CHAPTER 4 . Sedimentary Organic Matter Preservation and Atmospheric O2 Regulation
113
KeilI99S). Bioturbation sparingly affects OET, because particles should be mixed into
and out of the thin oxic layer of continental margin sediments with comparable probabilities. OET incorporates the sediment accumulation rate as a prime determinant
and is a direct indicator of oxic exposure within a sediment, as opposed to a remote
proxy such as bottom water O2 concentration (Betts and Holland 1991). The first published test of this hypothesis was the demonstration by Hartnett et al. (1998) that the
efficiency of organic carbon burial in surface sediments along the western Pacific
margin is indirectly related to the log of OET, as would be expected for an oxic
remineralization mechanism exhibiting first-order kinetics. This study indicated that
burial efficiency (preservation rate divided by delivery rate to the sea floor) appears
to be sensitive to oxic conditions on time scales of weeks to months, and thus that oxic
degradation is important on short as well as long time scales.
A more detailed study of the role of O2 in organic matter preservation was later
carried out for sediments depositing along a transect off the Washington State coast,
USA (Hedges et al.1999). In this study, %OC, surface area, C/N, biochemical (individual
amino acids, and carbohydrates) compositions and O 2 penetration depths were measured in 16 sediment cores. For six of these cores, 013C compositions and 14C-based
deposition rates were also determined. The measured elemental and stable carbon
isotope compositions both indicate that all these sediments are predominantly marine derived and compositionally uniform with depth. Due to off-shore increases in
O 2 penetration depth and attending decreases in sediment accumulation rates, OETs
increase exponentially off-shore in the six 14C-dated cores (Fig. 4.6a). Exposure periods ranged from decades on the continental shelf and upper continental slope, to hundreds of years on the lower continental slope, to approximately 1 000 years at the most
off-shore sampling site. A corresponding plot of OC/SA vs. log OET gives a fit to a
straight line with a slope corresponding to an approximate half-life of 150 years
(Fig. 4.7). Although this result is largely constrained by the intrinsic time scale represented by the studied cores (Middelburg et al. 1993), a major fraction of the organic
matter in these sediments appears to be remineralized under oxic conditions with a
time constant on the order of 100 years.
To more extensively test the inference of progressive off-shore oxic degradation,
the freshness of the organic matter in these deposits was tested by two different biochemical indicators and by an assessment of the percentage of total pollen grains showing physical evidence of degradation. Both molecular indicators of organic matter
"freshness" (%(BALA + GABA), and the percent of glucose within the aldose suite)
indicated that the remnant organic matter in farther off-shore sediments is more degraded. In contrast, progressive OC depletion and degradation were not observed
downcore at individual sites. Organic matter degradation was therefore effectively
complete below the shallow «3 cm) oxic surface horizons of these deposits. These
observations indicate that most of the sedimentary organic matter remaining along
the Washington continental margin is susceptible to remineralization under oxic conditions, but is degraded slowly, if at all, in the absence of molecular O2, This finding,
and parallel results from the MAP turbidites, point towards a consistent oxic effect
over a range of time scales. Such "02 sensitivity" does not necessarily hold for all organic matter, most of which (e.g. polysaccharides and proteins) is easily fermented
and rapidly remineralized regardless of redox conditions (e.g. Lee 1992; Canfield 1989,
1994). Oxygen sensitive organic matter appears to concentrate in the latter stages of
113
KeilI99S). Bioturbation sparingly affects OET, because particles should be mixed into
and out of the thin oxic layer of continental margin sediments with comparable probabilities. OET incorporates the sediment accumulation rate as a prime determinant
and is a direct indicator of oxic exposure within a sediment, as opposed to a remote
proxy such as bottom water O2 concentration (Betts and Holland 1991). The first published test of this hypothesis was the demonstration by Hartnett et al. (1998) that the
efficiency of organic carbon burial in surface sediments along the western Pacific
margin is indirectly related to the log of OET, as would be expected for an oxic
remineralization mechanism exhibiting first-order kinetics. This study indicated that
burial efficiency (preservation rate divided by delivery rate to the sea floor) appears
to be sensitive to oxic conditions on time scales of weeks to months, and thus that oxic
degradation is important on short as well as long time scales.
A more detailed study of the role of O2 in organic matter preservation was later
carried out for sediments depositing along a transect off the Washington State coast,
USA (Hedges et al.1999). In this study, %OC, surface area, C/N, biochemical (individual
amino acids, and carbohydrates) compositions and O 2 penetration depths were measured in 16 sediment cores. For six of these cores, 013C compositions and 14C-based
deposition rates were also determined. The measured elemental and stable carbon
isotope compositions both indicate that all these sediments are predominantly marine derived and compositionally uniform with depth. Due to off-shore increases in
O 2 penetration depth and attending decreases in sediment accumulation rates, OETs
increase exponentially off-shore in the six 14C-dated cores (Fig. 4.6a). Exposure periods ranged from decades on the continental shelf and upper continental slope, to hundreds of years on the lower continental slope, to approximately 1 000 years at the most
off-shore sampling site. A corresponding plot of OC/SA vs. log OET gives a fit to a
straight line with a slope corresponding to an approximate half-life of 150 years
(Fig. 4.7). Although this result is largely constrained by the intrinsic time scale represented by the studied cores (Middelburg et al. 1993), a major fraction of the organic
matter in these sediments appears to be remineralized under oxic conditions with a
time constant on the order of 100 years.
To more extensively test the inference of progressive off-shore oxic degradation,
the freshness of the organic matter in these deposits was tested by two different biochemical indicators and by an assessment of the percentage of total pollen grains showing physical evidence of degradation. Both molecular indicators of organic matter
"freshness" (%(BALA + GABA), and the percent of glucose within the aldose suite)
indicated that the remnant organic matter in farther off-shore sediments is more degraded. In contrast, progressive OC depletion and degradation were not observed
downcore at individual sites. Organic matter degradation was therefore effectively
complete below the shallow «3 cm) oxic surface horizons of these deposits. These
observations indicate that most of the sedimentary organic matter remaining along
the Washington continental margin is susceptible to remineralization under oxic conditions, but is degraded slowly, if at all, in the absence of molecular O2, This finding,
and parallel results from the MAP turbidites, point towards a consistent oxic effect
over a range of time scales. Such "02 sensitivity" does not necessarily hold for all organic matter, most of which (e.g. polysaccharides and proteins) is easily fermented
and rapidly remineralized regardless of redox conditions (e.g. Lee 1992; Canfield 1989,
1994). Oxygen sensitive organic matter appears to concentrate in the latter stages of