HEDGES, JOHN I., WAYNE A. CLARK, AND GREGORY L. COWIE

Limnol. Oceanogr., 33(5), 1988, 1116-l 136
0 1988, by the American
Society of Limnology
and Oceanography,
Inc.
Organic matter sources to the water column and surficial
sediments of a marine bay
John I. Hedges,’ Wayne A. Clark, and Gregory L. Cowie
School of Oceanography, WB-10, University of Washington, Seattle 98 195
Abstract
Sediment trap and net plankton samples were collected monthly for a year at three depths in a
marine bay (Dabob Bay, Washington). These materials and subsamples from a sediment box core
were analyzed for lignin oxidation products as well as elemental and stable carbon isotope compositions. The sediment core was compositionally uniform over its entire 50-cm length. The
elemental and lignin compositions of the sediment trap and core samples indicate nitrogen-rich
(atomic C : N = 7.5) plankton-derived organic matter mixed with vascular plant debris.
At most, vascular plant debris accounts for 10% (nonwinter months) to 35% (winter months) of
the total organic carbon in the upper water column (30 m) sediment trap samples and consists
predominantly of gymnosperm wood along with some nonwoody gymnosperm tissues and angiosperm woods. Bulk land-derived organic matter in Dabob Bay contains a maximum of 50% vascular
plant debris and comprises an average of one-third of the total organic carbon in the sediment
trap samples and two-thirds of the total organic carbon in the underlying sediments. Lignin in the
sediment trap and core samples shows evidence (from elevated vanillic acid:vanillin ratios) of
white-rot fungal degradation before (but not after) introduction to the water column at the study
site. Vascular plant debris introduced to the bay has already lost almost half of its initial bulk
polysaccharide. Glucose yields are particularly low whereas rhamnose and fucose are obtained in
excess of expected yields and must have additional sources.
Lignin and neutral sugars together account for ~20% of the total organic carbon in the sediment
trap and core samples. Overall, the sediments of Dabob Bay compositionally resemble the gymnosperm wood-rich particulate material introduced to the overlying water column during winter
and poorly record the input of plankton and other types of vascular plant debris during nonwinter
months.
Most of what is known about the sources
of organic matter to ancient and modern
aquatic environments
has been obtained
from the analysis of sedimentary deposits
(Eglinton and Murphy
1969; Tissot and
Welte 1978). Sediments have been a rich
source of organic geochemical information
because they provide large, time-averaged
samples of known sequence from a great
variety of depositional environments
and
’ To whom correspondence should be addressed.
Acknowledgments
This research was supported by NSF grants OCE 8219294 and OCE 84-21023. Contribution 1767 from
the School of Oceanography, University of Washington.
We thank Rita Homer for plankton characterizations, Jeff Stem for elemental analyses, and Coastal
Science Laboratories for isotopic measurements.- Roy
_
Carpenter and Carl Lorenzen shared equipment for the
field studies and John Ertel aided on numerous cruises.
Gray Drewry and Phil Crawford proved invaluable
boat captains over the course of I5 cruises. This manuscript benefited from reviews by John Ertel, Karen Weliky, Cindy Lee, Michael Peterson, and three anonymous reviewers.
,
geologic ages. Due to the wide range of stabilities and transport mechanisms exhibited
by different organic substances, however,
even modern sedimentary records are biased
toward refractory biogenic components and
specific hydrodynamic
classes of particles
(Moore 1969; Prahl 1985). Information
concerning short-term variations in source
intensity and composition also can be attenuated or lost due to sediment mixing.
The most extensive sedimentary records
have been laid down over geologic time in
high energy coastal marine environments
(Tissot and Welte 1978) where differential
transport, selective degradation, and postdepositional mixing of marine and terrigenous organic materials are especially pronounced.
One means of evaluating the extent to
which1 sedimentary records reflect actual biological and geographic sources is to directly
compare the compositions of organic materials settling through the water column
with those preserved in the underlying sediments. Such studies usually involve anal-
1116
Organic matter sources
20’
123OW
40’
20’
1224
20’
123*W
40’
20’
1220
0’
Fig. 1. Dabob Bay and its relation to Puget Sound,
Washington. The inset shows the station site.
yses of sediment trap and core samples from
a given site and have now been carried out
in marine environments
for a variety of
compound classes including hydrocarbons
(Bates et al. 1984, 1987), sterols (Gagosian
et al. 1983), amino acids (Montani and
Okaichi 1985), neutral sugars (Cowie and
pigHedges 1984a), and photosynthetic
ments (Taguchi 1982; Furlong and Carpenter 1988). In almost all cases, distinct compositional
differences have been found
between settling and accumulating organic
materials. These differences often can be attributed either to short-term changes in organic matter sources or to selective degradation in the water column or at the watersediment interface. In only a few comparisons has it been possible, however, to distinguish these variables by yearlong sediment trap studies that take into account
seasonal variability
and establish the relative magnitudes of the water column and
sediment accumulation fluxes (e.g. Prahl and
Carpenter 1979; Prahl et al. 1980; Taguchi
1982; Bates et al. 1984). Similar comparisons have not as yet been reported for lignin,
a major phenolic constituent of vascular land
plants, nor evaluated in light of synoptic
data from other organic source indicators.
We report here a study of the lignin, elemental, and stable carbon isotope compositions of particulate organic materials in
the water column and surface sediments of
a coastal marine bay (Dabob Bay, Wash-
1117
ington). One goal of this study is to test for
short-term changes in the diagenetic state
and sources of particulate organic matter in
the water column of the bay as they relate
to local processes. In addition, these sources
will be compared to those indicated by the
underlying sediments in order to test how
well the sedimentary record matches the
seasonally averaged organic input both in
the types and amounts of materials present.
Finally, the lignin, stable carbon isotope,
elemental and previously published neutral
sugar (Cowie and Hedges 1984a) compositions of these samples are compared as
source indicators and a major biochemical
inventory is determined. A complementary
paper discussing the fluxes and reactivities
of lignins, carbohydrates, total organic carbon, and nitrogen in Dabob Bay follows.
Site description
Dabob Bay is a northward extending arm
(max depth, 195 m) of Puget Sound that is
separated from Hood Canal by a sill at 120
m (Fig. 1). All the samples described here
were collected at a station 110 m deep in
the northern bay that has been the site of
almost continuous sediment trap studies
over the last decade. Dabob Bay has been
used as a study site because it is similar to
the local open coastal ocean with respect to
plankton composition,
bloom periodicity,
and control of sedimentation by zooplankton fecal pellets (Winter et al. 1975; Shuman
1978; Bennett 1980). The bay also is removed from direct river influence (Ebbesmeyer et al. 1975) and is sufficiently deep
(> 100 m) to study processes in the water
column. Although its deep waters remain
oxic year-round (Ebbesmeyer et al. 1975),
the bay has gentle horizontal advection (~4
cm s-l: Kollmeyer
1965) below the thermocline that favors relatively efficient collection of particulate materials by sediment
traps (Staresinic et al. 1978; Lorenzen et al.
198 1; Butman et al. 1986). The underlying
elastic sediments are highly bioturbated
(Bennett 1980; Carpenter et al. 1985) and
become reducing within a few centimeters
of the surface, as is typical of many coastal
deposits.
A major portion of the published data on
the comparative compositions and fluxes of
1118
Hedges et al.
organic matter in the water column and sediments of coastal marine environments
comes from sediment trap studies in Dabob
Bay. Analyses of sediment core and monthly sediment trap samples from this site have
already been reported for aromatic and aliphatic hydrocarbons as well as organic carbon, nitrogen, and stable carbon isotopes
(Prahl and Carpenter 1979; Bennett 1980;
Prahl et al. 1980). The neutral sugar compositions of most of the samples to be described here also have been published
(Cowie and Hedges 1984a). Additional information
on primary production
rates
(Welschmeyer and Lorenzen 1985a,b), pigment and elemental fluxes (Bennett 1980;
Welschmeyer
and Lorenzen
1985a,b;
Downs and Lorenzen 1985), and sediment
compositions and accumulation rates (Furlong and Carpenter 1988) is also available.
Sampling procedure
Methods used to collect the plankton,
sediment trap, and sediment core samples
are described in detail by Cowie and Hedges
(1984a). Briefly, bulk “phytoplankton”
was
collected monthly for a year (July 198 1-July
1982) by matched noon and midnight vertical tows with a 64-pm-mesh net over the
upper 50 m of the water column. One sample split was frozen for stable carbon isotope
analysis and a second was stored in Formalin for counting. The remaining bulk
sample was separated by wet sieving into
1,700-, 850-, 300-, and 64-pm size fractions
which were frozen.
Self-closing, Hg-poisoned sediment traps
(Shuman 1978; Lorenzen et al. 198 1) were
deployed between plankton collection dates
(14 samplings total) in four-trap arrays
moored at 30, 60, and 90 m at the 110-mdeep study site. Individual traps had an id.
of 16 cm, a height-to-diameter
ratio of 3 : 1,
and were deployed with baffles (13-mmsquare apertures) positioned flush with the
cylinder mouth. Diagonal pairs of Hg-poisoned and untreated sediment traps were
included in three early deployments to test
the effects of poisoning, and duplicate pairs
of poisoned samples also were collected
during winter 1982 at all three water depths
to assess overall compositional
reproducibility. After large, physically intact zoo-
plankton “swimmers” were separated, particulate: material was recovered from all traps
by continuous centrifugation at 15,000 rpm
and washed with distilled water to remove
salt.
An undisturbed 50-cm-long sediment box
core was collected (2 1 July 198 1) at the trap
deployment
site and subsampled with a
plastic tube (8-cm i.d.). Subsamples from
the smaller core were immediately removed
at 2-cm intervals within a N,-filled glove
bag and centrifuged under N2 to recover pore
waters., which were analyzed for Fe, Mn,
N02-, N03-, NH4+, POd3-, SiO,, and alkalinity as described by Murray et al. (1978).
Sample preparation and analysis
Plankton, sediment trap, and sediment
core samples were freeze-dried for 2-4 d.
Sediment core samples were not corrected
for salt, which accounts for an average of
5% of total sample mass and would reduce
mass-based concentrations
proportionately. Weight percentages of organic carbon
(OC), inorganic carbon (IC), and total nitrogen (N) were determined with a Carlo
Erba model 1106 CHN analyzer using vapor phase acidification to remove and quantify carbonates (Hedges and Stern 1984).
Stable carbon isotopic distributions
were
determined on dried carbonate-free samples by Coastal Science Laboratories (Austin, Texas) and are reported here in 6l 3C
notation as the per mil (Q/00)relative deviation from the PDB carbon standard (Degens
1969).
Lignin-derived
phenols were produced by
Cu0 oxidation (Hedges and Ertel 1982) and
analyzed as their trimethylsilyl
(TMS) derivatives by gas chromatography
on two
30-m by 0.25-mm-i.d. fused silica capillary
columns coated with either SE-30 (100%
dimethyl polysiloxane)
or DB- 170 1 [86%
dimethyl-( 14%)-cyanopropylphenyl
polysiloxane] liquid phase. Dual analyses were
necessary because nonlignin Cu0 reaction
products from some lignin-poor
sediment
trap samples coeluted on the SE-30 column
with ferulic acid, p-hydroxybenzaldehyde,
and acetosyringone. Overall yields of these
three compounds were determined from the
DB- 170 1 analysis, after scaling to the final
SE-30 results on the basis of vanillic acid
Organic matter sources
1119
Table 1. Symbol definitions and units (when appropriate).
V
S
C
A
28
P
PO
s: v
$d: Al)v
(Ad : Al)s
oc
TC
i:;
:
6°C
w
(0:E)v
(0:E)t
AVPD
%VPD
%Ter
c
u
Total yield (mg) of vanillin, acetovanillon, and vanillic acid per 100 mg of sample organic carbon
Total yield (mg) of syringaldehyde, acetosyringone, and syringic acid per 100 mg of sample organic carbon
Total yield (mg) of p-coumaric and ferulic acid per 100 mg of sample organic carbon (total cinnamyl phenol yield)
Total yield (mg) of the above eight lignin-derived phenols per 100 mg of sample organic carbon
(A=V+S+C)
Total yield (mg) of the above eight lignin-derived phenols per 10 g of bulk sample
Total yield (mg) of g-hydroxybenzaldehyde, p-hydroxyacetophenone, and p-hydroxybcnzoic acid
per 100 mg of sample organic carbon
Total yield of p-hydroxyacetophenone per 100 mg of sample organic carbon
Weight ratio of total syringyl to total vaniilyl phenol yield
Weight ratio of total cinnamyl to total vanillyl phenol yield
Weight ratio of vanillic acid to vanillin
Weight ratio of syringic acid to syringealdehyde
Organic carbon
Inorganic carbon (carbonate)
Total nitrogen (organic plus inorganic)
Atomic ratio of carbon to nitrogen
Weight ratio of nitrogen to carbon
Per mil relative deviation of the 13C: 12Cratio of a sample from the 13C: 12Cof the Pee Dee
Belemnite (PDB) standard
Weight ratio of the observed yield of a compound from a sample normalized to the yield expected from only the vascular plant component
Weight ratio of the observed yield of a compound from a sample normalized to the yield expected from both the vascular plant and fresh plankton components
Calculated A of the vascular plant debris in a sample (obtained in a ternary mixing with S: V
and C: V as input data)
Estimated percentage of the total organic carbon in a sample that exists in the form of vascular
plant debris
Percentage of total organic carbon that is terrestrially derived
Spearman’s rank correlation procedure
Mann-Whitney U-test
which seldom
pounds.
coelutes
with
other
com-
Lignin biomarker methods
Lignins are phenolic polymers which occur uniquely in vascular land plants, where
they constitute up to 30 wt% of the mass of
woody tissues (Sarkanen and Ludwig 197 1).
Cu0 oxidation of lignin yields simple phenols that can be used to characterize lignin
in a wide variety of natural samples including soils (Ertel and Hedges 1984), sediments
(Hedges and Mann 19793; Hedges et al.
1982) and natural waters (Meyers-Schulte
and Hedges 1986; Ertel et al. 1986). The
total yield (in mg per 100 mg OC) of the
three vanillyl (V), three syringyl (S ), and
two cinnamyl (C) lignin-derived
phenols
(defined as A; Hedges and Mann 1979a)
serve as an approximate indicator of the
relative lignin content of organic mixtures.
The main symbols and their definitions are
listed in Table 1. The corresponding total
yield of the same phenols normalized to 10
g of bulk sample (Z8) is roughly proportional to absolute lignin content. The weight
ratios S : V and C : V are useful parameters
for characterizing lignins from different types
of vascular plant tissues (Hedges and Mann
1979a). Finally, field and laboratory studies
give evidence that ratios of vanillic acid to
vanillin, (Ad : Al)v, and syringic acid to syringaldehyde, (Ad : Al)s, can reflect the extent to which a lignin sample has been oxidatively
degraded by white-rot
:fungi
(Hedges et al. 1982, in press; Ertel and
Hedges 1984; Ertel et al. 1986).
Statistics
All intervals about arithmetic means are
shown here as f 1 SD and all confidence
intervals are at the 95% (P = 0.05) level
1120
Hedges et al.
unless otherwise indicated. All linear correlations are made with the orthogonal leastsquares method (Weisberg 1980) because
error is involved in both measurements.
Nonparametric
statistical tests are used
throughout because their assumptions are
less restrictive. The two specific tests that
were used and the abbreviations by which
they will be identified are: Spearman’s rank
correlation procedure (TP)for correlation between pairs of variables in one group (Zar
1974) and the Mann-Whitney
U-test for difference in variability
between two groups
(Sokal and Rohlf 198 1).
horizon. All other dissolved constituents
generally increased sharply below 1 cm, but
exhibited minima between 5 and 10 cm,
possibly due to irrigation.
Sediment core and trap samples-Elemental and stable carbon isotopic compo-
sitions of the sedimentary organic matter
varied little over the length of the 50-cm
box core (Table 2). Compositions were particularly uniform for the O-22-cm samples
(n = 10) for which the variabilities in %OC
(2.56&-O. LO), (C:N)a (12.0&0.3), and 613C
(- 22.1+ 0. 17~) were comparable to the corresponding analytical precisions (Hedges
and Stem 1984). No fine-grained carbonate
Results
was detected in any sediment core sample.
Net plankton samples-Vascular plant
Percent OC’ and atomic C: N values for
debris was not evident microscopically
in
the winter deployment of duplicate 30-,
the net plankton samples, which also did
60-, and 90-m poisoned sediment traps were
not include small (~64 pm) plankton. The
determined with average percent deviations
diatom Chaetoceros concavicornis domifrom the mean of IL 2.4 and IL 1.3%. The
nated (> 80% abundance) the 64-300~pm
average deviation in 613C between the dusize fractions through summer and early fall
plicate 60-m traps was *0.2?&~ Average
198 1, when the net plankton had an atomic
percent deviations from the mean for the
carbon to total nitrogen ratio,, (C: N)a, of nine pairs of Hg-poisoned and unpoisoned
6.3 and a 613C of -26.0%0. The net tow
sediment traps (three deployments at three
depths) were +4.9% for %OC and * 5.1%
samples collected during late fall through
for (C : N)a. Although there was no apparent
winter (14 October-l 1 February) contained
trend in %OC between poisoned and unonly small numbers of copepods and other
small zooplankton. A sample from the end poisoned traps, eight of the nine poisoned
of this period had a (C : N)a of 5.1 and a samples had lower (C : N)a values.
613C of -24 .6%0.
Over the study period, the OC contents
of the Hg-poisoned sediment trap samples
The 19 March 1982 collection was domranged from 2.5 to 13.5% with consistently
inated (N 90%) by the diatom Thalassiosira
low values (3.OOkO.3 1) throughout the water
decipiens. Another chain-forming
diatom,
Cerataulina bergonii, predominated (> 95Oo) column (Fig. 2a) for materials collected durFebruary).
in the: 2 April sample which had a 613C of ing winter (10 November-12
During most of the rest of the year %OC
- 19.5Ym. Phytoplankton
were absent from
the 14 May tow sample, whereas Phaeocys- became progressively smaller with increastis pouchetii and Nitzschia seriata were ing depth in the water column. Percentages
of ino:rganic carbon (Table 3) were appreequally abundant in the 23 April sample.
Skeletonema costatum (- 50%), N. seriata ciable (0.2-3.0%) only during spring and
(-20%),
and mixed Chaetoceros species summer months and also decreased with
water depth. Atomic C : N values (Fig. 2b)
(-20%) were found in the 10 June collection which had a (C: N)a of 5.4 and a g13C ranged from 6.5 to 14.7 and generally mirrored YoOC trends. (C : N)a ratios averaged
of - 22.0?&
13.0 f 0.9 for the nine winter samples and
Sediment pore waters-The ambient botexhibited no consistent relationship to detom water contained only trace (~2 PM)
ployment depth.
levels of Fe, Mn, N02-, and NH,+ along
A complete time series in 613C was dewith typical concentrations of POd3- (3 PM),
termined only for the midwater column (60
N03- (35 PM), and SiO, (83 PM). Dissolved
m) tra.ps (Fig. 2c), which exhibited values
nitrate was present only at trace concentrations throughout the 50-cm deep core, and in a relatively narrow range of - 2 1.9 to
- 23.5!&1 from summer through winter 198 1,
nit&e was detectable only in the O-l-cm
Organic matter sources
1121
MONTH
JASONDJFMAMJJ
16
(4
T
T
12 -
T
M
T
0-
T
T
M
B
M
B
M
4J
fi
B
BB
Te
%PpTBB
I
0
~~oomooooo-6w-c‘! c‘! c‘! c? h! c‘! c‘! c‘! c‘! “! N. h! c‘! ru 9
000000000000000
M
T
q
I
I
I
I
I
I
(b)
T
14 -
I4
12 -
Ii
10 -
T
v
BT
MM
T
8
T
AL----
T
TM
I
6
BP
la
I
i@
I
I
- 19
I
c1
I
9
(cl
M
-.
1
-25
-
M
P
I
0
P
I
I
I
I
100
200
DAYS AFTER 1 JULY
I
300
1981
I
4 0
Fig. 2. Weight percent organic carbon (%OC),
atomic carbon to nitrogen ratio, (C: N)a, and stable
carbon isotope composition (Bi3C, per mil variation
relative to the PDB standard; Craig 1953) vs. time for
particulate sediment trap samples from Dabob Bay.
Corresponding means (+- 1 SD) for 10 samples from
the 0-22-cm horizon of a sediment core taken at the
study site are illustrated by the pair of horizontal lines
in each plot. The 613Cvalues of net (64-300 pm) plankton samples (P) collected from the upper water column
at the same site are given in panel c. Other abbreviations: T-top (30 m), M-middle (60 m), and Bbottom (90 m) sediment trap samples.
a maximum of - 19.3% in March 1982, and
values of -20.1 to -2 1.4Ym thereafter. The
range in S13Cfor synoptic top, middle, and
bottom trap samples collected at three different times of the year was consistently
< l%+ with no clear trends vs. depth. Overall, elemental and stable carbon isotope
compositions of winter trap samples were
1122
Hedges et al.
similar to those of the underlying surface
sediments (Fig. 2).
Lignin compositions also changed little
over the length of the Dabob Bay core (Table 2) Within the 0-22-cn1 sediment horizon, variability in lignin compositional parameters was within the analytical precision
of + 10% (Hedges and Ertel 1982). Average
percent deviations of individual
lignin parameters for samples from poisoned duplicate traps ( 15 January-l 2 February 1982)
deployed at 30, 60, and 90 m were also
within + 10% except for C : V (IL 14%).
The S : V ratios of the Dabob Bay sedi.ment trap samples varied little with time or
deployment depth (Fig. 3a) .and had an
overall average of 0.26 -t-O.03 (n = 40). C: V
values were uniformly low (0.05 kO.01) at
all water depths during winter months (Fig.
3b), but increased (~0.08) during spring
1982 and summers of both 198 1 and 1982.
P : V values also were elevated (> 0.16) during nonwinter months (Fig. 3c) at which time
they decreased with increasing water depth.
The average S : V (0.21 +O.O l), C : V
(0.07+0.01), and P: V (0.12t0.01)
ratios
of the surface (O-22 cm) sediment were significantly (U) lower than those obtained
from both nonwinter and (in the case of S : V
and P: V) winter sediment trap samples.
Total absolute yields (mg) of lignin phenols per 10 g dry weight of sample (28).
ranged from about 2 to 11 throughout the
year (Fig. 4a) and were uniformly
high
(8.9+ 1.O) during winter at all three trap deploynlent depths. 28 increased steadily from
summer 198 1 into the winter maximum and
then decreased toward a second minimum
in summer 1982. During nonwinter months
28 values were typically higher for deeper
sediment traps (Fig. 4a).
Organic-carbon-normalized
yields of total lignin-derived phenols (A) also were high,
3.OkO.3 mg (100 mg OC)-’ in all winter
sediment trap samples (Fig. 4b). During the
rest of the year, A values ranged from about
0.2 to 1.9 and exhibited trends with season
and deployment depth similar to those observed for 28. The average 28 (6.OkO.4) and
A (2.3 + 0.1) values of the underlying surface
sedin1ents were significantly (U) lower than
the corresponding averages for the winter
sedinlent trap samples. Acid-to-aldehyde
MONTH
JASONDJFMAMJJ
-. _-
‘1
L
0.30
M
TTMM’TT
El
a
(4
BP
B
B
61’
BM
TT
4-Q
(#j 0.20
I
!
T
T”----
0.10
!
’ ’ l ’ I 1 .
O----I
T
M
(b)
T
0.20 -
T
M
M
-T
3.0
T
T
B
-T
2.0 -
(cl
T
$
T
1.0 MM
B
OO
T
T
MM
T
T
MBkmp.
I
100
MT
I
T
I
I
200
DAYS AFTER
1 JULY
Bb!
T
14
BE
I
300
I
400
1981
Fig. 3. S : V, C : V, and P: V vs. time for sediment
trap samples from Dabob Bay. The sediment ranges
(horizontal lines) are the means _t1 SD for the O-22cm depth interval. Symbols and abbreviations as in
text and Fig. 2.
ratios within the vanillyl,
(Ad : Al)v, and
syringyl,
(Ad : Al)s,
families
averaged
0.35kO.04 and 0.333-0.09 and were significantly associated (r,) with each other, but
not with time of year or depth of sediment
trap deployment (Table 3).
Discussion
Organic matter sources-Initial
observations about the sources of organic matter
to the water column and sediments of Dabob Bay can be drawn from the broad compositional characteristics of the sediment
trap and core samples. For example, the
measurable yields of lignin-derived
phenols
Organic matter sources
1123
MONTH
from all sediment core and trap samples
JASONDJFMAMJJ
(Fig. 4) indicate that vascular land plant de- ’
Iii
bris was present in the water column
10
(4
throughout the year and has accumulated
T t;lv
B
8
in the underlying sediments for about 100
‘7
M
I!!
yr (avg sediment accumulation rate = 0.5
M-T-BT
6 -B
T-Mcm yr-I: Carpenter et al. 1985). Secondly,
8
e-,8
B
8
the production
of syringyl and cinnamyl
4 .M M T
T Y
compounds by all samples (Fig. 3) demT
2
onstrates that both angiosperm and nonwoody vascular plant tissues, respectively,
must be continuously present (Hedges and
P
Mann 1979a). Finally, the low (C : N)a (< 15)
B 6
and A (< 3.5) values of all the sediment trap
u
and core samples (Figs. 2,4) relative to most
types of vascular plant tissues (Hedges et al.
1986) indicate that nitrogen-rich
organic
matter from sources other than vascular
plants is always present.
The nitrogen-rich organic component of
the sediment trap and core samples can be
100
200
300
400
better defined by a plot of %OC vs. %N (Fig.
DAYS AFTER 1 JULY 1981
5) which gives an excellent fit to a straight
Fig. 4. 28 and A vs. time for sediment trap samples
line (r2 = 0.96) over a wide concentration
from Dabob Bay. Other details as in text and Fig. 2.
range. The line extrapolates to a negative
%N value at zero %OC, indicating that essentially all the nitrogen in the samples is sion line all correspond to samples from top
organic (Hedges et al. 1986). The slope of (30 m) traps that were collected during nonthe line (a C : N weight ratio of 6.3), which
winter months. The lower (%OC < 3.5, %N
corresponds to an atomic C : N of about 7.5,
< 0.35) portion of the trend consists almost
indicates the composition of the nitrogenexclusively of data for winter trap samples
rich organic component. The higher values
(all depths). Thus, the elemental composi(%OC > 10.5, %N > 1.50) along the regrestion of the sediment trap samples can be
4:j!F,F;!,
, y$ 1
14
12
10
4
2
I
0’
.O
I
0.4
I
I
I
0.8
I
1.2
%N
I
I
1.8
I
I 0
Fig. 5. Weight percent organic carbon (%OC) vs. weight percent total nitrogen (%N) in sediment trap samples
from Dabob Bay. The sediment range is the mean f 1 SD for the 0-22-cm depth interval. For the regression
linc: slope = 6.29, Y-intercept = 1.09, r2 = 0.96.
2.3
3.1
0.0
0.0
0.0
0.1
0.1
0.2
0.1
0.3
0.0
0.7
0.7
0.5
0.8
0.4
0.0
0.0
0.0
0.2
0.2
0.2
0.1
0.1
0.2
0.4
0.1
5.30
7.66
10.40
6.52
6.56
3.21
3.19
2.98
6.15
5.58
7.90
9.25
5.98
3-21 Jul
21 Jul-20 Aug
20 Aug-15 Sep
15 Sep-15 Ott
15 Ott-10 Nov
10 Nov-16 Dee
16 Dee-15 Jan
15 Jan-l 2 Feb
19 Mar-2 Apr
2-23 Apr
23 Apr-14 May
14 May-10 Jun
10 Jun-8 Jul
%IC
8.89
12.50
13.50
11.00
6.86
2.50
3.32
2.49
2.88
7.03
6.04
11.20
12.10
6.90
%OC
3-21 Jul
21 Jul-20 Aug
20 Aug-15 Sep
15 Sep-15 Ott
15 Ott-10 Nov
10 Nov-16 Dee
16 Dee-15 Jan
15 Jan-12 Feb
12 Feb-19 Mar
19 Mar-2 Apr
2-23 Apr
23 Apr-14 May
14 May-10 Jun
10 Jun-8 Jul
Period
0.80
1.15
1.20
0.82
0.90
0.28
0.32
0.27
0.86
0.64
1.08
1.34
0.82
1.59
1.72
1.95
1.55
0.82
0.20
0.31
0.21
0.31
0.99
0.70
1.54
1.80
0.98
%N
7.72
7.77
10.1
9.24
8.54
13.4
11.6
12.9
8.35
10.2
8.53
8.05
8.56
6.52
8.48
8.08
8.28
9.76
14.6
12.5
13.8
10.8
8.31
10.11
8.48
7.84
8.21
(C : N)a
-21.9
-23.6
-23.5
-22.9
-22.5
-23.5
-23.0
-23.2
- 19.3
-20.1
-21.4
-21.4
-20.6
-21.9
-23.5
-22.9
-23.1
6°C.
PO
Trap degth
0.44 0.026
0.23 0.017
0.25 0.023
0.27 0.015
0.27 0.018
0.33 0.048
0.30 0.038
0.32 0.040
0.27 0.024
0.25 0.012
0.44 0.031
0.41 0.028
0.28 0.017
0.3 1 0.021
~-ran
-a-y depth
0.36 0.023
0.30 0.017
0.21 0.011
0.26 0.017
0.28 0.017
0.31 0.043
0.32 0.034
0.35 0.038
0.19 0.009
0.24 0.021
0.45 0.020
0.36 0.020
0.23 0.017
P
s
= 30 m
0.15 0.05
0.13 0.04
0.19 0.05
0.34 0.08
0.64 0.18
2.48 0.74
1.97 0.54
2.36 0.67
1.42 0.37
0.27 0.06
0.78 0.22
0.37 0.08
0.21 0.04
0.30 0.07
= 60 m
0.60 0.15
0.46 0.10
0.39 0.08
0.64 0.18
0.77 0.23
2.44 0.72
1.99 0.48
2.25 0.58
0.34 O.i2
0.77 0.21
0.57 0.13
0.41 0.09
0.39 0.11
V
0.04
0.01
0.01
0.04
0.05
0.12
0.10
0.13
0.03
0.08
0.15
0.08
0.08
0.02
0.03
0.01
0.03
0.03
0.10
0.08
0.11
0.08
0.04
0.10
0.10
0.05
0.07
C
0.78
0.57
0.48
0.86
1.04
3.27
2.57
2.95
0.50
1.05
0.85
0.57
0.58
0.22
0.19
0.26
0.45
0.85
3.31
2.59
3.13
1.87
0.37
1.09
0.55
0.30
0.44
A
P: v
4.15
4.40
5.03
5.59
6.84
10.51
8.21
8.80
3.05
5.88
6.70
5.30
3.48
0.60
0.65
0.54
0.40
0.36
0.13
0.16
0.16
0.55
0.31
0.78
0.87
0.58
1.98 2.92
2.43 1.74
3.46 1.30
4.96 0.78
5.80 0.43
8.29 0.13
8.59 0.15
7.79 0.14
5.40 0.19
2.59 0.93
6.58 0.57
6.18 1.10
3.68 1.29
3.03 1.03
23
0.25
0.22
0.21
0.28
0.29
0.30
0.24
0.26
0.35
0.27
0.22
0.21
0.27
0.32
0.28
0.28
0.24
0.29
0.30
0.28
0.28
0.26
0.24
0.28
0.21
0.19
0.22
s: v
0.07
0.03
0.03
0.07
0.06
0.05
0.05
0.06
0.10
0.11
0.26
0.18
0.21
0.15
0.20
0.04
0.07
0.04
0.04
0.04
0.05
0.06
0.15
0.12
0.27
0.23
0.22
C: V
0.36
0.43
0.41
0.32
0.31
0.33
0.36
0.33
0.35
0.34
0.35
0.38
0.40
0.38
0.41
0.42
0.35
0.35
0.38
0.38
0.33
0.33
0.35
0.34
0.38
0.41
0.38
(Ad : Al)v
0.29
0.38
0.45
0.31
0.29
0.26
0.29
0.28
0.34
0.29
0.33
0.37
0.33
0.21
0.45
0.33
0.35
0.34
0.27
0.34
0.30
0.34
0.50
0.29
0.37
0.67
0.29
(Ad : Al)s
Table 3. Lignin and elemental compositions of Dabob Bay sediment trap samples vs. water depth. Abbreviations as in Table 2, except %IC-weight
% inorganic carbon, and Po-p-hydroxyacetophenone in the same units as P.
Organic matter sources
1125
explained by the presence of two types of
particulate material: one of which has an
elemental composition
similar to the underlying sediment plus a second type which
is rich in organic matter with an average
(C : N)a of near 7.5 and is introduced via
the upper water column during nonwinter
months.. Similar trends in elemental composition have been reported for sediment
trap materials from various other coastal
marine environments
(Ansell 1974; Hargrave and Taguchi 1978; Chester and Larrance 1981; Taguchi 1982).
Plots of the individual
carbon-normalized lignin parameters V, S, and C vs. the
weight ratio of nitrogen to organic carbon,
(N : C)w, can be used to test the previous
mixing model and to determine whether the
nitrogen-rich component is lignin-free. For
this application the parameter (N : C)w is
preferable to conventional
nitrogen-normalized ratios which vary nonlinearly with
V, S, and C during simple mixing.
Plots of V, S, C, and A vs. (N : C)w all
give good fits to straight lines with significant negative slopes (Fig. 6). The intercepts
of all four lines lie between 0.15 and 0.16,
which corresponds to an atomic C : N of
7.3-7.8. These linear trends indicate that
mixing of nitrogen-poor vascular plant remains with a lignin-free organic component
of atomic C : N = 7.5 produces the lignin
and elemental concentration
trends observed in the Dabob Bay samples. The compositional characteristics of the nitrogen-rich
component [(C : N)a = 7.5, A = 0, %OC >
13.51, along with the timing and apparent
shallow depth of its introduction in the water
column (Fig. 2b), are all consistent with a
planktonic source and exclude a primarily
allochthonous origin. This conclusion is in
agreement with other sediment trap studies
at the same site, which indicate strong direct
relationships between the flux of total organic carbon and the flux of plankton-derived hydrocarbons (Prahl et al. 1980) and
pheopigments (Downs and Lorenzen 1985)
through the midwater column during nonwinter months. Such patterns also appear
to be typical of greater Puget Sound (Bates
et al. 1984; Baker et al. 1985) and other
coastal marine regions (e.g. Chester and
Larrance 198 1; Taguchi 1982).
qedges et al.
1126
3.0
(N:C)w
(N:C)w
Fig. 6. Total V, S, C, and A (inset) vs. the weight ratio of total nitrogen to organic carbon, (N: C)w, in
sediment trap samples from Dabob Bay. All lignin parameters are defined in the text. For the regression lines:
V-slope = -36.9, X-intercept = 0.15, r2 = 0.81; S-slope = - L0.4, X-intercept = 0.15, r2 = 0.80; C-slope =
-1.99, X-intercept = 0.16, r2 = 0.36; A-slope = -48.3, X-intercept = 0.15, r2 = 0.81.
All the Dabob Bay sediment trap and core
samples produce p-hydroxyl phenols (P : V
= 0.1-3.0) in patterns similar to those exhibited by the cinnamyl compounds (Fig.
3). The three p-hydroxyl
phenols (p-hydroxybenzaldehyde,
p-hydroxyacetophenone., and p-hydroxybenzoic
acid) are produced in appreciable amounts (P : V= 0.050.80) by the Cu0 oxidation of most vascular
plant tissues except angiosperm
woods
(Hedges 1975; Chang and Allan 1971) and
sometimes have been included in parameters used to estimate lignin concentrations
(e.g. Leo and Barghoorn 1970; Gardner and
Menzel 1974). However, p-hydroxybenzaldehyde and p-hydroxybenzoic
acid also have
been found among the Cu0 reaction products of a blue-green alga, a brown alga, a
fungus, and the amino acid, tyrosine (Hedges
1975) none of which produce measurable
amounts of p-hydroxyacetophenone
(Hedges
1975).
As a test of their respective origins, total
yields of all three p-hydroxyl
phenols and
the individual
yield of p-hydroxyacetophenone were normalized to 100 mg OC
(parameters P and PO, respectively) and
plotted vs. (N : C)w as was previously done
for r/; S, and C (Fig. 6). PO exhibits a significant
(r,) inverse relationship
with
(N : C)w and gives a reasonably good fit to
a straight line (r2 = 0.48) with a significantly
(rS) negative slope and an intercept near 0.19.
In contrast, P fluctuates between 0.2 and
0.5 and shows no association with nitrogen
content (Y,). Therefore, p-hydroxyacetophenone is largely lignin derived, whereas
the bulk of p-hydroxybenzaldehyde
and
p-hydroxybenzoic
acid comes from other
sources such as plankton. Thus, use ofp-hydroxyl phenols as lignin indicators can be
problematic without supporting evidence for
this source.
Vascular plant sources of the Dabob Bay
sediment core and trap samples can be evaluated by comparing their lignin compositional parameters (Tables 2, 3) to those of
different broad categories of fresh land plant
tissues. For such comparisons, the diagenetic state of the lignin-bearing
material is
an important consideration because microbial degradation can lead to substantial alterations of original lignin and carbohydrate
compositions (Crawford 198 1; Hedges et al.
1985; Ertel et al. 1986). In particular, whiterot fungal degradation of angiosperm wood
characteristically
increases the acid : aldehyde ratios in both the vanillyl and syringyl
phenol families and concomitantly
reduces
A and S: V (Hedges et al. in press).
The observation that the acid : aldehyde
ratios of the sediment trap samples [(Ad:
Organic matter sources
1127
1.2
1.0
0.8
$ 0.8
.
TT
o.o+Qcqbm.
0.0
0.4
I
0.8
I
I
1.2
MT
c:v
Fig. 7. S : V vs. C : V for sediment trap and core samples from Dabob Bay. Abbreviations: A-angiosperm
woods; a-nonwoody angiosperm tissues; G-gymnosperm woods; g-nonwoodygymnosperm tissues. All ranges
are averages k 1 SD. Other symbols and abbreviations as in text and Fig. 2. Representative lignin compositions
for the various types of vascular plant tissues are from Ertel and Hedges (1985).
Al)v = 0.35 -I 0.04, (Ad: Al)s = 0.33t-0.09,
n = 401 (Table 3) are uniformly elevated
above the range of 0.1-0.2 characteristic of
most fresh vascular plant tissues (Ertel and
Hedges 1984; Hedges et al. 1986) suggests
that the remnant lignin has been subjected
to white-rot fungal degradation (Hedges et
al. in press). Elevated acid : aldehyde ratios
at all water .depths and throughout the year
demonstrate that these compositions
are
fixed before introduction to the upper water
column at the study site. Because most fungi
are obligate aerobes and can be inhibited by
high water content (Levi 1973), the fungal
degradation evident in lignin from the sediment trap samples probably occurred on
land before introduction to the bay (see also
Hedges et al. 1986; Hamilton and Hedges
1988). The essentially constant acid : aldehyde ratios observed throughout the Dabob
Bay core (Table 2) suggest that in situ degradation of sedimentary lignin by white-rot
fungi has not occurred to an appreciable extent over the lOO-yr interval (Carpenter et
al. 1985) represented by this sequence, but
do not rule out other types of microbial degradation.
The (Ad: Al)v values of the Dabob Bay
core and trap samples (0.29-0.43) are near
the range (0.28-0.30) obtained for physically intact vascular plant fragments in a
modern Washington continental shelf sed-
iment (Ertel and Hedges 1985), but distinctly lower than corresponding ratios for bulk
soil organic matter (0.6-0.8) and for humic
substances in soils (0.6-2.5) and dissolved
in freshwater (0.8-2.3) (Ertel and Hedges
1984; Ertel et al. 1984, 1986). Thus, the
lignin in the Dabob Bay trap and sediment
core samples appears to occur primarily in
vascular plant debris, as opposed to more
highly degraded humic substances. Similar
conclusions have been drawn for ligninbearing materials in other modern coastal
sediments (Hedges and Parker 1976; Ertel
and Hedges 1985) and are supported by the
large quantities of finely dispersed vascular
plant remains that can be sieved from sediments at the study site (pers. obs.).
Because the lignin in the Dabob Bay samples is not extensively altered, estimates can
be made of the types and amounts of vascular plant debris they contain. Qualitative
interpretations of vascular plant sources can
be made from a plot of the S : V and C : V
values of individual sediment trap samples
(Hedges and Mann 1979a). On such a plot
(Fig. 7), the compositional
points for all
winter and most low-carbon nonwinter trap
samples fall in a vanillyl phenol-rich domain (S : V = 0.2-0.3; C: V = 0.03-0.08)
as is typical of many sedimentary lignin
mixtures
from the Washington
region
(Hedges and Mann 1979a; Hedges et al.
1128
Hedges et al.
1984; Ertel and Hedges 1985; Prahl 1985).
All compositional points for the other samples trend toward the nonwoody gymnosperm end member. This decrease in S : V
with increasing C: v is expressed exclusively by nonwinter sediment trap samples
from the upper (30 or 60 m) water column
(Table 3).
The direction of the S : v vs. C: V data
trend (Fig. 7) is different from any previously reported for the Washington region
(previous references). The observed trajectory, however, can be produced by combining a gymnosperm wood-angiosperm
wood
mixture (S : V g 0.23, C: v s 0) at varying
proportions with nonwoody gymnosperm
tissues such as conifer needles (S: I’ = 0,
C: V = 0.5; Table 4). In contrast, no combination of nonwoody angiosperm tissues
(such as tree leaves or grasses) with softwood or hardwood can account for the observed high C : I/ at low S : I/ characteristic
of the spring and summer samples. Thus,
the Dabob Bay samples appear to contain
at least three broad types of vascular plant
tissues and are the first local sedimentary
mixtures to give evidence for the specific
presence of nonwoody gymnosperm tissues.
A simple mixing model (Ertel and Hedges
1985) can be used to roughly estimate the
percentages (on an organic carbon basis) of
the various types of vascular plant tissues
in the Dabob Bay samples as well as the
total weight percent of carbon present as
vascular plant debris (VPD). In this model
the weight percentages of different fresh tissue types (in ternary mixtures) which best
fit the sedimentary S : I’ and C: I’ values
are calculated along with the percentage of
lignin-free organic carbon needed to dilute
the A of this hypothetical mixture (AVPD)
down to the A of the natural sample. This
model treats sedimentary vascular plant tissues as being well preserved and thus likely
underestimates angiosperm components due
to preferential syringyl phenol degradation
and overestimates vascular plant carbon due
to selective polysaccharide loss (Hedges et
al. 1985, in press).
When the lignin model is run for ternary
mixtures of gymnosperm and angiosperm
wood with either nonwoody gymnosperm
or angiosperm tissues, the calculated con-
centrations of gymnosperm wood, angiosperm wood, and total vascular plant debris
are all relatively insensitive (within k 10%)
to the choice of the nonwoody tissue type.
Therefore, based on the previously presented arguments against a major nonwoody angiosperm component, only the results for mixtures of nonwoody gymnosperm
tissues with gymnosperm and angiosperm
woods are discussed.
The calculated major components of vascular plant tissue mixtures in Dabob Bay
sediment trap samples (Table 4) are gymnosperm woods (46 & 16%), nonwoody
gymnosperm tissues (43 + 19%) and angiosperm woods (1 1 + 3%). High levels of both
gymnosperm
(55-60%) and angiosperm
wood ( lo- 15%) occur in all fall and winter
samples. Wood levels are lowest in spring
when nonwoody gymnosperm tissues typically predominate, particularly in the upper
water column. This seasonal pattern may
reflect the combined effect of a pulsed input
of nonwoody angiosperm tissues in spring
as well as preferential mobilization of coarser
wood-rich debris during winter storm events
(Prahl 1985; Hedges et al. 1988).
The estimated AVPD values of the previously described plant tissue mixtures range
from about 4.5 for spring to almost 8.5 for
winter trap samples (Table 4). The corresponding percentages of the sedimentary total organic carbon in the 30-m samples that
are represented by vascular plant debris
[%VPD = (A/AvPD) x 1001 range from a
maximum of 30-40% in winter to <5% in
summer (Fig. 8a). The calculated compositional characteristics of the sediment core
samples closely resemble those of the winter
trap materials, except that estimated levels
of angiosperm woods are slightly lower due
to the smaller S : I’ of the sediments. The
factor of 10 range in A values compared to
the AVPD range of 2 for sediment trap samples (Table 4) clearly indicates that the seasonal variation in A (Fig. 4b) is primarily
driven by dilution of vascular plant debris
with plankton remains.
The A of the total land-derived organic
matter in these samples can be estimated
by extrapolating the regression line in Fig.
6 to the (N : C)w of the terrigenous organic
fraction. Bennett (1980) has demonstrated,
Organic matter sources
1129
Table 4. Plant end-member mixing model results for trap and sediment samples from Dabob Bay. Abbreviations: A-angiosperm woods; G-gymnosperm woods; g-nonwoody gymnosperm tissues; %VPD-% total
carbon present as vascular plant debris; AVPD-A
value of the pure tertiary tissue mixture; %Ter-percentage
of total OC that is terrestrially derived.
Period
%A
3-21 Jul
21 Jul-20 Aug
20 Aug-15 Sep
15 Sep-15 Ott
15 Ott-10 Nov
10 Nov-16 Dee
16 Dee-15 Jan
15 Jan-12 Feb
12 Feb-19 Mar
19 Mar-2 Apr
2-23 Apr
23 Apr-14 May
14 May-10 Jun
10 Jun-8 Jul
11
8
14
11
15
16
15
14
13
8
10
5
5
6
3-21 Jul
21 Jul-20 Aug
20 Aug-15 Sep
15 Sep-15 Ott
15 Ott-10 Nov
10 Nov-16 Dee
16 Dee-15 Jan
15 Jan-12 Feb
19 Mar-2 Apr
2-23 Apr
23 Apr-14 May
14 May-10 Jun
10 Jun-8 Jul
11
12
11
13
14
15
12
13
14
11
6
6
8
3-21 Jul
21 Jul-20 Aug
20 Aug-15 Sep
15 Sep-15 Ott
15 Ott-10 Nov
10 Nov-16 Dee
16 Dee-15 Jan
15 Jan-12 Feb
19 Mar-2 Apr
2-23 Apr
23 Apr-14 May
14 May-1OJun
10 Jun-8 Jul
13
10
10
12
13
15
13
13
14
14
10
12
9
30-m traps
SD
60-m traps
SD
90-m traps
SD
Trap overall
SD
Sediment*
SD*
11
4
11
3
12
2
11
3
9
1
* Calculated
for the 0-22-cm
sediment
%ci
%
Trap depth 30 m
27
62
21
71
60
26
49
40
27
58
59
25
24
61
58
28
53
34
30
62
34
56
13
82
77
18
18
76
Trap depth 60 m
51
38
69
19
22
67
50
37
51
35
57
28
58
30
54
33
38
48
38
51
14
80
25
69
19
73
Trap depth 90 m
56
31
54
36
51
39
57
31
30
57
57
28
61
26
59
28
56
30
53
33
43
47
40
48
32
59
Averages and standard deviations
40
49
19
22
46
43
17
20
52
36
9
10
46
43
16
19
37
54
2
2
depth interval
(n = 10).
%VPD
AVPD
%Ter
4
4
3
6
10
40
31
39
25
6
17
12
7
9
6.0
5.3
8.2
7.2
8.2
8.3
8.3
8.1
7.7
5.8
6.3
4.4
4.7
4.9
6
5
7
13
24
95
74
89
53
11
31
16
9
13
11
7
6
11
14
40
33
38
7
16
18
11
11
7.3
8.4
8.1
7.5
7.7
8.1
7.8
7.7
7.0
6.6
4.6
5.3
5.2
22
16
14
25
30
93
73
84
14
30
24
16
17
17
7
9
12
16
40
36
36
15
22
24
14
13
7.8
7.2
7.2
7.7
7.8
8.2
8.0
8.0
8.0
7.8
6.7
6.8
6.0
39
14
19
25
37
93
83
83
34
49
46
26
23
15
13
17
12
20
11
17
12
33
2
6.7
1.5
7.0
1.2
7.5
0.6
7.1
1.2
7.2
0.1
32
32
35
28
44
26
37
29
65
4
1130
Hedges et al.
based on inorganic sedimentary constituents, that the Dosewallips River (Fig. 1) is
likely the major source of terrigenous detrital materials to Dabob Bay. Other smaller
rivers emptying into southern Hood Canal
drain basins of similar relief and vegetation.
Two samples of suspended particulate material collected by Bennett (1980) in summer and winter from the lower Dosewallips
River had (C : N)a values of 13.5 and 17.4,
respectively, which correspond to an average (N : C)w of 0.076. Extrapolation
of the
A vs. (N : C)w plot (Fig. 6) to this value yields
a A of 3.5.
The maximal fraction of vascular plant
debris in the terrigenous component of each
sample (estimated by dividing 3.5 by the
corresponding AVPD) averages about half for
all trap and core samples. This result suggests that a major portion of the bulk terrigenous organic carbon is present in other
forms such as soil organic matter, charcoal,
or fossil substances associated with sedimentary rocks. Because the (C : N)a range
of the particulate organic matter discharged
by the Dosewallips River is distinctly lower
than values (25-500) typical of vascular
plant tissues (Hedges et al. 1986) the riverine vascular plant debris must either carry
considerable immobilized nitrogen (Gosz et
al. 1976) or be mixed with relatively nitrogen-rich materials such as soil organic matter (atomic C : N g 10-l 2; Meybeck 1982;
Hedges et al. 1986).
The percentage of total terrestrially derived organic carbon (%Ter) in the individual samples can be estimated by dividing
their measured A value by 3.5. This calculation involves the assumption that the lignin-bearing and bulk particulate organic
matter discharged by local rivers maintain
a constant ratio throughout transport and
deposition.
The resulting percentages of
bulk terrigenous organic carbon for individual trap samples (Table 4) run about twice
the previously discussed %VPD levels with
values consistently exceeding 70% at all
depths in the winter and dropping below
20% in the upper water column during
bloom periods (Fig. 8b). Average estimated
percentages of terrigenous organic carbon in
the 30-, 60-, and 90-m sediment traps are
about 30,35, and 45%, with a sediment core
MONTH
8
JASONDJFMAMJJ
j
4o
n
30
g
20
4
g
lo
3
g
g
0
100
80
PUNT
DEBRIS
2
3
60
3
0
40
E
2
20
0
SEDIMENT
100
200
300
DAYS AFTER 1JULY 1981
Fig. 8. a-The calculated maximal percentages of
total organic carbon occurring as gymnosperm woods
(G), nonwoody gymnosperm tissues (g), and angiosperm woods (A) vs. time; b-The percentages of total
terrigenous organic carbon (present in chemically recognizable plant debris and other forms) vs. time within
the 30-m sediment trap and bottom sediment samples
from Dabob Bay.
0
average of 65% (Table 4). The conclusion
that most of the particulate organic material
accumulating in the sediments of Dabob Bay
is of terrestrial origin agrees with both aluminum-normalized
carbon flux calculations (Bennett 1980) and the paucity of mar-me-derived hydrocarbons in the deposit
(Prahl et al. 1980). The compositional data
alone, however, do not indicate whether the
elevated concentrations of lignin and bulk
terrigenous organic matter within the sediment result from mixing or digenesis (see
Hedges et al. 1988).
Correlations with other source indicators -Stable carbon isotope compositions
have found wide application as indicators
of the relative amounts of marine- and terrestrially derived organic materials in a variety of coastal marine environments
(Fry
Organic matter sources
and Sherr 1984). In particular, excellent
agreement between A and 613C as indicators
of terrigenous organic carbon has been reported for modern sediments from the
southern Washington
continental
shelf
(Hedges and Mann 1979b).
The Dabob Bay sediment trap samples
exhibit almost a 4%0 annual range (-23.6
to - 19.3YL~)in 613C(Fig. 2c), which includes
the average sediment
core value of
-22.1 +O. I?&. These 613C values do not exhibit a statistically significant association (rS),
however, with either A (Fig. 4b) or (C : N)a
(Fig. 2b). This lack of correlation between
613C and the latter two internally consistent
(Fig. 6) source indicators suggests that the
stable carbon isotope compositions of the
Dabob Bay samples do not result from simple mixing of a 13C-depleted terrigenous
component with 13C-rich autochthonous organic matter, as is often observed (Fry and
Sherr 1984). A similar conclusion was drawn
by Prahl et al. (1980) based on trends in
613C vs. C : N and pristane during an earlier
(1977-1978) sediment trap time series at
the same site and depth.
The isotopic data for the four seasonal
net tow samples collected during the 198 l1982 study (Fig. 2c) indicate that the variability of 13C : 12C within local net plankton
may partially explain the lack of a clear association of 613C with other source indicators in the sediment trap and core samples.
Microscopic observation of all four bulk net
tow samples indicated that they were essentially free of vascular plant remains. However, the 613C values ranged from -26.0?&
for a fall sample rich in C. concavicornis to
- 19.5Ym for a spring sample comprised almost exclusively of C. bergonii. The 6.57~
range in 613Cbetween these two diatom-rich
mixtures is comparable to the “usual” difference between marine- and terrestrially
derived organic carbon in many temperate
coastal marine environments (Degens 1969;
Fry and Sherr 1984). Comparable carbon
isotopic variations in marine phytoplankton have been observed at the other temperate marine sites (Fry and Sherr 1984;
Gearing et al. 1984).
The Dabob Bay sediment trap materials
generally exhibit more positive 613C values
than net plankton collected during the same
1131
season, but still clearly reflect the ~Y&Jchange
in 613C between the winter and spring net
plankton (Fig. 2~). Thus net plankton appear to strongly affect, but not completely
control, the isotopic composition of the sediment trap samples. Either preferential loss
of 12C from net plankton-derived
organic
matter or an additional unsampled source
of isotopically “heavy” marine organic carbon (possibly in the <64-km size fraction
missed by the sampling net) is needed to
produce the generally observed enrichment
of 13C in the trap vs. the net plankton samples.
Carbohydrate-lignin relationships- Neutral sugar compositions have been reported
for many of these same samples and compared to patterns obtained from various
marine and vascular plant sources (Cowie
and Hedges 1984a). The high yields of ribose and fucose and the low yields of total
neutral sugars and a-cellulose-derived
glucose that were obtained are typical of plankton and bacteria and indicate that the carbohydrate components of the Dabob Bay
sediment trap and core samples are primarily marine derived. Although trends toward the distinctly separate compositional
ranges of fresh vascular plant tissues are observed in the winter trap and sediment core
samples (Cowie and Hedges 1984a), associations (r,) between A and the carbohydrate
parameters used to discriminate marine vs.
terrigenous sources (e.g. ribose- and fucoserelated parameters) are weak. In addition,
S : V is not associated (rS) with %(xylose),
or mannose : xylose as would be expected
for angiosperm sources. Also %(mannose),
and mannose : xylose do not exhibit a significant
association
with either
I’ or
%VPD, both of which should be directly
related to levels of gymnosperm tissues
(Cowie and Hedges 1984a).
These generally poor associations are surprising considering the high levels of vascular plant debris (Fig. 8) in the core and
winter sediment trap samples and the factor
of 5-l 0 higher yields of neutral sugars that
are typically obtained from fresh vascular
plant tissues vs. marine plankton (Cowie
and Hedges 1984a). On the basis of the extensive polysaccharide losses previously reported for peats (Hatcher et al. 1983) and
1132
Hedges et al.
sediment-buried woods (Hedges et al. 1985),
it is reasonable to expect that the vascular
plant debris in the Dabob Bay samples also
may have lost a major portion of their original carbohydrates.
MONTH
JASONDJFMAMJJ
n
6,
-1
(4
X
X
X
from the vascular plant component of the
Dabob Bay samples can be made by comparing the total yield of an individual neutral sugar from a sample to the amount that
should be obtained solely from the fresh
vascular plant components (Hamilton and
Hedges 1988). Such observed-to-expected
ratios for vascular plant sources, (0 : E)v,
can be determined for individual sugars by
dividing the actual yield by the sum of the
fractions of individual
plant tissues (Table
4) multiplied by their average yield of the
same sugar (Cowie and Hedges 1984a). Any
sugar whose (0 : E)v ratio is substantially
< 1 must be depleted in the vascular plant
component
(see Hamilton
and Hedges
1988).
For simplicity,
(0 : E)v ratios are presented only for individual neutral sugars in
the sediment core (O-22-cm average) and
30-m sediment trap samples, the latter of
which exhibit the extreme compositional
contrasts observed in the sample set (Table
3). The nine neutral sugars fall into two
groups based on their (0 : E)v ratios (Fig.
9a). Group 1 consists of glucose, lyxose,
mannose, and xylose which all are produced
from the sediment core and winter trap
samples in smaller amounts than expected
from vascular plant sources alone. All four
of these sugars are produced in characteristically high yields by vascular plant tissues
(Sjostrijm 198 1; Cowie and Hedges 1984a).
The mean (0 : E)v ratios over the 3 months
of winter for glucose (0.36), lyxose (0.39)
mannose (0.49), xylose (0.82), and total carbohydrates (0.62) correspond to average depletions of about 65, 60, 50, 20, and 40%,
respectively. The corresponding depletions
within the underlying surficial sediment (Fig.
9a) are comparable and well within the range
of alteration found previously for sedimentburied woods (Hedges et al. 1985).
The (0 : E)v ratios of lyxose, mannose,
and xylose in sediment trap samples from
the nonwinter months are consistently > 1
(Fig. 9a). Glucose ratios for the 30-m sed-
xG
MX
A test for preferential polysaccharide loss
Y
M
-----G-z-
1
G
X
x--x--,-
t
M
G
M
x
E
M
--G&--
hii
x
0
!
I
A
F
(b)
10 -
R
R
2
I
OO
I
I
100
DAYS
I
I
200
AFTER
I
1 JULY
I
300
1981
1
A
SEDIMENT
Fig. 9. Observed to expected (0 : E) yield ratios for
selected sugars vs. time in 30-m sediment trap and
sediment core samples. a. Ratio of observed yield to
the expected yield from the vascular plant component
alone, (0 : E)v. b. Ratio of observed to expected yields
from the vascular plant and plankton components
combined, (0 : E)t. Abbreviations: G-glucose; Mmannose; X -xylose; A- arabinose; F- fucose; Rrhamnose; T-galactose.
iment trap samples, however, are < 1 during
periods of spring and autumn when appreciable amounts of plankton-derived
organic
matter are present (Fig. 8). Glucose, therefore, must be depleted in the plankton component as well. Evidence that glucose, a
common storage sugar, is particularly susceptible to degradation has also been observed for anoxic sediments (Hamilton and
Hedges 1988) as well as suspended particulate (Ittekkot et al. 1982) and sediment
trap materials (Cowie and Hedges 1984a;
Hamilton and Hedges 1988).
The reason that the (0 : E)v ratios of the
four group 1 sugars are not < 1 during nonwinter months probably is because the low
overall VPD concentrations in these samples (Fig. 8) greatly reduce the potential to
detect carbohydrate depletion in the terrigenous component. Based on the uniform
(Ad : Al)v values observed for all the sediment trap samples (Table 3), it seems reasonable to expect that the woody plant de-
Organic matter sources
1133
MONTH
2oJ
r
A
S
0
N
M
M
‘PZdLlGI;IN
0
POLYSA
$? 16
c
t
0
A
100
DAYS
200
AFTER
1 JULY
300
1981
J
IARIDE
SEDIMENT
Fig. 10. Calculated weight percentages of the total organic carbon contributed by lignin and polysaccharide
vs. time for 30-m sediment trap and surface (O-22 cm) sediment samples. The factors used to convert neutral
sugar yields to the carbon equivalents are based on the wt% of carbon in the molecules, the water loss involved
in polymerization to polysaccharides, and the corresponding average recovery efficiencies reported by Cowie
and Hedges (1984b, their table 4). The factors used for converting lignin phenol yields to total lignin carbon are
based on the wt% of carbon in the molecules, the fractional carbon recovery vs. an intact phenyl propane unit
of equal methoxy content, and theoretical production efficiencies of 30, 90, and 100% for vanillyl, syringyl, and
cinnamyl phenols (Chang and Allan 197 1).
bris introduced during nonwinter months
will have suffered similarly extensive carbohydrate losses.
A testfor “excess”sources among the other (group 2) sugars can be made by dividing
the actual yield of an individual
sugar by
the maximum total yield that could be expected from fresh vascular plant material
and plankton combined (Hamilton
and
Hedges 1988). To determine this second total ratio, (0 : E)t, the maximal plankton
contribution
was determined by multiplying the fraction of nonvascular plant carbon
in the samples (Fig. 8a) by the average yields
of individual neutral sugars from Dabob Bay
phytoplankton
(Cowie and Hedges 1984a).
These calculations
give maximum
estimates of expected carbohydrate yields because preferential loss of lignins vs. neutral
sugars is rare in nature (Crawford 198 1;
Tanoue et al. 1982; Hedges et al. in press),
and the vascular plant component actually
is carbohydrate-depleted
(previous discussion). In addition, all nonvascular plant material is assumed to be present as plankton
even though substantial amounts of fossil
or highly degraded soil organic materials
apparently are present (Fig. 8) and can be
expected to be relatively carbohydrate poor
(Swain 1969). Finally, in order to allow for
the possibility that the vascular plant tissues
(< 50% of total sample carbon) might contain little or no polysaccaride, an (0 : E)t of
2 (corresponding to a pure phytoplankton
source) is taken as the lower bound for an
excess source for each sugar.
Three of the group 2 sugars (galactose,
rhamnose, and fucose) all have (0 : E)t ratios > 2 and thus are produced in substantial
excess of expected maximal plankton plus
vascular plant yields from all core and 30-m
trap samples (Fig. 9b). The degree of excess
production
is especially pronounced for
rhamnose and fucose, which have average
(0 : E)t ratios near 8 and 5 and therefore
come predominately
from unidentified
sources. Strong excess sources for these two
deoxy sugars are also indicated by similar
treatment of compositional data for plankton, sediment trap, and core samples from
Saanich Inlet (Hamilton and Hedges 1988).
Although the biological origins of the group
2 sugars are yet to be determined, bacteria
can yield high relative concentrations
of
rhamnose and fucose (Cowie and Hedges
1984a; Kenne and Lindberg 1983) and are
likely candidates. Whatever the biological
sources, the fact that the 30-m trap samples
1134
Hedges et al.
exhibit high (0 :E)t ratios (Fig. 9b) indicates that the excess sugars are derived either
from land or the upper water column.
A major biochemical inventory--It is possible from the present data set (Tables 2, 3;
Cowie and Hedges 1984a) to calculate the
maximal fraction of the total organic carbon
in the Dabob Bay samples that can be accounted for as lignin and polysaccharide and,
by difference, the minimal fraction that must
be present in other forms. These calculations are applied to the 30-m sediment trap
samples and the underlying sediment (O-22
cm) in order to evaluate the widest range of
compositions. The factors used to convert
molecule mass to biopolymer carbon are
given in the caption to Fig. 10. On average,
total polysaccharides account of 9% of the
total organic carbon in the sediment trap
samples and 6% of total carbon in the surface sediment (Fig. 10). Glucose, the predominant neutral sugar, contributes about
a third of the total polysaccharide carbon.
In comparison, lignin accounts for about
0.5-7% of the bulk organic carbon, with
highest values in the winter trap and sediment core samples. Approximately
90% of
the total lignin carbon in the gymnosperm
wood-rich Dabob Bay samples is present in
vanillyl (guaiacyl) structural units. Together, chemically recognizable polysaccharides
and lignins comprise about 5-20% of the
total organic carbon in the sediment trap
samples with average levels near 10% for
both the sediment trap and core samples
(Fig. 10).
The previous calculations leave about 90%
of the total organic carbon unaccounted for
at the molecular level. The maximal amount
of the remaining uncharacterized
organic
carbon that could be present as protein can
be estimated by multiplying the weight percent of bulk nitrogen by the average weight
ratio of C : N (3.3) in phytoplankton protein
(DiTullio and Laws 1983). This calculation
provides only an upper bound for protein
carbon because a major portion of the total
nitrogen likely is present in other forms such
as humic substances (Lee and Cronin 1982;
Henrichs et al. 1984). These maximal estimates of protein carbon in the sediment
trap samples still range from only 25 to 60%
with a mean value near 40, as compared to
a value of 35% for the surface sediment horizon. The resulting maximal estimates of
total carbon in the form of major biochemicals (polysaccharides + lignin + protein)
average about 50% for the 30-m sediment
trap samples and 45% for the underlying
sediment. Thus even with these extremely
generous estimates, less than half of the total
organic carbon can be accounted for on average as major biochemicals. The bulk of
the organic carbon in the Dabob Bay sediment trap and core samples must be present
in other forms such as lipids, fossil organic
matter, and hydrolysis-resistant
polymers.
Overview
Large seasonal fluctuations occur in the
types and relative amounts of terrigenous
and marine-derived particulate organic materials in the water column of Dabob Bay.
Both the lignin and polysaccharide components of the vascular plant debris, which
comprises roughly half of the terrigenous
component, have been measurably degraded before introduction to the bay. Little evidence for any of these observations can be
drawn solely from the detailed analysis of
a sediment core taken at the study site. Even
these recently deposited sediments provide
an incomplete and biased record of the
sources and modes of introduction
of particulate organic materials in this coastal marine environment.
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Submitted: 13 July 1987
Accepted: 23 September I987
Revised: 23 June 1988