CHAPTER 6 . Diagenesis of Organic Matter at the Water-Sediment Interface
149
that low bottom water oxygen concentrations retard decomposition, which then leads
to accumulation of OC in sediments. The relative importance of these two factors -
the supply of OC to the sea floor vs. bottom water oxygen concentration - has been
hotly debated (e.g. Emerson and Hedges 1988; Calvert and Pedersen 1992; Lee 1992;
Paropkari et al. 1992), and the debate has yet to be resolved (e.g. Keil and Cowie 1999;
Cowie et al.1999; Granesham et al.1999). Nonetheless, there is reason to expect a coupling between the OC rain rate and water column oxygen, since higher availability of
OC that can be remineralized leads to greater consumption of oxygen that produces
suboxia and anoxia. An additional factor may be involved. Tegelaar et al. (1989) proposed that refractory macromolecules comprise the bulk of preserved organic matter. By this hypothesis, it is this recalcitrant macromolecular material that dominates
the sedimentary OC that cannot be characterized at the molecular level (Wakeham
et al. 1997; Hedges et al. 2000). In support of this concept is increasing evidence for a
variety of biomacromolecules in algae and vascular plants that are resistant both to
biodegradation and chemical analyses (de Leeuw and Largeau 1993). Thus, iliere may
be molecular -structural factors iliat influence ilie intrinsic reactivity of organic substances.
Bacteria are major primary agents of early diagenesis (Deming and Baross 1993).
They contain unique and versatile enzymes that can act on labile and refractory organic matter to satisfy their nutritional requirements. Acting in concert with each oilier
and higher organisms, they produce powerful degradative systems beyond those of
individual organisms. The classic description of diagenesis involves a stratification of
sediments according to available oxidants that bacteria utilize in degrading organic
matter to produce energy (Fig. 6.2). In reality, the situation is complicated by the fact
that physiologically different types of bacteria degrade different classes of organic
matter using specific oxidants, and there is considerable overlap and interaction of
the zonation of the various respiratory functions, especially on the microscale. Because
bacteria are unable to assimilate molecules larger than about 600 daltons, they must
first hydrolyze complex molecules into simpler ones that can cross cellular membranes.
End products from one functional group of bacteria are often substrates to be used
by other groups in complex symbiotic relationships.
Recent research has expanded the number of mechanisms that may be involved in
organic matter diagenesis to help explain changes in biochemical composition that
occur during decomposition and preservation (reviewed by Hedges and Keil1995). A
sorption preservation hypothesis has been developed in which intimate association
of organic matter with mineral grains protects OC from degradation (Mayer 1994a,b;
Keil et al. 1994a, and earlier papers cited therein). Sorption of organic matter to mineral surfaces occurs in nature and is widely used to explain both the behaviour of hydrophobic xenobiotics in nature and why otherwise labile compounds are not always
rapidly degraded in sediments. The new findings show that there is covariation of %OC
and sediment surface area (SA) for surface sediments from a wide variety of continental margin environments (Fig. 6.3), suggesting stabilization of intrinsically labile
organic matter if it is intimately associated with mineral grains. Mayer (1994a,b) originally hypothesized that organic matter coats mineral grains in continental margin
sediments in a uniform "monolayer equivalent" coating of 0.5-1.0 mg OC m- 2 • Sediments from non-continental margin areas have different %OCISA relationships. Some
fraction of the organic matter might be found in mesopores that protect it from degradation, because the mesopores are smaller than the hydrolytic exoenzymes released
149
that low bottom water oxygen concentrations retard decomposition, which then leads
to accumulation of OC in sediments. The relative importance of these two factors -
the supply of OC to the sea floor vs. bottom water oxygen concentration - has been
hotly debated (e.g. Emerson and Hedges 1988; Calvert and Pedersen 1992; Lee 1992;
Paropkari et al. 1992), and the debate has yet to be resolved (e.g. Keil and Cowie 1999;
Cowie et al.1999; Granesham et al.1999). Nonetheless, there is reason to expect a coupling between the OC rain rate and water column oxygen, since higher availability of
OC that can be remineralized leads to greater consumption of oxygen that produces
suboxia and anoxia. An additional factor may be involved. Tegelaar et al. (1989) proposed that refractory macromolecules comprise the bulk of preserved organic matter. By this hypothesis, it is this recalcitrant macromolecular material that dominates
the sedimentary OC that cannot be characterized at the molecular level (Wakeham
et al. 1997; Hedges et al. 2000). In support of this concept is increasing evidence for a
variety of biomacromolecules in algae and vascular plants that are resistant both to
biodegradation and chemical analyses (de Leeuw and Largeau 1993). Thus, iliere may
be molecular -structural factors iliat influence ilie intrinsic reactivity of organic substances.
Bacteria are major primary agents of early diagenesis (Deming and Baross 1993).
They contain unique and versatile enzymes that can act on labile and refractory organic matter to satisfy their nutritional requirements. Acting in concert with each oilier
and higher organisms, they produce powerful degradative systems beyond those of
individual organisms. The classic description of diagenesis involves a stratification of
sediments according to available oxidants that bacteria utilize in degrading organic
matter to produce energy (Fig. 6.2). In reality, the situation is complicated by the fact
that physiologically different types of bacteria degrade different classes of organic
matter using specific oxidants, and there is considerable overlap and interaction of
the zonation of the various respiratory functions, especially on the microscale. Because
bacteria are unable to assimilate molecules larger than about 600 daltons, they must
first hydrolyze complex molecules into simpler ones that can cross cellular membranes.
End products from one functional group of bacteria are often substrates to be used
by other groups in complex symbiotic relationships.
Recent research has expanded the number of mechanisms that may be involved in
organic matter diagenesis to help explain changes in biochemical composition that
occur during decomposition and preservation (reviewed by Hedges and Keil1995). A
sorption preservation hypothesis has been developed in which intimate association
of organic matter with mineral grains protects OC from degradation (Mayer 1994a,b;
Keil et al. 1994a, and earlier papers cited therein). Sorption of organic matter to mineral surfaces occurs in nature and is widely used to explain both the behaviour of hydrophobic xenobiotics in nature and why otherwise labile compounds are not always
rapidly degraded in sediments. The new findings show that there is covariation of %OC
and sediment surface area (SA) for surface sediments from a wide variety of continental margin environments (Fig. 6.3), suggesting stabilization of intrinsically labile
organic matter if it is intimately associated with mineral grains. Mayer (1994a,b) originally hypothesized that organic matter coats mineral grains in continental margin
sediments in a uniform "monolayer equivalent" coating of 0.5-1.0 mg OC m- 2 • Sediments from non-continental margin areas have different %OCISA relationships. Some
fraction of the organic matter might be found in mesopores that protect it from degradation, because the mesopores are smaller than the hydrolytic exoenzymes released