TRANSFER AND BIOGEOCHEMISTRY OF ORGANIC MATTER AT SEDIMENT—WATER INTERFACE ON THE NORTHWESTERN

MEDITERRANEAN MARGIN

R. BUSCAIL* and F. GADEL

Laboratoire de Sedimentologie et Geochimie Marines, UA CNRS 715, Uninersite de Perpignan, 66025 Perpignan, France

ABSTRACT

The northwestern margin of the Gulf of Lions is cut by numerous canyons, areas of advective fluxes trans-ferring matter from the adjacent continental shelf towards the Mediterranean basin. With reference to the biogeochemical cycle of organic carbon in the marine environment, the deposit of organic matter was studied from the coastal zone near a river mouth (Tet), down to the base of the slope. The importance of continental or marine sources was stressed by biogeochemical contents of surficial deposits. The physico-chemical parameters and various organic compounds (total org C, hydrolyzable org C, N, amino acids, carbohydrates, phenolic com-pounds and humic substances) were studied for the different environments.

The input of organic compounds at the sediment—water interface was simulated by injection of 14C labelled organic molecules in sea water overlying surficial sediments. An amino-acid (14 (U) glutamic acid) was used to simulate a biodegradable marine organic molecule reaching the sea floor, whereas continental degraded com-pounds were represented by a lignin derived (14C(U) catechol). After 6-day laboratory incubations under in situ simulated conditions, in a dark thermostat-regulated chamber (13°C, oxidizing conditions), the dynamic of or-ganic matter exchanges between overlying water and sediment was established. Global budget is based on the following: (1) 14CO2 released during 6-day kinetics (7% with glutamic acid, 0.8% with catechol); (2) 14C remain- ing 74 0 14C , ing ih the water: dissolved organic C which averages 15 /o of the initial and mineral C fraction (0.1-3 /o); (3) 140 activity incorporated in sediment: a direct relationship can be observed between injected 14C catechol and the oxidation stage of the sediment. As far as glutamic acid is concerned, a higher fraction is recovered as 14CO2 which decreases the active 14C available for migrating in the sediment.

Moreover, the 14C activity in sediment was counted for different geochemical fractions for evaluating the relative part of biochemical and geochemical processes. This evaluation shows that distribution varies depend-ing on the nature of injected organic molecules, and is principally influenced by oxido-reduction potential, water content and geochemical characteristics of the natural organic matter. Site differentiation seems to be inde-pendent from the quantity of benthic microflora and corresponds better to the degree of heterotrophic activity.

Keywords: organic matter, biochemical compounds, humic substances, biogeochemical processes, 14C labelled compounds, sediment—water interface, northwestern Mediterranean margin.

INTRODUCTION where it is associated and diluted by marine organic material from plankton and ben-

Terrestrial organic matter, lignin den- thos. Indeed, Hunt (1970) and Hedges and ved, partially trapped on the continental Parker (1976) have demonstrated that most shelf is carried towards the deep areas terrestrial organic matter is trapped on the

* Corresponding author

114 ORGANIC MATTER AT SEDIMENT-WATER INTERFACE

shelves and is redistributed on the abyssal plains by turbidity currents. The continen-talcontribution to deep sea should be minor in areas where normal hemi-pelagic sedi-mentation occurs (Fontugne and Duplessy, 1986). Organic geochemistry of recent sedi-ments from the continental margin off southern New England (SEEP program) has shown a significant transport of sedi-ments from the continental shelf to the slope and rise. The distribution of the vari-ous organic compounds corresponds well to a simple linear mixing model where slope material is considered as derived directly from the shelf organic matter (Steinberg et al., 1987).

The northwestern Mediterranean mar-gin is cut by numerous canyons — areas of advective fluxes transferring matter from the adjacent continental shelftowards deep sea basin (Heussner et al, 1987). Fluxes of organic matter transported towards deep areas have already been studied (Monaco et a1.,1987). The nature of organic material is characteristic of its terrestrial origin on the coastal zone and becomes progressively more of a marine type offshore. However, terrigeneous organic matter is shown to set-tle inside the axial channel (from 500 to 1500 m) of the shelf-edge canyon (Lacaze-Duthiers) than along the flanks, or in the deep-sea fan (Buscail et al., 1988).

At the sediment–water interface, the sedimentary organic matter is exposed to the chemical and biological degradation processes of early diagenesis. Microbial mineralization and assimilation by benthic activity are important in the bioturbated zone (Aller, 1982). Formation of humic mat-erials which transform initial compounds is the main factor of decay (Pelet, 1984).

On the northwestern Mediterranean margin, the amount and physicochemical

nature of deposited organic matter were analyzed to appreciate the contribution of terrestrial or marine origin. Finally, label-led organic molecules were injected in sea water overlying sediment–water interfaces sampled in the Tat prodelta and in the slope. Incorporation, degradation and ex-change processes of organic matter were studied for the various environmental con-ditions.

MATERIALS AND METHODS

The sedimentary organic matter is studied at different sites of the margin. Un-disturbed sediment–water interfaces were sampled with a Reineck box-corer (636 cm2). The following sites were sampled in October: the Tet prodelta (26 m) on the con-tinental shelf, near the river mouth (Tat); on the slope, five bathymorphological sites were sampled in the Lacaze-Duthiers can-yon: the head (345 m), the axial channel (axis at 850 m and a side at 650 m), the ad-jacent open slope (interfluve, 400 m) bet-ween Lacaze-Duthiers and Pruvot canyons. The deep-sea fan, turbiditic accumulation area at the base of the slope (1740 m), was sampled in July (Fig. 1) .

Oxidation-reduction potential (Eh in mV) and pH were obtained using a pH meter (combined and platinum electrodes Ingold) immediately after box-corer sam-pling. Water content/dry weight sediment were measured on microcores obtained with 2 ml truncated syringes (Chassefiere et al., 1985) . The oxidized surface layer (0-1 cm) is collected, immediately frozen and freeze-dried in the laboratory. Subsamples were collected by vertical insertion of transpa-rent plastic tubes (7 cm diameter and 20 cm long) into the sediment through the over-lying water. On the oceanographic vessel,

3° 30' E

3° 40'E

3° 50E

4°0~E

4°IO'E 42°40'

3°20'E . ~

ORGANIC MATTER AT SEDIMENT—WATER INTERFACE

N A

42° 50

l

ißt ~~ Tet Prodelta

3° o!E

HEAD

Cap Bear 42°30 - ~

SIDE

AXIS

i

42°20'- C Cap de Creus

LACAZE-DUTHIERS CANYON

0 5 10 15 20Km

42°10

Fig. 1. Bathymetric chart of the northwestern Mediterranean (Canals and Got, 1986) showing the location of the study area.

115

subsamples were packed in ice and trans-ported within a few hours to the laboratory for 14C labelled substrate experiments.

Analyses

The geochemical characteristics of surfi-cialsediments were determined by analysis of different organic compounds. The anal-ytical procedure is indicated in Fig. 2. Org-anicand mineral carbon content/dry weight sediment were obtained by combustion in a Leco analyzer (after acidification of the samples with H3PO4 for total organic carb-

on (TIC). Hydrolyzable carbon (% of TIC) is given by the fraction hydrolyzed with 61 HC1 for 16 h at 110° C. Hydrolysis was per-formed in a Pyrex screw-cap tube with Tef-lon liner. Total nitrogen was measured by the Kj eldahl method using a Biichi auto-matic analyzer. Amino acids were analyzed by the colorimetric method. Absorption of products of the amino acids—ninhydrin reaction was measured at 570 nm with a Technicon automatic analyzer (Stevenson and Cheng, 1970). Carbohydrates were evaluated in a glucose equivalent by the method of Dubois et al. (1956) adapted for

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ORGANIC MATTER AT SEDIMENT-WATER INTERFACE

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ORGANIC MATTER AT SEDIMENT-WATER INTERFACE

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ORGANIC MATTER AT SEDIMENT-WATER INTERFACE 117

sediment by Liu et al. (1973). Lignin is oxidized quantitatively by alkaline nitro-benzene to yield products that include the phenolic aldehydes: vanillin (Van), syrin-gealdehyde (Syr) determinated by capillary GC (Pocklington and MacGregor, 1973).

Humic substances were extracted by al-kaline solvents (0.1 N NaOH-1% Na4P207; Debyser and Gadel, 1983). Organic carbon budget was established on the different fractions of the geochemical extractions. Dissolved organic carbon (DOC) was deter-mined in acid soluble fraction (2 N HCl), total humic substances and fulvic acids. A continuous flow system based on UV oxida-tion and CO2 quantification with a flame ionization detector was used for DOC measurement (Cauwet, 1984). The results of elementary analyses of humic and fulvic acids (C, H, N, 0, S) are plotted in a Van Krevelen (1961) diagram. Functional groups were determined by infrared spec-troscopy (Perkin-Elmer spectrophotometer - Kbr pellets were used) in order to obtain more information about the nature of humic and fulvic acids. 1H-NMR spectra were car-ried out using a Broker WP80 apparatus (Fourier transform).

These evaluations can be related to ben-thic population estimates. Total benthic meiofauna and viable bacteria countings (plate counts on solid 2216 medium) were made in the first centimeter of the surf"icial layers (L. Guidi, pers. comm.).

Experimentation

The input of organic compounds at the sediment-water interface was simulated by injection of 14C labelled organic compounds in water overlying two identically pre-servedsamples for each site. An amino acid (14C(U) glutamic acid) and a lignin derived

(14C(U) catechol) were used to simulate the input of marine and continental organic matter (1 ml of each added with an activity of 1 mC ml-1). Glutamic acid is the major free amino acid in interstitial water of mar-ine sediments, probably as a result of prod-uction and consumption by bacteria (Hen-richs and Farrington, 1987).

Incubations of 6 days were carried out at the in situ temperature (21°C on the Tet prodelta, 13°C in the canyon) in a dark thermostat-regulated chamber. Oxidizing conditions were preserved by flushing with air for CO2 retrieval. The 14CO2 produced by oxidative processes was collected daily in ethyleneglycol monethylether/ethanolam-ine (scintillation grade 7:2 V/I). At the end of incubation 14C radioactivity was counted in water and in soluble fractions extracted from sediment (0-12 cm) with a liquid scin-tillation counter (Intertechnique SL 2000-Lumagel solvent). 14C remaining in the water was identified in two fractions: organic dissolved 14C and bicarbonates (H14CO3-) after acidification of water. 14C

distribution in sediments was evaluated in geochemical fractions by successive acid and alkali extractions as natural organic matter was extracted (Fig. 2). In order to determine hydrolyzable and residual 14C humin, the residue of extraction was sub-mitted to hydrolysis for 16 h at 110°C (6 N HCl). The residue of hydrolysis was com-busted in an induction furnace (Leco) and 14CO2 was counted after retrieval.

RESULTS AND DISCUSSION

Distribution and nature of organic matter

On the Tet prodelta (26 m) organic mat-ter shows quantitative and qualitative seasonal variations resulting from autum-

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4

Eh

(mV

) pH

ORGANIC MATTER AT SEDIMENT-WATER INTERFACE

0.10 0.31 o.67 1.64 0.39 1.06

1.9o 1.46 1.79 0.07 0.54 0.47

0.08 0.27 0.65 0.06 1.48 0.34

0.14 2.30 1.08 5.64 0.34 1.68

0.22 2.57 1.73 5.70 1.82 2.02

2.1o 1.66 3.51 0.19 2.05 1.04

ORGANIC MATTER AT SEDIMENT-WATER INTERFACE 119

TABLE 2

Organic carbon contents in the different geochemical fractions extracted from surficial sediments in the Tet Prodelta and in the Lacaze-Duthiers canyon. Data are given in mg org C/g dry weight sediment

Total org Acid

Total humic Fulvic

Humic Hydrolyzable Residual

carbon soluble substances acid

acid humin humin

in mg org C/g dry weight sediment

Sites Prodelta 4.4 Head 6.0 Side 7.7 Axis 7.6 Interfluve 4.8 Fan 4.6

nal input of flooding rivers. The transport of terrestrial organic material from land to the continental shelf is detected by the terrigeneous marker-lignin in organic mat-erial trapped in the Tet prodeltaic deposits (Table 1). Inputs are discontinuous and sea-sonal as shown by the ten-fold variation in concentrations of lignin-derived phenolic aldehydes for surficial sediments (Buscail et al., in press). Organic material is rich in sugars, probably due to the occurrence of cellulose-lignin complexes which induce a low proportion of hydrolyzable organic car-bon (Table 1). On the other hand, the material transported in a divided state res-ults from the pedogenetic effects on land (organic matter absorbed as humic substan-ces). Humic acids are rich in nitrogenous and hydrogenous compounds (Table 3), very soluble and quite hydrolyzable but they rep-resent only 6% of TIC (Table 2). The part of particulate organic matter in humin is very high (94% of TIC). In bottom sedi-ments, mineralization effects decrease the organic matter content especially that of nitrogenous compounds (C/N ratio: 14) . During spring and summer, deposits have a higher content of carbohydrates and ami-

no acids due to planktonic blooms. However organic matter mineralization rates are very high in the upper centimeter of prodeltaic sediments. In our calculations we use organic carbon flux in bottom layer, sedimentation rate, and water content to determine organic carbon mineralization rate in the first centimeter of deposits. Based on this model calculation, minerali-zation (40 g C/m2 • year) is found to repre-sent 80% of the organic carbon flux reach-ing the sea floor, the last 20% corresponding to the surface sediment organic carbon ac-cumulation rates (10 g C/m2 • year). Hen-richs and Farrington (1987) have calculated these rates in sediments of a similar bathy-metric area (Buzzards Bay): in the upper meter of oxic to suboxic sediments, 14 C/m2 year-1 are mineralized and 36 g C/m year 1 are accumulated. It has to be noted that our results correspond only to the first oxic centimeter of the Tet prodelta sedi-ment.

On the slope the amount of organic mat-ter in deposits (Tables 1 and 2) increase along the axial channel of the canyon. This can be related to high organic carbon fluxes: 20 to 200 mg Org C/m2 day 1 (Monaco et al.,

Humic acids: Tet Prodelta Head Side Axis Interfluve Fan

5.79 5.95 30.24 - 49.43 1.35 0.096 5.47 5.47 27.52 2.43 44.64 1.47 0.105 5.16 4.80 31.36 1.60 47.88 1.29 0.086 4.69 3.40 - - 35.64 1.58 0.082 4.84 4.24 32.48 1.63 48.72 1.19 0.074 4.43 3.61 - - 32.40 1.64 0.095

0.466 - 0.462 0.020 0.491 0.012

0.012 0.500

120 ORGANIC MATTER AT SEDIMENT-WATER INTERFACE

TABLE 3

Elementary analysis of humic and fulvic acids extracted from surficial sediments in the Tet Prodelta and in the Lacaze-Duthiers canyon

H N O S C H/C N/C 0/C S/C in % dry weight of HA

atomic ratios

Fulvic acids: Tet Prodelta* 5.12 2.35 Head 5.40 3.68 Side 4.81 3.19 Axis 2.63 0.91 Interfluve 4.14 2.70

45.44 37.76 37.60

2.40 2.27 2.49

1.66

21.24 2.40 0.077 35.88 1.81 0.088 32.16 1.79 o.085 9.72 3.24 0.080 22.44 2.21 0.103

1.307 0.034 0.789 0.024 0.876 0.029

0.028

*Average of 5 seasonal data

1987), important benthic activity and high sedimentation rate. Offshore the increasing marine character is emphasized by a de-crease in the C/N ratio, some increase of hy-drolyzable organic carbon, carbohydrates and amino acids contents. Carbohydrates represent 12 to 29% of TIC. Amino acids follow the same trend (25 to 50%) and the highest values are found in fan deposits. On the other hand, the interfluve is charac-terized by a low amount of organic matter, a higher C/N ratio, a lower solubility and an abundant residual humin which emphasize the high decaying of organic matter. Deep sea fan deposits correspond to the final stage of organic matter after its transfer in the water column (1740 m): low content of organic matter, low solubility of humic com-pounds, increase of residual humin (Table 2). However, they show an increase in hyd-rolyzable products due to the abundance of nitrogenous compounds from autochtonous origin.

The evolution of the characteristic atomic ratios of humic acids shows that the values of H/C and N/C are relatively high in the canyon head, the axis and the deep-sea fan as op-posed to the values in the interfluve (Table 3, Fig. 3). The results of infrared spectroscopy of humic acids (Fig. 4) emphasize the relative-ly high content in carbohydrates (1050 cm 1 band) and nitrogenous compounds (amid band 1540 cm 1) for prodelta deposits with fresh terrestrial organic material and for the deep-sea fan sediments with marine organic matter. In the canyon axis, carbohydrate con-tents are higher than in the interfluve, as shown by infrared spectroscopy data (1050 cm 1 band) . This has to be related to the higher degradation in the interfluve area, characterized by a low sedimentation rate.

The 1-HNMR analysis is used to specify the contribution of aliphatic and aromatic fragments to the chemical structure of humic substances (Neyroud and Schnitzer, 1974); Wilson and Goh, 1977; Hatcher et al.,

i I 1 i

DEEP SEA FAN

INTERFLUVE

AB

SO

RB

AN

CE

0 CM-1 I50 0 I 00 j i i i i i J••__ _ J i i

Position (cm H ) of the stretching vibrations

ORGANIC MATTER AT SEDIMENT—WATER INTERFACE 121

Fan

0 Axis

0

Head

Prodelta

a

O Side

H/C

1.7 —

1.6 —

1.5 —

1.4 —

1.3 —

1.2 —

1.1 --

Head

Prodelta

a

Q Side

Interfluve * Interfluve

marine OM

terrestrial OM

N/C 0.11 0.10 0.09 0.08 0.07 0.06 0.40 0.45 0.50 0.55 0.60

O /C

Fig. 3. Variations of the H/C, N/C and O/C ratios of humic acids from surficial sediments of the Tet prodelta and the Lacaze-Duthiers canyon.

Fig. 4. Infrared spectroscopy of humic acids from surficial sediments of the Tet prodelta and the Lacaze-Duthiers canyon.

(a) • 002 ® DOC

10 Mineral C

® Soluble q Humin

I t I t

80 100 % 0

t I 20

t i

Interfluve

Axis

Side

Head

Prodelta

I I 40 60

122 ORGANIC MATTER AT SEDIMENT—WATER INTERFACE

CO2 Water Sediment

(b) Fan

Interfluve

Axis

Side

Head

Prodelta

• 002 ® DOC 8 Mineral C q Soluble q Humin

I I • i 80 1 0 0 %o

Fig. 5. Budget of the total injected 14C activity in the experimental microcosms after 6 days of incubation. (a) 14C(U) glutamic acid, (b) 14C(U) catechol. CO2: 14CO2 recovered during 6 days; DOC: dissolved organic 14C and mineral C (H14CO3) remaining in the water; 14C acid and alkali soluble fractions, 14C hydrolyzable and residual humin fractions extracted from sediments.

I t I I 0 20

40 60

1980). In the prodelta it shows a few aroma-tic peaks even though the aliphaticity is high. In the deepest zone, highly branched aliphatic chains (prevailing of the CH3

groups on the CH2-CH2 groups) suggests an important role played by the lipid fraction, principally due to zooplankton and zoo-benthos (Huc, 1980; Gadel and Torn, 1988).

ORGANIC MATTER AT SEDIMENT-WATER INTERFACE 123

Kinetics of incorporation of 14C organic molecules

The two labelled organic compounds, 14C glutamic acid and 14C catechol, induced dis-tinct responses to the simultaneous experi-ments conducted on the same sampled interface (Buscai1,1986) . The global budget of the initial 14C activity was divided into three main fractions: 14CO2, 14C remaining in the water, and 14C activity incorporated in sediment (0-12 cm depth) (Fig. 5). They represent the relative importance of bio-chemical and geochemical processes occur-ring at the sediment—water interface (Fig. 5a and b). 14CO2 results from biological respiratory and oxidative processes of or-ganic matter. The quantity of 14CO2 re-leased during 6-day kinetics is higher (7% in average) for 14C glutamic acid than for 14C catechol (about 1% retrieval of the ini-tial activity). Andreux et al. (1977) have ob-tained similar results during auto-oxida-tion kinetics of the catechol-glycine system

14 selectively labelled with C in a mull humus medium. The COON groups of amino acids are decarboxylated actively, whereas catechol is characterized by a very low mineralization rate due to high biologi-cal stability. Moreover, one molecule of catechol can release only one molecule of CO2. The mineralization of 14C glutamic acid was maximum in the canyon axis bot-tom (11% at 850 m and 14% at 650 m). The release of 14CO2 through microbial and meiofaunal mineralization is affected by such factors as oxidation-reduction poten-tial, water content and composition of natural organic compounds reaching the sea floor (Table 1A and B). In the case of glutamic acid injection, the proportion of CO2 collected during 6 days is correlated linearly with water content (r = 0.71) and

redox potential value (r = 0.90) at the inter-face. Catechol is not involved in such pro-cesses. In this case, only the increase in thickness of the oxidized layer (2 cm at the head of the canyon, 12 cm on the deep-sea fan) induces an increase in the total CO2 released during 6-day kinetics (r = 0.89).

The kinetics of 14CO2 retrieved during incubations in these microcosms have shown different mineralization rates. Two types of 14CO2 cumulative curves have been identified (Fig. 6a and b). Curves are linear only for catechol incubation and correspond to a regular decarboxylation process. The sigmo~dal curve is typical of the 14C gluta-mic acid biodegradation, probably due to growing bacteria populations (one order of magnitude for viable bacteria counts in sur-ficial sediments between zero time and 6-day incubation; L. Guidi, pers. comm.). Two of the cumulative curves (one at the canyon head with catechol and one in the axis for glutamic acid) show a very active oxidation during the first 24 hours, with 60% of carb-on atoms transformed into CO2. After 24 hours the quantity of retrieved CO2 varies depending on the nature of the injected or-ganic molecule and the sampling site (Fig. 6). In the case of catechol, the relationship between abundance of benthic microflora and meiofauna and CO2 is linear (r = 0.76 and r = 0.98). This can be explained by a toxical effect or by the recalcitrance of cate-chol molecules which can inhibit organisms after a 24 hour contact.

14C remaining in the water is divided into two fractions. As far as 14C dissolved organic is concerned it averages 10 to 15% of the initial activity. The part of dissolved organic products released in overlying water by benthic activity is measured by the 14C glutamic acid activity countings. The 14C released from catechol shows a very

Tet Prodelta 120

9.8% of initial

1 0 2 3 4 5 6 7 Days

Head 120 —

100 -

80 -

60 -

40 -

20 -

0 . 1

0 1

Axis 120

100

80

60

40

20

0 0 1

Fan 120

100

80

60

40

20

0

1.2% of initial

1 • 1 1 1 · 1 I

2 3 4 5 6 7 Days

ORGANIC MATTER AT SEDIMENT—WATER INTERFACE

cum

ulat

ed

%

cum

ula

ted

%

cum

ula

ted

%

2 3 4 5 6 7 Days

2 3 4 5 6 7 0 1 2 3 4 5 6 7 Days Days

Fig. 6a. Mineralization of the 14C labelled organic molecules: cumulative curves (in %) of the recovered 14CO2 during incubations in microcosms. (a) 14C(U) glutamic acid.

1

high value (34%) over the deep-sea fan sur-face. It is due to the fact that in deep envi-ronments, the lack of bioturbation prevents catechol being incorporated into surficial

sediments. As far as 14C mineral fraction (H14CO3+) is concerned, it decreases from 3% to 0.1% (Fig. 5) as pH decreases at the interface of the various sites (Table 1A).

cum

ulat

ed %

100

80

40

60

20

0

11% of initial

124

13.8% of initial

100

~ ° 80 ~ F á 60 ~ E 40 u

5 6 Days

cum

ula

ted

%

Interfluve 120

0 1 3 4 6 Days

7

cum

ulat

ed %

100

80

40

60

20

0

100

~ 80 o d ~~ 60 ~ ~ 40 u

1 2 3 4 5 6 7 8 Days

Axis

120

0.25% of initial

100

~ ° 80 -a a b 60 ~ ~ 40 u

cum

u lat

ed %

100

~ o ~ 80 ~ ~ ~ E 60 ~~

40

20 0 2 3 4 6 7

Days

ORGANIC MATTER AT SEDIMENT-WATER INTERFACE

125

Tet Prodelta 120

2% of initial

0

1

3 4 5 6 7 Days

Interfluve 120 0.7% of initial

100 O ° 80 '~~ a ~ ° 60 ~ ~ 40 u

20

0 (b) 0

1

2

4 5 6 Days

0

1

3 4 5 6 Days

Fig. 6b. Mineralization of the 14C labelled organic molecules: cumulative curves (in %) of the recovered 14CO2 during incubations in microcosms. (b) 14C(U) catechol.

14C Activity incorporated in sediment (0— As far as glutamic acid is concerned, a

12 cm) depends on the relative importance of relationship can be established between

biochemical and geochemical processes in- biotic and abiotic processes. The larger is the

fluencing early diagenesis of organic matter. fraction retrieved as 14CO2, the less is the

126 ORGANIC MATTER AT SEDIMENT—WATER INTERFACE

potential amount of 14C radioactivity which can migrate in the sediment (Fig. 5). Conse-quently 14C catechol migration appears more important. Site differentiation seems to be in-dependent of the quantity of benthic microflora, and rather corresponds to the de-gree of activity. This activity depends on the physico-chemical parameters of the sedimen-tary environment and the quality and quan-tity of organic matter reaching the seafloor. In the axial channel of the Lacaze-Duthiers canyon, suspended matter collected near the bottom by sediment-traps (Ecomarge Prog-ram, Monaco et al., 1987) contains from 2 to 4% organic carbon with a high hydrolyzable

fraction content (50% of TIC). On the other hand, the sediment-trap material collected at the head of the canyon contains a lower proportion (TOC < 1%) and is less hydroly-zable (20-30% of TIC), i.e., less labile for as-similation by benthic organisms (Buscail and Monaco, 1987). Flux variability can also part-ly explain the degree of activity: org C and N fluxes average 90 mg/m2 day and 20 mg/m2 day at the head of the canyon whereas the lower values obtained at 650 m (20 mg org C/m2 day 1 and 2 mg N/m2 day 1 are due to a dilution effect toward the deepest areas.

In the experimental microcosms, an en-riched millimetric layer at the interface (0—

BIOGEOCHEMICAL FRACTIONS IN SURFICIAL LAYERS

GLUTAMIC ACID

Fig. 7a. Distribution of the 14C activity in the different biogeochemical fractions. (a) 14C(U) glutamic acid. Ac. sol: acid soluble (2NHc1); HA; humic acid; FA: fulvic acid; Res. hum: residual humin; Hyd. hum: hydrolyzable humin.

ORGANIC MATTER AT SEDIMENT—WATER INTERFACE 127

5 mm) contains an average of 7% of injected 14C glutamic acid and even 12% on the in-terfluve where pH is lower (6.7). For 140 catechol injection, the highest values (20%) correspond to the input of natural phenolic compounds in form of ligneous debris (exist-ence of phenol—phenol type reactions). The layers below the millimetric top contain a very low proportion of initial 14C activity, often smaller than 1% of the initial amount.

In addition, 14C activity in sediment was counted for different geochemical fractions, obtained after successive acid and alkali ex-tractions (in eight layers of each interface from 0 to 12 cm). Only the results of inter-

face layers (0-5 mm) are presented in this study (Figs. 5 and 7a and b). The amount of CO2 retrieved after acidification of surficial sediments is very low, mainly for 14C catechol incubations. The more the natural amino acids are present, the less the added 14C glutamic acid is used at sediment inter-face. An inverse relationship exists between the quantity of natural amino acids and 14C. 14C acid soluble fraction in sediment increases when glutamic acid is injected in seawater. Analysis of amino acids (HPLC and liquid scintillation counting) confirms the total biodegradation of initial injected glutamic acid. Almost all detected amino

BIOGEOCHEMICAL FRACTIONS IN SURFICIAL LAYERS

Fig. 7b. Distribution of the 14C activity in the different biogeochemical fractions. (b) 14C(U) catechol (in % of introduced 14C). Ac. sol: acid soluble (2 N HC1); HA; humic acid; FA: fulvic acid; Res. hum: residual humin; Hyd. hum: hydrolyzable humin.

128 ORGANIC MATTER AT SEDIMENT-WATER INTERFACE

acids result from glutamic and aspartic acids metabolism: arginine, alanine, leucine, lysine, ornithine. However in canyon head and axis where most of benthic population is counted (1300 and 680 ind./10 cm 2 for total meiofauna) an amino acid from urea cycle (citrulline) is found. Final-ly, part of initial catechol can be included in the acid soluble fraction as a result of catechol oxidation (possibly as quinones; Andreux et al., 1977) .

Phenomena of humification are more ac-tive in the axial channel (side and axis) with higher organic carbon content and higher solubility of humic fractions (Fig. 5) . Poly-merizations of natural humic substances are more important with 14C catechol than with 14C glutamic acid. In the case of injec-tion of catechol, high FA/HA ratios confirm active oxydative processes which increase the amount of smaller molecules. Anions can be formed in alkaline marine environ-ment. Abundant cations in the surficial sediment such as Fe3+, Cu2+ and 1n4+

favour the formation of organo-metallic complexes, soluble in alkaline medium. Surficial layers at the head, axis and side of the canyon are very rich in Mn (Buscail and Monaco, 1987), which presents a catalytic effect of complexation with 14C catechol anionic forms; consequently these sites show noticeable 14C humic acid fractions (Fig. 7a and b).

In the case of injection of 14C glutamic acid, the initial molecules are transformed mainly into an abundant hydrolyzable humin fraction. This fraction contains com-bined amino acids (HPLC determination) which are good indicators of the occurrence of metabolic processes. It is also related to high carbon bioclast contents in head and interfluve sediments (Table 1A). In this case, injected glutamic acid can be absorbed

on the carbonate matrix and better extract-ed by acid hydrolytic treatment (Carter, 1978).

The residual humin fraction (geo-polymers) results from processes ofphysico-chemical insolubilization. With glutamic acid a linear correlation exists between residual humin fraction and carbohydrate content (Table 1B). Maillard (1917) has demonstrated that amino acids and sugars form black compounds by condensation: melano~dins. Aldehydes can participate also to analog reactions of geopolymeriza-tion (Andreux et al., 1977). Thus, the high proportion of phenolic aldehydes found in the Tet prodelta can explain the important residual humin fraction in the cast of 14C catechol injection.

CONCLUSION

Analysis of organic matter emphasizes active biogeochemical processes both on the prodelta and in the near canyon down to the deep areas. An active input of terrestrial or-ganic matter is observed during autumn and rapidly deposited on the prodeltaic area. This induces an aromatic character of the deposits which, on the contrary, show an increasing content in nitrogenous and aliphatic compounds during the warm period. In the canyon, despite the funneling of turbiditic flows, the impact of terrestrial input decreases progressively and is sig-nificant only inside the axis of the channel. Organic matter is rapidly buried and better preserved in the channel. On the contrary, it is transformed easily on the interfluve ex-posed for a long time to exchanges with overlying waters owing to a low sedimenta-tion rate. Prediagenetic effects are also evi-dent in the deep areas where settling through the water column involves a

ORGANIC MATTER AT SEDIMENT—WATER INT ERFACE 129

progressive degradation of organic matter prior to deposition.

The distribution of radioactivity in the specific organic fractions is somewhat simi-lar to that of natural organic compounds. Depending on their nature, labelled organic molecules generated during degradation processes are associated with the different natural organic fractions. For example, an increase of fulvic acids is observed in oxidiz-ing environments for natural and labelled organic compounds. On the other hand, humin-type condensed fractions increase in these experiments compared with those previously carried out only on sediment separated from overlying water (Gadel et al., 1983). This indicates that exchanges with overlying water can eliminate simple molecules at the interface.

The sedimentological characteristics of deposits can also favour some geochemical processes. Fine grain sediments (side and axis) seem to increase the incorporation of labelled compounds in humin. In the same way an increase in the hydrolyzable frac-tion in humin can be related to high carbo-nate content inducing adsorption processes.

ACKNOWLEDGEMENTS

This work forms part of the ECOMARGE program being carried out on the north-western Mediterranean margin (Gulf of Lion-France) and an outline was presented at the International Conference of Oceanol-ogy "Ecosystemes de marges continent-ales", June 1987, Perpignan (France). This research was carried out with the co-opera-tion of biologists: Dr. J. Soyer, Dr. F. de Bovee and Mrs. L. Guidi of the Arago Lab-oratory, Banyuls sur Mer (France), for ben-thic microflora and meiofauna data. We thank Dr. R. Daumas and Mrs. P. Rault of

the Ecologie et Biochimie Microbiennes des Milieux Marins Laboratory, University of Marseille (France) for collaboration on HPLC amino acids analysis.

We are also grateful to the crew of the re-search vessels: "Professeur Georges Petit" and "Catherine-Laurence" for their assis-tance.

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