Distribution and quality of sedimentary organic matter on the Aquitanian margin (Bay of Biscay)

Deep-Sea Research II 46 (1999) 2249}2288

Distribution and quality of sedimentary organic matter on the Aquitanian margin (Bay of Biscay) Henri Etcheber *, Jean-Claude Relexans
, Majida Beliard , Olivier Weber , Roselyne Buscail, Serge Heussner De& partement de Ge& ologie et Oce& anographie, Universite& de Bordeaux I, UMR CNRS 5805, Avenue des Faculte& s, 33405 Talence, France
¸aboratoire d1Oce& anographie Biologique, ;niversite& de Bordeaux I, Avenue des Faculte& s, 33405 Talence, France Centre de Formation et de Recherche sur l+Environnement Marin, Universite& de Perpignan, CNRS ERS 1745, Avenue de Villeneuve, 66860 Perpignan Cedex, France Received 20 February 1998; received in revised form 25 July 1998; accepted 30 July 1998

Abstract During the ECOFER experiment (French ECOMARGE program), sur"cial sediments were sampled on the Aquitanian margin with box corers and analyzed to determine the quantity and quality of organic matter. Sediments from the margin are enriched in organic carbon (mean value 1.35%) in comparison to deep-sea and shelf sediments, due to a "ne grain-size sedimentation. As sedimentation rates are high, the margin appears to be an organic depocenter. Some preferential organic enrichment zones were identi"ed in the Cap-Ferret Canyon. There is a supply of continental material from the Gironde estuary, but marine contribution seems more possible than Adour or spanish rivers. No seasonal variations of organic matter were observed at the surface of sediments, suggesting mineralization processes of labile organic matter: average organic carbon consumption was evaluated to 9.0 g C m\ yr\. Rapid biological mineralization processes are lower than on the Mediterranean margin, mainly related to signi"cant di!erences in water temperature. The great width of the canyon, its distance from the continent, and the current circulation pattern prevent any precise recording of the variable organic inputs to the sediment and favor nephelomK d transport, resuspension and shelf break processes, which wipe out any print of fresh material input. An organic carbon budget indicates that an equilibrium between organic inputs and organic mineralization#accumulation is not obtainable. The supply of suspended matter could have been minor during the year in question, and sedimentation rates are still imprecise.  1999 Elsevier Science Ltd. All rights reserved.

* Corresponding author. 0967-0645/99/$ - see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 4 5 ( 9 9 ) 0 0 0 6 2 - 4

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1. Introduction Coastal zones and margins are the most fertile areas in the ocean and the major settling zone of suspended material originating from continental sources or primary production (Monaco et al., 1990a,b; Wollast, 1990; Walsh, 1991). It is necessary to understand better all the processes that govern organic matter behavior in these areas, in its particulate (Rowe et al., 1986,1988,1994; Jahnke et al., 1990; Buscail et al., 1990,1997) or dissolved form (Suzuki and Tanoue, 1991). One of the major problem is to identify whether the storage of organic matter in sediment is permanent or temporary (Bender and Heggie, 1984; Emerson et al., 1985; Emerson and Hedges, 1988; Rowe et al., 1991). Numerous studies have been carried out in coastal zones and margins. Best results have been obtained by taking a multidisciplinary approach. Information obtained on suspended matter #uxes and hydrological circulation (Biscaye et al., 1988,1994) associated with sedimentation rates and organic carbon data (Anderson et al., 1988,1994), during SEEP (Shelf Edge Exchange Processes)-I and SEEP II experiments, were combined to understand the main trends of organic matter sedimentation. Organic carbon budgets, however, are signi"cant only if they are based on relevant data about the organic geochemical characteristics (Venkatesan et al., 1988; Buscail and Germain, 1997) and/or on biomass and biological activity in sediments (Relexans et al., 1996). In the ECOFER } ECOsyste`me du canyon du Cap-FERret } experiment (part of the French ECOMARGE } ECOsyste`me de MARGE continentale } program), the study of the Aquitanian margin, close to the Gironde system, was undertaken in order to answer some questions: E This margin is supposed to receive abundant continental #uxes and to have a very complex water circulation. Does it function like other margins? E Does the Cap-Ferret Canyon have a peculiar distribution of sedimentary organic matter? What is the in#uence of the vicinity of the continent, of the size or the morphology of the canyon on this distribution? E What is the C budget?  A multidisciplinary approach, based on our expertise acquired in the Mediterranean margin, was developed during the ECOFER experiment. The study of sedimentary organic carbon quality and quantity is coupled with a study of the benthic response to hydrological circulation and particulate #uxes. The Aquitanian margin has not been studied before, and therefore few or no data on C content, water circulation, etc. were available. Consequently we needed to acquire  all the `basica parameters before identifying more speci"c analyses. Our data were compared with the results obtained on other margins, which have led to some relevant conclusions.

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2. Material and methods 2.1. Sampling sites and sample collection Samples were collected from 1989 to 1993, on the continental margin of the Bay of Biscay in the Cap-Ferret Canyon (44330}44355 N; 2300}2355 W) and its surrounding area (44330}45340 N; 1330}3330 W). We intended to investigate the speci"c role of the canyon on the suspended material transfer to the ocean. The various bathymorphological sites of the canyon were sampled (Fig. 1): the shelf and upper slope (90}500 m); the upper canyon (500}850 m); the middle canyon (850}2500 m) incised by two branches, northern and southern (900}1700 m) separated by an inter#uve (1500}2000 m), which merges at 2300 m; the lower canyon (2500}3000 m). Five major cruises (ECOFER 1}5; Table 1) were conducted at di!erent seasons (Spring, Summer and Autumn) in the Cap-Ferret Canyon and several coastal cruises (SUPRABATH, 2}4 days long) in the surrounding area. PPS 3 sediment traps (Heussner et al., 1990; Heussner et al., 1999) were deployed for 14 months (June 1990}August 1991) on two mooring lines (mooring sites MS1 and MS2, Fig. 1) to evaluate the total mass and organic #uxes over the Cap-Ferret Canyon. Various box corers were used to take into account the grain-size characteristics of the sea #oor: Reineck and Smith-Mac Intyre box corers for sandy}silty sediments (90}770 m water depth); Flucha and Usnel box corers or SMBA Multicorer, which allow a good preservation of the uppermost layer, for muddy samples. 95 sediment cores from the

Fig. 1. Major morphological sites of the Cap-Ferret Canyon area (Bay of Biscay) and mooring sites (MS1}MS2).

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Table 1 Major ECOFER cruises and sediment trap moorings in the margin of the Bay of Biscay Moorings

ECOFER ECOFER ECOFER ECOFER ECOFER

1 2 3 4 5

Cruises

From

To

From

To

07/17/1989 05/20/1990 10/25/1990 05/06/1991 *

11/02/1989 09/29/1990 04/05/1991 08/10/1991 *

06/19/1989 05/03/1990 10/11/1990 04/30/1991 08/08/1991

07/17/1989 05/18/1990 10/26/1990 05/14/1991 08/27/1991

Cap-Ferret Canyon were obtained and analyzed (Fig. 2); 24 additional cores were sampled in the surrounding area (Fig. 3). Organic matter content was determined on short cores (10}30 cm long) sampled carefully, frozen on board, then cut into 1 cm slices. The top 2 cm of some cores also were cut into 1 mm slices. All the samples were freeze-dried in the laboratory. The quality of sedimentary organic matter was studied on some representative cores of the major sites of the margin (Fig. 1): the upper canyon, the middle canyon (MS1) and the lower canyon (MS2). Biogeochemical parameters were obtained at three levels (0}1 cm; 2}3 cm; 5}6 cm) of these typical cores (frozen and cut aboard). Biological experiments were carried out on board in the surface levels of cores a few hours after their retrieval. 2.2. Analyses Grain size was analyzed using a Malvern Laser Di!raction Particle Sizer (Type 2600), which gives the distribution frequency for 32 size classes ranging from 2 to 180 lm (Singer et al., 1988). The organic carbon (C ) content was determined on dry weight sediment by  combustion in an LECO CS 125 analyzer (Cauwet et al., 1990). Samples were acidi"ed in crucibles with 2N HCl to destroy carbonates, then dried at 603C to remove inorganic C and most of the remaining acid and water. The analyses were performed by direct combustion in an induction furnace, and the CO formed was determined  quantitatively by infrared absorption. Hydrolyzable organic carbon was evaluated from the fraction hydrolyzsed by 6 N HCl (1103C, 16 h), (Buscail et al., 1990). Hydrolysis was performed in a Pyrex screw cap tube with Te#on liner. The residual organic carbon (ROC) was measured by combustion (LECO analyzer) on the sediment after hydrolysis: TOC-ROC"HOC and HOC/TOC%"%HOC. Proteins (P) were analyzed by OPA #uorescence after hydrolysis by 6 N HCl (1103C, 16 h), calibrated with glycine (Parsons et al., 1984a; Delmas et al., 1990). Carbohydrates (C) were measured in aqueous extracts (10 min at 1003C) according to the procedure of Dubois et al. (1956), improved by Montreuil and Spik (1963), with

Fig. 2. Sampling location of surface sediments (box cores) in the Cap-Ferret Canyon, (ECOFER cruises).

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Fig. 3. Sampling location of surface sediments (box cores) on the margin of the Bay of Biscay (SUPRABATH cruises).

glucose as a standard. Lipids (L) were determined by weighing the dry residue of methanol}chloroform extracts after complete evaporation of the solvent (Bligh and Dyer, 1959). These protocols were chosen for two reasons: they are well suited for analyzing marine sediment samples, and allow comparisons with data previously obtained in the same way on sediments from surrounding estuarine and coastal environments (Relexans et al., 1992a,b; Laane et al., 1987; Lin and Etcheber, 1994). The sum of protein, carbohydrate and lipid contents (P#C#L) was assimilated to the easily extractable macromolecular organic matter of sediment (Khripouno! and Rowe, 1985; Laane et al., 1987; Mayer et al., 1988; Relexans et al., 1992a,b). Electron Transport System (ETS) activity (Packard, 1971; Christensen and Packard, 1977) is a measure of the potential dehydrogenase activity of all respiring micro-organisms and meiofauna. ETS activity was measured aboard, usually in the top centimeter of the sediment, using the method developed by Owens and King (1975). A new procedure based on the use of micro-titration plates (96 holes of 400 ll), developed by our laboratory (Relexans, 1996), was applied using a special colorimeter equipped with a 492 nm "lter. This method requires very small amounts of homogenate and chemicals, which allows several replicates.

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Pro"les of porewater oxygen were measured aboard by micro-minielectrodes (Revsbech et al., 1980; Reimers et al., 1984; Helder and Bakker, 1985; Jorgensen and Revsbech, 1985; Reimers and Smith, 1986; Revsbeck and Jorgensen, 1986). Only cores with an undisturbed sediment-water interface were used for oxygen pro"le determination. As pro"les tend to change with the experiment duration (Reimers et al., 1984; Silverberg et al., 1987), only single pro"les were performed on each core but two cores were usually measured at each station. Subsamples were kept at in situ temperatures in a refrigerated box until the oxygen pro"le was completed, i.e. less than 3 h after core retrieval. The polarographic oxygen sensor (POS) equipped with a needle electrode (Helder and Bakker, 1985) was lowered from the overlying water into the sediment by means of a motor-driven micromanipulator with a 0.1 mm resolution. The measurements were made without stirring to prevent atmospheric contamination. The response from the POS was assumed to be linear between the value of the current recorded in the oxygenated overlying water (where oxygen concentration was titrated by the Winkler method) and the zero-oxygen value at the depth where the response from the POS remained constant. The drift of the electrode response during measurement was generally less than 10%, as evaluated by comparing the response from the POS in the overlying water at the beginning and at the end of the experiment. Total oxygen consumption at the sediment}water interface was calculated according to Fick's "rst law of di!usion applied to sediment (Berner, 1980): j"! D ;dC/dz ,  X where J is the oxygen #ux, dC/dz the oxygen concentration gradient at the interface (z"0), the porosity in the same layer, and D the bulk sediment di!usion coe$cient  for oxygen. was assimilated to the weight loss of a wet sediment (0}1 cm) after complete drying at 603C. D is assumed to be K\;D with m"2.5 (Ullman and   Aller, 1982) for values between 0.7 and 0.88, which corresponds to the values found for our samples. D is the free solution di!usion coe$cient at in situ temperature  (Broecker and Peng, 1974). Because oxygen pro"le experiments were performed aboard, it is necessary to take into account a possible alteration of the oxygen distribution during core retrieval. Errors can be minimized by determining dC/dz at the interface (i.e. the level most sensitive to shifts in environmental conditions). The gradient was assumed to be linear from bottom water oxygen concentration to the concentration within sediment at the shallowest depth at which atmospheric contamination was low to non-existent (Reimers et al., 1984). As discussed by Jahnke et al. (1989), the possible contamination by atmospheric oxygen should be limited to about 4}5 mm, since our pro"les were performed within 3 h after core retrieval. The thickness of the disturbed surface resulting from core handling has been estimated to 2 mm. A 7 mm depth beneath the surface was chosen for determining the bottom of the gradient in our calculations. For most parameters (C , P#C#L, ETS activity, O ), signi"cance tests   (Dagnelie, 1973) were used for comparing the values obtained in di!erent sites. At the MS1 station during ECOFER 1 cruise, incubation experiments with labelled carbon (Buscail, 1986) were performed on subsamples, collected from Usnel box corers by inserting transparent plastic tubes into the sediment. The input of natural

443 443 443 443 443 443 453 443 453 443 443 453 453 443 453 443 453 453 443 443 443 443 443 443 443 443 443 443 443 443 443

KR 93019 KR 93020 KR 93006 KR 92024 KR 92016 KR 92015 KR 93015 KR 93021 KR 93029 KR 93005 KR 93033 KR 93014 KR 93008 BSMC 03 KR 93028 BSMC 04 KR 93024 KR 93016 BSMC 05 KR 93004 BSMC 06 BSMC 11 KR 93003 BSMC 12 KR 92020 BSMC 10 BSMC 12(2) BSMC 08 KR 92027 BSMC 09 KR 93007

30.96 31.11 34.75 36.97 33.33 33.41 23.87 30.86 00.65 34.57 52.37 23.36 33.74 34.96 00.38 34.02 07.02 10.61 32.87 36.29 32.00 35.00 36.06 37.00 34.95 33.87 37.00 32.76 37.48 32.69 36.74

Lat. N

Sample

13 13 23 23 23 23 33 23 23 23 23 33 33 23 23 23 23 33 23 23 23 23 23 23 23 23 23 23 23 23 23

35.27 54.05 02.86 00.99 04.47 03.60 12.99 04.55 20.25 05.43 12.57 15.77 28.11 07.02 21.04 06.97 39.94 02.34 07.11 06.26 07.06 08.00 07.87 08.00 08.60 08.12 08.00 08.10 02.75 08.31 08.96

Long. W 90 126 164 168 169 170 180 180 180 196 242 251 255 255 257 260 269 270 286 299 310 315 330 345 365 372 373 390 394 406 410

Depth (m) 0.20 0.14 0.23 0.21 0.27 0.24 0.20 0.23 0.11 0.27 0.11 0.13 0.09 0.32 0.43 0.30 0.09 0.25 0.25 0.26 0.22 0.32 0.20 0.75 0.31 0.58 0.29 0.40 0.68 0.34 0.16

C (%)  KR 93017 KR 93013 KR 93009 KR 50 KR 93023 KTB 088 KR 93001 KR 92022 KR 93032 KTB 103 KTB 132 KTB 161 KR 93027 KTB 131 KR 93002 KR 93012 KR 92019 FLU 92002 KR 93018 KTB 154 FLU 91502 KTB 126 KR 93026 FLU 92012 KR 93030 KR 93022 KJ 51 KJ 40 FLU 92005 KTB 105 KTB 055

Sample 453 453 453 443 453 443 443 443 443 443 443 443 443 443 443 453 443 443 453 443 443 443 443 443 443 453 443 443 443 443 443

10.40 22.85 33.24 44.62 06.66 50.67 37.13 36.41 52.30 49.61 37.72 44.57 59.84 35.90 38.37 22.98 38.19 38.86 10.00 36.03 34.85 36.48 58.53 34.80 52.18 06.23 36.94 34.05 35.00 48.56 38.53

Lat. N 33 33 33 23 23 23 23 23 23 23 23 23 23 23 23 33 23 23 33 23 23 23 23 23 23 23 23 23 23 23 23

02.75 18.94 29.62 07.65 41.66 07.69 10.01 10.41 13.10 11.58 07.98 08.14 21.76 03.96 11.80 19.26 12.90 13.09 04.19 12.86 12.47 04.41 24.49 12.61 13.92 42.79 14.39 12.94 14.00 10.97 17.08

Long. W 440 440 440 440 443 445 460 471 478 490 513 515 539 555 611 612 686 688 700 710 720 720 732 739 750 767 775 805 854 875 875

Depth (m) 0.49 0.34 0.39 0.77 0.34 1.17 0.13 0.22 0.38 0.68 0.37 1.08 1.38 0.71 0.38 0.48 0.73 0.33 0.70 0.67 0.84 1.08 0.50 1.22 0.77 1.24 1.23 1.24 1.09 1.08 1.34

C (%) 

Table 2 (a) Organic carbon content (% of dry weight) in sur"cial sediments (0}1 cm). Di!erent box corers were used for sampling: Reineck (KR), Smith and McIntyre (BSMC), Flucha (FLU), Usnel (KJ), Multitube (KTB) 2256 H. Etcheber et al. / Deep-Sea Research II 46 (1999) 2249}2288

443 443 443 443 443 443 443 443 443 443 443 443 443 443 443 443 443 443 443 443 443 443 443 443 443 443 443 443 443

KTB 109 KTB 111 FLU 92004 KTB 155 KTB 125 KJ 39 KTB 085 FLU 92003 KTB 136 FLU 92001 FLU 91501 FLU 92010 FLU 91500 KTB 082 KTB 123 KJ 38 KTB 124 KJ 48 KTB 120 KTB 081 KTB 146 KTB 171 KJ 26 KJ 29 KJ 20 KTB 170 KTB 080 KJ 22 KJ 54

45.16 45.07 34.78 36.92 37.59 34.30 49.06 39.70 40.06 34.84 37.83 34.77 37.56 47.59 39.69 35.23 39.60 43.56 39.80 45.98 42.94 46.92 46.25 46.03 45.78 35.29 45.19 45.73 41.84

Lat. N

Sample

23 23 23 23 23 23 23 23 23 23 23 23 23 23 23 23 23 23 23 23 23 23 23 23 23 23 23 23 23

09.17 09.40 15.58 17.44 04.51 15.83 09.80 15.35 08.00 17.94 19.81 18.86 22.23 10.13 06.28 19.97 06.22 11.34 06.05 10.61 12.42 23.04 23.62 23.81 22.98 29.90 13.03 23.67 20.14

Long. W 880 950 995 1015 1050 1084 110 1141 1170 1173 1250 1275 1332 1400 1415 1422 1431 1500 1670 1700 1750 1930 1948 1969 1975 1975 1990 1997 2025

Depth (m) 1.43 1.56 1.36 1.30 1.12 1.30 0.98 1.19 1.57 1.20 1.20 1.31 1.13 0.97 1.10 1.49 1.03 1.60 1.23 1.16 1.51 1.44 1.26 1.34 1.33 1.41 1.30 1.38 1.26

C (%)  KTB 138 KTB 167 KTB 159 KJ 10 KTB 074 KTB 071 KJ 07 KTB 075 KJ 52 KJ 46 KJ 21 KJ 03 KJ 47 KJ 05 KJ 55 KTB 070 KTB 166 KJ 42 KTB 065 KTB 061 KTB 062 KJ 31 KJ 33 KJ 34 KJ 14 KJ 45 KJ 57 KJ 57

Sample 443 443 443 443 443 443 443 443 443 443 443 443 443 443 443 443 443 443 443 443 443 443 443 443 443 443 443 443

41.66 41.97 42.16 43.28 43.62 43.92 44.37 43.66 43.85 43.82 43.86 43.64 43.62 43.87 43.22 43.09 53.29 46.90 45.64 44.53 44.98 46.85 46.08 46.44 46.82 45.76 46.30 46.24

Lat. N 23 23 23 23 23 23 23 23 23 23 23 23 23 23 23 23 23 23 23 23 23 23 23 23 23 23 23 23

13.24 49.39 51.43 16.92 16.88 17.09 18.84 17.26 21.54 18.17 20.31 18.73 19.57 18.43 19.58 28.60 45.18 36.13 36.55 38.01 37.22 37.12 36.99 37.86 37.88 37.90 40.50 40.57

Long. W 2045 2148 2160 2270 2300 2300 2309 2315 2325 2335 2350 2373 2386 2398 2410 2750 2860 2962 2980 2985 2985 3000 3008 3016 3016 3047 3070 3075

Depth (m) 1.41 1.48 1.00 1.43 1.42 1.51 1.30 1.65 1.39 1.85 1.33 1.57 1.37 1.50 1.34 1.54 1.26 1.22 1.50 1.07 1.25 1.24 1.38 1.29 1.63 1.39 1.44 1.20

C (%) 

(b) Organic carbon content (% of dry weight) in sur"cial sediments (0}1 cm). Di!erent box corers were used for sampling: Reineck (KR), Smith and McIntyre (BSMC), Flucha (FLU), Usnel (KJ), Multitube (KTB)

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organic matter at the sediment}water interface was simulated by the injection of C-labelled diatoms (Navicula incerta) in the overlying water (1 ml added with an activity of 1 lCi ml\). Incubations lasted, respectively, 4, 24 h and 6 days at in situ temperature (33C) in a dark thermostat-regulated chamber where oxidizing conditions were maintained by air #ushing. The CO produced by mineralization processes was collected every 12 h in ethyleneglycol monoethylether/ethanolamine (scintillation grade 7 : 2 v/v). The 143C radioactivity was counted at the end of each incubation in both overlying water and sediment. Dissolved C (DOC) was quanti"ed in the initial bottom water (¹) and after 4, 24 and 144 h of incubation using the UV-persulfate method (Cauwet, 1984). Dissolved free and combined amino acids (DFAA and DCAA) were determined using high performance liquid chromatography (HPLC) in the overlying water (Mopper and Lindroth, 1982). Individual amino acids were quanti"ed as O-phthaldialdehyde (OPA) derivatives, directly in the water sample for the DFAA and after 16 h hydrolysis in 6 N HCl at 1103C for the DCAA.

3. Results 3.1. Distribution of surxcial CMPE content We consider all the data obtained whatever the season. The distribution of C concentrations (% of dry weight, Table 2) in sur"cial sediments (0}1 cm) is summarized in Figs. 4 and 5. Low and variable C concentrations, ranging from 0.09 to 1.17% (average: 0.33%), were found on the shelf and the upper slope (90}500 m water depth). C contents were higher and more variable in the upper Cap-Ferret Canyon (500}850 m water depth), ranging from 0.33 to 1.38% (average: 0.84%). In the middle canyon (850}2500 m water depth), contents were higher and more homogeneous than in the upper part (average: 1.35%), varying from 0.97 to 1.63%. However, two restricted areas of this canyon showed di!erences: the major upper channels (northern and southern branches: 900}1700 m water depth) were characterised by low C content (0.98}1.23%, average 1.08%). The highest values of the Cap-Ferret Canyon area were observed at the inter#uve (1500}2000 m water depth) and at the merging site of northern and southern branches (2300 m water depth), (1.30}1.85%; average 1.56%). In the lower canyon (2500}3000 m water depth), C contents were of the same order of magnitude as in the middle canyon (average 1.35%). Surface sedimentary C contents for the various ECOFER cruises (Table 3) did not show any statistically signi"cant seasonal trend, even when the "rst mm of sediment was studied (based on the study of 25 cores). At all sites (merging, middle or lower canyon), intra-seasonal variations were of the same order of magnitude as inter-seasonal variations. 3.2. Vertical distribution of CMPE content At stations shallower than 850 m and in the northern and southern branches of the canyon, no signi"cant downcore variations of C content was recorded between the

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Fig. 4. Distribution of C (% of dry weight) in the upper 1 cm of sediment on the margin of the Bay of  Biscay.

Fig. 5. Sur"cial C contents (% of dry weight; 0}1 cm) as a function of water depth. 

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Table 3 Seasonal variations of organic carbon content (% of dry weight) in sur"cial sediments (0}1 cm and 0}1 mm(!)) Cruises

Middle canyon 850}2500 m

Con#uence (MS1) 2300 m

ECOFER 1 Summer

KJ 20 KJ 22 KJ 26 KJ 29 KJ 38 Kj 39

1.33 1.38 1.26 1.14 1.49 1.30

ECOFER 2 Spring

KJ 48

1.60 (1.65)

(1.40) (!) (1.38) (1.26) (1.65) (!)

ECOFER 3 Autumn ECOFER 5 Summer

KTB KTB KTB KTB KTB KTB KTB KTB KTB

80 81 82 85 120 123 125 136 155

1.30 1.16 0.97 0.98 1.23 1.10 1.12 1.57 1.30

(1.35) (1.20) (!) (!) (!) (1.15) (!) (!) (1.35)

KJ KJ KJ KJ KJ

03 05 07 10 12

1.57 1.50 1.30 1.43 1.33

Lower canyon (MS2) 3000 m (1.55) (1.55) (!) (1.57) (1.45)

KJ KJ KJ KJ

14 31 33 34

1.63 1.24 1.38 1.29

(1.75) (1.32) (!) (!)

KJ 46 KJ 47

1.85 (1.65) 1.37 (1.39)

KJ 42 KJ 45

1.22 (1.40) 1.39 (!)

KJ 52 KJ 54 KJ 55

1.39 (!) 1.26 (!) 1.34 (1.35)

KJ 57 KJ 69

1.44 (1.60) 1.20 (!)

KTB KTB KTB KTB

1.51 1.42 1.65 1.41

KTB 61 KTB 62 KTB 65

1.07 (!) 1.25 (!) 1.50 (1.60)

71 74 75 138

(1.80) (!) (1.80) (1.43)

0}6 cm and one-cm intervals (Fig. 6a). A slight decrease of 15% however, was, observed between the "rst mm of sediment and the deeper layers (example of KTB 82 in Fig. 6a). At all the other stations, the vertical decrease in C content was signi"cant  (Fig. 6b). C values dropped by a 40% average (between 30 and 60%) from the  surface to 5}10 cm. The strongest decrease was observed within the upper 2 cm, where sur"cial C content decreased by 20% on average (15}40%). The decrease in  C concentration was not related to the particle size distribution since the mean  grain size did not vary signi"cantly throughout the cores (Fig. 6b). The study of the C content in the various particle size fractions (Fig. 7) revealed  some general trends: E at any level, 85% of the C was contained in the "nest fraction ((16 lm), because  this fraction was the most abundant and characterised by the highest C concen trations; E the vertical decrease in C content occurred in all sedimentary fractions, with the  exception of the '63 lm fraction which was related to the weak contribution of this fraction.

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Fig. 6. Vertical variations of C (% of dry weight) in sediments from the Aquitanian margin: (a) upper  canyon; (b) middle and lower canyon.

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Fig. 7. C distribution in di!erent particle size fractions of sediment from the Aquitanian margin. 

3.3. Quality of sedimentary organic matter 3.3.1. Macromolecules and hydrolyzable C MPE At all stations, the three biogeochemical parameters (protein, carbohydrate and lipid contents) were higher in surface levels than in deeper layers (Table 4). The P/C/L ratios remained relatively constant (5/1/1.5) at 0}1 and 2}3 cm levels. Over the whole area, the average P#C#L content was 2200 lg g\ (range between 1895 and 2815 lg g\; Table 5) in sur"cial sediments (0}1 cm). This represents 8}10% of the sedimentary organic matter (calculated as C content;2). Sim ilarly, the average hydrolyzable C content was 7800 lg g\, which represents  50}60% of the organic carbon. In marine sediment, amino acids, sugars, amino-sugars and ammonium can represent the most important part of the acid-hydrolyzable fraction. The labile character of the organic matter is determined in the deposit by the variation of this hydrolyzable fraction. A signi"cant increase in P#C#L was recorded at the merging of the northern and southern branches (Site MS1), (Fig. 8a), where the hydrolyzable C contents were also the highest (Table 5).  The proteinic macromolecules are known to be biologically available and seasonally variable (Henrichs and Farrington, 1979; Stanley et al., 1987; Buscail et al., 1990). Special attention was paid to the protein content of the very sur"cial sediment layer (0}1 mm) for assessing better the seasonal variations of organic matter quality, but no signi"cant variation was recorded (Table 6).

805 1084 1422 1948 1969 1975 1997 2270 2309 2373 2398 3000 3008 3016 3016

KJ KJ KJ KJ KJ KJ KJ KJ KJ KJ KJ KJ KJ KJ KJ

40 39 38 26 29 20 22 10 07 03 05 31 33 34 14

Depth (m)

Sample

1300 1500 1275 1470 1410 1380 1495 1485 1755 1620 1550 1465 1485 1410 1410

1470 1170 1300 1225 1580 1560 1550 1340 1670 1480 1560 1275 1200 1325 1310

1055 1195 930 805 1045 1335 1045 1400 1375 1485 1220 1305 1045 1170 1180

245 220 215 265 300 265 380 380 345 340 320 275 220 240 315

0}1 cm

5}6 cm

0}1 cm

2}3 cm

C

P

200 160 170 190 195 235 225 210 180 195 175 205 210 225 195

2}3 cm 165 225 120 165 255 165 245 195 255 185 220 355 190 290 215

5}6 cm 425 385 405 495 375 465 465 540 715 535 495 330 405 405 520

0}1 cm

L

260 240 240 345 365 355 340 375 270 240 275 285 255 285 375

2}3 cm 85 175 40 55 125 185 85 265 110 70 115 170 185 175 210

5}6 cm 1.23 1.25 1.52 1.26 1.36 1.33 1.43 1.42 1.39 1.49 1.41 1.22 1.32 1.26 1.53

0}1 cm

C (%) 

Table 4 P, C, L (lg g\ dry weight) and organic carbon content (% of dry weight) at di!erent levels of sediments from the Cap-Ferret Canyon

1.19 1.15 1.18 1.16 1.14 1.28 1.33 1.26 1.21 1.08 1.30 1.13 1.13 1.21 1.16

2}3 cm

1.04 1.15 0.81 0.83 0.87 0.98 1.07 1.02 1.06 1.07 0.9 1.05 1.07 1.11 1.02

5}6 cm

H. Etcheber et al. / Deep-Sea Research II 46 (1999) 2249}2288 2263

Depth

(800 m 800}2000 m 2300 m 3000 m

Station

Upper Canyon Middle Canyon Con#uence (MS1) Lower Canyon (MS2)

1 6 4 4

Number of cores

1970 215$185 2510$350 2120$145

0}1 cm 1930 1910$240 1990$160 1785$165

2}3 cm

P#C#L (lf g\)

1305 1360$270 1725$235 1600$240

5}6 cm * 2 1 2

Number of cores

* 6890$580 10025 7670$110

0}1 cm

* 4060$310 33333 5850$90

5}6 cm

Hydrolyzable C (lg g\) 

Table 5 P#C#L and hydrolyzable organic carbon content (lg g\ dry weight) at di!erent levels of sediments from the Cap-Ferret Canyon ($95% CI)

2264 H. Etcheber et al. / Deep-Sea Research II 46 (1999) 2249}2288

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Fig. 8. Macromolecular organic matter (lg g\ of dry weight) in sediments from the Aquitanian margin.

A downcore decrease in P#C#L and hydrolyzable C content was observed  at all the stations (Fig. 8b): sur"cial values dropped by 30 and 35%, respectively, in surface (0}1 cm) and deeper (5}6 cm) layers. Two observations should be discussed: (i) the decrease in P#C#L content was only 10% of the organic matter decrease, while the decrease in hydrolyzable C was 80% of the C decrease;   (ii) at 10 cm in depth within the sediment, where organic matter is thought to be mainly refractory, signi"cant amounts of P#C#L and hydrolyzable C were  still found.

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Table 6 Seasonal variations of protein content (lg g\ dry weight) in sur"cial sediments (0}1 mm) from the Cap-Ferret Canyon Cruises

Middle canyon 1000}2000 m

Con#uence (MS1) 2300 m Mean CI 95%

ECOFER 1 Summer

KJ20 KJ22 KJ26 KJ29 KJ38 KJ39

1380 1495 1470 1410 1275 1500

1420$95

ECOFER 3 Autumn

ECOFER 5 Summer

KTB80 1850 KTB81 1640 KTB82 1005 KTB85 1540 KTB120 1650 1530$200 KTB123 1650 KTB125 1245 KTB136 1695 KTB155 1545

Lower canyon (MS2) 3000 m

Mean CI 95% KJ03 KJ05 KJ07 KJ10

1620 1550 1755 1485

KJ52 KJ53 KJ54 KJ55 KJ56

1465 1585 1275 1455 1430

1650$210

1440$165

KTB71 1710 KTB74 1850 KTB75 2060 1790$405 KTB138 1535

Mean CI 95% KJ14 KJ31 KJ33 KJ34

1410 1465 1485 1410

1445$60

KJ 1485 KJ 1195 KJ59 1605

1540$180

KTB60 1295 KTB61 1195 KTB65 1620

1370$625

3.3.2. ETS activity The mean value of ETS activity for the upper centimeter of sediment was 9.6 ll O g\ h\ (at 203C) and ranged from 1.4 to 20.9 ll O g\ h\ (Table 7a).   Spatial variations between the various morphological areas (Fig. 9a) were not statistically di!erent but showed some trends. In the upper and middle canyon, ETS activities were highly variable and lower than those registered in the northern and southern branches. The highest values were found at the inter#uve. MS1 and MS2 stations were characterized by homogeneous data, slightly lower than the mean value. No seasonal variations were observed between seasons at the two mooring stations. 3.3.3. Oxygen proxles The di!erences between the values obtained at the di!erent stations were not statistically signi"cant. However, oxygen consumption showed a trend of slight decrease from the upper canyon to the deepest stations (Table 7b and Fig. 9b). The highest values (305 nmole O cm\ d\) were found in the 1000}1500 m water depth  interval, but not in any speci"c morphological area. Southern and northern branches

H. Etcheber et al. / Deep-Sea Research II 46 (1999) 2249}2288

2267

were characterized by high values (except for core KTB 126), and both high and low values were found in the inter#uve. Oxygen #uxes, which are calculated from oxygen pro"les, are relevant indices of recycled carbon in sediment. They also are used to determine the sediment depth where oxygen is completely depleted by benthic community respiration. The average depth of oxygen penetration in the studied area was 2.0 cm ($0.3, 95% IC). No signi"cant di!erence was found between the morphological regions. The maximum depth of oxygen penetration was found outside the main axis of the canyon (KTB 170 and 171, at 4.1 and 4.4 cm, respectively). 3.3.4. Incubation experiments with labelled carbon At sediment}water interface (SWI), the global budget of the initial C activity was divided into three main fractions: CO , C in overlying water and C incorporated  in the sediment. A very low quantity of CO was released after the injection  of C-labelled diatoms. It reached 0.05, 0.3 and 1% of the initial activity after, respectively, 4, 12 and 144 h incubation time (Fig. 10a). In contrast, dissolved C activity in the aqueous phase represented about 10}12% of the initial activity and remained constant whatever the incubation time. A large proportion (90%) of the injected C activity was incorporated into the sediment. After a 144 h incubation, the migration depth inside sediment was about 10 cm. The DOC content in the overlying waters (Fig. 10b) varied in parallel with the incubation time: after 4 h, the DOC concentration was similar to ¹ (1.75 mg ll\). It  reached 30 mg ll\ after 24 h, and did not present any signi"cant change during the following days (28 mg ll\ after 144 h). The concentrations of dissolved amino acids in the overlying waters clearly increased during the 144 h incubation time (Figs. 11 and 12): DCAA, which were more abundant than DFAA (ratio 1 to 100), were produced 4}5 times faster during the "rst 24 h (initial concentration;1.65) than during the following days (initial concentration;0.35). Concentrations (expressed in lM l\) of individual DCAA increased (Fig. 11a). The results expressed in lmol% revealed that a decrease of some of them (e.g. alanine, lysine, c-aminobutyric acid) while leucine remained constant with time (Fig. 11b). The DFAA concentrations increased, except for alanine, isoleucine and c-aminobutyric acid (Fig. 12a). The major components were glutamic acid, serine, glycine, threonine and alanine, which represented together 60% of the total DFAA (Fig. 12b).

4. Discussion 4.1. Role of the Aquitanian margin in organic matter transfer The Aquitanian margin can be equated to an C depocenter for at least two  reasons. On the one hand, sur"cial sediments (Fig. 3) of this margin contain more C (1.35%) than sediments from the shelf ((0.5%) or deep ocean (average value:  0.5}1%, Premuzic et al. (1982)). This has been observed already in other continental

Mean CI 95%

ECOFER5

KTB88 KTB126 KTB131 KTB103 KTB105 KTB111 KTB161 KTB132 KTB154

8.9 4.6

11.1 15.2 8.6 4 7.55 20.85 6.2 1.4 5.4

a. ETS activity (kl O2 g\1 h\1 at 203C) ECOFER3

500}1000

KTB80 KTB85 KTB123 KTB125 KTB136 KTB155

1000}1500

13.4 8.2

12.8 10.1 12.15 8.75 29 7.7

KTB80 KTB81 KTB138 KTB120 KTB146 KTB170 KTB171

KJ54

1500}2000

Depth interval (m)

9.6 4.6

2.9 3.4 13.65 12.7 15.85 9.85 8.9

8.7

KTB71 KTB74 KTB75

KJ52 KJ53 KJ56

2300

8.9 1.7

8.5

8.9 5.7 8.5

Table 7 ETS activity (ll O g\ h\ at 203C) and oxygen consumption (nmole O cm\ d\) in the sediments from the Cap-Ferret Canyon  

KTB61 KTB65

KJ57 KJ58 KJ59 KJ60

3000

7.7 1

7.4 8.9

8.7 7.3 6.2 7.5

2268 H. Etcheber et al. / Deep-Sea Research II 46 (1999) 2249}2288

Mean CI 95% g C m\ yr\

ECOFER8

KTB88 KTB126 KTB131 KTB103 KTB105 KTB111 KTB161 KTB132 KTB154

240 30 8.9

280 220 310 210 210 200 260 230 220

b. Oxygen consumption (nmole O2 cm \1d\1) ECOFER3

KTB82 KTB85 KTB123 KTB125 KTB136 KTB155

305 25 11.3

280 280 320 300 330 320

KTB82 KTB85 KTB123 KTB120 KTB146 KTB170 KTB171

KJ54

235 50 8.5

220 250 210 340 210 200 170

260

KTB71 KTB74

KJ52 KJ53 KJ56

225 30 8.4

210 210

250 230 190

KTB61

KJ57 KJ58 KJ59 KJ60

215 50 7.9

260

220 210 220 150

H. Etcheber et al. / Deep-Sea Research II 46 (1999) 2249}2288 2269

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Fig. 9. (a) ETS activity (ll O g\ l\); (b) oxygen consumption (O nmoles cm\ d\) in sediments from   the Aquitanian margin.

margins: northeast of Taiwan (KEEP area; Lin et al. (1992)), south of New-England (SEEP area; Rowe et al. (1988)), Middle Atlantic Bight (SEEP II area; Anderson et al. (1994)), northwestern Mediterranean margin (Buscail et al., 1990; Buscail and Germain, 1997). On the other hand, sedimentation of the suspended material on the shelf is restricted to a few mud-patches (Lesueur et al., 1989), whereas high apparent sedimentation rates were measured in the margin (1000}2300 m). The values depend on the element used for measurement: 2}4 mm yr\ for Pb data (Radakovitch and Heussner, 1999) and 0.2}0.5 mm yr\ for C data (Arnold, personal communication).These values decrease eastward by a factor of 3 at site 2 (3000 m). The combination of high organic contents and high sedimentation rates makes this Aquitanian margin an area of preferential C accumulation. However, mineraliz ation processes have a direct in#uence on the buried C budget as will be discussed  hereafter. As shown in Fig. 13a, the enrichment in C of the Aquitanian margin is mainly  related to "ne grain-size sediments. The close relationship between the highest

H. Etcheber et al. / Deep-Sea Research II 46 (1999) 2249}2288

2271

Fig. 10. C `Navicula incertaa (diatoms) incubations: (a) budget of C fractions resulting from mineralization (CO ), release in the overlying water and integration in the deposit after 4 h, 24 h and 6 days;  (b) evolution of the DOC released in overlying waters.

C contents and the "nest grained sediments in marine environments is well known  (Cammen, 1982; Duchaufour et al., 1984; Relexans et al., 1992a,b; Anderson et al., 1994; Buscail et al., 1995). For example, on the Mediterranean margin, lowest C concentrations have been observed at the head of the Lacaze-Duthiers canyon  (Buscail and Gadel, 1991), where sediments are siltier (60%'40 lm) than elsewhere in the canyon (10%'40 lm).

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H. Etcheber et al. / Deep-Sea Research II 46 (1999) 2249}2288

Fig. 11. Dissolved combined amino acids in overlying waters of a time series incubation (Atlantic deep-sea sediment}water interface): (a) DCAA concentrations (lmole l\); (b) DCAA in mol%.

A mud-line, de"ned as the depth below the shelfbreak where the proportions of "ne-grained material no longer increase signi"cantly (Stanley et al., 1983), was found on the Aquitanian margin at 600 m (Fig. 13b). This limit is the erosion-deposition boundary beneath which the proportion of C increases in response to the deposition  of silty and/or clayey sediment. The sedimentological characteristics of sur"cial sediments (Cremer et al., 1999), are related to the hydrodynamic conditions, which are the major factor controlling grain-size distribution. On the Aquitanian margin, intense settling is induced both by thermo-haline seasonal fronts, observed on remote sensing images and located either at mid-shelf or near the shelf-break (Castaing et al., 1999; Froidefond et al., 1999) and by shelf nepheloid structures, directly in#uenced by internal tides and an along-slope current (Palanques and Biscaye, 1992; Durrieu De Madron, 1994).

H. Etcheber et al. / Deep-Sea Research II 46 (1999) 2249}2288

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Fig. 12. Dissolved free amino acids in overlying waters of a time series incubation (Atlantic deep-sea sediment}water interface): (a) DFAA concentrations (lmole l\); (b) DFAA in mol%.

4.2. Specixcity of the Cap-Ferret Canyon system The Cap-Ferret Canyon is a morphological anomaly in the Aquitanian continental slope. Fine-grained material mainly settles in the canyon. But some clear di!erences appear inside the canyon, in areas exposed to di!erent energy conditions, which directly in#uence suspended matter settlement. In comparison to adjacent areas, the northern and southern branches were found to be characterized by coarser sediments, and lower C contents, suggesting lower organic sedimentation. The vertical gradi ent of C content was limited to the 0}1 mm layer of sediment, and sediment oxygen  consumption and ETS activities were the highest due to the probable supply of labile organic matter. Therefore, the canyon branches can be considered to act as `channelsa, favoring the circulation of "nest particulate matter. Intense mineralization in the very sur"cial layer is assumed to prevent C accumulation.  The inter#uve is exposed to sediment winnowing by dynamic agents/currents and internal waves (Cremer et al., 1999): high C and CaCO contents suggest lower  

2274

H. Etcheber et al. / Deep-Sea Research II 46 (1999) 2249}2288

Fig. 13. Relationship between mean grain size (lm), depth and C content (%) in sediments from the  Aquitanian margin.

continental #uxes and a higher in#uence of marine material than in the other parts of the canyon. At the merging site (MS1), the deep water circulation is reduced (Durrieu et al., 1999). A signi"cant enrichment in C contents was found associated to high accumu lation rates (Radakovitch and Heussner, 1999), which makes this area a preferential depocenter where the lability of the organic fraction (P#C#L) is among the highest of the whole canyon system. The comparison of C content and sedimentation rates between the MS1 and MS2  sites indicates clearly decreased organic accumulation with increasing water depth. This observation is supported by the study of organic particulate #uxes, which decrease towards the open ocean (Radakovitch and Heussner, 1999). Preferential organic enrichment or accumulation areas has already been observed on other margins: on the northeastern TamK wan and the northwestern Mediterranean margins (Lin et al., 1992; Buscail et al., 1997), it takes place at 1000 m water depth. It appears deeper (1800}2300 m water depth) on the Aquitanian margin. More subtile di!erences in organic matter settling appear when the morphological complexity increases and the canyon system narrows. For example, the supply and accumulation of organic material is quite di!erent on the two sides of the main axis of the Mediterranean Lacaze-Duthiers Canyon, regardless of their orientation in the Liguro-Provencal current (Buscail and Germain, 1997). In conclusion, the hydrological conditions, morphological features, size of the canyon system and its location in relation to continental sources have a direct impact on the distribution of organic sedimentation and the existence of preferential organic enrichment and/or accumulation zones.

H. Etcheber et al. / Deep-Sea Research II 46 (1999) 2249}2288

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4.3. Origin of the sedimentary organic matter in the Cap-Ferret Canyon The particulate material that supplies the sediment}water interface has schematically a double origin: (i) marine from primary production and subsequent transfers along the trophic web; (ii) continental, due to advective transport of particles from the shelf and/or resuspension of sur"cial sediment of the margin. Their carbon content, organic matter quality (e.g., degradability) and time variability di!er. In the Cap-Ferret Canyon area, there are two main continental sources: the Gironde Estuary, and the Adour and spanish rivers (Castaing and Jouanneau, 1987). There is no evidence of a direct supply from the Gironde system by a bottom nepheloid layer, the processes being di!usive and occasional (Ruch et al., 1993; Castaing et al., 1999). This material contains 1.5% C , essentially refractory organic  matter (Lin and Etcheber, 1994). The circulation in the Bay of Biscay, characterized by a predominantly northward along-slope current, suggests that particles are supplied by the Adour and spanish rivers system, whose 3.0% C content is also refractory.  In order to evaluate the respective contribution of each component to the sedimentary organic fraction, particulate organic quality and #uxes in the water column have to be taken into account. The results of sediment trap experiments exhibit a typical margin pattern (Heussner et al., 1999); i.e. #uxes increase with depth at sampling sites and decrease o!shore (mass #uxes: 540 g m\ yr\ at MS1 and 170 g m\ yr\ at MS2, 2300 and 3000 m depth respectively). This pattern reveals the importance of advective transport. Seasonal peaks of mass and carbon #uxes were noted even in near-bottom sediment traps: a winter peak (without carbon content increase), which may be attributed to changes in the regime of lateral advection, and a summer peak (with carbon content increase), which would correspond to a supply more or less delayed from surface spring bloom. The mean level of primary production (similar at the two sites MS1 and MS2) is relatively low: about 0.4 g C m\ d\ (Laborde et al., 1999) on annual basis, which may reach 3}4 times this value during bloom episodes. The mean contribution of primary production to POC #uxes in near bottom sediment traps can be inferred from di!erent equations (Suess, 1980; Parsons et al., 1984b; Betzer et al., 1984). POC #uxes can be estimated at 2.8 (SD: 0.7) g C m\ yr\ (vs. 14.9 g C m\ yr\ of total POC) at site MS1 and 2.2 (SD: 0.6) g C m\ yr\ (vs. 5.3 g C m\ yr\ of total POC) at site MS2. During a bloom, the increase in POC #uxes from the surface waters can be estimated at about 3.5 times the annual value. These #uxes would increase the C content from 2.8 to 4% at MS1,  and from 3.35 to 6.5% at MS2. Such episodic POC enrichments were found in the material sampled by near-bottom sediment traps during spring and/or summer (Heussner et al., 1999). However, no signi"cant seasonal variations of abundance or quality of organic matter was found in the sediments from the Cap-Ferret Canyon. Several hypotheses can be proposed for explaining this observation: (i) marine organic matter is quickly mineralized because it is highly labile; (ii) the sampling of the upper 0}1 mm sediment layer is not precise enough to delineate seasonal variation (signi"cance of the thickness selected for sampling; discrepancy between the periodicity of carbon #uxes and

2276

H. Etcheber et al. / Deep-Sea Research II 46 (1999) 2249}2288

our sampling capacity); (iii) there is a benthic nepheloid layer whose resuspended material can dilute the fresh marine inputs. If we consider that the 2.8 g C m\ yr\ marine contribution at MS1 near-bottom sediment trap is realistic, the organic continental fraction should be equal to 12.1 g C m\ yr\, i.e. about 510}520 g of continental material for a 20}30 g total marine fraction. Hence, the percentage of C in continental suspended matter would  be 2.35%. The contribution of the organic fraction due to the Gironde particles (1.5% C content) and southern rivers (3% C content) can be evaluated to 45 and 55%,   respectively, from the following equation: 2.35"X;1.5#>;3, X#>"1 where X is the Gironde contribution and > the Southern rivers contribution. Such C contents (2.35%) are never re#ected in 0}1 mm sediment layer, even if this  continental C is supposed to be refractory. Several explanations can be proposed:  (i) the marine contribution is higher than calculated by the equations; in this case, contribution of southern rivers to the organic matter supplies decreases; (ii) the continental organic material, supposed to be refractory, is actually partially decomposed at the sediment}water interface (1 cm can be between a few months and 3}4 years according to the datation methods selected); (iii) old sediments (benthic nepheloid layer, resuspension) dilute recent suspended matter input. The results from the Mediterranean margin were quite di!erent (Fig. 14): a clear seasonal variability of the sedimentary organic matter quality was observed in the Lacaze-Duthiers Canyon, corresponding to the variability of organic matter bottom #uxes (Buscail et al., 1990). The amount of C increased progressively in the upper 

Fig. 14. Seasonal variability of organic matter in sur"cial sediments (0}1 cm) from the Mediterranean Lacaze-Duthiers Canyon (650 m depth).

H. Etcheber et al. / Deep-Sea Research II 46 (1999) 2249}2288

2277

1 cm deposit during autumn and winter. When comparing summer to the following spring, an extra milligram of C was found to be stored per gram of dry sediment  [this increase was calculated from 40 values obtained from four subsamplings per seasonal sampling (10)]. During the spring, organic matter was increasingly labile and the content in hydrolyzable C was twice as high as in summer; amino acids  increased by a factor of 4 and sugars by a factor of 1.4. These increasing values were well correlated with C #uxes measured for a near-bottom sediment trap. Fluxes  increased by a factor of 10 during this period (21}217 mg C m\ d\).  Such seasonal variations can be explained by the proximity of land, well-identi"ed sources of material (succession of phytoplankton blooms and direct continental supply due to high river discharge), and water circulation along the slope (the Liguro Provencal Current). The characteristics of the Cap-Ferret Canyon system are strictly inverse: large distance of the canyon from the main continental sources, which prevents any direct supply of material to the canyon (Jouanneau et al., 1999; Castaing et al., 1999), complex hydrology in the Bay of Biscay, with seasonal changes of the major current directions and the extension of several nepheloid structures in the canyon area (Castaing et al., 1999; Durrieu De Madron et al., 1999), and possible sediment resuspension in the canyon (Radakovitch and Heussner, 1999). 4.4. Sedimentary organic matter mineralization: inyuence of water temperature Mineralization processes at the water}sediment interface are due to enhanced biological activity on margins (De Bovee et al., 1990; Buscail and Guidi-Guilvard, 1993). In the Cap-Ferret Canyon, our study intended to estimate the C budget  involved in mineralization processes to display some typical reactions, to characterize the particulate labile organic fraction that is involved preferentially, and to compare the intensity of processes with those observed in other areas, whose bottom water temperatures are quite di!erent. Oxygen consumption values have been converted into carbon mineralization estimates, using a respiratory coe$cient evaluated to 0.85 (Hargrave, 1973). On average, 9 g C m\ yr\ was mineralized in the Cap-Ferret Canyon; the highest value 11.3 g C m\ yr\ was found in the 1000}1500 water depth interval; MS1 and MS2 sites were characterized by 8.4 and 7.9 g C m\ yr\, respectively. The uncertainties that can a!ect each term of the calculation of oxygen consumption from porewater pro"les imply that our results should be interpreted with caution. Comparison of data on oxygen consumption by sediment communities, measured with in situ methods and with shipboard methods (Fig. 15) show that both methods give results of the same order of magnitude. Therefore, the mean value of oxygen consumption can provide an estimation of carbon recycling (9 g C m\ yr\ on average). Mineralization processes of organic matter on the Aquitanian margin is illustrated by DAA measurements in the overlying waters, which give information on the kinetics of degradation and on the various metabolic cycles, involving mainly bacteria (Henrichs and Farrington, 1979; Henrichs, 1980; Jorgensen et al., 1980; Lee

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Fig. 15. Comparison of methods used to measure sediment community oxygen consumption (SCOC) in various areas from 500 to 5000 m depth. Open symbols: shipboard methods using oxygen electrodes (SPaci"c Ocean; Pamatmat, 1971; Reimers et al., 1984; Cole et al., 1987; Indian Ocean; Helder, 1989). Dark symbols: In situ benthic chambers (NE Atlantic Ocean: Jahnke et al., 1989; NW Atlantic Ocean: Smith and Teal, 1973; Smith and Cli!ord, 1976; Wiebe et al., 1976; Smith, 1978; Smith et al., 1978; Hinga et al., 1979; NE Paci"c: Smith, 1974; Smith et al., 1979; Smith and Hinga, 1983; Reimers and Smith, 1986; Berelson et al., 1987; Bender et al., 1989; Archer and Devol, 1992). # symbols: In situ microelectrodes (Reimers et al., 1986; Reimers and Smith, 1986; Reimers, 1987; Archer and Devol, 1992).

and Cronin, 1984; Cunin et al., 1986; Simon and Azam, 1989; Burdige and Martens, 1990): E the sediment reactivity to the input of labile planktonic organic matter is revealed by the high increase of DCAA contents during incubations, and proves that proteic synthesis and excretion processes are very active; E the increase of serine and glycine, abundant in diatom cell walls, and the increase of glutamic acid, which can be excreted by algae, revealed the degradation of diatoms; E tryptophane (decay product of AA metabolism by benthic fauna), taurine (an excretion product, which witnesses metazoan activity), glutamic acid (dominant in the intracell part of bacteria), ornithine (indicative of arginine degradation by bacteria) showed increasing concentrations, proving the general benthic fauna activity;

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E in contrast, respiratory processes decreased with time indicated by the decrease in dissolved free alanine, c-aminobuyric acid and isoleucine, while leucine (evidence of bacteria proteic synthesis) remain stable over 6 days of incubation. In order to get more information on the sedimentary particulate organic matter involved in mineralization processes, we have evaluated the budget of sedimentary labile C assimilated to:  (i) the quantity of C disappearing between the surface layer and the upper 5 cm of  sediment (level at which C values are constant, corresponding to long-term  buried organic matter); (ii) the quantity of hydrolyzable C disappearing between the surface layer and the  upper 5 cm of sediment; (iii) the quantity of P#C#L disappearing between the surface layer and the upper 5 cm of sediment. These quantities of C were estimated on a volume representing a surface of 1 m and  a sediment thickness of 5 cm. The concentrations of these contituents (per g dry sediment) were converted into contents per surface unit: X m\"Concentration g\;(1!')o;10, where ' is the porosity, and o the density of dry sediment (2.6), integrated over 5 cm depth in sediment. The labile C contents measured by the C gradients, hydrolysable C and    P#C#L were estimated to be 57, 46 and 5 g C m\, respectively. Hydrolysable C , which corresponds to nearly 80% of the disappeared organic  content represents well the mineralized labile organic fraction. In contrast, P#C#L corresponds to only 10% and cannot be assimilated to the total labile organic content. These parameters are relevant for studying fresh organic matter (phytoplankton, bacteria, etc.), but are limited for studying detrital organic material. Analytical methods must be chosen carefully because all results are not equivalent. For instance, the P/C ratios obtained in this study do not correspond to those usually found in the literature, where P and C concentrations are in the same order of magnitude (Romankevich, 1984; Buscail and Germain, 1997). This is mainly due to the strength of the extraction procedure used (H O for C and 6 N HCl for P).  We compared incubation experiments with labelled C diatoms on the Atlantic and Mediterranean margins (Buscail, 1992; Buscail and Guidi-Guilvard, 1993). The main di!erence (Fig. 16a) is the lower CO and metabolites proportions released in  the overlying water in the Cap-Ferret Canyon (1 and 10%) than in the Rhodanian Canyon (6 and 30%). Metabolic processes responsible for dissolved organic matter release are consequently less developed on the Atlantic side, and the proportion of C integrated in the Cap-Ferret Canyon sediment is higher than that in the Rhodanian Canyon. The DOC content in overlying water (Fig. 16b) indicates a rapid response of the sediment}water interface on the Mediterranean margin (4 h), a mineralization of the metabolites after a day, and a continuous DOC emission during the following days. In

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Fig. 16. Comparison of C `Navicula incertaa (diatoms) incubations on the Atlantic and Mediterranean margins.

the Cap-Ferret Canyon, the response to the organic input is delayed (24 h), and stable DOC concentrations suggest that the release processes do not persist. The mineralization activity seems less on the Aquitanian margin than on the Mediterranean margin. This observation can be related to the fact that the ratio between C content in suspended matter of sediment trap (3% on average on the  Atlantic margin and 2% on the Mediterranean margin) and sur"cial sediment (1.8% in the "rst mm at the site MS1 and 0.6% on the Mediterranean margin) is 2 in the Cap-Ferret Canyon and 4 on the Mediterranean margin. This lower degradation activity at the Atlantic sediment}water interface is assumed to be related to the low

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bottom water temperature (33C), compared to 133C in the Mediterranean sea, and to a less abundant benthic biomass: 10 viable bacteria (CFU ml) and 600 ind. 10 cm\ (meiofauna) in the Atlantic Ocean (DINET, personnal communication) compared to 10 viable bacteria and 800 ind. 10 cm\ (meiofauna) in the Mediterranean sea (De Bovee et al., 1990). 4.5. A tentative C

MPE

budget in the Cap-Ferret Canyon

An C budget in the Cap-Ferret Canyon is proposed, which is based upon data  from the average annual vertical particulate #uxes, sedimentation rates, and C contents of sur"cial sediments (0}1 cm) from the MS1 and MS2 sites.  Data retrieved from sequential sediment traps give an estimate for the average total annual mass #uxes of 540 and 170 g m\ yr\ for mooring sites 1 and 2, respectively, (Heussner et al., 1999). Near-bottom trap Pb #uxes were compared to the #uxes theorically required for supporting the excess Pb inventory in sediment (Radakovitch and Heussner, 1999). We concluded that 80 and 85% of the measured near-bottom #uxes reach the surface sediment at the sites MS1 and MS2. Consequently, 14.9 and 5.7 g C m\ yr\ have been incorporated into the sediment. These data are of the same order of magnitude as C consumption calculated by  studying the oxygen consumption of sedimentary community in the area (8.4 and 7.9 g C m\ yr\ at MS1 and MS2 sites). Most of the C input to the sediment is  considered as mineralized at the MS1 site, whereas the supply at MS2 site is not su$cient for covering mineralization processes. Sedimentation rates determined by the Pb method at these two sites are however very high: about 0.25 and 0.08 cm yr\ (Radakovitch and Heussner, 1999). Such values correspond to C accumulation of 23.3 and 6.9 g C m\ yr\, which are  higher than the #uxes estimated by sequential sediment trap experiments. With data from C data (0.5 and 0.2 mm yr\ at MS1 and MS2 sites), the C accumulation is  calculated to 4.5 and 1.7 g C m\ yr\. Several explanations can be proposed to elucidate the discrepancy between the vertical C #uxes and consumption-accumulation processes obtained with the Pb  method: (i) an over estimation of sedimentation rates by the Pb method, which can be expected from the high bioturbation observed in this area (Gerino et al., 1999); (ii) and/or an under-estimation of C #uxes given by the sediment trap experiments  due to: E a bad recovery rate of particulate matter #uxes in the water column by sediment traps; however, previous sediment trap experiments were always successful (Heussner et al., 1990); E major physical processes of sedimentary transport and resuspension in the near-bottom layer; the deepest sediment trap being situated above the nepheloid layer, this phenomenon would not be recorded in the traps. Such bottom transport is described by Radakovitch and Heussner (1999), but Durrieu De Madron et al. (1999) suggest that deep currents are not strong enough to generate signi"cant resuspension;

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E very low input of terrestrial material due to a major drought during the experiment period (discharge of the Garonne#Dordogne Rivers: 450 m s\ instead of the usual 1000 m s\). However, even normal, continental input is not su$cient to balance the carbon budget; (iii) and/or a time-scale discrepancy between sediment trap measurements (#uxes measured in a short time interval) and sedimentation rate data (based on several tens to a hundred years of accumulation). If we select the sedimentation rates given by the C method, the C budget is nearly  balanced at MS1 site: 14.9K8.4#4.5. However, the inputs are in su$cient at MS2 site: 5.7(7.9#1.7. The C budget in the Cap Ferret Canyon has still to be re"ned: C supplies to the   bottom may have been unusually low as a consequence of very dry period associated with low continental supply; a possible contribution of material brought by bottom transport under the sediment trap level has not been estimated; our present knowledge of C accumulation is not satisfactory since the estimation based on Pb is  four}"ve times higher than that by C method. Nevertheless, we can draw some conclusions about the role of this margin in the fate of organic matter: (i) this area can be considered as an organic carbon depocenter; (ii) active mineralization processes take place at the sediment water interface and (iii) mineralization processes limit the accumulation of organic matter.

5. Conclusions Major features can be identi"ed on the Aquitanian margin from the study of sedimentary organic matter. (1) The combination of high sedimentation rates and C contents in its sediments  indicates a preferred C deposition on the Aquitanian margin. Fine-grained  sedimentation in this area (presence of a mud-line at 600 m) is related to hydrodynamic processes (thermo-haline seasonal fronts, shelf nepheloid structures controlled by internal tides and along-slope currents). (2) The morphological heterogeneity of the Cap-Ferret Canyon system induces di!erences in organic matter sedimentation: the northern and southern branches of the canyon act as circulation channels (restricted organic sedimentation and intense mineralization processes). In contrast, the MS1 site is an C accumula tion area because of weak currents and "ne sediments. The comparison with Mediterranean canyons, where organic matter sedimentation is quite heterogeneous locally, reveals that size of the canyon, its distance from the continent, and hydrological circulation modify organic sedimentation characteristics. (3) The origin of the organic matter has not been proved. The high C contents  observed in near-bottom sediment trap material are probably due more to a marine contribution (labile organic matter) than to a possible Adour-spanish rivers supply (refractory organic matter).

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The fact that C contents do not vary seasonally in the bottom sediment and  values are signi"cantly lower than in the sediment trap suggests that: E marine contribution is dominant (which is in contrast with our calculated estimation using equations) and mineralized at the sediment}water interface; E and/or resuspension and nepheloid transport can wipe out any print of fresh material. (4) The study of particulate and dissolved sedimentary organic fraction indicates that mineralization processes are active: signi"cant vertical decrease of C contents in  sediment, changes in DOC contents and labile organic fraction (amino acid components). The average oxygen consumption is evaluated as 9 g C m\ yr\. Mineralization appears to be less developed than in the Mediterranenan Sea, due to a di!erence in water temperature. (5) A precise budget of C has not been yet estimated. During the experiment, the  organic carbon supply to the bottom was likely lower than for a normal year (14.9 and 5.7 g C m\ yr\ at MS1 and MS 2 sites, respectively). Sedimentation rates are too uncertain for proposing a budget of organic carbon accumulation (23.3 or 4.5 g C m\ yr\ at MS1 site; 6.9 or 1.7 g C m\ yr\ at MS2 site) according to the methods of dating.

Acknowledgements This work was part of margin ecosystem studies (French ECOMARGE program), "nancially supported by the INSU-CNRS (SDU sector). We thank the crews of the R.V. Suroit, Noroit and Cote d1Aquitaine for their helpful technical assistance and B. Deniaux for the help in preparing the manuscript.

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