FROELICH, P. N. Analysis of organic carbon in marine sediments

FROELICH, P. N. Analysis of organic carbon in marine sediments

564
Notes
ganics in lakes, p. 119-143. Zn S. D. Faust and
J. V. Hunter
[eds.], Organic compounds
in
aquatic environments.
Dekker.
DOBBS, R. A., R. H. WISE, AND R. B. DEAN. 1972.
The use of ultra-violet
absorbance for monitoring the total organic carbon content of water
and wastewater.
Water Res. 6: 1173-1180.
GHASSEMI,M.,AND
R.F. CHRISTMAN. 1968. Properties of the yellow organic acids of natural
waters. Limnol. Oceanogr. 13 : 583-597.
LEVESQUE, M. 1972. Fluorescence
and gel filtration of humic compounds. Soil Sci. 113: 346-
OTSUKI, A., AND R. G. WETZEL. 1973. Interaction
of yellow organic acids with calcium carbonate
in freshwater. Limnol. Oceanogr. 18: 490493.
-,
AND -.
1974. Calcium and total alkalinity budgets and calcium carbonate precipitation of a small hard-water lake. Arch. Hydrobiol. 73: 14-30.
SMART, P. L., B. L. FINLAYSON, W. D. RYLANDS,
AND C. M. BALL. 1976. The relation of fluorescence to dissolved organic carbon in surface
waters. Water Res. 10: 805-811.
SPAIN, J. D., AND S. C. ANDREWS. 1970. Water
mass identification
in a small lake using conserved chemical constituents.
Proc. 13th Conf.
Great Lakes Res. 1970: 733-743.
SWIFT, R. S., AND A. M. POSNER. 1971. Gel chromatography of humic acid. J. Soil Sci. 22: 237249.
TAN,
K. H., AND J. E. GIDDENS.
1972. Molecular
weights and spectral characteristics
of humic
and fulvic acids. Geoderma 8: 221-229.
WASSERMAN,
A. E. 1965. Absorption
and fluorescence of water-soluble
pigments produced by
four species of Pseudomonas. Appl. Microbial.
13: 175-180.
WETZEL,
R. G., AND A. OTSUKI.
1974. Allochthonous organic carbon of a marl lake. Arch. Hydrobiol. 73: 31-56.
353.
LEWIS, W. M., AND D. CANFIELD. 1977. Dissolved
organic carbon in some dark Venezuelan waters
and a revised equation for spectrophotometric
determination
of dissolved
organic carbon.
Arch. Hydrobiol.
79: 441445.
AND J. A. TYBURCZY. 1974. Amounts and
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of dissolved organic compounds from some freshwaters
of the southeastern U.S. Arch. Hydrobiol.
74: 8-17.
MANTOURA, R. F., AND J. P. RILEY. 1975. The analytical concentration
of humic substances from
natural waters. Anal. Chim. Acta 76: 97-106.
OGURA, N., AND T. HANYA. 1966. Nature of ultraviolet absorption of sea water. Nature 2 12: 758.
-,AND
-.
1967. Ultra-violet
absorption
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organic matters. Int. J. Oceanogr. Limnol.
91-102.
Limnol.
Oceanogr.,
25(3), 1980, 564-572
0 1980, by the American
Society of Limnology
Analysis
of organic
and Oceanography,
Inc.
carbon in marine
Abstract-A
method is presented for measuring organic carbon in both carbonate-rich
and carbonate-poor
marine sediments.
Samples are sonicated with phosphoric acid to remove carbonates. The spent acid supernatant
is analyzed for dissolved organic carbon, the
solid residue for total carbon; their sum yields
the organic carbon content of the sample. The
technique
is free from carbonate
interferences, involves no losses due to acid solubilization,
and has excellent
precision
(better
than -+0.02% C,,,) and accuracy (better than
22%).
Three general techniques
have been
developed for the measurement of organic carbon in marine sediments. In the
first, wet oxidation of organic carbon is
effected by a strong oxidant. In some
l This research
DES-76-02318.
Submitted: 4 June 1979
Accepted: 2 November 1979
was supported
by NSF
grant
sediments’
cases, the remaining
oxidant (originally
added in known excess) is measured by
back-titration
(e.g. Allison 1935; Duursma 1961; Froelich et al. 1971). In other
cases, the evolved CO, is measured directly (e.g. Hartmann et al. 1976; Mills
and Quinn 1979). Second, organic carbon
is measured as the difference in total carbon before and after dry combustion
at
some high temperature
(usuallv
450”600°C) (e.g. Trask 1932; Radar and Grimaldi 1961; Betzer et al. 1974; Heath et
al. 1977). In the third technique, the sample is leached with acid to eliminate
CaCO,, and the residue solid is analyzed
for total carbon (e.g. Hiilsemann
1968;
Gibbs 1977).
Each of these is susceptible to systematic errors. Wet oxidation may be affected
by redox interferences
(oxidation
of reduced species other than organic carbon:
565
Notes
Nino et al. 1969), uncertainties
concerning the redox state of sedimentary organic carbon, and inability
to oxidize refractory organic materials completely by wet
oxidation.
Techniques
of difference
on
combustion may suffer from the inability
to separate inorganic and organic carbon
by dry combustion
(Gibbs 1977; see below) and from the statistical problem of
subtracting
one large number
from
another to obtain a small, uncertain residual (in carbonate-rich
sediments). Analyses of total carbon after acid leaching
may be systematically
low due to loss of
acid-soluble
organic carbon during carbonate dissolution
(Roberts et al. 1973;
Heath et al. 1977; see below).
To circumvent
such problems, I have
developed a new technique to measure
organic carbon in marine sediments. This
technique has been used to analyze samples from the Atlantic and Pacific oceans
(Table 1; Froelich 1979).
I thank J. G. Q uinn for the use of his
Oceanography
International
carbon analyzer and M. E. Q. Pilson for the use of
his Carlo Erba CHN/OS microanalyzer.
G. Mills assisted in the DOC analyses. N.
Luedtke perftirmed
the neutron activation calcium analyses.
The general approach is to remove
CaCO, from a dried sediment sample by
treatment with H,PO+ The solid residue
is then analyzed for total carbon by instrumental
CHN microanalysis
and the
acidic supernatant for acid-soluble organic carbon by a modified dissolved organic
carbon (DOC) method (Menzel and Vaccare 1964; Kerr 1977). The sum of the two
determinations
equals the organic carbon
content of the sediment.
Samples are air- or freeze-dried,
powdered, and oven-dried at 110°C for several hours. Mills and Quinn (1979) found
no loss of organic carbon from samples
dried at 110°C for 4 h.
All glassware should be Pyrex, fired at
550°C overnight to remove traces of organic carbon. Reagents used in the DOC
analysis are as described by Strickland
and Parsons (1968), with the following
modifications.
An organic carbon-free
1
M (6%) H,PO, solution is prepared by
Table 1. Locations
and water depths
samples analyzed for organic carbon.
5GCl
10GCl
14GCl
27GCl
28GCl
7BC15B2
9BC26B2
Atlantic (G-76-5)
2”51.8’N,6”42.7’W
1”05.1’N,8”11.6’W
0”00.1’s,12”19.3’w
4”59.1’N,5”06.0fW
4”55.9’N,5”05.4’W
Pacific (M-INMD77-I)
8”49.0’N,103”59.8’W
6”33.2’N,92”45.4’W
of core
4,563
4,956
4,170
80
229
m
m
m
m
m
3,116 m
3,568 m
adding
10 g of potassium
persulfate
(K&O,)
and 60 ml of concentrated (85%)
H,PO, to 1 liter of tapwater. This solution
is refluxed gently for about 4 h and then
cooled and stored in a tightly stoppered,
prefired, all-glass bottle. Organic carbonfree water is prepared in the same manner.
A 200-ppm-C primary DOC standard is
prepared by adding 0.9500 g of reagent
grade sucrose to 2 liters of C-free water.
Working standards can be prepared easily by spiking the primary standard directly into ampoules containing 5 (or 10)
ml of DOC-free
H,PO,. Acetanilide
or
nicotinic
acid (Natl. Bur. Std. reference
materials) are used to calibrate the response of the CHN analyzer.
About 200 mg of powdered, dried sample is weighed accurately into a 40-ml
centrifuge tube. Thirty milliliters
of the
1 M H,PO, solution is added slowly until
the reaction ceases and then more rapidly
to wash down the walls of the tube. The
tube and its contents are suspended in a
sonication bath for about 30 min. Sonication drives CO, from solution and thus
speeds the reaction
CaCO, + 2H+ + Ca”+ + H,O + CO,.
The tubes are then centrifuged
until the
supernatant is clear.
Aliquots (5 or 10 ml, depending on concentration)
are pipetted into DOC ampoules. After addition of 0.2 g of K&O,
the sample is bubbled with N, for 6 min
to remove dissolved
inorganic
carbon.
The ampoules are then sealed and heated
to 110°C for at least 1 h. Substantial practice is needed to achieve reliable sealing
Notes
566
Table 2. Comparison
of carbon and nitrogen
standards against NBS acetanilide
with the Carlo
Erba 1106.
m
NBS acetanilide:
(n =8)
C senstivity:
6.624 x 10e4 pg-C/unit-CO,-area
N sensitivity:
1.764 x 1O-3 pg-N/unit-N,-area
theoretical
%C = 71.09
theoretical
%N = 10.36
0.8
so
00
OI
NBS nicotinic
%C found
theoretical
%N found
theoretical
0.4
0.0
0
0.8
0.4
w,:
1.2
w,
Fig. 1. Plot of residual weight after carbonate
dissolution
(W,:W,) vs. weight expected if only
CaCO, were dissolved [l - (%CaCO,/lOO)]. The 1: 1
line represents expected relationship.
Lines +3%
and -3% represent systematic gain (or loss) of 3%
of material other than CaCO,. For example, dashed
line represents weight loss due to washout of interstitial sea salts (~2%).
and consistent
blanks. Standards and
blanks are carried through the same procedure. Acidification
of samples is unnecessary because enough excess acid remains after decalcification
(see Menzel
and Vaccaro 1964; Strickland and Parsons
1968; Kerr 1977; Goulden and Anthony
1978).
The DOC concentrations
were measured with an Oceanography International model 0524 carbon analyzer which detects CO, with a nondispersive
infrared
detector and integrates
the CO, peak
areas automatically.
The DOC should be
analyzed in duplicate if the acid-soluble
fraction
(%SOC/%C,,p)
is likely to be
>15% or so of the total organic carbon.
Otherwise, single analyses are sufficient,
if ampoule breakage (and sample loss)
during heating is minimal.
The unused spent acid solution is carefully decanted from each centrifuge tube.
The residue is resuspended and washed
with organic carbon-free water to remove
most calcium salts and excess H,PO*, the
rinse discarded after a second centrifuging, and the tubes are dried at 110°C. The
discarded rinse should be checked occa-
acid: (n =3)
= 59.22kO.37
= 58.54
= 11.32k0.11
= 11.38
sucrose: (n =3)
%C found = 42.07kO.10
theoretical
= 42.11
%N found = 0.08-+0.002
theoretical
= 0.00
sionally for DOC to prevent unintentional carbon loss. The DOC levels in this
rinse were always ~0.5% of the total organic carbon. After the sediment is quantitatively
recovered, weighed, and powdered, aliquots are weighed into sample
boats and analyzed for total carbon by
CHN analyzer.
Tin sample boats are used in a Carlo
Erba model 1106 CHN/OS
analyzer
which flash combusts carbon to CO, in an
oxygenated Cr,O, reactor at 1,600”C. This
instrument recovers >99% of the carbon
in graphite, and thus presumably
combusts even the most refractory organic
materials (Pella and Colombo 1973). The
CO, is separated by gas chromatography
and detected by thermal conductivity
with automatic electronic integration.
In
general, this analysis (CHN) is not done
in duplicate, except where values are extremely low and additional
precision is
desired.
Total carbon (organic carbon plus inorganic carbon in carbonates) is obtained
by analyzing
an unacidified
sample by
CHN analyzer.
Organic carbon is obtained by summing soluble organic carbon (SOC) and
residual organic carbon (ROC).
%C,,, = %SOC + %ROC
%SOC = lt;;
I
(1)
(2)
567
Notes
Table 3.
Carbon and nitrogen
responses
of the Carlo Erba 1106: NBS acetanilide
C
Core
analyzed
Boat batch
No. std.
lOGC1
lOGC1
lOGC1
lOGC1
Test
Test
Operating
N
sensitivity*
3
5
3
3
4
2
Ref G-l
Test
5GCl
14GCl
5GCl
14GCl
27GCl
27GCl
standard.
9.963
9.976
10.140
9.984
9.761
9.863
conditions
2.772
2.688
2.750
2.687
2.647
2.548
altered
5
8
3
3
2
3
5
3
6.630
6.624
6.587
6.399
6.387
6.411
6.528
6.531
1.838
1.764
1.767
1.723
1.715
1.745
1.817
1.812
9BC26B2
7BC15B2
C
4
6.619
1.818
9BC26B2
7BC15B2
C
4
6.340
1.814
9BC26B2
7BC15B2
28GCl
C
3
6.552
1.769
28GCl
C
4
6.517
1.785
9BC26B2
7BC15B2
28GCl
C
6
6.459
1.811
28GCI
D
4
6.571
1.785
Means 2 SE of means
-
(6)
9.9420.13
6.511kO.096
It 1.4%
(14)
(20)
C.V.
* In (pg-C/unit-CO,-area)
x
lo-*
or (pg-N/unit-N,-area)
x
2.68-cO.08
1.783kO.038
+2.4%
10e3.
has been determined
independently
(Fig. 1). Almost all the data fall within
+3% of the expected 1:l line, suggesting
(3) that the W,:W, term can be safely replaced with 1 - (%CaCO,/lOO) if CaCO,
is known, eliminating
the need for a secwhere DOC is mass of dissolved organic
carbon in 30 ml of spent acid (in pg-C),
ond weighing
of the residual
solids.
W, is initial weight of sample (in mg), W, However, small random errors in %CaCO,
is weight of sample residue after decal(or in W,:W,) are magnified in the calcification (in mg), and (CHN) is concenculated Co,, when %CaCO, is >80%. In
tration of carbon in the residual decalcisediments containing >9O% CaCO, analfied sample (in ,ug*mg-I). The ratio W,:W,
yses must be accurate to better than + 1%
(Eq. 3) is equal to 1 - (%CaCO,/lOO) if to make this replacement
safely. In sedonly CaCO, is lost upon acidification.
iments containing >95% CaCO,, WR may
This assumption
has been tested in a be difficult
to recover and weigh accuwide variety of sediments containing
O- rately. This problem can be minimized
90% CaCO,, where the CaCO, content
by prior taring of the centrifuge tubes and
a/ROC
= cCHN) ’ wR
0
10 x w*
zz (CHN) 1 _ %CaC03
10
100
[
1
Notes
568
Table 4.
Carbon
and nitrogen
blanks in tin sample boats with the Carlo Erba 1106.
N
C
Boat batch
No. blanks
A
B
C
D
8
5
5
5
Means ? SE of means
n=4
n=2
Cleaning
method
Ccl,, acetone
Hexane, acetone
Hexane, acetone
Hexane, acetone
by increasing the weight of sample and
volume of acid.
Washout of interstitial
salts will account for about a 2% weight loss not due
to CaCO, dissolution (see Fig. 1). No correction has been made here for this error.
Below are given operational blanks, instrumental
sensitivities,
and checks on
the internal consistency
of some of the
data collected with the technique.
Carbon
standards
must be crosschecked against each other to establish
consistency
between
the instrumental
CHN and DOC analyses (Table 2). The
Table 5. Carbon blanks (early blanks were high
due to inexperience
in the ampoule sealing step)
and responses with the Oceanography
International
0524 carbon analyzer (DOC): Sucrose standard.
No.
blanks
Blank*
/a-c
No.
std.
Sensitivity!
10GCl
1
4.15
5
4.176
5GCl
3
3.13-t0.88
8
4.050
14GCl
4
2.54kO.59
7
3.847
27GCl
8
1.0350.27
3
4.017
9BC26B2
7BC15B2
28GCl
6
1.59kO.62
4
3.876
9BC26C2
7BC15B2
28GCl
8
1.71 -t-O.61
5
4.000
28GCl
7
1.32k0.26
5
3.994
(6)$
-
1.8920.79
-
(7)
-
3.994&0.110
k2.8940
Means
-+SE of
means
C.V.
* lo-ml volumes for all blanks.
t In (pg-C/unit-CO,-area)
X 10e3.
$ Omitting
10GCl.
10.58*0.88*
10.24?0.85*
1.15+0.22
0.9520.35
1.05*0.14
1.31kO.28
-
* High N blanks due to inadvertent
use of impure
oxygen in 0, pulse
A and B have been omitted from calculation
of nitrogen
means.
Core
analyzed
1.29+0.11
1.35kO.12
1.6350.21
0.95+0.16
mode.
“Zero
oxygen”
was used
for batches
C and
D. Batches
measured carbon values are within + 1%
of theoretical values.
Long term (2 month) stability of the
Carlo Erba 1106 instrument
is excellent
(Table 3): its responses varied by only
about +1.4% for carbon, mostly due to
small daily fluctuations
in operating conditions. The instrument
is reportedly capable of +0.3% on a short term (singlerun) basis (Pella and Colombo
1973).
Thus the residual carbon values (CHN)
reported here are accurate to better than
* 1%.
Blanks for instrumental
CHN microanalysis were generally about 1.3 + 0.3
@g-C, although for any one batch of tin
sample boats, the variability
in blanks
was consistently
smaller, between +-0:l
and +0.2 ,ug-C (Table 4).
These data indicate that the overall accuracy of the residual organic carbon data
(CHN) is better than +l%, with a precision of better than kO.3 ,ug-C.
Long term variability
in the response
of the Oceanography
International
0524
(DOC analyses) averaged about +2.8%
(Table 5). Short term variability
(due to
instrument
drift) is only slightly better
than *2%, though this is difficult to demonstrate. DOC blanks averaged 1.9 + 0.8
,ug-C (0.19 * 0.08 mg. liter-‘). With care,
DOC blanks cl.0 + 0.4 pg-C (0.10 2 0.04
mg-liter-l)
can be obtained routinely (G.
Mills pers. comm.).
The accuracy of the DOC analyses
(DOC) is believed to be better than +2%,
with a precision of about kO.8 pg-C.
The overall reliability
of the method
can be estimated with the above data and
one additional
piece of information:
the
569
Notes
fraction of organic carbon soluble in acid
(%s0c/%c,,,)
increases
roughly
with
%CaCO, (Fig. 2). This allows estimation
of the maximum uncertainty
(cumulative
error) in the final calculated
Co,, as a
function of %CaCO,, taking account of
variabilities
in blanks and accuracy for
the component DOC and CHN analyses.
In sediments containing
>80% CaCO,,
the accuracy is limited by the reliability
in W,:W, (or %CaCO,), regardless of the
organic carbon concentration.
In less calcareous sediments analyses of highly organic samples are limited primarily
by
the accuracy in residual carbon measurements (CHN).
For less organic sediments, the accuracy is limited by the residual
carbon
blank
(CHN).
For
sediments
with little
CaCO,, %SOC/
%Cor, is small, whereas for highly calcareous sediments the weighing error is relatively large. Thus, the blank and variability
in DOC measurements
do not
contribute
significantly
to errors in calculated C,,, values.
These estimates of precision
and accuracy in the component parts of the procedure indicate that the overall accuracy
of the technique
is about -+2%, with a
precision
better than *0.02% Corg, for
sediments with <90% CaCO,.
Replicate analyses of one open-ocean,
highly calcareous core-top sample (Ref.
G-l, 90% CaCO,) yielded a SD of about
+0.004% co,, , equivalent
to a C.V. of
about 22% (Table 6). Replicate analyses
of this same sample by Mills and Quinn
(1979) yielded a value within 0.011% Co,,
of this determination
(bottom of Table 6).
Table 6.
Replicate
analyses
WI
Run
of reference
WE
mg
1
2
3
4
202.0
181.4
126.3
66.7
18.0
15.8
11.0
5.7
DOC
pg.30 ml-’
80
of Ref. G-l by Mills
-
m
50
-
40
-
v
v
0
5GCIo
IOGCI
14GCI
27GCI
28GCI
1582
2682
0
*
8
073
l o
OOO
.
v
A
A
v
•I
20
IO
0
0
0.1
0.2
0.3
0.4
0.5
% sot
%%rcJ
Fig. 2. Plot of acid-soluble
organic carbon fraction (%SOC/%C,,J
vs. %CaCO,, showing increase
in acid-soluble
fraction with increase in %CaCO,.
These data are consistent with my estimates of accuracy and precision.
One way to check for systematic errors
in carbon data from sediments is to see
whether inorganic carbon values are consistent with calcium values, since almost
all calcium and inorganic carbon are in
CaCO,. Calcium is plotted against inor-
CHN
mg-’
%SOC
%ROC
19.46, 19.88
19.20
19.83, 20.37
20.31
0.066, 0.069
0.069
0.069
0.060,0.070
0.173,0.177
0.167
0.173, 0.177
0.173
0.243
0.236
0.244
0.238
0.067-+0.004
0.173+0.004
0.240+0.004
(4)
(6)
and Quinn
vv
v..
*
v
w
v
.* ? 0
60
$
.“I
-
70-
MeanskSE
of means (n)
Analysis
v
v
sample G-l.
w
133, 140
126
87
40,47
90
1979: 0.251kO.007
(3)
(6)
%Co,
570
Notes
% ORGANIC
0
0
I
Cu
I
0
0
G76-5-IOGCI
0
E20
2.0
I
I
0
PI
IO
CARBON
1.0
kO
0
0
0
0
2
4
6
% c
8
IO
INORG
Fig. 3. Inorganic
carbon (Cotal - C,,,) plotted
vs. calcium in same samples. Calcium determined
by instrumental
neutron activation
analysis. Solid
line-theoretical
line for CaCO, diluted with CaDashed line-theoretical
and Cinorg-free material.
line for CaCO, diluted with Cinorp-free aluminosilicate containing 2% calcium (see text).
ganic carbon for all samples analyzed for
total and organic carbon and for calcium
in Fig. 3 (core locations given in Table
1). In general, the data lie along the solid
line that would be expected if CaCO,
were being diluted only by Ca- and Cinorgfree phases. If the dilution
is by an inorganic carbon-free but calcium-containing aluminosilicate
phase, the data will
lie slightly above the solid line (the dashed
line shows the influence
of a diluting
phase contributing
2% calcium on a total
sediment basis). It seems clear that the
method yields a consistent measure of inorganic carbon. Thus, the organic carbon
values cannot contain large systematic
errors.
During the development
of this method, it was possible to infer something of
the nature of organic carbon in marine
sediments.
Several experiments
were
done to determine if C,,, and Cinorg could
be separated by ignition (difference-oncombustion
technique).
First, total and
organic carbon were measured in samples from a core in the eastern equatorial
Atlantic
(G-76510GC
1). Each sample
was then split and ashed either in a muf-
la.
VO
w30
I3
O.
MV
0
a
v
0
Cl
v
0
0
v
0
40
;
i
50
1
1
0
0
0
0
v
v
0
0
v
0
v
0
Fig. 4. Organic carbon vs. depth in a core from
the eastern equatorial
Atlantic:
0-unashed,
Vashed at 250°C for 24 h, A-ashed
at 500°C for 24
h, O-low-temperature
ashed (oxygen plasma) at
250 W for 24 h.
fle furnace at 250°C or 500°C for 24 h or
in a low-temperature
asher (LFE Corp.,
model 505) at 250 W for 24 h. The results
(Fig. 4) indicate that about 66 + 9% of
the organic
carbon is combusted
at
250°C about 90 + 4% at 5OO”C, and about
72 of: 8% by low-temperature
ashing. This
is consistent with Gibbs’ (1977) finding
that complete combustion of organic carbon at temperatures below 1,OOO”C is difficult. Second, a single aliquot of the uppermost
sample in core lOGC1 was
repeatedly
pyrolized
at increasing temperatures. The results (Fig. 5) show the
absence of a bimodal CO, release that
would be expected if organic carbon and
inorganic carbon could in fact be separated by thermal degradation.
(Pyrolysis
571
Notes
was performed
with a Chemical
Data
System 900/382 extended
pyroprobe/
CHN analyzer.
Pyrolysis
products are
converted to CO,, which is detected after
gas Chromatographic
separation by thermal conductivity.
Unfortunately
the instrument cannot be calibrated reliably in
terms of either carbon or temperature.
Thus, the CO, and temperature
values
have only relative significance.) It is commonly thought that biogenic calcite does
not decompose below about 800°C. I suspect, however, that the temperature
of
decomposition
is reduced in naturally occurring biogenic calcites because they inMagnesium
carbonclude magnesium.
ates begin giving
up CO, at 400°C.
Apparently
naturally
occurring
organic
and inorganic carbon cannot be separated
accurately
by difference-on-ignition
techniques because their thermal degradation temperatures overlap.
Several investigators
have cited evidence that some fraction of the organic
matter in calcareous sediments is intimately associated with biogenic calcite
and is solubilized
(and often discarded)
during acid decalcification
procedures
(Roberts et al. 1973; Heath et al. 1977).
Indeed, Fig. 2 suggests that the fraction
of acid-soluble
organic carbon is greater
in more calcareous sediments. The acidsoluble fraction is probably a complex
mixture
of marine humic compounds
plus carbon peptized by acid from the
organic matrix of calcite tests. Thus SOC
need not be associated exclusively
with
CaCO,. It is clear, however, that discarding the acid wash during carbonate dissolution steps can result in the loss of 515% of the organic carbon in low-carbonate sediments and up to 45% or more in
carbonate-rich,
Co,,-poor sediments. Roberts et al. (1973) have also found acid solubilization
of up to 40% of the organic
carbon in modern shallow-water
carbonate sediments. Attempts to circumvent
this loss by acidification
followed
with
evaporation
to dryness result in the inclusion
of nonstoichiometric
calcium
salts or excess acid, leading to physical
problems in drying and reweighing.
Unless the entire decalcified sample is ana-
G76-5-10
GCI
(O-2cm)
r-
0
400
PYROPROBE
800
1200
SE TING, OC
Fig. 5. Pyrolysis temperature
scan of one surface sample to attempt thermal separation of organic
and inorganic carbon (see text). Sample contains
0.06% C,,, and 7.53% Cinorp.
lyzed in bulk without splitting
and reweighing, serious errors can be incurred
in C,,, determinations
of carbonate-rich
sediments.
My technique allows organic carbon to
be measured with an accuracy of 22%
and a precision
of about 20.02%. It is
suitable for use with both carbonate-poor
and carbonate-rich
sediments and eliminates or minimizes systematic errors that
have plagued other methodologies.
I emphasize particularly
that organic
carbon cannot be determined
by difference-on-ignition
due to incomplete
oxidation of organic carbon at temperatures
<l,OOO"C, while at the same time preventing volatile carbon loss from inorganic carbonates at temperatures ~-500°C.
Similarly, since a significant fraction of
the organic carbon in marine sediments
is acid-soluble,
particularly
in carbonatemethods
in which the
rich deposits,
spent acid supernatant is discarded after
decalcification
will underestimate
organic carbon.
P. N. Froelich2
Graduate School of Oceanography
University
of Rhode Island
Kingston 0288 1
References
ALLISON, M. 1935. A titrametric
mination of organic carbon
40: 311-318.
method for deterin soils. Soil Sci.
2 Present address: Department
of Oceanography,
Florida State University,
Tallahassee 32306.
Notes
572
BETZER, P. R., K. L. CARDER, AND D. W. EGGIMANN.
1974. Light scattering and suspended
particulate
matter on a transect of the Atlantic
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Suspended solids in water. Plenum.
DUURSMA, E. K. 1961. Dissolved
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FROELICH, P. N. 1979. Marine phosphorus
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Ph.D. thesis, Univ. Rhode Island,
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-,
B. GOLDEN, AND 0. H. PILKEY. 1971. Organic carbon in sediments of the North Carolina continental
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oxidation
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HEATH, R. T., T. C. MOORE, AND J. P. DAUPHIN.
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@ 1980, by the American
Society of Limnology
and Oceanography,
A sampler for cohesive sediment
benthic boundary layer1
Abstruct-A
sampling apparatus for collecting sediment suspensions
from the benthic
boundary
layer has been developed
and
tested. Samples are collected
in evacuated
pressure-resistant
bottles by activation
of solenoid valves used to release the vacuum. The
behavior
of the apparatus
with respect to
water velocity is assessed from direction and
inclinometer
sensors attached
to its main
frame. The apparatus has been tested at a
depth of 30 m in the Bristol Channel in water
velocities of l-2 rn. s-l. Profiles of suspended
sediment in the boundary layer are derived
from gravimetric
analysis of the samples collected.
l This work was supported
B4/02.
by N.E.R.C.
grant F60/
holes on the continental
margin off Florida.
U.S. Geol. Surv. Prof. Pap. 581-B. 10 p.
KERR, R. A. 1977. The isolation and partial characterization
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NINO, K., K. 0. EMERY, AND C. M. KIM.
1969. Organic carbon in sediments of Japan Sea. J. Sediment. Petrol. 39: 1390-1398.
PELLA, E., AND B. COLOMBO. 1973. Study of carbon, hydrogen and nitrogen determination
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Am. Pet. Inst.
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Submitted:
16 February
Accepted: 8 October
1979
1979
Inc.
in the
Accurate assessment of the concentrations of suspended solids in the benthic
boundary layer is needed to estimate the
transport
of solids. Concentrations
of
these boundary layer solids can be higher
than those of the overlying
waters and
therefore make highly significant
contributions to sediment flux. Suspended solids concentrations
are determined by two
main methods: by in situ devices based
on light transmission, i.e. silt meters, and
by the gravimetric
analysis of samples
collected
by water samplers.
Optical
measurements
are limited
by particle
and concentrationsize, composition,
the last producing
a disproportionately