158
S. Wakeham
lipids. Sterols were generally more stable than fatty acids and displayed the largest
reduction in degradation rate when oxygen was absent. These trends could help account for the fact that unsaturated lipids are particularly vulnerable to loss, and ratios of fatty acid:sterol decrease during diagenesis as observed in field studies
(e.g. Kawamura et al.1980; Sun and Wakeham 1994; Canuel and Martens 1996). Remarkably, degradation patterns for lipids common to the two phytoplankton used by Harvey
and Macko (1997) (Thallasiosira weissflogii and Synechococcus sp.) were not always
similar, suggesting that factors other than molecular structure might affect the degradation rate. Furthermore, the proportion of total lipids that could be identified and
quantified as individual compounds decreased with the degree of degradation, with
the result that only a small fraction (18-24%) of lipids could be identified after the
majority of pac and lipids were degraded. This loss of identifiable biochemicals has
been described previously (e.g. Wakeham et al.1997; Hedges et al. 2000), to the extent
that the origin, reactivity and fate of a large amount of OC remains obscure.
Diagenesis not only results in absolute loss of organic compounds via degradation,
but it is accompanied by changes in ratios of compounds as a function of their relative lability or due to in-growth of diagenetic products at the expense of precursors.
Organic matter degradation is accompanied, for example, by decreases in weight percentages of glucose among total aldoses, but increases in percentages of the deoxy
sugars, rhamnose and fucose (Hamilton and Hedges 1988; Cowie et al. 1992). Increases
in mole-percentages of the non-protein amino acids f3-alanine plus y-aminobutyric
acid (Henrichs et al. 1984; Cowie et al. 1995) also occur. In a detailed study of amino
acids in sediments from the Peru upwelling region, Henrichs et al. (1984) detected high
concentrations of p-aminoglutaric acid, a nonprotein isomer of glutamic acid. Although
a specific source was not identified, increasing ratios of f3-aminoglutaric acid:glutamic
acid with depth in sediment cores (Fig. 6.8) strongly suggested an in situ bacterial
source.
Among lipids, ratios of unsaturated to saturated fatty acids typically decrease down
core (Fig. 6.9), because as noted above, unsaturated fatty acids that are derived from
phytoplankton are more susceptible to degradation in sediments than are saturated
acids (Kawamura et al.1980; Sun and Wakeham 1994; Canuel and Martens 1996). From
profiles such as these, it is uncertain whether the unsaturated fatty acids are oxidized
partially and removed from the fatty acid "analytical window:' remineralized completely to CO2, or hydrogenated in situ to their unsaturated analogs. Branched-chain
fatty acids (e.g. iso- and anteiso-C 1S and C 17 ) that are common in bacterial cell membranes (Kaneda 1991) are often found to increase in relative abundance in sediments
as diagenesis progresses. Enrichments in branched-chain fatty acids are widely used
as indicators of bacterial alteration of organic matter (Perry et al. 1979). However, the
reliance on branched-chain fatty acids as bacterial indicators must be tempered by
the fact that many aerobic bacteria do not produce these compounds, so in situations
where aerobic microbial decomposition is intense, branched-chain fatty acids may not
be particularly abundant (Wakeham 1995). Branched-chain fatty acids at best may be
semiquantitative indicators of bacterial biomass but are poor indicators of bacterial
activity.
Sterols are popular biomarkers, because as a group they represent a wide variety of
molecular structures that are remarkably useful as source indicators and because alterations to the sterol skeleton can be readily followed with the analytical tools avail-
S. Wakeham
lipids. Sterols were generally more stable than fatty acids and displayed the largest
reduction in degradation rate when oxygen was absent. These trends could help account for the fact that unsaturated lipids are particularly vulnerable to loss, and ratios of fatty acid:sterol decrease during diagenesis as observed in field studies
(e.g. Kawamura et al.1980; Sun and Wakeham 1994; Canuel and Martens 1996). Remarkably, degradation patterns for lipids common to the two phytoplankton used by Harvey
and Macko (1997) (Thallasiosira weissflogii and Synechococcus sp.) were not always
similar, suggesting that factors other than molecular structure might affect the degradation rate. Furthermore, the proportion of total lipids that could be identified and
quantified as individual compounds decreased with the degree of degradation, with
the result that only a small fraction (18-24%) of lipids could be identified after the
majority of pac and lipids were degraded. This loss of identifiable biochemicals has
been described previously (e.g. Wakeham et al.1997; Hedges et al. 2000), to the extent
that the origin, reactivity and fate of a large amount of OC remains obscure.
Diagenesis not only results in absolute loss of organic compounds via degradation,
but it is accompanied by changes in ratios of compounds as a function of their relative lability or due to in-growth of diagenetic products at the expense of precursors.
Organic matter degradation is accompanied, for example, by decreases in weight percentages of glucose among total aldoses, but increases in percentages of the deoxy
sugars, rhamnose and fucose (Hamilton and Hedges 1988; Cowie et al. 1992). Increases
in mole-percentages of the non-protein amino acids f3-alanine plus y-aminobutyric
acid (Henrichs et al. 1984; Cowie et al. 1995) also occur. In a detailed study of amino
acids in sediments from the Peru upwelling region, Henrichs et al. (1984) detected high
concentrations of p-aminoglutaric acid, a nonprotein isomer of glutamic acid. Although
a specific source was not identified, increasing ratios of f3-aminoglutaric acid:glutamic
acid with depth in sediment cores (Fig. 6.8) strongly suggested an in situ bacterial
source.
Among lipids, ratios of unsaturated to saturated fatty acids typically decrease down
core (Fig. 6.9), because as noted above, unsaturated fatty acids that are derived from
phytoplankton are more susceptible to degradation in sediments than are saturated
acids (Kawamura et al.1980; Sun and Wakeham 1994; Canuel and Martens 1996). From
profiles such as these, it is uncertain whether the unsaturated fatty acids are oxidized
partially and removed from the fatty acid "analytical window:' remineralized completely to CO2, or hydrogenated in situ to their unsaturated analogs. Branched-chain
fatty acids (e.g. iso- and anteiso-C 1S and C 17 ) that are common in bacterial cell membranes (Kaneda 1991) are often found to increase in relative abundance in sediments
as diagenesis progresses. Enrichments in branched-chain fatty acids are widely used
as indicators of bacterial alteration of organic matter (Perry et al. 1979). However, the
reliance on branched-chain fatty acids as bacterial indicators must be tempered by
the fact that many aerobic bacteria do not produce these compounds, so in situations
where aerobic microbial decomposition is intense, branched-chain fatty acids may not
be particularly abundant (Wakeham 1995). Branched-chain fatty acids at best may be
semiquantitative indicators of bacterial biomass but are poor indicators of bacterial
activity.
Sterols are popular biomarkers, because as a group they represent a wide variety of
molecular structures that are remarkably useful as source indicators and because alterations to the sterol skeleton can be readily followed with the analytical tools avail-