Aerobic respiration in pelagic marine sediments? - Science Direct

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aerobic respiration of organic matter in pelagic sed- iments has been to investigate changes occurring in the organic matter itself (e.g., Stevenson and Cheng,.
G.mchimica CI Gumachimica Acfa Vol. 46, pp. I 101-I 120 0 persamOn Ltd. 1982. Psi&cd in U.S.A.

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Aerobic respiration in pelagic marine sediments? VARIS GRUNDMANIS* and JAMFS W. MURRAY School of Oceanography, University of Washington, Seattle, Washington 98195 (Received January 9, 1981; accepted in revised form February 10, 1982)

Abstract-Analyses for dissolved oxygen, nitrate and total CO2 in the interstitial water have been combined with solid phase sediment analyses of carbon and nitrogen to calculate the rates of reaction and stoichiometry of decomposing organic matter in central Equatorial Pacific pelagic sediments. The diagenesis is dominated by aerobic respiration and nitrification. Organic carbon and total nitrogen decrease exponentially with depth in both red clay and carbonate ooze sediments. In addition, there is a correlation between surface organic carbon and total nitrogen with distance from the equator. Fixed NH, is relatively constant with depth and constitutes 12 to 64% of the total nitrogen. The remainder is considered to be organic nitrogen. The C/N ratio of the decomposing organic matter was obtained using three approaches. Using the correlations of organic carbon with total nitrogen or organic nitrogen the molar ratios varied from 3.4 to 18.1,The average of all stations was 12.6 using total nitrogen and 13.7 using organic nitrogen. The Redfield ratio is 6.6. Approaches using interstitial water chemistry gave lower ratios. The average value using correlations between dissolved oxygen and nitrate was 8.1. The same approach using total COt and nitrate gave an average of 9.1. Due to difficulties in unambiguously interpreting the solid phase data we favor the ratios obtained from the pore water analyses. The rate of organic matter decomposition can be obtained from model calculations using the dissolved oxygen and solid organic carbon data. Most gradients occur in the upper 10 to 20 cm of the sediments. Assuming that bioturbation is more important than sedimentation we have calculated first order rate constants. The average values using organic carbon and dissolved oxygen was 3.9 kyr-’ and 4.2 kyr-’ respectively using a biological mixing coefficient of 100 cm* kyr-‘. These rate constants decrease in direct proportions to the mixing coefficient. I. INTRODUCTION

is an extensive oceanographic literature on the diagenetic oxidation of organic matter in organic rich, anoxic marine sediments (see for example, Berner, 1974; Goldhaber et al., 1977; Bender et al., 1977; Murray et al., 1978, 1980; Emerson et al., 1980). The stoichiometry and rates of reaction in oxic marine sediments has not been as thoroughly studied (Berner, 1979, 198 1). This is a conspicuous gap because most pelagic marine sediments fall into this category. Only sediment in the “hemipelagic” zones that border the continents, shallow sediments on rise crests and some eastern Equatorial regions become anoxic in the upper tens of centimeters. This distribution reflects the fact that high biological productivity and rapid sedimentation rates enhance the preservation of organic carbon (Toth and Lerman, 1977; Heath et al., 1977; Mtiller and Stress, 1979). In most pelagic regions the surface productivity is low and the sedimentation rate is slow and thus most of the labile organic matter reaching the sea floor is oxidized at (or very close to) the sediment-water interface (Menzel, 1974; Emerson and Bender, 198 I ). The primary approach used previously to study aerobic respiration of organic matter in pelagic sediments has been to investigate changes occurring in the organic matter itself (e.g., Stevenson and Cheng, 1972; Mtiller, 19’75, 1977; Waples and Sloan, 1980; THERE

* Present Address: Hawaii Loa College Kaneohe, Oahu, Hawaii 96144 7 University of Washington Contribution No. 1252

Mtiller and Mangini, 1980). The concentrations of organic carbon and nitrogen decrease with depth until some low background level is reached. Typically soiid sediments have low C/N ratios which are believed to be due to large amounts of fixed NH: relative to organic nitrogen (Stevenson and Cheng, 1972; Muller, 1977) and organic nitrogen compounds sorbed within clay minerals (Mtiller, 1977). The model of Suess and Miller (1980) incorporates previous results which demonstrate that detrital organic matter settling through the water column undergoes strong elemental fractionation by preferential removal of N- and P-containing organic compounds (see also Rittenberg et al., 1955; Holm-Hansen et al., 1966; Gordon, 1971; Bishop et al., 1977). At the sediment-water interface a portion of the detritus is converted into biomass by benthic organisms, which concentrate nitrogen and phosphorus relative to carbon. This reconstituted organic matter serves as an energy source for benthic heterotrophic organisms. When organic carbon and total nitrogen analyses are corrected for fixed NH4 and sorbed organic matter the C/N ratio of the decomposible organic matter is found to be close to the Redfield value of 6.6 (Muller, 1977). The rates of these respiration reactions have been calculated primarily by assuming a balance between burial by ~dimentation and a first order organic carbon decomposition term. The value of the first order rate constants (k,) depend on the depth interval investigated. Muller and Mangini (1980) found k, to range from 3.2 to 27.5 X 10m6yr-’ in the upper meter

1101

1102

V. GRUNDMANIS

1400

AND J. W. MURRA’r

calculate

140°

180’

40”

the stoichiometry

of the organic

matter

being aerobically oxidized in the upper tens of centimeters of pelagic sediments and the rates at which these reactions proceed. 40”

II. THE STUDY AREA Our sampling region was chosen to be representative of pelagic aerobic environments. We wanted to include both red clay and carbonate ooze sediments as well as a transect across the equator where the influence of increasing surface biological productivity would be detected. Based on these criteria we joined R/V Knorr cruise 73, Leg VII (JulyAugust 1978) in the central Equatorial Pacific (Fig. 1). The exact station locations and depths are given in Table I. Those stations marked with an asterisk were not sampled for interstitial waters due to time limitations or because the sediments were too consolidated for the in situ sampler to penetrate. The sediments at station marked “RC” in Table 1 consisted of pelagic red clays while those marked “C” contained carbonate.

III. SAMPLING AND ANALYTICAL METHODS

140*

160’

180’

100~

1

1400

I

FIG. 1. Stations occupied during R/V Knorr, Leg VII,

July 1978.

of pelagic sediments. Waples and Sloan ( 1980) found that k, decreased to 0.8 to 1.4 X low6 yr-’ in the deeper portions of DSDP-Leg 58 sediments. Most of these studies of organic matter diagenesis have not included the related interstitial water chemistry. Hartmann et 01. (1973, 1975) reported data from the sites studied by MUlier and co-workers. They found that interstitial nitrate concentrations invariably increase with depth suggesting an aerobic environment. Although the carbon and nitrogen content of the sediments themselves were measured for this study the primary emphasis was on the interstitial reactants and products involved in the diagenetic oxidation of sedimentary organic matter. The most important of these measurements was the measurement of dissolved interstitial oxygen. In most previous studies the presence or absence of oxygen has been inferred from the interstitial water chemistry of other chemical species, especially nitrate and manganese (Hartmann er al., 1973; Bender et al., 1977; Emerson et al., 1980, Jahnke er al., 1981). The relationship of these other chemical species to oxygen was not investigated. Oxygen is the critical measurement because when present it is the therm~yn~ical~y favored terminal electron acceptor for the oxidation of organic matter by heterotrophic organisms (Stumm and Morgan, 1970). The diffusive flux of oxygen into the sediments at our study sites using these results has been previously described (Murray and Grundmanis, 1980). The main goal of this study is to use both solid phase and interstitial water analyses to

The in situ interstitial water sampler described in Murray and Grundmanis (1980) was used to obtain gas-tight interstitial water samples. The sampler is a modified version of the in situ sampler described by Sayles et al. (1976). At all stations, except A and E, calibrated 4 ml sampling loops were placed in line between the sampling port and the slave cylinders so that they were filled during collection of the interstitial water in situ. At stations A and E, the sample loops were filled after the sampler had been returned to the deck of the ship. Sinpie oxygen samples were obtained at every sampling depth at stations A, E, F, G, H, and P. Duplicate samples were obtained at every other sampling depth at stations I, J, and K. Immediately after sampler retrieval the gas sample valves were closed, removed from the sampler and stored under water to minimize gas exchange in ease of ieaking valves. Oxygen analyses were begun immediately after removal of all the sample valves from the sampler. All gas analyses were performed on a Hewlett-Packard dual column gas chromatograph equipped with a sample stripping system and a Carle thermistor detector. The 4 ml sample was stripped, dried over silica gel and passed over a molecular sieve SA column (6’ X I/4”, 100-120 mesh) at a gow rate of 120 ml min-‘. The gas ~hromatograph was standardized with a Carle gas sampling valve with the sample loop filled at ambient temperature (measured to 0. I “C) and ambient pressure (measured at 0.01 inch Hg) with the appropriate gas. Compressed air was used for 02. Pure argon was used for argon. The standardization was done by flushing the contents of the Carle gas valve through 4 ml of pre-stripped seawater. Argon and oxygen are not separated from each

Table 1

station ii A

G H I K

L* M* e R

Station locations, water depth and sediment type at srarions occupied during R/V Knorr 73, LeS VII. Stations marked with an asterisk were not sampled ior inferstitlal waters.

Lat.

LonP.

Wafer depth Sediment type betersf

OFX'13.9'N169%4.1'E 07'27.3% 177'58.8'E 07O23.4'N 17Y033.1'E 07°19.2'N 177='13.4'W 05'3O.Y'N 175'52.1'W 03'40.4'N 174'54.3'W 01°45.8'N 173'32.9% 00'02.6'5 171'04.2'W 02'26.9'5 168'04.7'W 03'39.5'S 166'37.9'W 05'56.7'5 164*03.2'W OY910.1'S 166*32.9% _--~

4239 5269 5067 3572 5352 4765 5361 5469 5047 5283 4822 3770

c RC RC c

RC C RC RC RC RC RC C -~~ -

1103

AEROBIC RESPIRATION IN PELAGIC MARINE SEDIMENTS other, and thus a small correction for Ar was made, based on the solubility of Ar in seawater. Accuracy and precision for the dissolved O2 analyses were estimated to be about 2%. Aliquots for other analyses were taken from the sampler after removal of the gas valves. Nutrients (nitrite, nitrate, silica and phosphate) were measured on board ship using a four-channel Technicon AutoAnalyzer II system. Ammonia was not measured. Calorimetric manganese analysis (Brewer and Spencer, 1974) were done on samples from station G. None of the samples showed any manganese above the blank (10 pmole kg-‘) and manganese analyses were not done at any of the other stations. Gravity cores (obtained using a modified Benthos gravity corer) were taken at many of the same locations where we collected interstitial water samples. Dr. Kent Fanning (University of South Florida) generously provided squeezed sediment samples for analyses of solid carbon and nitrogen. The sediment samples were kept frozen and returned to the University of Washington. The carbon and nitrogen analyses were done using a Carlo Erba C-H-N elemental analyzer. The sediments were dried at 100°C until constant weight and then total C and total N were determined directly. Organic carbon was then determined by a method similar to Waples and Sloan (1980). About one gram of dried sediment was weighed and between 5 and 10 ml of concentrated HCI was added until effervescence ceased. The acid solution was not decanted so as not to loose soluble organic carbon (Froelich, 1980). The carbonate free sediment was then dried at 90°C, weighed and ground. Organic carbon was then analyzed on the C-H-N analyzer. The % calcium carbonate was calculated according to 8.33 (total C - organic C). The total N analyzed on the acid digested samples was in excellent agreement with analyses on the untreated sediments. For fixed NH. the oraanic N was first removed using KOBr (Silva and Brem&r, 1966). Then the residue wai totally digested in HCl and HF in teflon bombs. The solutions were analyzed calorimetrically for NH: (Matsunga and Nashimura, 1974). We did not analyze for exchangeable NH: as all previous work in this area has found it to be generally less than 3% of total N (Muher, 1977). The standard used for the Carlo Erba C-N analyses was acetanilid (71.1% C; 10.4% N) and the precision was 3.0% for C and 10% for N. IV. RESULTS

The interstitial water concentrations of oxygen, nitrate, silica, and total CO2 (calculated from pH and alkalinity) and the solid phase carbon and nitrogen analyses are presented in the appendix. Silicate analyses are used to verify that the in situ samples are valid and that bottom water has not been inadvertently sampled. Agreement between our silicate values and those of Dr. Kent Fanning on squeezed sediment cores collected during the same cruise was excellent. The oxygen, nitrate, and total CO2 concentrations vary systematically with depth and will be discussed extensively later. Interstitial water phosphate concentrations varied little and were less than the bottom water concentrations expected for this part of the Pacific Ocean. Even our port zero, which samples bottom water, was low in PO,,. Thus we suspect that phosphate was lost during sample collection by the in situ sampler and the data will not be presented here. The average porosity over the top 10 cm in these sediments is about 0.90 and the tortosity at z = 0 is 0 = 2.0 (C. Olson and F. L. Sayles, personal com-

munication). The average specific gravity of the solid phases ranges from 2.63 to 2.15, based on direct measurement. V. DISCUSSION

Heterotrophic organisms obtain energy via the respiration oxidation of organic compounds to COZ. In a very simplified way, microorganisms can be viewed as catalysts of the oxidative degradation of organic matter. A general equation describing this oxidative process can be written as (modified from Redfield et al., 1963): (CH,O),(NH~),(H~P04),

+

x02

---

xc02 + yNH, + ZHJPOI + xH20

(1)

The multiple arrows serve to emphasize that the overall process involves many different steps and a wide variety of microorganisms. This discussion is focused primarily on the carbon, nitrogen, and oxygen relationships. Although phosphorus is released during aerobic oxidation of sedimentary organic matter it can be involved in rather complex solid phase reactions with numerous mineral phases (e.g., Suess, 1979) and is therefore not necessarily a good indicator of the extent of organic matter oxidation. Therefore, phosphorus will not be included in this discussion. Ammonia is released during aerobic oxidation of organic matter, and is oxidized to nitrite by Nitrosomonas and then to nitrate by Nitrobacter. At some locations of oxic sedimentation, well defined maxima in the nitrogen intermediates NH:, NO; and urea have been observed within 10 cm of the sedimentwater interface (Suess et al., 1980, Jahnke et al., 1981). Eventually all of these intermediates are oxidized to nitrate which is the only major nitrogen species accumulating in aerobic sediments. The overall oxidation of ammonia to nitrate can be written as: NH, + 202 -HN03 + H20 (2) Combining this equation with Equation (1) we obtain an equation describing the aerobic oxidation of organic matter including ammonia oxidation (nitrification). (CH,O),(NH,),

+ (x + 2y)O2 xc02 + yHNOl

+ (x + y)H20

(3)

In the classic Redfield equation (Redfield et al., 1963) for the composition of plankton, the coefficients are x = 106, y = 16 for a C/N ratio of 6.63. In marine sediments the stoichiometry is not necessarily the same. In many modeling exercises the stoichiometry of the decomposing organic matter is assumed for simplicity to be. the same as in the Redfield model (e.g., Berner, 1977; Froelich et al., 1979; Emerson et al., 1980). Using the data presented here we can test this assumption and evaluate its limitations.

II04

V. GRUNDMANIS “1,

AND J. W. MURRAY

ORGANIC

CARBON

FIG. 2. The solid organic carbon profiles in the sediments from stations A, E, H, I, K, L, M, and R. Stations A, I, and R are carbonate ooze. The rest are red clay. The solid lines are model fits as discussed in the text. The profiles labeled B, are best fits to the carbon data. The lines labeled B,, were calculated using the values of B(02) obtained from a best fit of the O2 profiles in Fig. 6.

1. Stoichiometry

The C/N ratios in Pacific deep-sea sediments have been extensively studied by Mtiller, Suess and coworkers. These analyses have shown that in the solid phase many factors influence the C/N ratios and that the total organic carbon/total nitrogen ratios do not necessarily represent the C/N ratio of the decomposing matter (Mtiller, 1977). Solid phase analyses were carried out on the carbonate containing sediments from stations A, i, and R and the red clay sediments from stations E, H, K, L, and M. The compensation depth for the region of our samples is at approximately 4900 m. The organic carbon profiles at all stations showed a decrease with increasing depth in the sediments and appeared to be approaching a constant value (Fig. 2). This constant value may represent a non-metabolizable fraction, at least on the time scales discussed here. There were no systematic differences between the red clay and carbonate containing sediments. Mtiller (1977) and Mtiller and Mangini (1980) found similarly shaped organic carbon profiles with decreases that extended to 1 to 2 m below the surface. They used organic carbon/A1203 ratios as the criteria for constancy. An average ratio of 0.0054 was found to hold for the constant organic carbon depth range. They used this ratio to correct the total organic carbon for a non-metabolizable component that they proposed to be protected by clay minerals. We also conducted a limited number of aluminum analyses on cores H, K, and M (Table 2). The organic carbon/

A1203 ratio decreases with depth from about 0.060 at the surface to 0.014 at 50 cm. Our ratio is about a factor of two greater than the lowest ratios observed by Mttller and Mangini (1980). This implies that our cores were probably not long enough to reach the truly constant organic region identified by Miiller and Mangini (1980) and that our estimates of the non-metabolizable organic carbon fraction are possibly a factor of two too high. In this paper we will treat our constant carbon values as non-metabolizable and keep in mind that any conclusions we reach about rates pertain to early diagenesis in the top 50 cm. The organic carbon content of the surface sediments varies from 0.14% at station R to 0.68% at station K (Fig. 3). There is a systematic correlation between surface organic carbon and distance from the equator which reflects the higher biological productivity in the equatorial surface waters (Arrhenius, 1952). Mtiller and Suess (1979) have derived an equation for predicting the sedimentary organic carbon content from the annua1 average primary productivity rate (R), sediment rate (S), dry density (P,), and porosity (4). % organic carbon =

0~30.R.~“,30 f PAI

-#I

(4)

In the region of our samples productivity averages about 36 mg C rn--* yr-’ (Lisitzin, 1972) and the sedimentation rate ranges from 0.1 to 2.0 cm lOA3 yr-‘. With the lower values being more typical (Ku et al., 1968). Using these values in the above equation predicts surface organic carbon to range from 0.02%

AEROBIC Table 2

RESPIRATION

MARINE

1105

SEDIMENTS

Solid phase C, N, and Al 0 and analyses and ratios from selected depths at the red clay stations,Ii;2, and M.

station I c erg c : Total N station

IN PELAGIC

w f Fixed Nli;

A1203

II

0- 0.5 l- 1.5 3.5- 4 l- 8 29-30

0.583 0.546 0.401 0.344 0.183

0.056 0.053 0.042 0.040 0.026

station K o.s- I. 1.5- 2 3.5- 4 11-12 19-20 29-30 39-40 49-50

0.678 0.652 0.682 0.355 0.316 0.231 0.207 0.187

0.076 0.069 0.074 0.046 0.050 0.045 0.046 0.040

station n 0.5- 1 3- 3.5 11-12

0.433 0.312 0.197

0.037 0.028 0.017 -

0.015 0.014/0.019 0.015 C~.01?/0.018/0.016 0.011 0.011

0.007 0.011

0.012 0.009 0.012 0.011 0.011 0.011 0.010 0.012/0.0011

to 0.42% which is satisfactory considering the uncertainty in the pr~uctivity and sedimentation rates. Total nitrogen was measured on the same samples as organic carbon. The values are much lower than for carbon and the scatter is greater. Nevertheless, the profiles of total nitrogen and total organic carbon are similar. The total nitrogen values at the surface range from 0.14% to 0.01% and also show some symmetry across the equator (Fig. 3). The profiles also decrease with depth over the top 10 to 20 cm and appear to level off at constant non-zero values (Fig.

9.09 9.88

0.041 0.038 0.027 0.023 0.015

10.07 10.11 10.62

0.064 0.055 0.040 0.034 0.017

10.4 10.3 9.5 8.6 7.0

14.2 14.4 14.8 14.9 12.2

0.065 0.062 0.063 0.034 0.041 0.033 0.035 0.029

11.23 10.51 10.79 11.87 12.21 12.36 12.45 12.83

0.060 0.062 0.063 0.030 0.026 0.019 0.017 0.014

8.9 8.9 9.2 7.7 6.3 5.1 4.5 4.7

10.4 10.5 10.8 10.4 7.7 7.0

0.026 0.018 0.006

9.75 10.22 10.32

0.044 0.030 0.019

11.7 11.1 11.6

16.6 17.3 32.8

::I

4). Fixed NH: was measured at stations H, K, and M and was found to vary little over the top 50 cm (Table 2). This is consistent with the results of Muller (1977) and Mirller and Mangini (1980). At station K fixed NH: accounts for 12% to 25% of total N. At stations H and M the contribution reaches 42 and 64% respectively. The ratios of total organic carbon/ total nitrogen decrease with depth from around 10 at the sediment-water interface to as low as 2 at depth (Table 2 and Appendix). Plots of total nitrogen versus total organic carbon

0.12

*

0.10

& g : '

0.08

8 I? !!j 0.06 ul Z

D

I

I

I

5’N

0”

50s LATITUDE

FIG. 3. The solid organic carbon and total nitrogen in the surface sediments as a function of latitude. A, I, and R are carbonate ooze sediments. E, H, K, L, and M are red clay.

1106

V. GRUNDMANIS

AND J. W. MURRAY

O/o TOTAL 0.02

0.06

0.02 ----t---l-

0.06

N 0.02

0.06

0.02

0 06

FIG. 4. The profiles of total N and fixed NH, in the sediments at stations A, E, H, I, K, L, M, and R. The solid lines are for 7’0total nitrogen. The open circles at stations H, K, and M are the fixed NH: values. The difference between total N and fixed NH: should represent organic N. can yield information about the stoichiometry of the decomposing organic matter. Assuming a steady state input of organic matter with a constant C/N ratio, constant fixed (plus exchangeable) NH4, and constant non-metabolizable organic carbon, the slope of a linear C/N relationship in the sediments would be equal to the C/N ratio of the organic matter being oxidized. A non-linear C/N relationship would reflect a changing C/N ratio or variation in fixed NH4 or nonmetabolizable carbon. A non-zero intercept would indicate unavailable N or C depending on the sign. The total organic carbon/total nitrogen plots are shown in Fig. 5. For all cases except stations I and R these two variables are strongly correlated (r r 0.96). The intercept in each case is on the total N axis. This suggests that there is some inert nitrogen in the sediments, probably fixed NH4 in the clays, since aerobic sediments contain little exchangeable ammonia (Stevenson and Cheng, 1972; Mtiller, 1977). The C/N ratios obtained from the slopes in Fig. 5 were converted to atomic ratios and these values are also shown in the figure. The values range from 3.4 to 18.1 with the highest values in the equatorial region. At all stations except A the molar ratios are greater than the Redfield value. For stations H, K, and M the points are also shown for organic carbon versus organic nitrogen (total nitrogen corrected for fixed ammonia). The ratios of C/N,,, are 15.5.

14.0, and 11.6 respectively. At station H the corrected regression goes through zero suggesting that the original intercept on the total N axis was due to fixed NH:. At station K, correction for fixed NH; still leaves a residual total N intercept that may represent exchangeable NH:. At station M the intercept is on the carbon axis suggesting nitrogen poor organic matter protected by clays. Later we will compare these values to those obtained using ZCOz, Or, and N03. Of the methods we will discuss the greatest uncertainty in calculating the C/N of the decomposable organic matter lies with the approach using the solid phase carbon and nitrogen analyses. This is due to the corrections that have to be made for non-metabolizable carbon and nitrogen. 2. Rates of decomposition

The shapes of the organic carbon and total nitrogen profiles in the non-carbonate containing sediments can be modeled to obtain information about the rate of organic matter decomposition. Under steady state conditions, assuming a constant porosity and sedimentation rate, and first order kinetics for the oxidation of organic matter, the profiles of organic carbon (and nitrogen) should be described by the following equation (Berner, 1980):

1107

AEROBIC RESPIRATION IN PELAGIC MARINE SEDIMENTS Q6

1

I

I

0.5 -

0.3

CIN,=3.4

0.02

I.06

0.06

3.06

%

rl

0.02

TOTAL

--_&I 0.02

0.06

0. IO

C

N

FIG. 5. Total N versus organic carbon at stations A, E, H, I, K, L, M, and R. The lines labeled C/ NT are the btst fit linear regressions and the molar ratios obtained from the slopes are given in the figure. At stations I and R the correlations were not significant and lines were not drawn. For stations H, K, and M data points and regressions are shown for organic N versus organic C. Organic N is defined as total N-&d NH:.

where K is the bioturbation mixing coefficient, w is the sedimentation rate and kc is the constant first order rate constant for the oxidation of utilizable organic matter (G). The solution of Equation (5) for the boundary conditions: z=o: z-co:

G=G, G+O

leads to the expression G(z) = Go ex

w - (d

+ 4kcK)“2 2K

>I z

t6)

where z is the depth in the sediments (positive downward) and G,, is the organic carbon concentration (5%)at the sediment-water interface_ In all of the cores the concentration of organic carbon levels off and approaches a positive non-zero value. This means that either there is a “background” concentration of non-metabolizable organic matter,

or some other required chemical is depleted before all of the carbon is gone. A “background” concentration of non-metabolizable carbon is probably a reasonable interpretation since sedimentary organic matter is known to condense into large inert polymers like kerogen and humic material. Adsorption of nitrogen rich phosphorus poor organic matter by clay minerals also contributes substantially to the nonmetabolizable fraction (Mttller and Mangini, 1980; Suess and Mtiller, 1980). The presence of this nonmetabolizable carbon can be included in Equation (6) to obtain: G+)

= G., + G,,., exp(&)

(7)

where G&)

= Grimm + G,(z)

(8)

and B = w - (wr + 4k‘.K)“2 e 2K

(9)

V. GRUNDMANIS

1108 Table 3

Parameters for the fit of G (z)= G +(; exp (BcZ) to the solid organic carbonl'data. n?hrm'O metabolizable carbon at z.= O(G ) and the non-metabolizable carbon (Gm'y are defined in the$text. Bc can be approxi%!&ed by (kc/K) . kc is a first order constant for organic matter oxidation, K is the sediment mixing coefficient and Z is the depth in the sediments (positive downwards). Values of k were calculated assuming K = 100 cm2 kyr -1 = G

Station ii A E li I K L M R

I

G

nm

(X) 0.090 0.170 0.200 0.215 0.200 0.080 0.180 0.045

/ 0.165 0.256 0.390 0.385 0.500 0.420 0.345 0.100

/ (cmCl ) ,/ / -0.23 j ’ -0.06 I -0.15 I -0.25 I

'B

-0.28 -0.20 average

! j

k (ky& 5.3 0.5 2.2 6.2

7.8 4.0 3.9

G,,,, is the metabolizable organic carbon at the sediment-water interface. The subscripts m and nm have been added to the notation in Equation (7) to emphasize the difference between metabolizable and non-metabolizable carbon. The data from all stations were fit to Equation (7) by minimizing the variance of the data points from an exponential line described by Equation (7). A summary of the various parameters for each of the stations is given in Table 3. The exponential coefficient, E,, varies from -0.06 cm-’ to -0.28 cm-‘. Most of the decrease in organic carbon occurs within 10 cm of the sediment-water interface. In this region the downward flux due to bioturbation is much more important than sedimentation. Using the dimensionless number K/Lw it can be shown that for w less than 2 cm kyr-‘, biological mixing, over the depth from 0 to L cm, dominates the profile for K greater than 20 cm* kyr-‘. Guinasso and Schink (1975) have suggested that, K, the biological mixing coefficient, varies from 1 to 1000 cm* kyr-’ in deep sea sediments. Their analysis of microtektite data (Glass, 1969; Glass et al., 1973) indicates that abyssal sediments are mixed to depths as great as 40 cm with rates decreasing from 100 to 1 cm* kyr-‘. Mixing coefficients based on plutonium distributions are greater (100 to 400 cm* kyr-‘) but may be high because of plutonium migration in the sediments. Nozaki et al. (1977) used *“‘Pb and 14C to estimate a coefficient of 180 cm* kyr-’ on the Mid-Atlantic Ridge. Closer to our sites is a study of Peng et al. (1979) based on closely spaced radio-carbon measurements. They found a mixed layer of at least 7 cm with a mixing coefficient of 120 cm2 kyr-‘. Cochran and Krishnaswami (1980) considered 31 cm* kyr-’ to be the best estimate for DOMES sites A, B, and C in the central Equatorial Pacific. In summary, most recent studies of pelagic sediments have found a fairly narrow range of biological mixing coefficients that average 100 + 50 cm2 kyr-‘.

AND

J. W. MURKA\

Using this value for the biological mixing coetticient, the exponential coefficient, B, can be approximated by (kc/K )‘I*. Assuming K = 100 cmL kyr ’ we have calculated the first order rate constant, k,, for each of our stations. These values range from 0.46 to 7.8 kyr-’ (Table 3) and must be considered to have an uncertainty of at least 50%. The average value of 3.9 kyr-’ corresponds to a half life of about 250 years. If the biological mixing coefficient is 10 cm* kyr-’ the half life will be 2500 years. This is much faster than estimates by Miiller and Mangini ( 1980) of 25,000 to 2 17,000 years for organic carbon profiles extending over the top few meters. These authors assumed a balance between sedimentation and a first order reaction term and neglected sediment mixing. Waples and Sloan ( 1980) also assumed a first-order rate law for the top few 100 m of DSDP cores and obtained a k, of 1.4 myrr’ or a residence time of 714,000 years. Both of these estimates pertain to the sediments below the biological mixed layer. It is logical that they would be much slower as they apply to the refractory carbon that has escaped to the deeper layers of the sediments. B. Oxygen

Oxygen decreases with increasing depth in the sediments at every station sampled (Fig. 6). This decrease is indicative of oxygen consumption in the sediments below the sediment-water interface (Murray and Grundmanis, 1980). At most of the stations, the decrease in oxygen is quite rapid and most of the change in oxygen concentration is complete in the upper 10-I 5 cm. At some stations the decrease in oxygen is much more gradual and occurs over the entire sampling interval. Although there exist differences in the shapes of the oxygen profiles, they all have certain common features. The oxygen concentration does not reach zero at any depth in the sampling interval at any of the stations. Instead it levels off or appears to be approaching exponentially some constant non-zero value. This value varies from station to station, but is never less than 50 pmoles kg-‘. For purposes of comparing the shapes of the oxygen profiles with those of organic carbon and for calculating the rate constant for aerobic respiration in the sediments, exponential equations were fit to all of the oxygen data. Under steady state conditions, assuming constant porosity and sedimentation rate, and first order kinetics, the distribution of O2 in the sediments should he described by a balance between the downward diffusive flux and consumption by respiration (130~/&),,,. The respiration term can be approximated as a first order consumption proportional to the solid metabolizable organic carbon ( ko2G,) (Berner, 1974). Thus, Do2

$$ =ko,G,

(10)

where Do2 is the diffusion coefficient of O2 in the

AEROBIC RESPIRATION

IN PELAGIC MARINE

DISSOLVED

OXYGEN

(p

moles

SEDIMENTS kg-

1

200

. .

.

J I

.

K

.

1

I

1

P I

-

I

FIG. 6. Vertical profiles of dissolved oxygen in the interstitial water at stations A, E, F, G, H, I, J, K, and P. The solid lines represent the exponential curves that best fit the data.

sediments and ko2 is a first order rate constant. The equation describing the distribution of utilizable organic carbon, G,, was derived in the previous section. Combining these gives Do2 %

=

ko2Gm.a exp(B,z)

(11)

where B, was defined in Equation (9) and found to be approximated by (kc/K)‘/*. The solution to this equation can be expressed as: O*(z) = Ol, + A02 exp(B,z)

(12)

where Ao2

_

koGn,o

Do,Bf

~(1 - 4)a 12 - 10Oq1

(13)

and 0; is the constant oxygen concentration at depth. The factor p( 1 - d)(r/12 - 1OOd is necessary to convert organic carbon from 9%to moles cm;:. The term A02 (moles cm;:) is the total change in oxygen concentration from the interface to the depth where it levels off to some constant value. a is the stoichiometry factor relating oxygen to carbon as in Equation (3) and p is the density of solid sediments.

The oxygen data was fit to Equation (12) by minimizing the variance of the actual data points from the exponential curve. The values of O)z, AOr, and the values of B, obtained from the oxygen profiles [e.g., B,(02)] for each station are summarized in Table 4. The best fit exponential curves are drawn in Fig. 6. The value of the coefficient B determines the depth scale over which Ol, is reached. Larger values of B mean a more rapid approach to 05. As the data in Table 4 shows there are no systematic differences in A02 or B,(OJ between carbonate and red clay sediment. The values of A02 do appear to be higher in the equatorial region as were the concentrations of surface organic carbon (Fig. 3). If we assume, as we did for organic carbon, that bioturbation is more important than sedimentation for the downward flux of solid organic carbon, then again B, can be approximated by (kc/K)‘/*. The first order rate constants for organic carbon utilization (k,), calculated using a biological mixing coefficient of K = 100 cm* kyr-‘, are given in Table 4. The average value (n = 9) of kc obtained by modeling the O2 profiles is 4.2 kyr-‘. This is in excellent agreement

1110

V. GRUNDMANIS Table 4

AND J. W. MURRAY

The best fit parameters for the fit of the interstital oxygen data to an equation of the form: 02(z) = 0; + A02 exp (Bcz) where A02 is the total change in oxygen concentration from the interface to the depth where it levels off to some constant value, 0'. The values of Bc determined from the 02 profiles is given*as Bc(02). Values of kc were calculated assuming K = 100 cm* kyr-1.

Station Il A E F G H I J

Sediment Type

(wol:i

C RC RC C RC C RC RC C

K

P

kg-l)

131 113 137 90 108 80 77 57 117

(PO::

kg-l)

37 80 55 77 a7 122 125 138 60

;::"z: -0.17 -0.31 -0.089 -0.058 -0.17 -0.19 -0.20 -0.32 -0.18 average

with the average value (n = 8) of 3.9 kyr-’ obtained from the organic carbon profiles. Our assumed value of K = 100 cm2 kyr-’ can be shown to be realistic as follows. Starting with Equation (13) and assuming that the oxygen consumption and carbon utilization are linked by the same reactions, thus ko2 = k,, we derive Ao

=

KGm,odl - 4)

2

Do,12. 100-~$

(14)

Rearranging this equation we can estimate an average value for K. This is an average value because K most likely decreases with depth (Guinasso and Schink, 1975). For example, at station H where A02 = 87 X 10m9 moles cme3, G,,0 = 0.39, C#I= 0.90, (Y = 1.28 and p = 2.6 gm cme3 and Do2 = 6.0 X lOA cm* set-’ (Murray and Grundmanis, 1980) we obtained K = 137 cm* kyr-‘. This is in good agreement with the range of values discussed earlier. At stations A, E, H, I, and K data on the organic carbon content of the sediments and the interstitial oxygen concentrations were both obtained. We can relate the two using Equations (7), ( 12), and ( 13). Differentiating (7) we obtain Equation ( 15). dGr ~ = PC&, exp@s) dz

Differentiating

(15)

( 12) we obtain (16). do2 = B,A02 exp(Bs) dz

(16)

These equations can be combined to give dGr cm.0 do2== and finally, substituting

(17)

( 13) into Equation ( 17)

(k;?) 2.89 9.61 0.79 0.33 2.89 3.61 4.00 10.2 3.2 4.2+3.5

where LYis the stoichiometric ratio relating O2 to carbon. The solid carbon/dissolved oxygen relationships are illustrated in Fig. 7 and the important parameters describing the best lirmar fit to the data are given in Table 5. The range of values of dGT/d02 for all five stations is only 209L with an average value of 3.14 X lo-“. The linear correlations are highly significant. The intercept of the best fit line at all five stations is negative, indicating that if all of the carbon was oxidized there would still be some oxygen remaining. Utilizable organic carbon is undoubtedly limiting at all locations in this part of the Pacific. The changes in porosity are small, thus the approximately linear relationship between dissolved oxygen and solid organic carbon suggests a constant ratio of D&k&-‘. C. Nitrate

During microbial degradation of organic matter ammonia is liberated to the interstitial waters. Under anaerobic conditions it accumulates in the interstitial water and adsorbs on the sediments (Rosenfeld, 1979; Murray et al., 1978; Boatman and Murray, 1982). Under aerobic conditions, however, ammonia is oxidized to nitrite and then nitrate by nitrifying bacteria and little ammonia or nitrite accumulates in the sediments under equilibrium conditions, The nitrate data presented in the appendix are consistent with the sediments being aerobic over the entire sampling interval at every station. The concentration of interstitial nitrate increases in the sediments to values greater than bottom water values (Fig. 8). The nitrate concentrations level off or appear to be leveliing off. In fact, they look very much like the mirror images of their respective oxygen profiles. The nitrate concentrations never begin to decrease after they level off. This indicates that denitaitication is not OCcurring in the top 50 cm of these sediments which

AEROBIC RESPIRATION

IN PELAGIC MARINE

SEDIMENTS

1111

u u

0.7

2 z o

0.6

$ 0.5

OXYGEN

(pmoles

kg-’ 1

FIG. 7. The relationship between solid organic carbon and dissolved oxygen in the interstitial water from stations A, E, H, I, and K. A and I are carbonate ooze sediments and E, H, and K are red clay. The points labeled B represent bottom water. The solid lines are the best fit linear regressions through the data.

is completely in agreement with the high oxygen concentrations. Under steady state conditions an equation similar to Equation (12) should describe the interstitial ni-

trate profiles. NO;(z)

= NO;’ + ANO; exp(&)

(19)

The parameters in Equation (19) for each of the stations are summarized in Table 6. In addition, Table 6 lists the values of B, calculated for the oxygen and organic carbon profiles as a comparison. The values of ANO; are largest in the equatorial region, the same as for A02 and percent organic carbon. The agreement between B,(NO;), B,(O,), and B,(carbon) is quite good for most of the stations. Since oxygen

consumption and nitrification are related processes the values should be the same. At two stations (E and K) the value of B for carbon is much less than those for O2 and N03. This may be an indication that the in situ sampler overpenetrated at those sites and thus we lost some of the shallowest pore water points. Examination of the O2 and NO3 profiles from these stations (Figs. 6 and 8) reveals that they decrease much more abruptly than most of the other profiles. When the value of B,(Oz) is used to fit the solid organic carbon profiles (Fig. 2) the calculations predict that organic carbon should decrease much faster than it actually does. Thus, at stations E and K we appear to have overestimated BC(Oz) and &(NOd.

I I I2

AND J. W. MURKAl’

V. GRUNDMANIS

Table

5

The best fit parameters describing the relationship between solid organic carbon and interstitial water dissolved oxygen as described by: dGT % organic carbon = do 0 + intercept 2 2 The correlation coefficients (r) are also given.

where cy = stoichiometric coefficients relating the given interstitial species to carbon D = diffusion coefficient of that species. Rearranging, we can calculate (Yoz/Nobthe oxygen/ nitrate stoichiometry. Thus: (21) The oxygen/nitrate relationships were calculated for each of the stations, by using the reduced major axis method (Till, 1974). A summary of the best fit lines for dOJdN0; are given in Table 7 along with the mean value and limits on ffoz/NoTcalculated from Equation (21) using 1.22 as the ratio of Do2/DNO; The average of 10.1 is slightly higher than the Redfield value of 8.6. The oxygen/nitrogen stoichiometry can be used to calculate the carbon/nitrogen stoichiometry. From Equation (3) the value of the stoichiometric ratio (Yoz/No;can be expressed as

~;

average

= 3.1420.70

x 10

The relationship between oxygen and nitrate can be described by (from Berner, 1977):

(20)

a02/NOS

NITRATE 2c)

40 I

/-

=

x-l-2Y

(22)

Y

moles kg-’ )

(p

40

20

+y.

.

. .

.

.

_

. . . . . . . ~ *

. .

.

.

.

.

7

.

. .

t

. .

I ; ;

A, :,

,,t 20

QO

*

l

E

-.A-

40 1

30

I .

50 I

.

20

40

. . .

40 /

I

I

.

. .

.

.

._ . . .

.

.

I

I

.

. . . .

.

20-

20

I

I

. .

.

H

/

. .

G

I !

I

.

.

1

F

:

I

. -1 --_-_L

.

. . . .

_

.

.

.

.

.

40-

1 60~

I

.

.

.

.

. I

J

.

1

I

.

. .

K /

. 1

p

: I

I

FIG. 8. Vertical profiles of nitrate in the interstitial water at stations A, E, F, G, H, I, J, K, and P.

.

i

AEROBIC

Table 6

RESPIRATION

IN PELAGIC

MARINE

SEDIMENTS

1113

The best fit parametersfor the fit of the interstitial nitrate data to the equation: NO;(Z) = NO; + ""0; exp (BcZ) where AN03 is the total increasein nitrateconcentration from the interfaceto the depth where it levelsoff at scme constant value, NO!,. The best fit ratio of B, from the dissolvedoxygen and carbonprofilesare shown for comparison. Bc (NO;)

NO;

Sediment @imoleskR-1) type

Station #

C RC RC C RC C RC RC C

A E F G H I J K P

This equation can be rearranged X - = Y

39.0 39.2 41.0 44.0 42.3 46.7 51.1 46.9 39.4

8.6 9.4 9.3 10.3 17.1 19.5 15.9 9.8

Bc CO21

(cm-11 0.15 0.12 0.11 0.065 0.19 0.11 0.21 0.25 0.11

to give:

(cm-l) -0.17 -0.31 -0.089 -0.058 -0.17 -0.19 -0.20 -0.32 -0.18

-

2 =

%/NOT

-0.15 -0.25 -0.099

I

--I

d=Oz &co

aC’N - dN0; aOa/NOr

BC

-1 (cm ) -0.23 -0.060

(24)

DNor

(23)

The term x/y is the stoichiometry ratio relating carbon to nitrogen. The only assumptions inherent to this calculation are that the organic carbon is at an oxidation level equivalent to carbohydrate and that organic nitrogen is released as ammonia and completely oxidized to nitrate, The carbon/nitrogen stoichiometric ratio aC/No; was calculated using Equation (23) and the values of %,/NO; given in Table 7. The average value of 8.1 is slightly larger than the Redfield ratio of (YC/NcJ= 6.6 and lower than the values obtained earlier from our solid phase carbon and nitrogen data.

This equation was applied to the data from the red clay stations making the assumption that Dzcch =DHCOF A value of 0.62 was USC~ for DHCOy/DNO~ (Li and Gregory, 1974). The calculated values of ffC/N are given in Table 8. Again the average value is 9.1 which agrees well with the values 8.1 obtained from Or and N03. The ratio of CO2 produced to oxygen consumed can be calculated in a similar manner. The values are given in Table 8 and are compared with the Redfield ratio of 106/138 = 0.78. The values are also

Table 7

D. co, The total CO2 data can be combined with nitrate to derive a third way to calculate the carbon/nitrogen stoichiometry of the decomposing organic matter. Total CO2 calculated from pH and alkalinity agrees to within +2% of the measured total COz. The calculated total CO, from the red clay stations E, F, H, J, K, and P is presented in the Appendix. This calculated total CO1 data is comparable in precision to the dissolved oxygen and nitrate data, however, the relative increases are considerably less than the relative changes in oxygen and nitrate. Only values for total CO2 in red clay sediments are used in these calculations because there the increase in total CO2 should be due primarily to effects of respiration. In carbonate containing sediments the increase in total CO2 includes the effects of the reaction of CO2 produced during respiration with solid CaCO,. It is possible to calculate (Ye/Nfrom total CO,/ NO3 correlations in red clay sediments and compare these values with those calculated from Or/NO3 and from solid organic C/total N. An equation similar to (21) can be written for total CO, and NO1.

The stoichiometricfactors for oxygen consumed to nitrate produced (a calculatedfrom oxygen-nitrate02/NC3) correlationscorrectedfor differences in diffusioncoefficientsas described in the text. The carbon/nitrogen ~:~~~:a~~tr:~~factors(aC,N03) 'C/NO - a02/N0 - 2 are also The c 4 assic

Red 1 ield

ratios

shown.

are

shown

for comparison.

Station

# 1

2

‘C/N

-7.8 -10.7 -6.9 -8.5 -9.2 -7.7 -6.6 -8.5 -8.7

7.5+3.1 11.174.1 6.g2.1 8.4Tl.7 9.2T2.2 7.4Tl.O 6.1G.4 8.471.1 8.654.4

Average Redfield Ratio

8.6

6.6

V. GRUNDMANIS

1113 Table 8

Station

AND

J. W. MURRA\r

The stoichiometrii factors for total CO2 to nitrate producea (li and total CO2 produced to oxygen consumed (UC/~ )_ These facto:iN) are calculated from the total CO2-nitrate and trS tal C02-oxygen correlations corrected for differences in diffusion coefficients. The factor for oxygen consumed to total CO2 produced is also shown. The classic Redfield values are given for comparison.

ii i

L dNO.,

r

I .-

0.55 0.86 0.90 0.98 0.96 0.99

-0.45 -0.73 -0.86 -0.99 -0.99 -0.74

average

9.121.8

0.8070.22 0.960.08 0.85T0.11 1.04+0.22 0.90+_0.23

Redfield ratio

6.6

0.78

given as a&/c for comparison. The mean value of six stations in 0.90, however, the error range is large and easily includes the Redfield value. IV. CONCLUSIONS

Solid organic carbon, dissolved interstitial oxygen, and dissolved interstitial nitrate from red clay sediments in the central Equatorial Pacific form an internally consistent set of data, the first of its kind to be collected from pelagic sediments. All three constituents vary exponentially with depth, solid organic carbon and dissolved oxygen decreases while nitrate increases. The system is driven by the flux of metabolizable organic carbon down into the sediments. The “folding constants” of the exponential curves have approximately the same values for all three substances as would be expected if they were all controlled by the same chemical reaction which in this case is aerobic respiration with nitrification. The exponential “folding constants” are a function of the sedimentation rate, bioturbation mixing coefficient and the first order rate constant for organic matter oxidation. Near the sediment-water interface (O-1 5 cm), where most of the concentration changes occur, bioturbation is probably more important than sediTable 9

average

1.23

1.x+0.32

mentation for controlling the downward flux of reactive organic carbon. Without detailed studies of U-Th series isotopes or 14C the rate of bioturbation can only be estimated from literature studies of similar locations (e.g., Guinasso and Schink, 1975). Most recent studies of sediment mixing in pelagic sediments have determined mixing coefficients that average around 100 cm2 kyr-‘. Using this value we have calculated that the average value of the first order rate constant for organic carbon decomposition is 3.9 kyr-‘. This corresponds to a half life of about 250 years. A similar value was independently obtained by modeling the oxygen data. These values are valid for red clay as well as carbonate ooze sediments. The relationship between solid organic carbon and dissolved oxygen suggests that the oxygen consumption is limited by the availability of oxidizable organic carbon. Even if all the organic carbon present was oxidized dissolved oxygen would still be present. This finding can confidently be extended to most of the area of pelagic sediments that have long been supposed to be aerobic. Nitrate, which is a retatively easy measurement, would be the most diagnostic measurement to make in future studies that wish to verify that pelagic sediments are aerobic. The exten-

The carbon/nitrogen stoichiometry @C/N) of the decomposing organic matter calculated using: A) dzC02/dN03 in interstitial water; B) d02/dN03 in interstitial water; C) d erg C/d total N in solid sediments; and D) d org C/d org N in solid sediments.

A A E F G H I J K L M P

i

1.25 1.11 1.18 0.96

7.121.6 11.3k4.6 9.7k2.2 6.eO.7 8.9+_1.4

10.7+0.5 9.1kl.8

B 7.523.1 ll.lk4.1 6.4+2.1 8.Gl.7 9.2+2.2 7.471.0 6.1TO.4 8.451.1

C

D

3.4 8.5

14.7

15.5

15.3 18.1 15.5

14.0

12.6+5.5 -

13.752.0

11.6

8.6t4.4 8.151.5

AEROBIC RESPIRATION IN PELAGIC MARINE SEDIMENTS

sion of these conclusions below the top 30 cm of pelagic marine sediments, is uncertain because of the possibility of nonsteady state deposition organic carbon during the geologic past. The results for solid organic carbon and total nitrogen and dissolved oxygen, nitrate and total CO2 can be combined to calculate the stoichiometry of the decomposing organic matter. The ratio of oxygen consumed to total CO1 produced in red clay sediments averages 0.90 + 0.23 with error limits that overlap with the Redfield ratio of 0.78 which corresponds to a C/N ratio of 106/16. The ratio of oxygen consumed to nitrate produced averages 10.1 in both red clay and carbonate ooze sediments, however, again the error range overlaps with the Redfield ratio of 8.6. The ratio of organic carbon consumed to nitrate produced was calculated by several different approaches which are compared in Table 9. The C/N ratios calculated using 02, NOs, and total CO* data tend to agree with each other. The average values using these dissolved constituents are only slightly larger (about 30%) than the Redfield ratio of 6.6. The C/N ratios obtained from the solid phase analyses tend to be significantly higher at about twice the Redfield value. We have no simple explanation for the discrepancy other than to reiterate the fact that using the approach of the solid sediment analyses involves many more assumptions and corrections for unavailable carbon and nitrogen. The grand average of all values based on pore water analyses gives a C/ N ratio of the decomposing organic matter of 8.5 + 1.6. The error limits overlap with the classic Redfield ratios. The mean values, however, suggest that the decomposing organic matter may be slightly deficient in nitrogen so that the actual C/N ratio is 106/ 12 rather than 106/ 16. This suggests that a stoichiometric equation of the type (CHZO)VX(NH~)IZ + 13002 = 106CO2 + 12HNOx + 118H20

(25)

may be appropriate for modeling the early diagenesis of organic matter in central Equatorial Pacific sediments. The C/N ratio is smaller than the value of 106/8 proposed by Hartmann et al. (1973) and Sholkovitz (1973) for organic matter decomposing in hemipelagic sediments off the west coast of North Africa and in the Santa Barbara Basin respectively. The close agreement with the “classic” Redfield ratios in the central Pacific suggests that the oxidizable organic matter incorporated into the sediments is relatively close to the original stoichiometry for “fresh” organic matter. This is a surprising result when compared with sediment trap and large volume filtration studies of particulate matter in seawater. For example, Bishop et al. (1977) found C/N to increase from 7.74 in the photic zone to 9.86 at 388 m indicating a 20% preferential loss of nitrogen. Knauer et al. (1979) found C/N to increase from 14 at 75 m to 29 at 1050 m in open ocean sediment traps. Honjo ( 1980) also found the ratios in sediment

1115

traps to increase with depth. The ratios reported here tend to support the argument by Suess and Mtlller (1980) that organic matter synthesized by benthic organisms constitutes a major fraction of the metabolizable organic carbon in pelagic marine sediments. Acknowledgement-We are grateful for the invitation from Dr. F. L. Sayles to participate in R/V Knorr 33 Leg 7. Mr. Jeff Sawlan assisted us in sample collection and analyses. Dr. Kent Fanning, Mr. Jabe Breland, and Ms. S. L. Jones measured the nutrients using a four-channel Technicon auto-analyzer II system. The manuscript benefited from discussions with Ms. K. Kuivila and Mr. R. Jahnke and reviews by Drs. C. Martens, E. Suess and T. R. S. Wilson. Barbara Fulton typed the manuscript. This research was supported by NSF Grant No. GCE 78- 18746. REFERENCES Arrhenius G. (1952) Sediment cores from the east Pacific. In Reports of the Swedish Deep-Sea Exploration 19471948 (edited by H. Petterson), Vol. V, l-227. Bender M. L., Froelich P. N., Heath G. R., Fanning K. A. and Maynard V. (1977) Interstitial nitrate profiles and oxidation of sedimentary organic matter in the Eastern Equatorial Atlantic. Science 198, 605-609. Berner R. A. (1974) Kinetic model for the early diagenesis of nitrogen, sulfur, phosphorus and silicon in anoxic marine sediments. In The Sea (edited by E. D. Goldberg), Vol. 5, Wiley-Interscience, 427-450. Berner R. A. (1977) Stoichiometric model for nutrient regeneration in anoxic sediments. Limnof. Oceanogr. 22, 781-786. Berner R. A. (1979) A new look at biogenous material in deep sea sediments. Ambio Special Report No. 6, 5-10. Berner R. A. (1980) Early Diagenesis: a Theoretical Approach. Princeton University Press. 250 pp. Berner R. A. (1981) A new geochemical classification of sedimentary environments. J. Sediment Petrol. 51, 359365. Bishop J. K., Edmond J. M.. Ketten D. R., Bacon M. P. and Silker W. B. (1977) The chemistry, biology and vertical flux of particulate matter from the upper 400 m of the equatorial Atlantic ocean. Deep-Sea Res. 24, 51 I548. Boatman C. D. and Murray J. W. (1982) Modeling exchangeable NH: adsorption in marine sediments: process and controls of adsorption. Limnol. Oceanogr. 27, 99110. Brewer P. G. and Spencer D. W. (1974) Calorimetric determination of manganese in anoxic waters. Limnol. Oceanogr. 16, 107-I 10. Cochran J. K. and Krishnaswami S. (1980) Radium, thorium, uranium and *“‘Pbin deep-sea sediments and sediment pore waters from the north Equatorial Pacific. Amer. J. Sci. 280, 849-889. Emerson S. and Bender M. (1981) Carbon fluxes at the sediment-water interface of the deepsea: calcium carbonate preservation. J. Mar. Rex 39, 139-162. Emerson S., Jahnke R., Bender M., Froelich P., Klinkhammer G., Bowser C. and Setlock G. ( 1980) Earlv . diaaenesis in sediments from the eastern Equatorial Pacific. I. Pore water nutrient and carbonate results. Earth Planet. Sci. Lett. 49, 57-80. Froelich P. N. (1980) Analysis of organic carbon in marine sediments. Limnol. Oceanogr. 25, 564-572. Froelich P. N., Klinkhammer G. P., Bender M. L., Luedtke N. A., Heath G. R., Cullen D., Dauphin P., Hammond D., Hartmann B. and Maynard V. ( 1979) Early oxidation of organic matter in pelagic sediments of the eastern Equatorial Atlantic: suboxic diagenesis. Geochim. Cosmochim Acta 43, 1075-1090. Glass B. P. (1969) Reworking of deepsea sediments as s

1116

V. GRUNDMANIS

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J. W. MURRAY

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STATION A

% Total C

0.467 0.453 0.470 0.427 0.341 0.381 0.369 0.299 0.286 0.278 0.239 0.209 0.200 0.225 0.191 0.117

Depth (cm)

O-l l-2 2-3 3-4 4-5 5-6 7-8 lo-11 14-15 19-20 24-25 29-30 34-35 39-40 44-45 49-50

STATION E

9.53 9.47 9.51 9.90 9.89 10.18 10.32 10.32 10.65 10.71 10.05 8.81 7.59 8.91 9.38 9.76

% Total C

O-l 1-2 2-3 3-4 4-5 S-6 7-8 10-11 14-15 19-20 24-25 29-30 34-35 39-40 44-45 49-50

Depth (cm)

0.498 0.622

0.299 0.332 0.166 0.174 0.158 0.174 0.307 0.241

0.556 0.465 0.166

0.391 0.301 0.361 0.348 0.280 0.265 0.241 0.210

0.133 0.169 0.171

%C&O3

%erg c

0.407 0.378

77.1 76.6 77.4 80.9 80.9 83.5 84.6 84.6 87.5 88.2 82.7 72.2 61.8 72.8 77.2 80.3

%C&O3

0.237 0.238 0.179 0.155 0.138 0.116 0.121 0.123 0.102 0.088 0.088 0.112 0.138 0.133 0.079 0.087

%erg c

0.051 0.053 0.058 0.059 0.053 0.054 0.052 0.053 (0.091) 0.043 0.041 0.035 0.023 0.038 0.025

i

5.8 4.4 6.8

5.6 5.1

6.6 5.7 6.7 6;7 5.3

7.9 7.1

Org c Total N

4.1

0.033

% Total N

2.2

0.040

-

10.7 3.4 3.0 3.2 2.8 2.1 2.2 2.3

Org c Total N

0.022 0.071 0.060 0.049 0.049 0.037 0.054 0.053

% Total N

O-o.5 0.5-1.0 1.0-2.0 2.0-2.5 2.5-3.0 3-4 7-8 9-10 14-16 19-20 24-26 29-30 34-35 39-40 44-46 49-50

Depth (cm)

2.703 2.846 2.907 3.071 3.587 4.772 6.580 6.902 5.596 4.758 0.717 0.318 0.196 0.162 0.212 0.199

% Total C

0.222

0.490 0.562 0.514 0.498 0.451 0.356 0.217 0.261 0.211 0.222 0.217 0.215

4 Org c

0.583 0.543 0.546 0.406 0.401 0.338 0.344 0.271 0.242 0.249 0.228 0.183 0.228 0.217 0.178 0.205

0.661 0.646 0.600 8.527 0.488 0.419 0.399 0.346 0.286 0.285 0.251 0.207 0.226 0.205 0.213 0.238

0.0.5 0.5-1.0 1.0-1.5 2.5-3.0 3.5-4.0 5-6 7-8 9-10 15-16 19-20 24-25 29-30 34-35 39-40 44-45 49-50 STATION I

%0x-g c

% Total C

Depth (cm)

STATION H

I) Carbon-nitrogen analyses of solid sediments. Percent CaCO, was calculated according to (Cr% org) 8.3. When organic carbon is equal to or greater than total carbon CaCO, is listed as 0%.

APPENDIX COMPILATION OF THE DATA FROM CRUISE R/V KNORR 78, LEG VII RELEVANT TO DISCUSSIONS PRESENTED IN THIS PAPER

0.0

18.4 18.9 19.9 21.4 26.0 36.6 52.8 55.1 44.7 37.6 4.15 0.855

% C&O3

0.647 0.855 0.448 1.004 0.722 0.672 0.456 0.622 0.365 0.299 0.191 0.199 0.0 0.0 0.290 0.274

%c&o3

0.034

0.080 0.086 0.054 0.046 0.096 0.104 0.067 0.060 0.055 0.052 0.034 0.030

% Total !i

0.027

0.026

6.5

6.1 6.6 9.6 10.7 4.7 3.4 3.2 4.4 3.8 4.3 6.4 7.2

Total N

Org c

7.0

10.4 9.2 10.3 8.3 9.5 8.2 8.6 6.9 0.056 0.059 0.053 0.049 0.042 0.041 0.040 0.039

Org c Total N

% Total N

(cm)

39-40

19-20

14-15

?.5-8.C

4.0-4.5 5.0-5.5

3.0-3.5

2.0-2.5

0.5-1.0

o-o.5

corell1

Depth

STATION L

39-40

19-20 30-31

14-15

9.5-10

7.0-7.5

3.5-4.0

2.5-3.0

1.5-1.0 1.0-1.5 1.5-2.0

o-o.5

:ore li2

0.348 0,234 0.196 0.177 0.211 0.142 0.154 0.209 0.175

0.297

0.566 0.601 0.541 0.566 0.601 0.543 0.538 0.490 0.483 0.477 0.442 0.396

4 Total

C

X

0.086

0.097 0.074

0.085

0.146

0.178

0.366

0.400

0.422 0.500 0.419

0.499

Org C

0.739

0.374 0.664

0.764

0.730

0.988

0.921

0.747

1.19 0.838 1.03

0.847

%CaC03

N

7.7

c

co.013

6.6

5.7

7.5

6.5

13.3

11.8

15.4

11.9

13.5

0.035

0.026

c

Total N

Org

6.3 6.3 7.5 5.1 4.2 4.5 5.2 4.7

12.2

-

Org

Total N 8.9 9.7 9.2 8.9 8.2 9.2 7.9

0.034

0.037

%Total N

0.046 0.049 0.050 0.033 0.045 0.048 0.046 0.040 0.040

1.10 0.979 1.64 1.00 0.714 0.954 0.788 0.631 0.996

0.355 0.308 0.316 0.247 0.231 0.201 0.207 0.210 0.187

I

0.076 0.069 0.071 0.072 0.077 0.074 0.063

0.880 0.822 1.02 0.847 0.988 0.548 1.59

0.678 0.669 0.652 0.647 0.635 0.682 0.497

%Total

0.784 0.768 0.775 0.749 0.754 0.748 0.689 0.571 0.488 0.426 0.514 0.368 0.317 0.316 0.302 0,286 0.307

&co3

D-o.5 0.5-1.0 1.0-1.5 1.5-2.0 2.5-3.0 3.5-4.0 5-6 J-0 11-12 15-16 19-20 24-25 29-30 34-35 39-40 44-45 49-50

x

x erg c

x Total c

Depth (cm)

STATION K

O-l 1-2 2-3 3-4 4-5 5-6 6-J 7-a 6-9 9-10 11-12 13-14 15-16 17-18 19-20 24-26 29-30 34-35 38-39

Depth (cm)

STATION R

11.26 11.39 11.30 11.35 11.34 11.52 11.52 11.59 11.74 11.74 11.90 11.80 12.13 11.83 12.04 11.84 11.80 11.48 11.37

%Total C

0.177 0,176 0.182

0.058 0.056 0.047 0.049 0.043 0.047

0.068

0.140 0.098 0.109 0.069 0.085 0.080 0.062

4 Org c

--

97.7 99.5 97.9 97.6 94.9

100.2

97.7

95.1 95.8 97.1 96.9 98.6

93.7 92.9 93.6 93.4 94.9

92.3

%CaC03

0.95 0.57 0.59 0.77 0.51 0.35

0.197 0.180

0.89

0.312 0.238

0.526 0.489 0.420 0.405 0.353 0.266 0.251 0.270 0.238 0.224

0.46 1.29 1.14

0.524 0.433 0.385

0.579 0.509

C&O3

o-o.5 0.5-1.0 1.0-1.5 2.0-2.5 3.0-3.5 4.0-5.0 J-S 11-12 19-20 24-25 39-40 49-50

x

%Org c

%Total c

STATION El Depth (cm)

I

N

N

0.0064

0.0068

0.0044 0.0076 0.0057 0.0081 0.012 0.0053

0.010

Total

0.019

0.022 0.017

0.028

0.025 0.037 0.028

X Total

8.8

3.5

Org c Total h 14.0 22.3 14.3 12.1 10.5 6.7 11.7

9.3

10.S 11.6

li.1

20.9 11.7 13.7

Org c Total N

7 10 13 16

19 22

25 28

33 38

43 48 53

2

6 7

a 9

10 II

12 13 14

2 5

4

1

^ _

^

,. ., ._

-I

127 129 131 151 133 1.56 121 122 95 LO2 128 107 143 L20 117 135 98 150 I.22 99 92 91 116 130 114 152 124 99 134 LO1 89 78 80 117 119 152 LO1 114 126 128 148 99 82 82 140 134 88 111 123 108 117 90 116 136 136 93 109 79 98 96

102 135

124 108

159

_ _ 55 132 73 134 57 121

84 99 " _ 96 93 73 82 66

_,._

78 -

84 68

76 88

91 84

P

95 177

K

..

_

38

11

48 53

I.3 14

43

33

10

12

28

9

25

22

7 8

19

16

6

5

10 I3

.96 !07 ;33 .28

J

3 4

I 7

H

201 203 142 141 170 155 143 134 137 149 114 159 130 133

Bottom Water 168 193 192 167

G

2

0

F

4

E

1

A

Port I

Depth (4

II. Interstitialoxygen concentrations(~rmoles kg-l)

A

39.6 39.2 39.2 39.0 38.8

38.8 38.7 38.9 39.2 39.6

38.6 38.5 39.3 38.6 38.6

38.0

38.0

37.1

35.7

Bottom Water 33.1

0

Depth (cm)

37.3 36.6 37.1 38.4 36.8 36.1 39.2 38.9 37.1 38.2 38.2 39.1 37.6 39.0 39.9 38.9 39.7 39.4 40.2 39.1

30.3 35.0 34.7 37.3 37.3

30.9

E

38.0 38.2 38.2 38.5 39.2 38.9 39.5 37.1

32.6 32.7 36.9 36.9 37.6 37.8

F

39.7 39.9 40.7 40.7 41.1 41.2 42.2 42.5 41.6 41.8 42.8 42.5 42.5 42.7 42.6 42.6 43.3 43.9 43.8 43.6 43.8 43.8

34.6 34.8 35.9 35.6 40.0 39.7

G

42.6

42.1

42.1

42.2

41.9

41.5

41.6

41.7

41.2

40.6

39.8

40.2

38.8

32.0

H

Interstitialnitrate concentrations(pnoleskg-l)

Port #

III.

46.3 46.9 46.8

45.9

45.7 46-4 45.5

44.9 45.3 44.3

41.6 42.7 44‘0

G.4 42.3 43.1

39.3 39.6

40.1

29.6

I

50.9 51.7 50.8 51.2 so.9

51.2 51.2

51.0 50.8 51.2

50.7 51.1 49.5

48.3 46.5 48.8

46.6 46.7

43.4

31.6

J

46.7 46.8 46.7 47.0 47.3

47.0 47.1 46.2

46.4 46.0 45.7

45.7 46.0 46.1

45.7 45.6

43.9 43.3

43.3

K

39.9

39.4

38.8

39.4

39.1

38.5

38.2

37.9

37.9

37.3

36.4

34.8

29.6

P

250 253 245 266 277 287 299 306 315 307

239 253 257 266 261 266 276 277 280 283 283

213 237 232 253 249 254 266 268 271 280 275

216 221 234 235

243 240 250 256 251 262 269 272 271 280 270

129 231 243

141 151 222

123 204 217

124 190 226

Bottom Water 4 7 10 13 16 19 22 25 28 33 38 43 48 53

0 1 2 3 4 5 6 7 8 9 10 11 12 13 14

146 195 207

H

F

A

Depth (cm)

E

G

Interstitialsilica concentrations(ymnoles kg-lj

Port #

IV.

317 308 289 329 333 319 353 345 356 362 361

124 274 264

I

421 420 427 427

420 422 436 435

410 414

409

354 370

K

361 360 388 371 399

121 304 351

J

258 259 279 273 279 279 283 289 285 297 295

127 248

P

0 1 2 3 4 5 6 7 a 9 10 I1 12 13 14 -__

(cm)

Depth Sottom Water 4 7 10 13 16 19 22 25 28 33 38 43 48 53 ~-."^--

E

2277 2239 2243 2194 2256 2259 2273 2254 2270 2229

2271 2248 2261

J

K

2426 2439 2451

2455

2449

2530 2547 2600 2590 2619 2548 2588 2570 2587

2443 2562

2317 2354 2451 2431 2557

2437 2441 2447 2448 2467 2473 2486 2488

2441 2421

2254 2369

P

-1, , calculated

kg Interstitial total CO1 concentrations (i.rmoles from pH and alkalinity

Port I

V.