge et al. 1996). It is interesting to
note, that the same range of about 80% submicroscopic OM was estimated using a sedimentological
approach (Wagner 1993; Fig. 3). Weight-balanced
relative TOC contents of the sediment fraction
<2l1m, separated by settling velocity using the density of quartz, were determined on a set of glacial!
intergalcial sediments from the Norwegian Sea.
The results indicated that 45-75% of the bulk
TOC are bound to the grain size fraction <211m.
However, since organic particles have
much lower average densities (1.2-1.6 g cm 3 ) than
quartz, grain-size equivalents were calculated for
the smallest visible particle size of vitrinite, i.e. I
11m particle diameter, according to Stockes's Law.
Following this approach, submicroscopic OM is
associated with quartz grain size equivalents <0.49
11m, which is about a quarter ofthe total fraction <
2 f!m. If, for simplicity, it is assumed that OM is
equally distributed within sub-grain size fractions
<211m, the total range of submicroscopic OM is calculated to be 14% to 18%, which is fairly close to
the estimate proposed by Littke (1993).
There is a second concern against quantitative
organic petrology which addresses the influence of
different particle sizes if macerals are not counted
as volume but as grain-%. Counting of grain-%
gives the proportion of each particle type relative
to the total particle composition without consideration of its volume. Hence, the bigger the proportion of large organic particles, the more relevant
becomes the aspect of volume. There is a dilemma
if typical deep-sea samples are analyzed: Due to
the low TOC contents of these deposits macerals
555
Microscopic Approaches
There has been much discussion on the relative
significance of the various geochemical and optical approaches of OM and kerogen typing (Powell
et al. 1982; Tissot and Welte 1984; Scnftle et al.
1993). Optical studies comprise organic petrography (maceral composition determined under incident white and fluorescent light) and palynology
(palynomorph association studied with translucent
light). Both methods register the composition and
diagenetic stage of individual organic particles either as macerals (Stach et al. 1982; Taylor et al.
1998) or as palynomorphs (Tyson 1995). Despite
the general concerns against quantitative optical
approaches, there is a significant number of studies covering the whole range of old, mature rocks
to modern, immature sediments showing that quantitative optical data convincingly correlate to bulk
geochemical proxies (e.g. Stein et al. 1986; Jones
1987; Wiesneret al. 1990; Littke 1993; Wagner and
Hi:ilemann 1995; Wagner subm.; for reviews see
Senftle et al. 1993, and Tyson 1995). Outlines and
interpretation of marine pollen data are discussed
by Dupont (this volume).
With regard to the scope of this study the following discussion concentrates on the application
of organic petrologic studies to young, immature
marine deposits. Particulate OM in marine
sediments is broadly dispersed and generally not
abundant. Identification and quantification of
small-sized organic particles (macerals) in marine
sediments is therefore restricted by various factors,
such as size, abundance and nature of macerals.
The recognition of individual macerals is limited by
the maximum resolution oflight microscopy (111m
particle diameter). Optical quantification of submicroscopic OM is hardly possible, although fluorescent submicroscopic OM is still recognizable as
"matrix fluorescence". Few studies of modern and
past marine settings enable a critical evaluation of
quantitative microscopic data (Wiesner et al. 1990;
Littke 1993; Wagner 1993; Littke and Sachsenhofer 1994, Littke et al. 1997a; Wagner 1998,
1999). Littke (1993) argues thatthe amount oftotal OM which is invisible, i.e. submicroscopical, can
be quantitatively assessed by comparison of total
counted macerals and TOC content. Due to the
average densities of organic and mineral matter an
approximate relation of2: 1 is expected ifTOC is
completely represented by visible POM (Scheidt
and Littke 1989). Similar relations are reported from
Triassic and Cretaceous deep-sea sediments off
NW Australia (Rullki:itter et al. 1992) and late
Quaternary sediments of the Norwegian-Greenland
Sea (Wagner 1993). Based on a comprehensive set
of microscopic data Littke (1993) concluded that
more than 80% of the total OM occurs in form of
discrete, optically resolvable particles in most marine sediments. On the other hand, this percentage
may be considerably smaller if marine OM dominates, i.e. <10% off Oman and Peru (ten Haven
et al. 1990; Uickge et al. 1996). It is interesting to
note, that the same range of about 80% submicroscopic OM was estimated using a sedimentological
approach (Wagner 1993; Fig. 3). Weight-balanced
relative TOC contents of the sediment fraction
<2l1m, separated by settling velocity using the density of quartz, were determined on a set of glacial!
intergalcial sediments from the Norwegian Sea.
The results indicated that 45-75% of the bulk
TOC are bound to the grain size fraction <211m.
However, since organic particles have
much lower average densities (1.2-1.6 g cm 3 ) than
quartz, grain-size equivalents were calculated for
the smallest visible particle size of vitrinite, i.e. I
11m particle diameter, according to Stockes's Law.
Following this approach, submicroscopic OM is
associated with quartz grain size equivalents <0.49
11m, which is about a quarter ofthe total fraction <
2 f!m. If, for simplicity, it is assumed that OM is
equally distributed within sub-grain size fractions
<211m, the total range of submicroscopic OM is calculated to be 14% to 18%, which is fairly close to
the estimate proposed by Littke (1993).
There is a second concern against quantitative
organic petrology which addresses the influence of
different particle sizes if macerals are not counted
as volume but as grain-%. Counting of grain-%
gives the proportion of each particle type relative
to the total particle composition without consideration of its volume. Hence, the bigger the proportion of large organic particles, the more relevant
becomes the aspect of volume. There is a dilemma
if typical deep-sea samples are analyzed: Due to
the low TOC contents of these deposits macerals