[AMERICAN JOURNAL
OF
SCIENCE, VOL. 299, SEPT., OCT., NOV., 1999, P. 529–555]
SEDIMENTARY ORGANIC MATTER PRESERVATION: A TEST FOR
SELECTIVE DEGRADATION UNDER OXIC CONDITIONS
JOHN I. HEDGES*, FENG SHENG HU**, ALLAN H. DEVOL*,
HILAIRY E. HARTNETT*, ELIZABETH TSAMAKIS*, and RICHARD G. KEIL*
ABSTRACT. We report here a test of the hypothesis that the extent of organic
matter preservation in continental margin sediments is controlled by the average
period accumulating particles reside in oxic porewater immediately beneath the
water/sediment interface. Oxygen penetration depths, organic element compositions, and mineral surface areas were determined for 16 sediment cores collected
along an offshore transect across the Washington continental shelf, slope, and
adjacent Cascadia Basin. Individual amino acid, sugar, and pollen distributions
were analyzed for a 11 to 12 cm horizon from each core, and 14C-based sediment
accumulation rates and stable carbon isotope compositions were determined
from depth profiles within a subset of six cores from representative sites. Sediment accumulation rates decreased, and dissolved O2 penetration depths increased offshore along the sampling transect. As a result, oxygen exposure times
(OET) increased seaward from decades (mid-shelf and upper slope) to more than
a thousand years (outer Cascadia Basin). Organic contents and compositions were
essentially constant within individual sediment cores but varied consistently with
location. In particular, organic carbon/surface area ratios decreased progressively offshore and with increasing OET. Three independent compositional
parameters demonstrated that the remnant organic matter in farther offshore
sediments is more degraded. Both concentration and compositional patterns
indicated that sedimentary organic matter exhibits a distinct and reproducible
‘‘oxic effect.’’ OET helps integrate and explain organic matter preservation in
accumulating continental margin sediments and hence provides a useful tool for
assessing transfer of organic matter from the biosphere to the geosphere.
INTRODUCTION
Burial of organic matter in marine sediments directly links the global cycles of
carbon, oxygen, and sulfur over geologic time (Berner, 1982, 1989). The organic matter
preserved in sedimentary deposits is important as a progenitor of fossil fuels, a recorder
of Earth history, and the ultimate source of essentially all atmospheric O2 (Berner and
Canfield, 1989; Engel and Macko, 1993; Hunt, 1996). Although continental margins
account for only ⬃10 percent of total ocean area and 20 percent of total ocean primary
production (Killops and Killops, 1993), greater than 90 percent of all organic carbon
burial occurs in sediments depositing on deltas, continental shelves, and upper continental slopes (Berner, 1989). Thus, to understand the processes and environmental conditions that control organic matter burial on a global basis, it is fundamentally important to
study preservation along continental margins.
The question of what variables control organic matter preservation in marine
sediments is one of the most complex and controversial issues in contemporary
biogeochemistry (Berner, 1980; Calvert and Pedersen, 1992; Canfield, 1994; Emerson
and Hedges, 1988; Hedges and Keil, 1995; Henrichs, 1992; Henrichs and Reeburgh,
1987). Suggested important factors include primary production rate (Calvert and Pedersen, 1992), water column depth (Smith, 1978; Suess, 1980), organic matter sources
(Hedges, Clark, and Cowie, 1988; Schubert and Stein, 1996) and reaction histories
(Berner and Westrich, 1985), sediment transport processes (Lückge and others, 1996;
Pedersen, Shimmield, and Price, 1992), sediment accumulation rate (Henrichs, 1992;
Müller and Suess, 1979), bottom water oxygen concentration (Betts and Holland, 1991;
Demaison and Moore, 1980; Richards and Redfield, 1954), availability of O2 (Cai and
* School of Oceanography, Box 357940, University of Washington, Seattle, Washington 98195-7940
** Department of Plant Biology, 265 Morrill Hall, University of Illinois, 505 South Goodwin Avenue,
Urbanna, Illinois 61801
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John I. Hedges and others—Sedimentary organic matter preservation
Reimers, 1995; Hulthe, Hulth, and Hall, 1998; Reimers and others, 1992) and other
dissolved electron acceptors (Brandes and Devol, 1995; Soetaert, Herman, and Middelburg, 1996), cometabolization (Canfield, 1994), microbial dynamics (Lee, 1992; Smith,
Walsh, and Jahnke, 1992), mixing and irrigation by macrobenthos (Aller, 1982), redox
fluctuations (Aller, 1994; Aller and others, 1996), and sorption (Keil and others, 1994a;
Mayer, 1994a,b; Ransom and others, 1998). Identifying direct ‘‘causative’’ effects on
sedimentary organic carbon preservation that can be assigned to a particular process or
environmental condition is particularly challenging because many of the above factors
are interdependent and covary complexly in patterns that shift nonlinearly over time
(Middelburg, Klug, and van der Nat, 1993) and space (Jahnke, 1996; Reimers and others,
1992).
Here we report a specific test of the hypothesis that degradation under oxic
conditions controls long-term preservation of organic matter in sediments accumulating
along continental margins (Hedges and Keil, 1995). It has long been recognized that
open ocean sediments depositing beneath deep water columns are poor in organic
matter, with typical mass-normalized percentages of organic carbon (percent OC) in the
range of 0.1 to 0.3 (Premuzic and others, 1982). These organic concentrations are
roughly 10 percent of those observed in sediments of similar grain size depositing along
continental margins, a pattern that also holds when OC contents of sediments are
normalized to specific surface-area, as opposed to mass (Hedges and Keil, 1995; Weiler
and Mills, 1965). As a result of this extreme depletion in OC, coupled with slow overall
accumulation rates, pelagic sediments contribute less than 5 percent of modern-day
global organic carbon preservation (Berner, 1989).
While there are many explanations for the low OC contents of offshore sediments,
surface-oxidized marine turbidites demonstrate that long-term exposure to molecular
oxygen is sufficient alone to cause such depletions. Numerous deep-sea turbidites have
now been described (Thomson and others, 1993). The relict f-turbidite from the Madeira
Abyssal Plain (MAP) region off Northwest Africa (de Lange, Jarvis, and Kuijpers, 1987;
Weaver and Rothwell, 1987), however, has been best characterized organically (Prahl
and others, 1989, 1997). This initially homogeneous turbidite was emplaced about
140,000 yrs before present and then subjected to in situ oxidation for the first ⬃10,000
yrs after deposition (Buckley and Cranston, 1988). O2 eventually diffused ⬃0.5 m deep
into the ⬃4-m thick deposit. Comparative analyses of sedimentary horizons above
(oxidized, 0.1-0.2 percent OC) and below (unoxidized, 0.9-1.0 percent OC) the redox
boundary in two separate MAP cores indicated that ⬃80 percent of the original OC
(Cowie and others, 1995) and all optically recognizable pollen (Keil and others, 1994b)
were destroyed during long-term oxic exposure. Organic matter below the redox
boundary was moderately degraded, as is typical of upper continental margin deposits
(Cowie and others, 1995). In contrast, organic materials in the oxidized interval were
heavily degraded and compositionally resembled mixtures from deep-sea sediments
(Cowie and Hedges, 1994; Hedges and Keil, 1995).
Based on parallel offshore trends in pollen and organic contents along the continental margin of Washington State (Keil and others, 1994c, 1998) and other regions
(Premuzic and others, 1982; Reimers and others, 1992), it has been hypothesized that
oxic degradation also limits organic matter preservation in incrementally-depositing
surface marine sediments (Hedges and Keil, 1995; Reimers, 1989). The logic behind this
premise is that the average time of exposure of sedimentary organic matter to oxic
conditions in the surface of depositing sediments corresponds roughly to the depth of
penetration of O2 into porewaters divided by the average sediment accumulation rate at
the site (Hedges and Keil, 1995). Because O2 penetration depths generally increase
offshore, as sediment accumulation rates become slower, it follows that in situ O2
exposure times should increase farther offshore. In analogy to radiochemical decay, a
a test for selective degradation under oxic conditions
531
linear increase in time of exposure to porewater O2 should cause an exponential
decrease in the OC content of the sediment accumulating below the oxic surface horizon
(Hedges and Keil, 1995). Thus, offshore decreases in sedimentary organic content along
continental margins should be pronounced. This hypothesis was recently supported by
the observation that the burial efficiencies of OC in surface marine sediments accumulating off the coasts of Washington, California, and Mexico decrease sharply as O2
exposure times increase from days to millennia (Hartnett and others, 1998). This first
publication, however, did not include information on the depositional settings, sources,
vertical profiles, and surface area loadings of the sedimentary organic matter. More
critically, molecular and pollen data were not presented to test whether organic remains
in more slowly depositing offshore sediments are in fact more degraded. We present here
a comprehensive study of organic matter degradation in sediments depositing along the
Washington State margin and discuss the implications of strong evidence for more
extensive degradation in the presence of O2.
STUDY SITE
Patterns of sediment distribution along the convergent plate margin of Washington
State generally parallel the coastline in response to local input and transport processes
(Gross and others, 1972). The southern continental shelf (⬃0-200 m water depth) is 25 to
60 km wide and incised by marine canyons (fig. 1). The mid-shelf (⬃100 m) is covered by
a silt deposit that separates OC-poor modern sands inshore and relict sands offshore
(Nittrouer and Sternberg, 1981). Sediments in this region derive primarily from the
Columbia River (White, 1970) and are carried northward parallel to the shore by
resuspension during winter storms (Kachel and Smith, 1989). Finer silt- and clay-size
particles are winnowed out and transported offshore to the continental slope and
Cascadia Basin (fig. 1). The local continental slope (⬃200-2500 m) is contacted by a
Fig. 1. Washington margin study region. Circled numbers indicate that 14C-based sediment accumulation
rates were determined for that site.
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John I. Hedges and others—Sedimentary organic matter preservation
pronounced O2 minimum (⬍15 percent saturation) between water depths of ⬃500 to
2000 m (Carpenter, 1987). Slope sediments in this region are relatively rich in organic
carbon (⬎2 percent OC) and accumulate more slowly than on the adjacent shelf
(Carpenter and Peterson, 1989). Surface sediments of Cascadia Basin (⬃2500-3000 m)
are gradually depositing hemipelagic muds that contain less pollen and bulk organic
carbon than texturally similar deposits on the upper slope (Hedges and Keil, 1995).
Cascadia Basin is bound about 500 km to the west by the Juan de Fuca Ridge, beyond
which surface sediments deposit slowly and contain low organic carbon contents (0.1-0.3
percent OC) typical of open ocean deposits (Gross and others, 1972).
SAMPLE COLLECTION AND ANALYSIS
Sediments (fig. 1) were collected July 20 to August 17, 1994 during Cruise 94-07B of
the R/V Wecoma (Lambourn, Hartnett, and Devol, 1996). A 20 ⫻ 30 cm Soutar box
corer was used to recover sediment blocks up to 0.7 m deep that were then subsampled
on deck with 10-cm diam core tubes. Sediments were extruded from core tubes in a
N2-filled glove box and subsampled at 0.5 cm intervals from 0 to 2 cm, 1.0 cm intervals
from 2 to 10 cm, 2.0 cm intervals to 25 cm, and at 3.0 cm for deeper intervals. The bulk
subsamples, along with small portions for 14C analysis, were immediately frozen.
Dissolved oxygen profiles were determined by the whole-core squeezing method of
Lambourn, Devol, and Murray (1991), using an in-line polarographic oxygen microelectrode. Oxygen penetration depths into sediment porewaters were calculated (relative
precision ⫾15 percent) as described by Brandes and Devol (1995). Sediment porosities
were determined by water loss upon drying at 60°C, assuming a sediment density of
2.4 g/cm3 and a porewater salinity of 35 percent.
Nitrogen-specific mineral surface areas were determined using the one-point BET
method on a Quantachrome Monosorb surface area analyzer. Prior to determination,
sediments were cleaned of organic matter via hydrogen peroxide treatment (Mayer,
1994a; Keil and others, 1994c). The average precision for the samples reported here was
⫾3 permil.
Elemental compositions were measured with a Carlo Erba model 1106 CHN
analyzer (Hedges and Stern, 1984). Weight percentages of organic carbon (percent OC)
were determined (relative precision ⫾2 percent) following vapor phase acidification.
Weight percentages of total nitrogen (percent TN) and total carbon were determined
similarly, but without acidification. Weight percentages of inorganic carbon (percent IC)
were calculated as the difference between total carbon and organic carbon. The 14C
contents of all bulk sedimentary organic samples were determined at the National Ocean
Sciences AMS Facility (Woods Hole Oceanographic Institution). Samples were combusted (after acidification) to CO2, which was converted to graphite and analyzed along
with NBS Oxalic Acid I and II standards (plus process blanks). Counting errors in the
reported 14C contents (app. 1) were estimated by NOSAMS and averaged ⫾2 percent of
the measured ‘‘age.’’ The stable carbon isotopic composition in a fraction of the same
CO2 was determined by NOSAMS and is reported here as the permil deviation (␦13C)
from the PDB standard material (Stuiver and Polach, 1977). This measurement has a
precision of ⫾0.1–0.2 permil.
Pollen analyses were made on 1 cm3 of sediment taken from the 11 to 12 cm horizon
of selected cores. All sediments were sieved through a 7-µm mesh screen to remove fine
particles, after which pollen was isolated followed standard techniques (Faegri and
others, 1989). A known amount of Eucalyptus pollen was added as an internal standard
to each sample to determine natural pollen concentrations. Pollen grains in up to five
slides of each sample were counted in oil immersion under 400x and 1000x magnification. Pollen sums ranged from 15 to 250 grains. Individual grains were classified as
a test for selective degradation under oxic conditions
533
‘‘degraded’’ if they exhibited any of the types of physical deterioration described by
Cushing (1967).
Neutral sugars (aldoses) were analyzed by the method of Cowie and Hedges (1984).
Dry samples were pretreated with 12 M H2SO4 at room temperature for 2 hrs to facilitate
glucose yield from cellulose. The acid was then diluted to 1.2 M and used to hydrolyze
individual samples for 3 hrs at 100°C. Hydrolysate solutions were neutralized to a pH of
6.5 with Ba(OH)2 and centrifuged to remove BaSO4. The supernatant was deionized by
passage through a column of mixed anion-cation exchange resins and rotoevaporated
nearly to dryness. The residue was dissolved in pyridine and anomerically equilibrated
with LiClO4 catalyst at 60°C for 48 hrs. The equilibrated aldoses were converted to
trimethylsilyl derivatives with BSTFA (Regis Chemical Company) and quantified with a
precision of ⫾5 to 10 percent by gas chromatography on a 30 m by 0.25 mm I.D. quartz
capillary column coated with DB-1 (J&W Scientific) methylsilicone liquid phase.
Amino acids were measured by high-pressure liquid chromatography using chargedmatched recovery standards and the general procedure described by Cowie and Hedges
(1992a). Aqueous hydrolyses were done in 6 N HCl under N2 for 70 min at 150°C. The
hydrolysis mixture was dried, dissolved in water, and the component amino acids were
converted to fluorescent o-phthaldialdehyde (OPA) derivatives with a Gilson model 231
automated injector. The OPA derivatives were injected after 1 min of reaction onto a
15 cm ⫻ 4.6 mm-I.D. column operated in reverse-phase mode with 5-µm C18 packing.
Using this method, individual amino acids can be measured with a precision of ⫾5 to 10
percent.
RESULTS AND DISCUSSION
To test definitively the hypothesis that degradation under oxic conditions exerts a
major control on organic matter preservation in surface sediments off the Washington
State coast, it is necessary to: (1) directly measure varying oxygen exposure times (OET)
at representative sites across the local continental margin, (2) determine (and properly
express) the corresponding sedimentary organic matter concentrations, (3) demonstrate
that the sedimentary deposits being compared contain similar types of preserved organic
matter and are vertically uniform, and (4) test whether any decreases in organic
concentrations that might attend increases in OET are supported by independent
measurements of advanced degradation. These issues will be sequentially addressed in
the following sections.
Offshore Trends in Physical Characteristics of Sediments
The southern Washington State margin (fig. 1) provides a uniformly varying coastal
environment well suited to the goal of relating organic preservation to physical setting.
Nine sediment cores were collected from a region of the Washington continental shelf
and slope that lies between 46.4 and 48.8°N latitude and extends ⬃20 to 50 km offshore.
Seven additional cores were taken along a transect that extended another 100 km
offshore through Cascadia Basin along approximately 46.7°N latitude. Water depths at
the sites where the ‘‘shelf/slope’’ cores were collected decreased almost linearly offshore
from ⬃100 to 2000 m, whereas depths along the ‘‘basin’’ sequence deepened gradually
from ⬃2500 to 2750 m (fig. 2A). This ‘‘shelf/slope’’ and ‘‘basin’’ terminology will be used
hereafter, with the latter set of deepwater samples being indicated in figures by bold fonts
(note that core numbers are not sequential with depth).
In the following discussion and illustrations most geographic trends are presented in
terms of distance offshore, as opposed to water depth. Distance was chosen simply
because it corresponds more straightforwardly to a chart. The same results could have
been presented as a function of water depth. Given local bathymetry (fig. 2A), the effect
of a depth-based format would be to stretch trends among nearshore stations and
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John I. Hedges and others—Sedimentary organic matter preservation
Fig. 2. Physical properties of sediment core samples: (A) Water depth versus distance offshore, (B) O2
penetration depth versus distance offshore.
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535
Fig. 2(C) Average sediment accumulation rate versus distance offshore, and (D) Calculated oxygen
exposure time (OET) versus distance offshore.
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John I. Hedges and others—Sedimentary organic matter preservation
compress those for offshore samples. None of the following major trends, or their
interpretation, would be appreciably altered by normalization to depth versus distance
offshore.
Oxygen penetration depths generally increase offshore (fig. 2B), from values of ⬃0.5
to 1.0 cm on the mid-shelf (Cores 4 and 8) and continental slope (Cores 11, 13, 2, 15, 6,
20, and 3), to a range of ⬃1.5 to 3.5 cm in Cascadia basin (Cores 5, 23, 19, 9, 17, 1 and
16). Core 16 has an unusually low O2 penetration depth for its extreme offshore location,
possibly because of recent local deposition of labile organic matter. The overall offshore
increase in O2 penetration below the water/sediment interface likely results from the
combined effects of lower primary production farther off the Washington coast (Perry,
Bolger, and English, 1989) and greater attenuation of organic particles raining through
longer water columns (Martin and others, 1987). Impingement of the oxygen minimum
zone between ⬃500 to 2000 m water depth (Carpenter, 1987) may explain the unusually
low O2 penetration depths measured in Cores 13, 6, 15, 20, and 3 from the upper slope.
Sediment accumulation rates were determined for the six cores listed in table 2 and
circled in figure 1. Because 4 to 7 measurements of 14C were required per core, additional
dating was prohibitively expensive for this study. The six profiled cores were selected on
the basis of their uniformity and greater length, with an eye toward representing a range
of water depths and distances offshore. Although 210Pb profiles were also measured for
almost all cores, the calculated accumulation rates were invariably greater than 14Cbased rates (when available) for the same deposits. Due to mixing and the relatively short
half-life of 210Pb, this difference became greater with increasing distance offshore. In
slowly accumulation sediments from Cascadia Basin, 210Pb-based values often exceeded
14C-counterparts by more than two orders of magnitude.
14C-based sediment accumulation rates decrease progressively offshore (fig. 2C)
from average values near 15 cm/ky on the upper slope (Cores 13 and 15) to 3 cm/ky in
the outer basin (Core 1). This seaward decrease is expected from previous studies in this
region (Carpenter, Peterson, and Bennett, 1982; Carson, 1971). All three of the deepest
cores (17, 1, and 16) from the outer basin penetrated a gray clay horizon at a depth of 15
to 20 cm (W94-B07 cruise log) whose age (at the surface) is on the order of 9000 to 13,000
yrs (Carson, 1973). The deepest dated sample (29-30 cm) from Core 1 was from within
the gray clay horizon (about 10 cm below the interface) and had a 14C age of 11,500 years
(app. 1). A 15 to 20 cm glacial/post-glacial boundary with an age of ⬃10,000 yrs
corresponds to an average sediment accumulation rate of roughly 2 cm/ky. This value is
close to the mean rate of 3 cm/ky determined from the 14C profile for Core 1 (table 2) and
to deposition rates (⬃2 cm/ky) reported for this offshore region by Carson (1973). Thus,
14C appears to be a reliable radionuclide for determining comparative average sediment
accumulation rates within this study region (fig. 1).
Oxygen exposure times, calculated by dividing the previously discussed O2 penetration depths by the corresponding 14C-based sediment accumulation rates, increase
exponentially away from the coast (fig. 2D). Calculated OET values range from decades
on the slope, to centuries at the inner basin, up to a millenium (Station 1) in outer
Cascadia Basin. The overall increase by a factor of about 30 results from a 6⫻ offshore
increase in O2 penetration and a corresponding 5⫻ decrease in accumulation rate.
Because these two contributing trends are nearly linear (fig. 2B,C), OET data for the
other 10 (undated) cores should follow a general trend similar to that in figure 2D.
Neither O2 penetration depth nor accumulation rate alone explains the observed
increase in OET, a parameter that integrates both in situ particle dynamics as well as O2
supply and demand (Hartnett and others, 1998).
Offshore Trends in Organic Content
Weight percentage of organic carbon is the traditional measurement of organic
matter content in both sediments and soils (Hedges and Oades, 1997). Full profiles
a test for selective degradation under oxic conditions
537
(12-19 samples per core) of percent OC, percent TN, and percent IC were determined
for each of the 16 Washington Margin sediment sequences, amounting to almost 250
analyses of each element overall. Inorganic carbon was a trace component (overall
average of 0.2 wt percent) of these sediments and will not be discussed further. As is
typical for much of the Washington margin (Carpenter, 1987; Hedges and Mann, 1979),
depth profiles of percent OC and percent TN were relatively featureless, except for the
few cores that penetrated OC-poor glacial gray clay at depth. The percent standard
deviations in percent OC and percent TN among sediments (excluding gray clays) from
individual cores averaged ⫾11 and ⫾22 percent, respectively, with the greater variability in percent TN resulting largely from the challenge of measuring its lower concentration. Due to this overall vertical uniformity, only mean percent OC, percent TN and
(C/N)a values are reported (and plotted) for each core (table 1).
Average mass-normalized organic contents of the Washington margin sediment
cores ranged from about 1 to 3 percent OC and 0.1 to 0.3 percent TN (table 1) and fall in
the range of earlier analyses (Gross and others, 1972; Hedges and Mann, 1979; Prahl and
others, 1994). Because percent TN closely parallels percent OC (later discussion), we will
focus here on the more abundant element. An offshore decrease in percent OC is
discernable in figure 3A. No sediment farther than 100 km from shore has values ⬎1.5 wt
percent. Cores 4, 8, and 2 exhibit lower organic contents than the other near-shore
deposits (fig. 3A). Sediments from these same three cores (table 1) also exhibit unusually
low (⬍0.7) porosites and surface areas (⬍20 m2 g), indicating that their component grains
are relatively coarse. Because (A) most organic matter in sediments from the Washington
margin is associated with mineral surfaces (Hartnett and others, 1998; Keil and others,
1994a,c), and (B) coarse minerals have small surface/mass ratios (Mayer, 1994a,b), the
low percent OC values of these samples probably result from their coarser textures.
Surface area-normalized organic ‘‘loadings’’ (mgOC/m2) are less susceptible to
textural influences than mass-normalized counterparts and in this case yield a smooth
offshore decrease in organic content (fig. 3B). In particular, Cores 8 and 4 are shown to
be the most organic-rich on a surface area basis. A plot of OC/SA versus distance
offshore reveals a break near 100 km (⬃2500 m water depth) that is consistent with other
studies in this region (Carpenter and Peterson, 1989; Prahl, 1985). Organic loadings
decrease rapidly offshore from the mid-shelf and across the slope but then diminish
gradually along the floor of Cascadia Basin (fig. 3B). Most OC/SA ratios of the slope
samples span the range of 0.5 to 1.0 mgOC/m2, as is typical of non-deltaic upper
continental margin sediments (Keil and others, 1994c; Mayer 1994a,b). In contrast, all
five cores from outer Cascadia Basin have average OC/SA ratios near 0.3, which
indicate substantially reduced preservation efficiencies. Representative profiles of OC/SA
down the six 14C-dated cores (fig. 4) illustrate the uniformity of organic carbon loading
below the upper few centimeters of sediment in these cores, although the deepest
analyzed horizons represent ages since deposition of roughly 2000 to 8000 yrs. This
uniformity suggests minimal organic matter degradation below the upper few centimeters of sediment (see later discussion). Such results can be considered robust, however,
only if the sources of organic matter preserved in these deposits is demonstrably similar
and independent evidence can be presented for an offshore increase in diagenetic
alteration.
Sedimentary Organic Matter Sources
Stable carbon isotope compositions of the organic matter in the six dated Washington margin cores provide information on the both the origin and spatial uniformity of the
associated organic matter. The variability of ␦13C values for organic matter in different
horizons of each dated sediment core averaged ⫾0.2 permil, which is close to analytical
reproducibility. Over the six separate sites (fig. 1), ␦13C averages of individual cores
ranged only from ⫺22.4 to ⫺23.0 permil, with no evidence of an offshore trend (table 2).
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John I. Hedges and others—Sedimentary organic matter preservation
Fig. 3. Average organic carbon contents of individual sediment cores given as (A) Weight percentages,
%OC, versus distance offshore, and (B) Organic surface area loadings, OC/SA, versus distance offshore.
Fig. 4. Vertical profiles of OC/SA down the six cores that were 14C-dated (fig. 1, app 1). Age11.5 represents the calculated age (yrs since deposition)
of the 11 to 12 cm sediment horizon routinely used for molecular-level biochemical analyses.
a test for selective degradation under oxic conditions
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John I. Hedges and others—Sedimentary organic matter preservation
Thus, the organic mixtures in these sediments are remarkably uniform in their stable
carbon isotope composition, and hence likely in their percentages of marine and
terrestrial organic matter. While assignments of endmember ␦13C values to marine
sedimentary mixtures are dangerous (Hedges and Prahl, 1993), respective averages of
⫺21.5 and ⫺25.5 permil for marine- and terrestrially-derived organic matter are
reasonably well established for the Washington margin (Hedges and Mann, 1979; Prahl
and others, 1994). Given these boundary conditions, sediments from the dated cores
appear to comprise roughly two-thirds marine and one-third terrestrial organic matter
(see also Prahl and others, 1994).
Atomic C/N ratios of the 16 sediment cores fall into two groups (table 1). Cores 4, 8,
and 15 from the mid-shelf and upper slope have (C/N)a ratios between 17 and 18
(mean ⫽ 17.3 ⫾ 0.3). The (C/N)a ratios of the other 13 cores fall without pattern in the
range of 10 to 13 (mean ⫽ 11.8 ⫾ 1.0) across the entire slope-basin span. Prahl and
others (1994) reported similar (C/N)a ratios for surface sediments from this region. A
plot of core-average percent OC versus percent TN illustrates the close relationship of
these two elements. Best-fit regression lines to the averaged elemental data are illustrated
in figure 5 for the three shallow (percent OC ⫽ 14.7*percent TN ⫹ 0.03, r2 ⫽ 0.998) and
13 ‘‘other’’ cores (percent OC ⫽ 9.28*percent TN ⫹ 0.13, r2 ⫽ 0.926). Both regression
lines have intercepts near the origin, indicating that inorganic nitrogen is a minor
component of TN, such that the numeric mean and slope-derived (C/N)a values are
representative of the bulk of the organic matter in these two size classes (Bergamaschi
and others, 1997).
The unusually high (C/N)a values of the three shallow-water sediments may be
associated with their depositional settings. Cores 4 and 8 are from the southern
Washington mid-shelf silt deposit, whose sediments are characterized by relatively high
Fig. 5. Average weight percentage organic carbon (%OC) versus weight percentage total nitrogen (%TN)
for all Washington margin sediment cores.
a test for selective degradation under oxic conditions
541
concentrations of vascular plant debris (Hedges and Mann, 1979). These deposits are
known to exhibit higher average (C/N)a values (⬃15) than more offshore slope and basin
(means 9-11) sediments (Prahl and others, 1994). Core 15 is from a submarine channel
associated with the Columbia River (figs. 1, 2A) and hence also may preferentially
receive vascular plant debris (Gross and others, 1972; Harmon, 1972). Size fractionation
of surface sediments from the Washington shelf and slope (Keil and others, 1994c; 1998)
demonstrate that the terrigenous organic components of sands and silts are enriched in
vascular plant remains [(C/N)a ⬎ 30], whereas more nitrogen-rich soil organic matter
[(C/N)a ⬍ 15] is the major terrigenous component of clays. Local vascular plant debris
and soil organic matter have similarly ‘‘light’’ (␦13C ⫽ ⫺25 to ⫺26 permil) stable carbon
isotope compositions (Keil and others, 1994c). Thus, by slightly increasing the ratio of
woody debris to soil organic matter within a constant total amount of terrigenous organic
material, it is possible to elevate the (C/N)a of the nearshore sediments without
appreciably changing their ␦13C values (for example, Core 15). Overall, the predominantly marine-derived organic mixtures from the Washington margin sediments (tables
1 and 2) appear sufficiently similar in organic composition to allow a reasonable
comparison of potential diagenetic influences throughout this region of contrasting OET.
Offshore Trends in Organic Matter Composition
If in fact the offshore drop in OC/SA (fig. 3B) is due to cumulative in situ
degradation near the sediment surface, then the biochemical compositions of these same
sediments should provide independent evidence of advanced organic alteration at more
offshore sites. To investigate this issue the amino acid and neutral sugar (aldose)
compositions of sediments collected from the 11 to 12 cm horizon of each core were
determined by hydrolysis and molecular level analyses. This intermediate sediment
interval was chosen to be below the depths of O2 penetration and rapid particle mixing,
TABLE 1
Locations and physical characteristics of sediment samples from the Washington State
continental margin
† Originally designated core numbers (in bold) are 200 units higher (for example, 201, 202 . . .), Spl# ⫽
the number of samples used to calculate whole core averages, * Indicates values averaged for whole core,
shaded rows correspond to 14C dated cores, ⌽ ⫽ porosity, IC ⫽ inorganic carbon, OC ⫽ organic carbon, TN ⫽
total nitrogen, C/N(a) ⫽ atomic organic carbon/total nitrogen (calculated directly from tabulated mean percent
OC and percent TN), Spl. ⫽ sample, SA ⫽ surface area, Penet. ⫽ penetration.
542
John I. Hedges and others—Sedimentary organic matter preservation
TABLE 2
Average ␦13C values, sediment accumulation rates and oxygen exposure times for six 14C-dated
sediment cores
Uncertainty intervals are standard deviations for ␦13C (defined in text), standard errors in line fitting for
accumulation rates, and calculated standard deviations for O2 exposure time (using the listed errors in the
accumulation rates and an estimated ⫾15% uncertainty in oxygen penetration depth measurements).
without being so far removed from the surface that comparisons to current O2 penetration depths and sedimentation rates would be unduly compromised. Corresponding
depth profiles of sugars and amino acids were also determined within selected cores to
examine the effects of prolonged in situ degradation that could bias comparisons among
cores of different ages. Amino acids and aldoses were chosen because both biochemical
types derive primarily from marine sources and can provide useful diagenetic information (Cowie and Hedges, 1994; Cowie and others, 1995).
Non-protein amino acids provide one of the most reliable biochemically-based
indicators of organic matter degradation. In particular, Cowie and others (1995) demonstrated that pronounced elevations in the mole percentages of ␤-alanine plus ␥-aminobutyric acid, percent (BALA ⫹ GABA), occur when sedimentary mixtures are subjected to
slow oxic degradation in deep-sea turbidites. In addition, percent (BALA ⫹ GABA) has
proven useful for comparing degradation stages in rivers (Hedges and others, 1994,
submitted) and coastal marine environments (Cowie, Hedges, and Calvert, 1992b; Keil
and others, 1998). A plot of percent (BALA ⫹ GABA) versus distance offshore (fig. 6A)
demonstrates a highly significant linear increase, [percent (BALA ⫹ GABA) ⫽
km*0.0291 ⫹ 1.64, r2 ⫽ 0.90]. In comparison to the intercept of 1.6 percent at zero
distance offshore, fresh organic matter has a percent (BALA ⫹ GABA) near 1 (Cowie
and Hedges, 1994). To test for progressive in situ degradation, depth profiles of percent
(BALA ⫹ GABA) were measured at six horizons each in Cores 9 and 19. The total range
of variability within both these Cascadia Basin cores was less than ⫾10 percent of the
corresponding mean (fig. 6A) and hence comparable in magnitude to analytical precision. Minimal vertical gradients in bioactive materials are typical of sediment cores from
the Washington margin (Carpenter, 1987; Hedges and Mann, 1979), partially as a result
of physical mixing of biologically active surficial sediments (Carpenter, Peterson, and
Bennett, 1982). Because 210Pb-based mixing depths average about 5 cm in these cores
(unpublished data), it is difficult to determine where in the upper 5 cm or so of these
deposits their diagenetic signatures are imprinted.
Glucose weight percentages (percent GLC) among total aldoses generally decrease
with organic matter degradation, whereas percentages of the 2 six-carbon deoxy sugars,
rhamnose (RHA) and fucose (FUC), increase (Cowie, Hedges, and Calvert, 1992;
Hamilton and Hedges, 1988; Hernes and others, 1996). Both these trends are observed
with increasing distance from the Washington coast (fig. 6B). All three derivative
a test for selective degradation under oxic conditions
543
parameters, percent (FUC ⫹ RHA), 100/wt percent GLC, and (FUC ⫹ RHA)/GLC,
increase toward deeper water. The simplest of these parameters, 100/percent GLC, has a
value near 1.5 to 2.5 for fresh plankton and vascular plant tissues (Cowie and Hedges,
1984; Hernes and others, 1996). Values of 100/percent GLC increase sharply offshore
from near 3 (⬃30 wt percent glucose) for sediments on the shelf and inner slope to 4.5 to
5.5 (⬃20 wt percent glucose) for those from outer Cascadia Basin (fig. 6B). The
observation that 100/wt percent GLC increases more sharply on the shelf and inner
slope than does percent (BALA ⫹ GABA) demonstrates that different indicator compounds can provide contrasting sensitivities to progressive diagenetic alteration (Cowie
and Hedges, 1994; Wakeham and others, 1997).
The variability of 100/wt percent GLC within a 6-sample sequence from Core 19
(fig. 6B) is comparable to analytical precision. Thus, in situ carbohydrate alteration also is
not apparent, even within the biologically active upper portion of this sediment core.
The overall lack of biochemical evidence for progressive degradation (fig. 6), or OC loss
(for example, fig. 4), throughout the varying lengths of the profiled cores indicates that it
is valid in this case to compare whole-core averages among sediment sequences of
different average age (table 2 and fig. 4). As would be expected if exposure to oxic
conditions is critical, the diagenetic differences evident in figures 3 and 6 appear to be
imprinted in the well-mixed upper few centimeters of the sediments. Although fermentation and sulfate reduction are possible throughout the top meter of Washington margin
deposits, these processes do not occur sufficiently fast to leave discernable depletions (fig.
4) or chemical imprints over the time intervals represented by the lengths of the
14C-dated cores (2000-8000 yrs). This evidence for minimal deep degradation is consistent as well with the general observation that ancient shales exhibit organic carbon
contents similar to those of modern fine-grained marine sediments that have accumulated below the diagenetically active upper 5 to 10 cm (Hunt, 1996).
Pollen affords an independent (nonchemical) method for assessing the relative
extents to which organic materials in these sediments have been exposed to diagenetic
alteration. Previous research has shown that pollen is sensitive to long-term oxic
degradation and completely disappears under extreme conditions of O2 exposure in
oxidized deep-sea turbidites (Keil and others, 1994b). This observation is striking
because pollen walls, sporopollenins, are among the most resistant biological remains
known and persist for millions of years in anoxic sediment and peat deposits (Brooks and
Shaw, 1978). Pollen grains, however, are sorted differentially during sedimentary
transport and tend to concentrate with silt-size mineral grains (Keil and others, 1998). For
this reason, and because of variable inputs over time, it is difficult in degradation studies
to assess changes in pollen abundances per unit volume of bulk sediment. It is feasible,
however, to apply the percentage of physically degraded pollen grains in a sample
(Cushing, 1967) as a qualitative degradation indicator.
Pollen was counted in the 11 to 12 cm intervals of nine of the Washington margin
sediment cores (fig. 6C). Pollen grains were observed in all the analyzed sediment
samples. While a detailed description of pollen compositions is beyond the scope of this
paper, the assemblages were primarily of mixed conifer types (Pinaceae, Pinus, Picea,
Tsuga) along with abundant alder (Alnus), as is typical of the Pacific Northwest region
over the last 6000 yrs (Heuser, 1985). The four continental shelf and slope samples
contained 100,000 to 250,000 pollen grains/cm3 of sediment, whereas all five sediments
from Cascadia Basin contained ⬍20,000 grains/cm3. The most prevalent forms of
physical alteration exhibited by the sedimentary pollen grains were diffuse surface
erosion, pitting, and perforation. The observed percentages of physically altered grains
increased from values near 20 percent on the continental slope to 40 to 60 percent in
Cascadia Basin. The observation that the basin sediments contained lower concentrations of more corroded pollen suggests that these recalcitrant terrestrially-derived
544
John I. Hedges and others—Sedimentary organic matter preservation
Fig. 6. Distance from shore of individual sediment cores versus average degradation indicators,
including: (A) Mole percent of ␤-alanine plus ␥-aminobutyric acid, %(BALA ⫹ GABA); (B) One hundred
times the inverse of weight percent glucose, 100/%GLC; and
a test for selective degradation under oxic conditions
545
Fig. 6(C) Percent of degraded pollen.
particles are also subject to severe in situ sedimentary degradation. It seems unlikely that
more degraded pollen would be subject to greater offshore transport or that such
extensive degradation would occur during relatively brief periods of marine transport.
Long term degradation on land, however, could account for the lower threshold of 20
percent degradation observed in all cores. Nevertheless, pollen adds a third, unique line
of compositional evidence in support of an in situ oxic effect in Washington coast
sediments.
OVERVIEW AND IMPLICATIONS
While the evidence presented is consistent with the hypothesis that the lower
organic contents of more offshore Washington margin sediments (fig. 3A, B) result from
longer in situ degradation under oxic conditions (fig. 2D), a number of overriding points
and interpretive constraints merit further discussion.
Relationships between organic parameters and with environmental variables other
than distance offshore have not been stressed to this point. It is mechanistically
informative, however, to test directly how well the amino acid- and carbohydrate-based
diagenetic parameters correlate with surface-normalized organic content. Plots of OC/SA
versus percent (BALA ⫹ GABA) and 100/percent GLC (fig. 7A and 7B, respectively)
illustrate that the average organic contents of the Washington margin cores decrease with
increasing degradation. Most of the discernable organic matter alteration and loss (figs. 3
and 6), however, occurs over a relatively short distance on the shelf/slope (20-50 km
offshore, 100-2500 m water depth), as opposed to along the 100-km long sequence of
cores on the floor of Cacadia Basin. In contrast, OET increases more among surface
sediments from Cascadia Basin (fig. 2D). The apparent insensitivity of degradation
extent to OET values greater than 200 yrs is particularly evident in a plot of OC/SA
546
John I. Hedges and others—Sedimentary organic matter preservation
Fig. 7. Organic carbon/surface area ratios (OC/SA) plotted versus: (A) %(BALA ⫹ GABA), (B) 100/
%GLC, and
a test for selective degradation under oxic conditions
547
Fig. 7(C) log of OET. See figure 6 for definitions of the two organic parameters.
versus log of OET (fig. 7C), where higher carbon loadings persist in the most offshore
sediments (Core 1) than would be expected for simple first-order decay.
These ‘‘long-tailed’’ decreases in organic matter versus time are the rule, rather than
the exception, for sediments of many types and ages (Boudreau, 1997; Westrich and
Berner, 1984). Such ever-slowing kinetics can be variously described with ‘‘multi-G’’
models for sums of different organic components degrading with contrasting first-order
rates (Berner, 1980, 1981; Jørgensen, 1978) or as ‘‘power’’ (Middelburg, 1989) or
‘‘reaction continuum’’ models (Boudreau and Ruddick, 1991), in which remineralization
rate constants decrease as a function of time (Tarutis, 1993). The present data set is too
small and geographically diverse to ‘‘backward project’’ multiple organic components
with characteristically different reaction rates (Van Liew, 1962). Even these six data,
however, indicate that the OC/SA trend is, as theoretically expected (Hedges and Keil,
1995), exponential in form. The best least-squares fit of the data (excluding Core 1)
indicates a half-life for oxic organic matter remineralization on the order of 100 yrs (fig.
7C, assuming first-order degradation). This dynamic, however, is largely constrained by
the time scale represented by the shelf/slope cores (Middelburg, Vlug and van der Nat,
1993) and does not rule out faster (or slower) kinetics (Boudreau and Ruddick, 1991). In
fact, Hartnett and others (1998) recently reported a negative exponential relationship
between sedimentary organic carbon burial efficiency and OET that extends from
centuries to days. In contrast to previous speculation that OETs beyond a threshold of
centuries to millennia might be needed to affect organic matter preservation along the
Washington margin (Hedges and Keil, 1995), ‘‘oxic effects’’ are now apparent at
exposure times of decades or less (Hartnett and others, 1998). Thus, selective degradation under oxic conditions need not be slow.
548
John I. Hedges and others—Sedimentary organic matter preservation
Uncertainties about the physical history of particulate materials comprising these
sediments necessarily cascade into parallel ambiguities as to the sites and mechanisms of
their alteration. For example, it is not clear when and how degradation characteristic of
more offshore sedimentary organic matter (figs. 3, 6) occurred. Since the component
mineral particles are predominantly clastic, they must (like pollen) be derived eventually
from land. After being carried to the coastal ocean by rivers or wind, these particles may
have been subject to multiple deposition/resuspension cycles in the course of offshore
transport to their present sites (Nittrouer and Sternberg, 1981). Thus, their total history of
exposure to O2 may be substantially greater than would be estimated from their final
(sampled) depositional site alone. Such multi-step transport should lead to more advanced degradation at more offshore sites, and thus produce degradation trends that
would be difficult to discriminate from simple a ‘‘oxic effect’’ at one point of deposition.
Several characteristics of sedimentary organic matter transport and degradation
within this (and other) depositional region, however, should mitigate large cumulative
effects of sequential transport. First, the sediments that comprise most of our data set
were sampled from water depths ⬎500 m (fig. 2A) where periods of sufficient energy for
sediment resuspension should be relatively rare. This notion of relatively quiescent
deposition is supported by uniformly high total surface areas (fig. 7) and porosities (table
1), indicative of a paucity of sand in all sediments from depths greater than 500 m.
Second, extensive resuspension and offshore relocation of sedimentary particles along a
benthic transport pathway should result in relatively constant deposition rates, or in the
extreme, more extensive offshore accumulation. In contrast, sediments of our study
region accumulate at rates that decrease progressively offshore (fig. 2C) and to depths
that are consistent with gradual accumulation at relatively constant rates (see 14C results).
Since Cascadia Basin is bound seaward by a continuous ocean ridge, sedimentary
particles are not likely to abridge this depositional record by escaping farther offshore.
Moreover, the OET values calculated for a particular site will only be directly affected if
resuspension erodes particles deeper than the oxic sediment horizon and if the resuspended particles are then transported to a deposition site with an appreciably different
O2 penetration depth or average accumulation rate. The relatively uniform compositions
down the sediment cores in this study (figs. 4 and 6), and the smooth geographic trends
among individual cores (figs. 2, 3, 6, and 7), all point toward relatively uniform
accumulation histories.
Bioturbation and irrigation can complicate interpretations of oxygen exposure
histories as well. Calculated oxygen exposure times should not be greatly affected by
bioturbation, which (if nonselective) should move particles into and out of the oxygenated surface horizon of marine sediments with comparable probabilities. Under conditions such as off the Washington coast, however, where bioturbation intervals can be
substantially deeper than O2 penetration depths, the number of times an individual
particle passes between oxic and anoxic zones will be much greater with bioturbation
than without (Aller, 1994). Periodic irrigation of anoxic porewaters with oxygenated
bottom water should have the same local effect. Given evidence that ‘‘re-exposure’’ of
organic matter from anoxic sediments to oxygenated conditions facilitates biodegradation (Hulthe, Hulth, and Hall, 1998), OET calculations based on steady state conditions
may substantially underestimate cumulative degradation under oscillating redox conditions (Aller, 1994). Thus periodic resuspension/redeposition and bioturbation/irrigation
events will both cause OET to conservatively estimate total biodegradation history. The
observation that OET provides a useful guideline for the extent of cumulative degradation in Washington margin sediments (fig 7C) suggests, but does not prove, that average
a test for selective degradation under oxic conditions
549
depositional conditions predominate over scattered periodic events in determining the
amounts and types of organic materials that are preserved. Significantly, all these
scenarios focus primarily on conditions and processes near the sediment/water interface,
as opposed to those occurring near the surface of the overlying water column.
The issue of where oxic degradation ‘‘stops’’ on the Washington margin is presently
unresolved. Although the organic matter accumulating on the floor of Cascadia Basin is
measurably altered (figs. 3, 6, and 7), it may not yet have reached full degradation
maturity. One observation supporting this notion is that the average percent OC (⬃1.1)
and OC/SA (⬃0.3 mgOC/m2) of Cascadia Basin sediments are distinctly higher than the
corresponding contents (percent OC ⬇ 0.1-0.2; OC/SA ⬇ 0.01-0.10 mgOC/m2) typical
of open ocean deposits (Hedges and Keil, 1995). In fact, Gross and others (1972) report
percent OC values less than 0.5 percent on the Tufts Plain to the west of the Juan de Fuca
Ridge. In addition, all the Cascadia Basin sediments contain ample pollen in contrast to
little or no sporopollenin in sediments from the oxidized MAP turbidite horizon and
open ocean (Keil and others, 1994a). Finally, the few deep-water sediments that have
been analyzed yield much higher abundances of non-protein amino acids (Lee, Wakeham, and Hedges, submitted; Whelan, 1977), a sensitive compositional indicator of
advanced oxic degradation (Cowie and others, 1995). The limited seaward extent of
Cascadia Basin, which is bound west of the sampling transect by the Juan de Fuca ridge
crest system (fig. 1), interrupts the typical offshore decrease in productivity, sediment
accumulation rate, and hence OET, that would characterize most continental margins.
Analyses of sediments from a transect extending farther offshore from an unbound land
mass will be necessary to test for more severe sedimentary degradation that might attend
porewater O2 exposure on time scales of thousands to hundreds of thousands of years
(Middelburg, Vlug and van der Nat, 1993).
To compare absolute extents of degradation among different sediments and depositional sites, consistent relationships between organic content and alteration are needed.
Given the varying sensitivities of biochemical-based degradation indicators over different stages (and types?) of alteration (fig. 7), multiple indicators of diagenetic stage may be
necessary. A plot of percent OC versus SA (fig. 8), however, does illustrate a potentially
useful pattern of organic matter loss and alteration at higher values of percent
(BALA ⫹ GABA) corresponding to greater OETs. In this format, OC/SA values of
Washington margin sediments ‘‘sweep’’ down clockwise with increasing degradation
along ‘‘rays’’ whose lengths depend on specific surface areas. While it is premature to
attempt a rigorous fit, percent OC values for a given surface area do decrease as percent
(BALA ⫹ GABA) values increase. Thus surface area and diagenetic state can be
combined to constrain the organic content of these sediments. This relationship may well
extrapolate downward to the low OC/SA and high percentages of non-protein amino
acids typical of extremely organic-poor open ocean sediments. Because the percent
(BALA ⫹ GABA) values of the upper shelf/slope sediments are already near 1, as is
typical of fresh organic matter, another parameter will be necessary to extend this
relationship to sediments with OC/SA values greater than one (Hartnett and others,
1998; Hedges and Keil, 1995).
Related to the issue of where organic degradation ‘‘stops’’ is the question of how
valid it is to compare degradation state among sediment cores of different average ages,
which result from analyzing comparable depth intervals among deposits accumulating at
contrasting average rates (fig. 2C). If appreciable degradation continues below the
bioturbated surface zone of different deposits, then organic materials in more slowly
accumulating sediments will be more degraded simply because they are on average
550
John I. Hedges and others—Sedimentary organic matter preservation
Fig. 8. A degradation ‘‘fan plot’’ of %OC versus specific surface area, SA, for average data from all 16
Washington margin cores and other reference samples. The corresponding trend in %(BALA ⫹ GABA) is the
best-fit line to the given diagenetic arc.
older, as opposed to having initially passed through the surface oxic zone more slowly.
Inspection of OC/SA ratios down the six cores chosen for 14C dating (fig. 4), however,
indicates little evidence for consistent in situ degradation below the upper few centimeters of sediment. This evidence for minimal in situ degradation below the surface
bioturbated layer is supported by the narrow range of variability and lack of a
discernable downward increase, in the diagenetic parameters measured over the lengths
of cores 9 and 19 (fig. 6A,B). Such featureless profiles are typical of the other cores in this
study (table 1) and most sediments from the Washington margin (Hedges and Mann,
1979; Prahl and others, 1994). The general observation that shales and modern marine
sediments of similar texture both contain an average of about 1 weight percent of organic
carbon (Hunt, 1996) argues against extensive organic matter losses from sedimentary
deposits on geologic time scales (Hedges and Keil, 1995). It appears, therefore, that the
processes that imprint the amount and types of organic preserved in these deposits occur
predominantly within the upper few centimeters of these sediment and thus are largely
independent of the overall length of the studied cores.
The mechanisms of ‘‘oxic degradation’’ indicated by the previous relationships are
not yet clear. While oxic conditions appear to be necessary in sedimentary porewaters
for extensive degradation to occur, this relationship does not necessarily mean that O2 is
the causative electron acceptor or enzyme intermediate. In fact, the vertical separation
between oxic, suboxic, and anoxic conditions in these continental margin sediments is so
narrow (and fluctuating) that any number of compounds could act as redox agents and
catalysts. It is also likely that specific microorganisms enzymatically facilitate key
551
a test for selective degradation under oxic conditions
electron exchanges, and that some macrobenthic organisms are especially effective in
creating physical conditions conducive to these reactions (Aller, 1994). Thus ‘‘oxic
degradation’’ is more appropriately viewed as a set of as yet unknown processes that
characteristically prevail under sedimentary conditions where O2 is present, as opposed
to a specific mechanism that directly involves molecular oxygen. Oxygen exposure time
simply incorporates benthic conditions and dynamics into a parameter that predicts the
extent of organic matter preservation near the sediment/water interface where the leak
of OC from the biosphere to geosphere is directly controlled. Future mechanistic insights
into preservation processes will no doubt lead to new and better predictors.
ACKNOWLEDGMENTS
We thank the Captain and crew of the R/V Wecoma for their assistance and
patience during Cruise 94-07B. Dean Lambourn, Crystal Thimsen, Jay Brandes, and Elli
Tsamakis helped with many aspects of sample collection and analysis. Constructive
comments by Drs. Robert Aller and Marc Alperin substantially improved this manuscript. This manuscript was completed while JH was a Fellow of the Hanse Wissenschaftskolleg, Delmenhorst, Germany. Our research was supported by NSF grant OCE
9401903 to JH and OCE 9116275 to AD and OCE-9402081 to RK.
APPENDIX 1
Isotope and age characteristics of individual sediment horizons from the six 14C-dated
sediment cores
Core
#
Sediment
Depth (cm)
␦13C
(‰)PDB
Age
(yr BP)
Age Error
(yr BP)
1
1
1
1
1
1
1
3
3
3
3
3
3
9
9
9
9
9
13
13
13
13
15
15
15
15
15
15
19
19
19
19
19
2.5
5.5
7.5
9.5
11.5
14.0
29.5
1.8
7.5
11.5
17.0
29.0
43.0
2.5
9.5
18.0
29.5
39.5
4.5
11.5
22.0
32.5
1.8
7.5
14.0
18.0
22.0
32.5
1.8
5.5
18.0
29.0
43.0
⫺22.7
⫺22.9
⫺22.9
⫺23.0
⫺22.9
⫺22.9
⫺23.4
⫺22.4
⫺22.2
⫺22.4
⫺22.8
⫺22.3
⫺22.1
⫺23.3
⫺22.5
⫺22.8
⫺22.9
⫺23.1
⫺24.2
⫺22.5
⫺22.1
⫺22.2
⫺22.9
⫺23.0
⫺22.9
⫺22.9
n.d.
⫺22.3
⫺22.5
⫺22.3
⫺22.4
⫺22.7
⫺24.8
4470
4590
4680
5220
5510
6100
11500
1600
2150
2370
2400
2660
2590
4280
5110
6750
9270
10550
1490
1390
2210
2710
785
805
1370
1740
1640
1610
3090
3280
4570
6330
8150
55
35
40
40
30
65
60
35
25
30
55
25
35
65
30
40
45
40
55
45
50
55
45
25
20
25
30
25
95
80
60
95
45
Bold data indicated core horizons used for sediment accumulate rate calculations.
552
John I. Hedges and others—Sedimentary organic matter preservation
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