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 organicmatter 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 weresampled on the Aquitanian margin with box corers and analyzed to determine the quantity andquality of organic matter. Sediments from the margin are enriched in organic carbon (meanvalue 1.35%) in comparison to deep-sea and shelf sediments, due to a "ne grain-size sedimenta-tion. As sedimentation rates are high, the margin appears to be an organic depocenter. Somepreferential organic enrichment zones were identi"ed in the Cap-Ferret Canyon. There isa supply of continental material from the Gironde estuary, but marine contribution seems morepossible than Adour or spanish rivers. No seasonal variations of organic matter were observedat the surface of sediments, suggesting mineralization processes of labile organic matter: averageorganic carbon consumption was evaluated to 9.0 g C m~2 yr~1. Rapid biological mineraliz-ation processes are lower than on the Mediterranean margin, mainly related to signi"cantdi!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 inputsto the sediment and favor nephelomKd transport, resuspension and shelf break processes, whichwipe out any print of fresh material input. An organic carbon budget indicates that anequilibrium between organic inputs and organic mineralization#accumulation is not obtain-able. The supply of suspended matter could have been minor during the year in question, andsedimentation 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

1. Introduction

Coastal zones and margins are the most fertile areas in the ocean and the majorsettling zone of suspended material originating from continental sources or primaryproduction (Monaco et al., 1990a,b; Wollast, 1990; Walsh, 1991). It is necessary tounderstand 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 isto identify whether the storage of organic matter in sediment is permanent ortemporary (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 resultshave been obtained by taking a multidisciplinary approach. Information obtained onsuspended 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 experi-ments, were combined to understand the main trends of organic matter sedimenta-tion. Organic carbon budgets, however, are signi"cant only if they are based onrelevant 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 } ECOsysteme du canyon du Cap-FERret } experiment (part ofthe French ECOMARGE } ECOsysteme de MARGE continentale } program), thestudy of the Aquitanian margin, close to the Gironde system, was undertaken in orderto answer some questions:

f This margin is supposed to receive abundant continental #uxes and to have a verycomplex water circulation. Does it function like other margins?

f Does the Cap-Ferret Canyon have a peculiar distribution of sedimentary organicmatter? What is the in#uence of the vicinity of the continent, of the size or themorphology of the canyon on this distribution?

f What is the C03'

budget?

A multidisciplinary approach, based on our expertise acquired in the Medi-terranean margin, was developed during the ECOFER experiment. The studyof sedimentary organic carbon quality and quantity is coupled with a studyof the benthic response to hydrological circulation and particulate #uxes. TheAquitanian margin has not been studied before, and therefore few or no data onC

03'content, water circulation, etc. were available. Consequently we needed to acquire

all the `basica parameters before identifying more speci"c analyses. Our data werecompared with the results obtained on other margins, which have led to some relevantconclusions.

2250 H. Etcheber et al. / Deep-Sea Research II 46 (1999) 2249}2288

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 ofBiscay in the Cap-Ferret Canyon (44330@}44355@ N; 2300@}2355@ W) and its surround-ing area (44330@}45340 N; 1330@}3330@ W). We intended to investigate the speci"c roleof the canyon on the suspended material transfer to the ocean. The various bathymor-phological 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) incisedby 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 (June1990}August 1991) on two mooring lines (mooring sites MS1 and MS2, Fig. 1) toevaluate the total mass and organic #uxes over the Cap-Ferret Canyon. Various boxcorers 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 waterdepth); Flucha and Usnel box corers or SMBA Multicorer, which allow a goodpreservation 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).

H. Etcheber et al. / Deep-Sea Research II 46 (1999) 2249}2288 2251

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

Moorings Cruises

From To From To

ECOFER 1 07/17/1989 11/02/1989 06/19/1989 07/17/1989ECOFER 2 05/20/1990 09/29/1990 05/03/1990 05/18/1990ECOFER 3 10/25/1990 04/05/1991 10/11/1990 10/26/1990ECOFER 4 05/06/1991 08/10/1991 04/30/1991 05/14/1991ECOFER 5 * * 08/08/1991 08/27/1991

Cap-Ferret Canyon were obtained and analyzed (Fig. 2); 24 additional cores weresampled in the surrounding area (Fig. 3).

Organic matter content was determined on short cores (10}30 cm long) sampledcarefully, frozen on board, then cut into 1 cm slices. The top 2 cm of some cores alsowere cut into 1 mm slices. All the samples were freeze-dried in the laboratory.

The quality of sedimentary organic matter was studied on some representativecores 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 atthree 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 fewhours after their retrieval.

2.2. Analyses

Grain size was analyzed using a Malvern Laser Di!raction Particle Sizer (Type2600), which gives the distribution frequency for 32 size classes ranging from 2 to180 lm (Singer et al., 1988).

The organic carbon (C03'

) content was determined on dry weight sediment bycombustion in an LECO CS 125 analyzer (Cauwet et al., 1990). Samples were acidi"edin crucibles with 2N HCl to destroy carbonates, then dried at 603C to removeinorganic C and most of the remaining acid and water. The analyses were performedby direct combustion in an induction furnace, and the CO

2formed 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 screwcap tube with Te#on liner. The residual organic carbon (ROC) was measured bycombustion (LECO analyzer) on the sediment after hydrolysis: TOC-ROC"HOCand 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 tothe procedure of Dubois et al. (1956), improved by Montreuil and Spik (1963), with


Fig

.2.

Sam

pling

loca

tion

ofsu

rfac

ese

dim

ents

(box

core

s)in

the

Cap

-Fer

ret

Can

yon,(

EC

OF

ER

cruises

).


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

glucose as a standard. Lipids (L) were determined by weighing the dry residue ofmethanol}chloroform extracts after complete evaporation of the solvent (Bligh andDyer, 1959). These protocols were chosen for two reasons: they are well suited foranalyzing marine sediment samples, and allow comparisons with data previouslyobtained in the same way on sediments from surrounding estuarine and coastalenvironments (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 tothe easily extractable macromolecular organic matter of sediment (Khripouno! andRowe, 1985; Laane et al., 1987; Mayer et al., 1988; Relexans et al., 1992a,b).

Electron Transport System (ETS) activity (Packard, 1971; Christensen andPackard, 1977) is a measure of the potential dehydrogenase activity of all respiringmicro-organisms and meiofauna. ETS activity was measured aboard, usually in thetop 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 colorimeterequipped with a 492 nm "lter. This method requires very small amounts ofhomogenate and chemicals, which allows several replicates.


Pro"les of porewater oxygen were measured aboard by micro-minielectrodes(Revsbech et al., 1980; Reimers et al., 1984; Helder and Bakker, 1985; Jorgensen andRevsbech, 1985; Reimers and Smith, 1986; Revsbeck and Jorgensen, 1986). Only coreswith an undisturbed sediment-water interface were used for oxygen pro"le determina-tion. 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 coreswere usually measured at each station. Subsamples were kept at in situ temperaturesin a refrigerated box until the oxygen pro"le was completed, i.e. less than 3 h after coreretrieval. The polarographic oxygen sensor (POS) equipped with a needle electrode(Helder and Bakker, 1985) was lowered from the overlying water into the sediment bymeans of a motor-driven micromanipulator with a 0.1 mm resolution. The measure-ments were made without stirring to prevent atmospheric contamination. The re-sponse from the POS was assumed to be linear between the value of the currentrecorded in the oxygenated overlying water (where oxygen concentration was titratedby the Winkler method) and the zero-oxygen value at the depth where the responsefrom the POS remained constant. The drift of the electrode response during measure-ment was generally less than 10%, as evaluated by comparing the response from thePOS in the overlying water at the beginning and at the end of the experiment.

Total oxygen consumption at the sediment}water interface was calculated accord-ing to Fick's "rst law of di!usion applied to sediment (Berner, 1980):

j"!/D4]dC/dz

z/0,

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

4the bulk sediment di!usion coe$cient

for oxygen. / was assimilated to the weight loss of a wet sediment (0}1 cm) aftercomplete drying at 603C. D

4is assumed to be /m~1]D

0with m"2.5 (Ullman and

Aller, 1982) for / values between 0.7 and 0.88, which corresponds to the values foundfor our samples. D

0is 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 mostsensitive to shifts in environmental conditions). The gradient was assumed to be linearfrom bottom water oxygen concentration to the concentration within sediment at theshallowest depth at which atmospheric contamination was low to non-existent(Reimers et al., 1984). As discussed by Jahnke et al. (1989), the possible contaminationby atmospheric oxygen should be limited to about 4}5 mm, since our pro"les wereperformed within 3 h after core retrieval. The thickness of the disturbed surfaceresulting from core handling has been estimated to 2 mm. A 7 mm depth beneath thesurface was chosen for determining the bottom of the gradient in our calculations.

For most parameters (C03'

, P#C#L, ETS activity, O2), 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 boxcorers by inserting transparent plastic tubes into the sediment. The input of natural


Tab

le2

(a)

Org

anic

carb

on

conte

nt(%

ofdr

yw

eigh

t)in

sur"

cial

sedim

ents

(0}1

cm).

Di!

eren

tbo

xco

rers

wer

euse

dfo

rsa

mplin

g:R

einec

k(K

R),

Smith

and

McI

nty

re(B

SMC

),Flu

cha

(FLU

),U

snel

(KJ)

,M

ultitube

(KT

B)

Sam

ple

Lat

.NLong

.WD

epth

(m)

C03'(%

)Sa

mple

Lat

.NLong

.WD

epth

(m)

C03'

(%)

KR

9301

9443

30.9

613

35.2

790

0.20

KR

9301

7453

10.4

033

02.7

544

00.

49K

R93

020

443

31.1

113

54.0

512

60.

14K

R93

013

453

22.8

533

18.9

444

00.

34K

R93

006

443

34.7

523

02.8

616

40.

23K

R93

009

453

33.2

433

29.6

244

00.

39K

R92

024

443

36.9

723

00.9

916

80.

21K

R50

443

44.6

223

07.6

544

00.

77K

R92

016

443

33.3

323

04.4

716

90.

27K

R93

023

453

06.6

623

41.6

644

30.

34K

R92

015

443

33.4

123

03.6

017

00.

24K

TB

088

443

50.6

723

07.6

944

51.

17K

R93

015

453

23.8

733

12.9

918

00.

20K

R93

001

443

37.1

323

10.0

146

00.

13K

R93

021

443

30.8

623

04.5

518

00.

23K

R92

022

443

36.4

123

10.4

147

10.

22K

R93

029

453

00.6

523

20.2

518

00.

11K

R93

032

443

52.3

023

13.1

047

80.

38K

R93

005

443

34.5

723

05.4

319

60.

27K

TB

103

443

49.6

123

11.5

849

00.

68K

R93

033

443

52.3

723

12.5

724

20.

11K

TB

132

443

37.7

223

07.9

851

30.

37K

R93

014

453

23.3

633

15.7

725

10.

13K

TB

161

443

44.5

723

08.1

451

51.

08K

R93

008

453

33.7

433

28.1

125

50.

09K

R93

027

443

59.8

423

21.7

653

91.

38BSM

C03

443

34.9

623

07.0

225

50.

32K

TB

131

443

35.9

023

03.9

655

50.

71K

R93

028

453

00.3

823

21.0

425

70.

43K

R93

002

443

38.3

723

11.8

061

10.

38BSM

C04

443

34.0

223

06.9

726

00.

30K

R93

012

453

22.9

833

19.2

661

20.

48K

R93

024

453

07.0

223

39.9

426

90.

09K

R92

019

443

38.1

923

12.9

068

60.

73K

R93

016

453

10.6

133

02.3

427

00.

25FL

U92

002

443

38.8

623

13.0

968

80.

33BSM

C05

443

32.8

723

07.1

128

60.

25K

R93

018

453

10.0

033

04.1

970

00.

70K

R93

004

443

36.2

923

06.2

629

90.

26K

TB

154

443

36.0

323

12.8

671

00.

67BSM

C06

443

32.0

023

07.0

631

00.

22FL

U91

502

443

34.8

523

12.4

772

00.

84BSM

C11

443

35.0

023

08.0

031

50.

32K

TB

126

443

36.4

823

04.4

172

01.

08K

R93

003

443

36.0

623

07.8

733

00.

20K

R93

026

443

58.5

323

24.4

973

20.

50BSM

C12

443

37.0

023

08.0

034

50.

75FL

U92

012

443

34.8

023

12.6

173

91.

22K

R92

020

443

34.9

523

08.6

036

50.

31K

R93

030

443

52.1

823

13.9

275

00.

77BSM

C10

443

33.8

723

08.1

237

20.

58K

R93

022

453

06.2

323

42.7

976

71.

24BSM

C12

(2)

443

37.0

023

08.0

037

30.

29K

J51

443

36.9

423

14.3

977

51.

23BSM

C08

443

32.7

623

08.1

039

00.

40K

J40

443

34.0

523

12.9

480

51.

24K

R92

027

443

37.4

823

02.7

539

40.

68FL

U92

005

443

35.0

023

14.0

085

41.

09BSM

C09

443

32.6

923

08.3

140

60.

34K

TB

105

443

48.5

623

10.9

787

51.

08K

R93

007

443

36.7

423

08.9

641

00.

16K

TB

055

443

38.5

323

17.0

887

51.

34


(b)

Org

anic

carb

on

conte

nt(%

ofdry

wei

ght)

insu

r"ci

alse

dim

ents

(0}1

cm).

Di!

eren

tbox

core

rsw

ere

use

dfo

rsa

mpl

ing:

Rei

nec

k(K

R),

Sm

ith

and

McI

nty

re(B

SMC

),Flu

cha

(FLU

),U

snel

(KJ)

,M

ultitube

(KT

B)

Sam

ple

Lat

.NLong

.WD

epth

(m)

C03'(%

)Sa

mple

Lat

.NLong

.WD

epth

(m)

C03'

(%)

KTB

109

443

45.1

623

09.1

788

01.

43K

TB

138

443

41.6

623

13.2

420

451.

41K

TB

111

443

45.0

723

09.4

095

01.

56K

TB

167

443

41.9

723

49.3

921

481.

48FL

U92

004

443

34.7

823

15.5

899

51.

36K

TB

159

443

42.1

623

51.4

321

601.

00K

TB

155

443

36.9

223

17.4

410

151.

30K

J10

443

43.2

823

16.9

222

701.

43K

TB

125

443

37.5

923

04.5

110

501.

12K

TB

074

443

43.6

223

16.8

823

001.

42K

J39

443

34.3

023

15.8

310

841.

30K

TB

071

443

43.9

223

17.0

923

001.

51K

TB

085

443

49.0

623

09.8

011

00.

98K

J07

443

44.3

723

18.8

423

091.

30FL

U92

003

443

39.7

023

15.3

511

411.

19K

TB

075

443

43.6

623

17.2

623

151.

65K

TB

136

443

40.0

623

08.0

011

701.

57K

J52

443

43.8

523

21.5

423

251.

39FL

U92

001

443

34.8

423

17.9

411

731.

20K

J46

443

43.8

223

18.1

723

351.

85FL

U91

501

443

37.8

323

19.8

112

501.

20K

J21

443

43.8

623

20.3

123

501.

33FL

U92

010

443

34.7

723

18.8

612

751.

31K

J03

443

43.6

423

18.7

323

731.

57FL

U91

500

443

37.5

623

22.2

313

321.

13K

J47

443

43.6

223

19.5

723

861.

37K

TB

082

443

47.5

923

10.1

314

000.

97K

J05

443

43.8

723

18.4

323

981.

50K

TB

123

443

39.6

923

06.2

814

151.

10K

J55

443

43.2

223

19.5

824

101.

34K

J38

443

35.2

323

19.9

714

221.

49K

TB

070

443

43.0

923

28.6

027

501.

54K

TB

124

443

39.6

023

06.2

214

311.

03K

TB

166

443

53.2

923

45.1

828

601.

26K

J48

443

43.5

623

11.3

415

001.

60K

J42

443

46.9

023

36.1

329

621.

22K

TB

120

443

39.8

023

06.0

516

701.

23K

TB

065

443

45.6

423

36.5

529

801.

50K

TB

081

443

45.9

823

10.6

117

001.

16K

TB

061

443

44.5

323

38.0

129

851.

07K

TB

146

443

42.9

423

12.4

217

501.

51K

TB

062

443

44.9

823

37.2

229

851.

25K

TB

171

443

46.9

223

23.0

419

301.

44K

J31

443

46.8

523

37.1

230

001.

24K

J26

443

46.2

523

23.6

219

481.

26K

J33

443

46.0

823

36.9

930

081.

38K

J29

443

46.0

323

23.8

119

691.

34K

J34

443

46.4

423

37.8

630

161.

29K

J20

443

45.7

823

22.9

819

751.

33K

J14

443

46.8

223

37.8

830

161.

63K

TB

170

443

35.2

923

29.9

019

751.

41K

J45

443

45.7

623

37.9

030

471.

39K

TB

080

443

45.1

923

13.0

319

901.

30K

J57

443

46.3

023

40.5

030

701.

44K

J22

443

45.7

323

23.6

719

971.

38K

J57

443

46.2

423

40.5

730

751.

20K

J54

443

41.8

423

20.1

420

251.

26


organic matter at the sediment}water interface was simulated by the injection of14C-labelled diatoms (Navicula incerta) in the overlying water (1 ml added with anactivity of 1 lCi ml~1). Incubations lasted, respectively, 4, 24 h and 6 days at in situtemperature (33C) in a dark thermostat-regulated chamber where oxidizing condi-tions were maintained by air #ushing. The 14CO2 produced by mineralizationprocesses was collected every 12 h in ethyleneglycol monoethylether/ethanolamine(scintillation grade 7 : 2 v/v). The 143C radioactivity was counted at the end of eachincubation in both overlying water and sediment. Dissolved C

03'(DOC) was quanti-

"ed in the initial bottom water (¹0) and after 4, 24 and 144 h of incubation using theUV-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 acidswere quanti"ed as O-phthaldialdehyde (OPA) derivatives, directly in the watersample for the DFAA and after 16 h hydrolysis in 6 N HCl at 1103C for the DCAA.

3. Results

3.1. Distribution of surxcial Corg

content

We consider all the data obtained whatever the season. The distribution ofC

03'concentrations (% of dry weight, Table 2) in sur"cial sediments (0}1 cm) is

summarized in Figs. 4 and 5. Low and variable C03'

concentrations, ranging from 0.09to 1.17% (average: 0.33%), were found on the shelf and the upper slope (90}500 mwater depth). C

03'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 themiddle canyon (850}2500 m water depth), contents were higher and more homogene-ous 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 lowC

03'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 themerging 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

03'contents were of

the same order of magnitude as in the middle canyon (average 1.35%).Surface sedimentary C

03'contents for the various ECOFER cruises (Table 3) did

not show any statistically signi"cant seasonal trend, even when the "rst mm ofsediment was studied (based on the study of 25 cores). At all sites (merging, middle orlower canyon), intra-seasonal variations were of the same order of magnitude asinter-seasonal variations.

3.2. Vertical distribution of Corg

content

At stations shallower than 850 m and in the northern and southern branches of thecanyon, no signi"cant downcore variations of C

03'content was recorded between the


Fig. 4. Distribution of C03'

(% of dry weight) in the upper 1 cm of sediment on the margin of the Bay ofBiscay.

Fig. 5. Sur"cial C03'

contents (% of dry weight; 0}1 cm) as a function of water depth.


Table 3Seasonal variations of organic carbon content (% of dry weight) in sur"cial sediments (0}1 cm and0}1 mm(!))

Cruises Middle canyon850}2500 m

Con#uence (MS1)2300 m

Lower canyon (MS2)3000 m

ECOFER 1 KJ 20 1.33 (1.40) KJ 03 1.57 (1.55) KJ 14 1.63 (1.75)Summer KJ 22 1.38 (!) KJ 05 1.50 (1.55) KJ 31 1.24 (1.32)

KJ 26 1.26 (1.38) KJ 07 1.30 (!) KJ 33 1.38 (!)KJ 29 1.14 (1.26) KJ 10 1.43 (1.57) KJ 34 1.29 (!)KJ 38 1.49 (1.65) KJ 12 1.33 (1.45)Kj 39 1.30 (!)

ECOFER 2 KJ 48 1.60 (1.65) KJ 46 1.85 (1.65) KJ 42 1.22 (1.40)Spring KJ 47 1.37 (1.39) KJ 45 1.39 (!)

ECOFER 3 KJ 52 1.39 (!) KJ 57 1.44 (1.60)Autumn KJ 54 1.26 (!) KJ 69 1.20 (!)

KJ 55 1.34 (1.35)

ECOFER 5 KTB 80 1.30 (1.35) KTB 71 1.51 (1.80) KTB 61 1.07 (!)Summer KTB 81 1.16 (1.20) KTB 74 1.42 (!) KTB 62 1.25 (!)

KTB 82 0.97 (!) KTB 75 1.65 (1.80) KTB 65 1.50 (1.60)KTB 85 0.98 (!) KTB 138 1.41 (1.43)KTB 120 1.23 (!)KTB 123 1.10 (1.15)KTB 125 1.12 (!)KTB 136 1.57 (!)KTB 155 1.30 (1.35)

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 82in Fig. 6a).

At all the other stations, the vertical decrease in C03'

content was signi"cant(Fig. 6b). C

03'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, wheresur"cial C

03'content decreased by 20% on average (15}40%). The decrease in

C03'

concentration was not related to the particle size distribution since the meangrain size did not vary signi"cantly throughout the cores (Fig. 6b).

The study of the C03'

content in the various particle size fractions (Fig. 7) revealedsome general trends:

f at any level, 85% of the C03'

was contained in the "nest fraction ((16 lm), becausethis fraction was the most abundant and characterised by the highest C

03'concen-

trations;f the vertical decrease in C

03'content occurred in all sedimentary fractions, with the

exception of the '63 lm fraction which was related to the weak contribution ofthis fraction.


Fig. 6. Vertical variations of C03'

(% of dry weight) in sediments from the Aquitanian margin: (a) uppercanyon; (b) middle and lower canyon.


Fig. 7. C03'

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 Corg

At all stations, the three biogeochemical parameters (protein, carbohydrate andlipid contents) were higher in surface levels than in deeper layers (Table 4). The P/C/Lratios 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~1 (rangebetween 1895 and 2815 lg g~1; Table 5) in sur"cial sediments (0}1 cm). This repres-ents 8}10% of the sedimentary organic matter (calculated as C

03'content]2). Sim-

ilarly, the average hydrolyzable C03'

content was 7800 lg g~1, which represents50}60% of the organic carbon. In marine sediment, amino acids, sugars, amino-sugarsand ammonium can represent the most important part of the acid-hydrolyzablefraction. The labile character of the organic matter is determined in the deposit by thevariation of this hydrolyzable fraction. A signi"cant increase in P#C#L wasrecorded at the merging of the northern and southern branches (Site MS1), (Fig. 8a),where the hydrolyzable C

03'contents were also the highest (Table 5).

The proteinic macromolecules are known to be biologically available and sea-sonally 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 sedimentlayer (0}1 mm) for assessing better the seasonal variations of organic matter quality,but no signi"cant variation was recorded (Table 6).


Tab

le4

P,C

,L(l

gg~

1dry

wei

ght)

and

orga

nic

carb

onco

nten

t(%

ofdry

wei

ght)

atdi!er

ent

leve

lsofse

dim

ents

from

the

Cap

-Fer

ret

Can

yon

Sam

ple

Dep

thP

CL

C03'(%

)(m

)0}

1cm

2}3

cm5}

6cm

0}1

cm2}

3cm

5}6

cm0}

1cm

2}3

cm5}

6cm

0}1

cm2}

3cm

5}6

cm

KJ

4080

513

0014

7010

5524

520

016

542

526

085

1.23

1.19

1.04

KJ

3910

8415

0011

7011

9522

016

022

538

524

017

51.

251.

151.

15K

J38

1422

1275

1300

930

215

170

120

405

240

401.

521.

180.

81K

J26

1948

1470

1225

805

265

190

165

495

345

551.

261.

160.

83K

J29

1969

1410

1580

1045

300

195

255

375

365

125

1.36

1.14

0.87

KJ

2019

7513

8015

6013

3526

523

516

546

535

518

51.

331.

280.

98K

J22

1997

1495

1550

1045

380

225

245

465

340

851.

431.

331.

07K

J10

2270

1485

1340

1400

380

210

195

540

375

265

1.42

1.26

1.02

KJ

0723

0917

5516

7013

7534

518

025

571

527

011

01.

391.

211.

06K

J03

2373

1620

1480

1485

340

195

185

535

240

701.

491.

081.

07K

J05

2398

1550

1560

1220

320

175

220

495

275

115

1.41

1.30

0.9

KJ

3130

0014

6512

7513

0527

520

535

533

028

517

01.

221.

131.

05K

J33

3008

1485

1200

1045

220

210

190

405

255

185

1.32

1.13

1.07

KJ

3430

1614

1013

2511

7024

022

529

040

528

517

51.

261.

211.

11K

J14

3016

1410

1310

1180

315

195

215

520

375

210

1.53

1.16

1.02


Tab

le5

P#

C#

Lan

dhy

droly

zable

org

anic

carb

onco

nten

t(l

gg~

1dry

wei

ght)

atdi!er

ent

leve

lsofse

dim

ents

from

the

Cap

-Fer

ret

Can

yon

($95

%C

I)

Sta

tion

Dep

thN

um

ber

P#

C#

L(l

fg~1)

Num

ber

Hyd

roly

zable

C03'(l

gg~

1)ofco

res

ofco

res

0}1

cm2}

3cm

5}6

cm0}

1cm

5}6

cm

Uppe

rC

anyo

n(

800

m1

1970

1930

1305

**

*

Mid

dle

Can

yon

800}

2000

m6

215$

185

1910

$24

013

60$

270

268

90$

580

4060

$31

0C

on#

uenc

e(M

S1)

2300

m4

2510

$35

019

90$

160

1725

$23

51

1002

533

333

Low

erC

anyo

n(M

S2)

3000

m4

2120

$14

517

85$

165

1600

$24

02

7670

$11

058

50$

90


Fig. 8. Macromolecular organic matter (lg g~1 of dry weight) in sediments from the Aquitanian margin.

A downcore decrease in P#C#L and hydrolyzable C03'

content was observedat 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 bediscussed:(i) the decrease in P#C#L content was only 10% of the organic matter decrease,

while the decrease in hydrolyzable C03'

was 80% of the C03'

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 C03'

werestill found.


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

Cruises Middle canyon Con#uence (MS1) Lower canyon (MS2)1000}2000 m 2300 m 3000 m

Mean Mean MeanCI 95% CI 95% CI 95%

ECOFER 1 KJ20 1380 KJ03 1620 KJ14 1410Summer KJ22 1495 KJ05 1550 1650$210 KJ31 1465 1445$60

KJ26 1470 1420$95 KJ07 1755 KJ33 1485KJ29 1410 KJ10 1485 KJ34 1410KJ38 1275KJ39 1500

ECOFER 3 KJ52 1465 KJ 1485Autumn KJ53 1585 KJ 1195 1540$180

KJ54 1275 1440$165 KJ59 1605KJ55 1455KJ56 1430

ECOFER 5 KTB80 1850 KTB71 1710 KTB60 1295Summer KTB81 1640 KTB74 1850 KTB61 1195 1370$625

KTB82 1005 KTB75 2060 1790$405 KTB65 1620KTB85 1540 KTB138 1535KTB120 1650 1530$200KTB123 1650KTB125 1245KTB136 1695KTB155 1545

3.3.2. ETS activityThe mean value of ETS activity for the upper centimeter of sediment was

9.6 ll O2

g~1 h~1 (at 203C) and ranged from 1.4 to 20.9 ll O2

g~1 h~1 (Table 7a).Spatial variations between the various morphological areas (Fig. 9a) were not statist-ically di!erent but showed some trends. In the upper and middle canyon, ETSactivities were highly variable and lower than those registered in the northern andsouthern branches. The highest values were found at the inter#uve. MS1 and MS2stations 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 proxlesThe di!erences between the values obtained at the di!erent stations were not

statistically signi"cant. However, oxygen consumption showed a trend of slightdecrease from the upper canyon to the deepest stations (Table 7b and Fig. 9b). Thehighest values (305 nmole O

2cm~2 d~1) were found in the 1000}1500 m water depth

interval, but not in any speci"c morphological area. Southern and northern branches


were characterized by high values (except for core KTB 126), and both high and lowvalues were found in the inter#uve.

Oxygen #uxes, which are calculated from oxygen pro"les, are relevant indices ofrecycled carbon in sediment. They also are used to determine the sediment depthwhere oxygen is completely depleted by benthic community respiration. The averagedepth of oxygen penetration in the studied area was 2.0 cm ($0.3, 95% IC). Nosigni"cant di!erence was found between the morphological regions. The maximumdepth of oxygen penetration was found outside the main axis of the canyon (KTB 170and 171, at 4.1 and 4.4 cm, respectively).

3.3.4. Incubation experiments with labelled carbonAt sediment}water interface (SWI), the global budget of the initial 14C activity was

divided into three main fractions: CO2, 14C in overlying water and 14C incorporated

in the sediment. A very low quantity of 14CO2

was released after the injectionof 14C-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 14C ac-tivity in the aqueous phase represented about 10}12% of the initial activity andremained constant whatever the incubation time. A large proportion (90%) of theinjected 14C activity was incorporated into the sediment. After a 144 h incubation, themigration depth inside sediment was about 10 cm.

The DOC content in the overlying waters (Fig. 10b) varied in parallel with theincubation time: after 4 h, the DOC concentration was similar to ¹

0(1.75 mg ll~1). It

reached 30 mg ll~1 after 24 h, and did not present any signi"cant change during thefollowing days (28 mg ll~1 after 144 h).

The concentrations of dissolved amino acids in the overlying waters clearly in-creased during the 144 h incubation time (Figs. 11 and 12): DCAA, which were moreabundant than DFAA (ratio 1 to 100), were produced 4}5 times faster during the "rst24 h (initial concentration]1.65) than during the following days (initial concentra-tion]0.35). Concentrations (expressed in lM l~1) 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 andc-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 C03'

depocenter for at least tworeasons. On the one hand, sur"cial sediments (Fig. 3) of this margin contain moreC

03'(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


Tab

le7

ETS

activi

ty(l

lO2

g~1

h~

1at

203C

)an

dox

ygen

consu

mption

(nm

ole

O2

cm~

2d~

1)in

the

sedi

men

tsfrom

the

Cap

-Fer

ret

Can

yon

Dep

thin

terv

al(m

)

500}

1000

1000}15

0015

00}20

0023

0030

00

a.E

TSac

tivi

ty(k

lO2

g~1

h~1

at203C

)EC

OF

ER

3K

J54

8.7

KJ5

28.

9K

J57

8.7

KJ5

35.

7K

J58

7.3

KJ5

68.

5K

J59

6.2

KJ6

07.

5

EC

OF

ER

5K

TB

8811

.1K

TB

8012

.8K

TB

802.

9K

TB

718.

5K

TB

617.

4K

TB12

615

.2K

TB

8510

.1K

TB

813.

4K

TB

74K

TB65

8.9

KT

B13

18.

6K

TB

123

12.1

5K

TB13

813

.65

KT

B75

KT

B10

34

KTB

125

8.75

KT

B12

012

.7K

TB10

57.

55K

TB

136

29K

TB14

615

.85

KT

B11

120

.85

KT

B15

57.

7K

TB

170

9.85

KT

B16

16.

2K

TB

171

8.9

KT

B13

21.

4K

TB15

45.

4

Mea

n8.

913

.49.

68.

97.

7C

I95

%4.

68.

24.

61.

71


b.O

xyge

nco

nsum

ptio

n(n

mol

eO

2cm

~1 d~

1 )EC

OF

ER

3K

J54

260

KJ5

225

0K

J57

220

KJ5

323

0K

J58

210

KJ5

619

0K

J59

220

KJ6

015

0

EC

OF

ER

8K

TB88

280

KTB

8228

0K

TB

8222

0K

TB

7121

0K

TB

6126

0K

TB

126

220

KT

B85

280

KT

B85

250

KT

B74

210

KT

B13

131

0K

TB

123

320

KT

B12

321

0K

TB10

321

0K

TB

125

300

KT

B12

034

0K

TB10

521

0K

TB

136

330

KT

B14

621

0K

TB11

120

0K

TB

155

320

KT

B17

020

0K

TB

161

260

KT

B17

117

0K

TB

132

230

KT

B15

422

0

Mea

n24

030

523

522

521

5C

I95

%30

2550

3050

gC

m~

2yr

~1

8.9

11.3

8.5

8.4

7.9


Fig. 9. (a) ETS activity (ll O2

g~1 l~1); (b) oxygen consumption (O2

nmoles cm~2 d~1) in sediments fromthe 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 andGermain, 1997). On the other hand, sedimentation of the suspended material on theshelf is restricted to a few mud-patches (Lesueur et al., 1989), whereas high apparentsedimentation rates were measured in the margin (1000}2300 m). The values dependon the element used for measurement: 2}4 mm yr~1 for 210Pb data (Radakovitch andHeussner, 1999) and 0.2}0.5 mm yr~1 for 14C data (Arnold, personal communica-tion).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 thisAquitanian margin an area of preferential C

03'accumulation. However, mineraliz-

ation processes have a direct in#uence on the buried C03'

budget as will be discussedhereafter.

As shown in Fig. 13a, the enrichment in C03'

of the Aquitanian margin is mainlyrelated to "ne grain-size sediments. The close relationship between the highest


Fig. 10. 14C `Navicula incertaa (diatoms) incubations: (a) budget of 14C fractions resulting from mineraliz-ation (14CO

2), 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.

C03'

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, lowestC

03'concentrations have been observed at the head of the Lacaze-Duthiers canyon

(Buscail and Gadel, 1991), where sediments are siltier (60%'40 lm) than elsewherein the canyon (10%'40 lm).


Fig. 11. Dissolved combined amino acids in overlying waters of a time series incubation (Atlantic deep-seasediment}water interface): (a) DCAA concentrations (lmole l~1); (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 foundon the Aquitanian margin at 600 m (Fig. 13b). This limit is the erosion-depositionboundary beneath which the proportion of C

03'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 controllinggrain-size distribution. On the Aquitanian margin, intense settling is induced both bythermo-haline seasonal fronts, observed on remote sensing images and located eitherat mid-shelf or near the shelf-break (Castaing et al., 1999; Froidefond et al., 1999) andby shelf nepheloid structures, directly in#uenced by internal tides and an along-slopecurrent (Palanques and Biscaye, 1992; Durrieu De Madron, 1994).


Fig. 12. Dissolved free amino acids in overlying waters of a time series incubation (Atlantic deep-seasediment}water interface): (a) DFAA concentrations (lmole l~1); (b) DFAA in mol%.

4.2. Specixcity of the Cap-Ferret Canyon system

The Cap-Ferret Canyon is a morphological anomaly in the Aquitanian continentalslope. Fine-grained material mainly settles in the canyon. But some clear di!erencesappear inside the canyon, in areas exposed to di!erent energy conditions, whichdirectly in#uence suspended matter settlement. In comparison to adjacent areas, thenorthern and southern branches were found to be characterized by coarser sediments,and lower C

03'contents, suggesting lower organic sedimentation. The vertical gradi-

ent of C03'

content was limited to the 0}1 mm layer of sediment, and sediment oxygenconsumption and ETS activities were the highest due to the probable supply oflabile organic matter. Therefore, the canyon branches can be considered to act as`channelsa, favoring the circulation of "nest particulate matter. Intense mineraliz-ation in the very sur"cial layer is assumed to prevent C

03'accumulation.

The inter#uve is exposed to sediment winnowing by dynamic agents/currents andinternal waves (Cremer et al., 1999): high C

03'and CaCO

3contents suggest lower


Fig. 13. Relationship between mean grain size (lm), depth and C03'

content (%) in sediments from theAquitanian margin.

continental #uxes and a higher in#uence of marine material than in the other parts ofthe canyon.

At the merging site (MS1), the deep water circulation is reduced (Durrieu et al.,1999). A signi"cant enrichment in C

03'contents was found associated to high accumu-

lation rates (Radakovitch and Heussner, 1999), which makes this area a preferentialdepocenter where the lability of the organic fraction (P#C#L) is among the highestof the whole canyon system.

The comparison of C03'

content and sedimentation rates between the MS1 and MS2sites indicates clearly decreased organic accumulation with increasing water depth.This observation is supported by the study of organic particulate #uxes, whichdecrease towards the open ocean (Radakovitch and Heussner, 1999).

Preferential organic enrichment or accumulation areas has already been observedon other margins: on the northeastern TamKwan and the northwestern Mediterraneanmargins (Lin et al., 1992; Buscail et al., 1997), it takes place at 1000 m water depth. Itappears deeper (1800}2300 m water depth) on the Aquitanian margin. More subtiledi!erences in organic matter settling appear when the morphological complexityincreases and the canyon system narrows. For example, the supply and accumulationof organic material is quite di!erent on the two sides of the main axis of theMediterranean Lacaze-Duthiers Canyon, regardless of their orientation in theLiguro-Provencal current (Buscail and Germain, 1997).

In conclusion, the hydrological conditions, morphological features, size of thecanyon system and its location in relation to continental sources have a direct impacton the distribution of organic sedimentation and the existence of preferential organicenrichment and/or accumulation zones.


4.3. Origin of the sedimentary organic matter in the Cap-Ferret Canyon

The particulate material that supplies the sediment}water interface has schemati-cally a double origin: (i) marine from primary production and subsequent transfersalong the trophic web; (ii) continental, due to advective transport of particles from theshelf 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: theGironde 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 bottomnepheloid layer, the processes being di!usive and occasional (Ruch et al., 1993;Castaing et al., 1999). This material contains 1.5% C

03', essentially refractory organic

matter (Lin and Etcheber, 1994). The circulation in the Bay of Biscay, characterized bya predominantly northward along-slope current, suggests that particles are suppliedby the Adour and spanish rivers system, whose 3.0% C

03'content is also refractory.

In order to evaluate the respective contribution of each component to the sedimen-tary organic fraction, particulate organic quality and #uxes in the water column haveto be taken into account. The results of sediment trap experiments exhibit a typicalmargin pattern (Heussner et al., 1999); i.e. #uxes increase with depth at sampling sitesand decrease o!shore (mass #uxes: 540 g m~2 yr~1 at MS1 and 170 g m~2 yr~1 atMS2, 2300 and 3000 m depth respectively). This pattern reveals the importance ofadvective transport.

Seasonal peaks of mass and carbon #uxes were noted even in near-bottomsediment traps: a winter peak (without carbon content increase), which maybe attributed to changes in the regime of lateral advection, and a summer peak(with carbon content increase), which would correspond to a supply more orless 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~2 d~1

(Laborde et al., 1999) on annual basis, which may reach 3}4 times this value duringbloom episodes. The mean contribution of primary production to POC #uxes in nearbottom sediment traps can be inferred from di!erent equations (Suess, 1980; Parsonset al., 1984b; Betzer et al., 1984). POC #uxes can be estimated at 2.8 (SD: 0.7)g C m~2 yr~1 (vs. 14.9 g C m~2 yr~1 of total POC) at site MS1 and 2.2 (SD: 0.6)g C m~2 yr~1 (vs. 5.3 g C m~2 yr~1 of total POC) at site MS2. During a bloom, theincrease in POC #uxes from the surface waters can be estimated at about 3.5 times theannual value. These #uxes would increase the C

03'content from 2.8 to 4% at MS1,

and from 3.35 to 6.5% at MS2. Such episodic POC enrichments were found in thematerial 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 organicmatter was found in the sediments from the Cap-Ferret Canyon. Several hypothesescan be proposed for explaining this observation: (i) marine organic matter is quicklymineralized because it is highly labile; (ii) the sampling of the upper 0}1 mm sedimentlayer is not precise enough to delineate seasonal variation (signi"cance of the thick-ness selected for sampling; discrepancy between the periodicity of carbon #uxes and


our sampling capacity); (iii) there is a benthic nepheloid layer whose resuspendedmaterial can dilute the fresh marine inputs.

If we consider that the 2.8 g C m~2 yr~1 marine contribution at MS1 near-bottomsediment trap is realistic, the organic continental fraction should be equal to12.1 g C m~2 yr~1, i.e. about 510}520 g of continental material for a 20}30 g totalmarine fraction. Hence, the percentage of C

03'in continental suspended matter would

be 2.35%. The contribution of the organic fraction due to the Gironde particles (1.5%C

03'content) and southern rivers (3% C

03'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

03'contents (2.35%) are never re#ected in 0}1 mm sediment layer, even if this

continental C03'

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) thecontinental organic material, supposed to be refractory, is actually partially decom-posed at the sediment}water interface (1 cm can be between a few months and 3}4years according to the datation methods selected); (iii) old sediments (benthicnepheloid layer, resuspension) dilute recent suspended matter input.

The results from the Mediterranean margin were quite di!erent (Fig. 14): a clearseasonal variability of the sedimentary organic matter quality was observed in theLacaze-Duthiers Canyon, corresponding to the variability of organic matter bottom#uxes (Buscail et al., 1990). The amount of C

03'increased progressively in the upper

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


1 cm deposit during autumn and winter. When comparing summer to the followingspring, an extra milligram of C

03'was found to be stored per gram of dry sediment

[this increase was calculated from 40 values obtained from four subsamplings perseasonal sampling (10)]. During the spring, organic matter was increasingly labile andthe content in hydrolyzable C

03'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 werewell correlated with C

03'#uxes measured for a near-bottom sediment trap. Fluxes

increased by a factor of 10 during this period (21}217 mg C03'

m~2 d~1).Such seasonal variations can be explained by the proximity of land, well-identi"ed

sources of material (succession of phytoplankton blooms and direct continentalsupply due to high river discharge), and water circulation along the slope (the LiguroProvencal Current).

The characteristics of the Cap-Ferret Canyon system are strictly inverse: largedistance of the canyon from the main continental sources, which prevents any directsupply 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 currentdirections and the extension of several nepheloid structures in the canyon area(Castaing et al., 1999; Durrieu De Madron et al., 1999), and possible sedimentresuspension 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 enhancedbiological 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

03'budget

involved in mineralization processes to display some typical reactions, to characterizethe particulate labile organic fraction that is involved preferentially, and to comparethe intensity of processes with those observed in other areas, whose bottom watertemperatures are quite di!erent.

Oxygen consumption values have been converted into carbon mineralizationestimates, using a respiratory coe$cient evaluated to 0.85 (Hargrave, 1973). Onaverage, 9 g C m~2 yr~1 was mineralized in the Cap-Ferret Canyon; the highest value11.3 g C m~2 yr~1 was found in the 1000}1500 water depth interval; MS1 and MS2sites were characterized by 8.4 and 7.9 g C m~2 yr~1, respectively.

The uncertainties that can a!ect each term of the calculation of oxygen consump-tion from porewater pro"les imply that our results should be interpreted with caution.Comparison of data on oxygen consumption by sediment communities, measuredwith in situ methods and with shipboard methods (Fig. 15) show that both methodsgive results of the same order of magnitude. Therefore, the mean value of oxygenconsumption can provide an estimation of carbon recycling (9 g C m~2 yr~1 onaverage).

Mineralization processes of organic matter on the Aquitanian margin is illustratedby DAA measurements in the overlying waters, which give information on thekinetics of degradation and on the various metabolic cycles, involving mainlybacteria (Henrichs and Farrington, 1979; Henrichs, 1980; Jorgensen et al., 1980; Lee


Fig. 15. Comparison of methods used to measure sediment community oxygen consumption (SCOC) invarious 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). Darksymbols: In situ benthic chambers (NE Atlantic Ocean: Jahnke et al., 1989; NW Atlantic Ocean: Smith andTeal, 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):

f the sediment reactivity to the input of labile planktonic organic matter is revealedby the high increase of DCAA contents during incubations, and proves that proteicsynthesis and excretion processes are very active;

f the increase of serine and glycine, abundant in diatom cell walls, and the increase ofglutamic acid, which can be excreted by algae, revealed the degradation of diatoms;

f tryptophane (decay product of AA metabolism by benthic fauna), taurine (anexcretion product, which witnesses metazoan activity), glutamic acid (dominant inthe intracell part of bacteria), ornithine (indicative of arginine degradation bybacteria) showed increasing concentrations, proving the general benthic faunaactivity;


f in contrast, respiratory processes decreased with time indicated by the decrease indissolved free alanine, c-aminobuyric acid and isoleucine, while leucine (evidence ofbacteria proteic synthesis) remain stable over 6 days of incubation.

In order to get more information on the sedimentary particulate organic matterinvolved in mineralization processes, we have evaluated the budget of sedimentarylabile C

03'assimilated to:

(i) the quantity of C03'

disappearing between the surface layer and the upper 5 cm ofsediment (level at which C

03'values are constant, corresponding to long-term

buried organic matter);(ii) the quantity of hydrolyzable C

03'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 C03'

were estimated on a volume representing a surface of 1 m2 anda sediment thickness of 5 cm. The concentrations of these contituents (per g drysediment) were converted into contents per surface unit:

X m~2"Concentration g~1](1!')o]104,

where ' is the porosity, and o the density of dry sediment (2.6), integrated over 5 cmdepth in sediment.

The labile C03'

contents measured by the C03'

gradients, hydrolysable C03'

andP#C#L were estimated to be 57, 46 and 5 g C m~2, respectively.

Hydrolysable C03'

, which corresponds to nearly 80% of the disappeared organiccontent represents well the mineralized labile organic fraction. In contrast, P#C#Lcorresponds to only 10% and cannot be assimilated to the total labile organiccontent. These parameters are relevant for studying fresh organic matter (phytoplank-ton, bacteria, etc.), but are limited for studying detrital organic material. Analyticalmethods 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 theliterature, where P and C concentrations are in the same order of magnitude (Roman-kevich, 1984; Buscail and Germain, 1997). This is mainly due to the strength of theextraction procedure used (H

2O for C and 6 N HCl for P).

We compared incubation experiments with labelled 14C diatoms on the Atlanticand Mediterranean margins (Buscail, 1992; Buscail and Guidi-Guilvard, 1993). Themain di!erence (Fig. 16a) is the lower CO

2and metabolites proportions released in

the overlying water in the Cap-Ferret Canyon (1 and 10%) than in the RhodanianCanyon (6 and 30%). Metabolic processes responsible for dissolved organic matterrelease are consequently less developed on the Atlantic side, and the proportionof 14C integrated in the Cap-Ferret Canyon sediment is higher than that in theRhodanian Canyon.

The DOC content in overlying water (Fig. 16b) indicates a rapid response of thesediment}water interface on the Mediterranean margin (4 h), a mineralization of themetabolites after a day, and a continuous DOC emission during the following days. In


Fig. 16. Comparison of 14C `Navicula incertaa (diatoms) incubations on the Atlantic and Mediterraneanmargins.

the Cap-Ferret Canyon, the response to the organic input is delayed (24 h), and stableDOC concentrations suggest that the release processes do not persist.

The mineralization activity seems less on the Aquitanian margin than on theMediterranean margin. This observation can be related to the fact that the ratiobetween C

03'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 theCap-Ferret Canyon and 4 on the Mediterranean margin. This lower degradationactivity at the Atlantic sediment}water interface is assumed to be related to the low


bottom water temperature (33C), compared to 133C in the Mediterranean sea, and toa less abundant benthic biomass: 104 viable bacteria (CFU ml) and 600 ind. 10 cm~2

(meiofauna) in the Atlantic Ocean (DINET, personnal communication) comparedto 105 viable bacteria and 800 ind. 10 cm~2 (meiofauna) in the Mediterranean sea(De Bovee et al., 1990).

4.5. A tentative Corg

budget in the Cap-Ferret Canyon

An C03'

budget in the Cap-Ferret Canyon is proposed, which is based upon datafrom the average annual vertical particulate #uxes, sedimentation rates, andC

03'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 totalannual mass #uxes of 540 and 170 g m~2 yr~1 for mooring sites 1 and 2, respectively,(Heussner et al., 1999). Near-bottom trap 210Pb #uxes were compared to the #uxestheorically required for supporting the excess 210Pb inventory in sediment(Radakovitch and Heussner, 1999). We concluded that 80 and 85% of the measurednear-bottom #uxes reach the surface sediment at the sites MS1 and MS2. Conse-quently, 14.9 and 5.7 g C m~2 yr~1 have been incorporated into the sediment.

These data are of the same order of magnitude as C03'

consumption calculated bystudying the oxygen consumption of sedimentary community in the area (8.4 and7.9 g C m~2 yr~1 at MS1 and MS2 sites). Most of the C

03'input to the sediment is

considered as mineralized at the MS1 site, whereas the supply at MS2 site is notsu$cient for covering mineralization processes.

Sedimentation rates determined by the 210Pb method at these two sites are howeververy high: about 0.25 and 0.08 cm yr~1 (Radakovitch and Heussner, 1999). Suchvalues correspond to C

03'accumulation of 23.3 and 6.9 g C m~2 yr~1, which are

higher than the #uxes estimated by sequential sediment trap experiments. With datafrom 14C data (0.5 and 0.2 mm yr~1 at MS1 and MS2 sites), the C

03'accumulation is

calculated to 4.5 and 1.7 g C m~2 yr~1.Several explanations can be proposed to elucidate the discrepancy between the

vertical C03'#uxes and consumption-accumulation processes obtained with the 210Pb

method:(i) an over estimation of sedimentation rates by the 210Pb method, which can be

expected from the high bioturbation observed in this area (Gerino et al., 1999);

(ii) and/or an under-estimation of C03'#uxes given by the sediment trap experiments

due to:f a bad recovery rate of particulate matter #uxes in the water column by

sediment traps; however, previous sediment trap experiments were alwayssuccessful (Heussner et al., 1990);

f major physical processes of sedimentary transport and resuspension in thenear-bottom layer; the deepest sediment trap being situated above thenepheloid layer, this phenomenon would not be recorded in the traps. Suchbottom transport is described by Radakovitch and Heussner (1999), butDurrieu De Madron et al. (1999) suggest that deep currents are not strongenough to generate signi"cant resuspension;


f very low input of terrestrial material due to a major drought during theexperiment period (discharge of the Garonne#Dordogne Rivers: 450 m3 s~1

instead of the usual 1000 m3 s~1). However, even normal, continental input isnot su$cient to balance the carbon budget;

(iii) and/or a time-scale discrepancy between sediment trap measurements (#uxesmeasured in a short time interval) and sedimentation rate data (based on severaltens to a hundred years of accumulation).

If we select the sedimentation rates given by the 14C method, the C03'

budget is nearlybalanced at MS1 site: 14.9K8.4#4.5. However, the inputs are in su$cient at MS2site: 5.7(7.9#1.7.

The C03'

budget in the Cap Ferret Canyon has still to be re"ned: C03'

supplies to thebottom may have been unusually low as a consequence of very dry period associatedwith low continental supply; a possible contribution of material brought by bottomtransport under the sediment trap level has not been estimated; our present know-ledge of C

03'accumulation is not satisfactory since the estimation based on 210Pb is

four}"ve times higher than that by 14C 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 ofsedimentary organic matter.(1) The combination of high sedimentation rates and C

03'contents in its sediments

indicates a preferred C03'

deposition on the Aquitanian margin. Fine-grainedsedimentation in this area (presence of a mud-line at 600 m) is related to hy-drodynamic processes (thermo-haline seasonal fronts, shelf nepheloid structurescontrolled by internal tides and along-slope currents).

(2) The morphological heterogeneity of the Cap-Ferret Canyon system inducesdi!erences in organic matter sedimentation: the northern and southern branchesof the canyon act as circulation channels (restricted organic sedimentation andintense mineralization processes). In contrast, the MS1 site is an C

03'accumula-

tion area because of weak currents and "ne sediments. The comparison withMediterranean canyons, where organic matter sedimentation is quite heterogen-eous locally, reveals that size of the canyon, its distance from the continent, andhydrological circulation modify organic sedimentation characteristics.

(3) The origin of the organic matter has not been proved. The high C03'

contentsobserved in near-bottom sediment trap material are probably due more toa marine contribution (labile organic matter) than to a possible Adour-spanishrivers supply (refractory organic matter).


The fact that C03'

contents do not vary seasonally in the bottom sediment andvalues are signi"cantly lower than in the sediment trap suggests that:f marine contribution is dominant (which is in contrast with our calculated

estimation using equations) and mineralized at the sediment}water interface;f 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 thatmineralization processes are active: signi"cant vertical decrease of C

03'contents in

sediment, changes in DOC contents and labile organic fraction (amino acidcomponents). The average oxygen consumption is evaluated as 9 g C m~2 yr~1.Mineralization appears to be less developed than in the Mediterranenan Sea, dueto a di!erence in water temperature.

(5) A precise budget of C03'

has not been yet estimated. During the experiment, theorganic carbon supply to the bottom was likely lower than for a normal year (14.9and 5.7 g C m~2 yr~1 at MS1 and MS 2 sites, respectively). Sedimentation ratesare too uncertain for proposing a budget of organic carbon accumulation (23.3 or4.5 g C m~2 yr~1 at MS1 site; 6.9 or 1.7 g C m~2 yr~1 at MS2 site) according tothe 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 theR.V. Suroit, Noroit and Cote d1Aquitaine for their helpful technical assistance andB. Deniaux for the help in preparing the manuscript.

References

Anderson, R.F., Bopp, R.F., Buesseler, K.E., Biscaye, P.E., 1988. Mixing of particles and organic constitu-ents in sediments from the continental shelf and slope o! Cape Cod: SEEP-I results. Continental ShelfResearch 8, 925}946.

Anderson, R.F., Rowe, G.T., Kemp, P.F., Trumbore, S., Biscaye, P.E., 1994. Carbon budget for themid-slope depocenter of the Middle Atlantic Bight. Deep-Sea Research II 41, 669}703.

Archer, D., Devol, A., 1992. Benthic oxygen #uxes on the Washington shelf and slope: a comparison of insitu microelectrodes and chamber #ux measurements. Limnology and Oceanography 37, 614}629.

Bender, M.L., Heggie, D.T., 1984. Fate of organic carbon reaching the deep sea #oor: a status report.Geochimica et Cosmochimica Acta 48, 977}986.

Bender, M.L., Jahnke, R., Weiss, R., Martin, W., Heggie, D.T., Orchardo, J., Sowers, T., 1989. Organiccarbon oxidation and benthic nitrogen and silica dynamics in San Clemente Basin, a continentalborderland site. Geochimica et Cosmochimica Acta 53, 685}697.

Berelson, W.M., Hammond, D.E., Johnson, K.S., 1987. Benthic #uxes and the cycling of biogenicsilica and carbon in two Southern California bordeland basins. Geochimica et Cosmochimica Acta 51,1345}1363.

Berner, R.A., 1980. In: Early diagenesis: A theoretical approach. Princeton University Press, pp. 273.


Betzer, P.R., Showers, W.J., Laws, E.A., Winn, C.D., Di Tullio, G.R., Kroopnick, P.M., 1984. Primaryproduction and particulate #uxes on a transect of the Equator at 153W in the Paci"c Ocean. Deep-SeaResearch 31, 1}11.

Biscaye, P.E., Anderson, R.F., Deck, B.L., 1988. Fluxes of particles and constituents to the eastern UnitedStates continental slope and rise: SEEP-I. Continental Shelf Research 8, 855}904.

Biscaye, P.E., Flagg, C.N., Falkowski, P.G., 1994. The Shelf Edge Exchange Processes experiment, SEEP-II:an introduction to hypotheses, results and conclusions. Deep-Sea Research Part II 41, 231}252.

Bligh, E.G., Dyer, W.J., 1959. A rapid method of total lipid extraction and puri"cation. Canadian Journal ofBiochemistry and Physiology 37, 911}917.

Broecker, W.S., Peng, T.H., 1974. Gas exchange rates between air and sea. Tellus 26, 21}35.Burdige, D.J., Martens, C.S., 1990. Biogeochemical cycling in an organic-rich coastal marine basin: 11. The

sedimentary cycling of dissolved, free amino acids. Geochimica et Cosmochimica Acta 54, 3033}3052.Buscail, R., 1986. Biogeochemical processes of incorporation, transformation and migration of organic

matter at the marine water-sediment interface: Simulation by 14C labelled compounds. OrganicGeochemistry 10, 1091}1097.

Buscail, R., 1992. DeH gradation de matieH re organique dissoute et particulaire l'interface eau-seH diment sur lesmarges du Golfe du Lion et du Golfe de Gascogne. Rapport Commission Internationale Mer MeH diter-raneH e 33, 379.

Buscail, R., Ambatsian, P., Monaco, A., Bernat, M., 1997. 210Pb, manganese and carbon: indicators offocusing processes on the Northwestern Mediterranean continental margin. Marine Geology 137,271}286.

Buscail, R., Gadel, F., 1991. Transfer and biogeochemistry of organic matter at the Sediment}waterinterface on the Northwestern Mediterranean Margin. In: Berthelin, J. (Ed.), Diversity of EnvironmentalBiogeochemistry. Developments in Geochemistry, 6. Elsevier Science Publishers, Amsterdam,pp. 113}130.

Buscail, R., Germain, C., 1997. Present-day organic matter sedimentation on the Northwestern Mediterra-nean Margin: importance of o!-shelf export. Limnology and Oceanography 42, 217}229.

Buscail, R., Guidi-Guilvard, L., 1993. Organic input and transformation at the water-sediment interface ofthe Northwestern mediterranean slope (Gulf of Lions). Annales Institut Oceanographique 69, 147}153.

Buscail, R., Pocklington, R., Daumas, R., Guidi, L., 1990. Flux and budget of organic matter in the benthicboundary layer over the Northwestern Mediterranean Margin. Continental Shelf Research 10,1089}1122.

Buscail, R., Pocklington, R., Germain, C., 1995. Seasonal variability of the organic matter in a sedimentarycoastal environment: sources, degradation and accumulation (continental shelf of the Gulf ofLions}northwestern Mediterranean Sea). Continental Shelf Research 15, 843}869.

Cammen, L., 1982. E!ect of particle size on organic content and microbial abundance within four marinesediments. Marine Ecology Progress Series 9, 273}280.

Castaing, P., Froidefond, J.M., Lazure, P., Weber, O., Prud'homme, R., Jouanneau, J.M., 1999. Relationshipbetween hydrology and seasonal distribution of suspended sediments on the continental shelf of the Bayof Biscay. Deep-Sea Research II 46, 1979}2001.

Castaing, P., Jouanneau, J.M., 1987. Les apports seH dimentaires actuels d'origine continentale aux oceH ans.Bulletin de l'Institut de GeH ologie du Bassin d'Aquitaine 41, 53}65.

Cauwet, G., 1984. Automatic determination of dissolved org. C in sea water in the sub-ppm range. MarineChemistry 14, 297}306.

Cauwet, G., Gadel, F., De Souza Sierra, M.M., Donard, O., Ewald, M., 1990. Contribution of the Rho( neRiver to org. C inputs to the Northwestern Mediterranean Sea. Continental Shelf Research 10,1025}1037.

Christensen, J.P., Packard, T.T., 1977. Sediment metabolism from the Northwestern African upwellingsystem. Deep-Sea Research 24, 331}343.

Cole, J.J., Honjo, S., Erez, J., 1987. Benthic decomposition of organic matter at a deep-water site in thePanama Basin. Nature 327, 703}704.

Cremer, M., Weber, O., Jouanneau, J.M., 1999. Sedimentology of box cores from the Cap-Ferret Canyonarea (Bay of Biscay). Deep-Sea Research II 46, 2221}2227.


Cunin, R., Glansdor!, N., Pierard, A., Stalon, V., 1986. Biosynthesis and metabolism of arginine in bacteria.Microbiological Reviews 50, 314}352.

Dagnelie, P., 1973. TheH ories et MeH thodes statistiques. Presses Agronomiques de Gemblaux, Gemblaux,tome 2, 378pp.

De Bovee, F., Guidi, L., Soyer, J., 1990. Quantitative distribution of deep-sea meiobenthos in theNorthwestern Mediterranean (Gulf of Lions). Continental Shelf Research 10, 1123}1145.

Delmas, D., Frikka, M.G., Linley, E.A.S., 1990. Dissolved primary amine measurement by Flow InjectionAnalysis with o-phthaldialdehyde: comparison with High-Performance Chromatography. MarineChemistry 29, 145}154.

Dubois, M., Gilles, K.A., Hamilton, J.K., Rebers, P.A., Smith, F., 1956. Colorimetric method for determina-tion of sugars and related substances. Analytical Chemistry 28, 350}356.

Duchaufour, H., Jocteur-monrozier, K., Pelet, R., 1984. Optical and geochemical studies of granulometriefractions from recent marine sediments. Organic Geochemisty 6, 305}315.

Durrieu De Madron, X., 1994. Hydrography and nepheloid structures in the Grand-Rho( ne canyon.Continental Shelf Research 14, 457}477.

Durrieu De Madron, X., Castaing, P., Ny!eler, F., Courp, T., 1999. Slope transport of suspended particulatematter on the Aquitanian margin of the Bay of Biscay. Deep-Sea Research II 46, 2003}2027.

Emerson, S., Fischer, K., Reimers, C., Heggie, D., 1985. Org. C dynamics and preservation in deep-seasediments. Deep-Sea Research 32, 1}21.

Emerson, S., Hedges, J.J., 1988. Processes controling the org C content of open ocean sediments.Paleoceanography 3, 621}634.

Froidefond, J.M., Castaing, P., Prud'homme, R., 1999. Monitoring suspended particulate matter #uxeswith the AVHRR/NOAA-II satellite. Application to the Bay of Biscay. Deep-Sea Research II 46,2029}2055.

Gerino, M., Stora, G., Weber, O., 1999. Evidence of bioturbation in the Cap-Ferret Canyon in deepnortheastern Atlantic. Deep-Sea Research II 46, 2289}2307.

Hargrave, B.T., 1973. Coupling carbon #ow through some pelagic and benthic communities. Journal ofFisheries Research Board of Canada 30, 1317}1326.

Helder, W., 1989. Early diagenesis and sediment-water exchange in the Savu Basin (Eastern Indonesia).Netherlands Journal of Sea Research 24, 555}572.

Helder, W., Bakker, J.F., 1985. Shipboard comparison of micro and mini electrodes for measuring oxygendistribution in marine sediments. Limnology and Oceanography 30, 1106}1109.

Henrichs, S.M., 1980. Biogeochemistry in dissolved free amino acids in marine sediments. Ph. D. Thesis,Woods Hole Oceanography Institute. Mass. Technol. Joint Program, 253pp.

Henrichs, S.M., Farrington, J.W., 1979. Amino acids in interstitial waters of marine sediments. Nature 279,319}322.

Heussner, S., Durrieu De Madron, X., Radakovitch, O., Beaufort, L. Biscaye, P.E., Carbonne, J., Delsaut,N., Etcheber H., 1999. Spatial and temporal patterns of downward particulate #uxes on the continentalslope of the Bay of Biscay (northeastern Atlantic). Deep-Sea Research II 46, 2101}2146.

Heussner, S., Ratti, C., Carbonne, J., 1990. The PPS 3 time-series sediment trap and the trap sampleprocessing techniques used during the ECOMARGE experiment. Continental Shelf Research 10,943}958.

Hinga, K.R., Sieburth, J.Mc N., Heath, G.R., 1979. The supply and use of organic material at the deep-sea#oor. Journal of Marine Research 37, 557}579.

Jahnke, R.A., Emerson, S.R., Reimers, C.E., Schu!ert, J., Ruttenberg, K., Archer, D., 1989. Benthic recycling ofbiogenic debris in the Eastern Tropical Atlantic Ocean. Geochimica Cosmochimica Acta 53,2947}2960.

Jahnke, R.A., Reimers, C.E., Craven, D.B., 1990. Intensi"cation of recycling of organic matter at the sea#oor near ocean margins. Nature 348, 50}54.

Jorgensen, B.B., Revsbech, N.P., 1985. Di!usive boundary layers and oxygen uptake of sediments anddetritus. Limnology and Oceanography 30, 111}122.

Jorgensen, N.O.G., Mopper, K., Lindroth, P., 1980. Occurence, origin and assimilation of free amino acidsin an estuarine environment. Ophelia suppl. 1, 179}192.


Jouanneau, J.M., Weber, O., Cremer, M., Castaing P., 1999. Fine-grained sediment budget on thecontinental margin of the Bay of Biscay. Deep-Sea Research II 46, 2205}2220.

Khripouno!, A., Rowe, G.T., 1985. Les apports organiques et leur transformation en milieu abyssala l'interface eau-seH diment dans l'OceH an Atlantique tropical. Oceanologica Acta 8, 293}301.

Laane, R., Etcheber, H., Relexans, J.C., 1987. The nutritive value of particulate organic matter in estuariesand its ecological implication for macrobenthos. Mitteilungen aus dem Geologisch-PalaK ontologischeInstitut Universitt Hamburg,. SCOPE UNEP Sonderband 64, 71}91.

Laborde, P., Urratia, J., Valencia, V., 1999. Seasonal variability of primary production in the Cap-Ferret Canyon area (Bay of Biscay) during the ECOFER cruises. Deep-Sea Research II 46,2057}2079.

Lee, C., Cronin, C., 1984. Particulate amino-acids in the sea: e!ects of primary productivity and biologicaldecomposition. Journal Marine Research 42, 1075}1097.

Lesueur, P., Weber, O., Marambat, L., Tastet, J.P., Jouanneau, J.M., Turon, J.L., 1989. Datation d'unevasiere de plate-forme atlantique au deH boucheH d'un estuaire. La vasiere a l'Ouest de la Gironde (France)est d'a( ge historique (Vieme siecle a nos jours). Comptes Rendus de l'AcadeHmie des Sciences Paris. II 308,935}940.

Lin, R.G., Etcheber, H., 1994. The degradability of particulate organic matter in the Gironde Estuary,France. Chinese Journal of Oceanology and Limnology 12, 106}113.

Lin, S., Liu, K.K., Chen, M.P., Chen, P., Chang, F.Y., 1992. Distribution of org. C in the KEEP areacontinental margin sediments. TAO 3, 365}378.

Mayer, L.M., Macko, S.A., Cammen, L., 1988. Provenance, concentration and nature of sedimentaryorganic nitrogen in the Gulf of Maine. Marine Chemistry 25, 291}304.

Monaco, A., Biscaye, P., Soyer, J., Pocklington, R., Heussner, S., 1990a. Particle #uxes and ecosystemresponse on a continental margin: the 1985}1998 Mediterranean ECOMARGE experiment. Continen-tal Shelf Research 10, 809}839.

Monaco, A., Courp, T., Heussner, S., Carbonne, J., Fowler, S.W., Deniaux, B., 1990b. Seasonality andcomposition of particulate #uxes during ECOMARGE I. Western Gulf of Lions. Continental ShelfResearch 10, 959}987.

Montreuil, J., Spik, G., 1963. MeH thodes colorimeH triques de dosage des glucides totaux: meH thode au pheH nolsulfurique. Monographie du Laboratoire de chimie Biologique, Lille (France), 5pp.

Mopper, K., Lindroth, P., 1982. Dial and depth variations in dissolved free amino acids and ammoniumin the Baltic Sea determined by shipboard HPLC analysis. Limnology and Oceanography 27,336}347.

Owens, T.G., King, F., 1975. The measurement of respiratory electron transport system activity in marinezooplankton. Marine Biology 30, 27}36.

Packard, T.T., 1971. The measurement of respiratory electron transport activity in marine phytoplankton.Journal of Marine Research 29, 235}244.

Palanques, A., Biscaye, P.E., 1992. Patterns and controls of the suspended matter distribution over the shelfend the upper slope south of New-England. Continental Shelf Research 12, 577}600.

Pamatmat, M.M., 1971. Oxygen consumption by the seabed IV. Shipboard and laboratory experiments.Limnology and Oceanography 16, 536}550.

Parsons, T.R., Maita, Y., Lalli, C.M., 1984a. A Manual of Chemical and Biological Methods for SeawaterAnalysis.. Pergamon Press, Oxford, pp. 173.

Parsons, T.R., Takahashi, M., Hargrave, B., 1984b. Biological Oceanographic Processes. 3rd edition.Pergamon Press, Oxford, pp. 330.

Premuzic, E.T., Benkovitz, C.M., Ga!rey, J.S., Walsh, J.J., 1982. The nature and distribution of organicmatter in the surface sediments of world oceans and seas. Organic Geochemistry 4, 63}77.

Radakovitch, O., Heussner, S., 1999. Fluxes and budget of 210Pb on the continental margin of the Bay ofBiscay (northeastern Atlantic). Deep-Sea Research II 46, 2175}2203.

Reimers, C.E., 1987. An in situ micropro"ling instrument for measuring interfacial pore water gradients;methods and oxygen pro"les from the North Paci"c Ocean. Deep-Sea Research 34, 2019}2035.

Reimers, C.E., Fischer, K.M., Merewether Jr., R., Smith, K.L., Jahnke, R.A., 1986. Oxygen micropro"lesmeasured in situ in deep ocean sediments. Nature 320, 741}744.


Reimers, C.E., Kalhorn, S., Emerson, S.R., Nealson, K., 1984. Oxygen consumption rates in pelagicsediments from the Central Paci"c: First estimates from microelectrodes pro"les. Geochimica Cosmo-chimica Acta 48, 903}910.

Reimers Jr., C.E., Smith, K.L., 1986. Reconciling measured and predicted #uxes of oxygen across the deepsea sediment-water interface. Limnology and Oceanography 31, 305}318.

Relexans, J.C., 1996. Measurement of the respiratory electron transport system (ETS) activity in marinesediments: state of the art and interpretation. I. Methodology and review of litterature data. MarineEcology Progress Series 136, 277}287.

Relexans, J.C., Deming, J., Dinet, A., Gaillard, J.F., Sibuet, M., 1996. Sedimentary organic matter andmicro-meiobenthos with relation to trophic conditions in the tropical northeast Atlantic. Deep-SeaResearch I 43, 1343}1368.

Relexans, J.C., Etcheber, H., Castel, J., Escaravage, V., Auby, I., 1992a. Response of biota to sedimentary organicmatter quality of the West Gironde mudpatch, Bay of Biscay (France). Oceanologica Acta 15, 639}649.

Relexans, J.C., Lin, R.G., Castel, J., Etcheber, H., Laborde, P., 1992b. Benthic respiratory potential withrelation to sedimentary carbon quality in seagrass beds and oyster parks in the tidal #ats of ArcachonBay. France. Estuarine, Coastal and Shelf Science 34, 157}170.

Revsbech, N.P., Jorgensen, B.B., 1986. Microelectrodes: their use in microbial ecology. In: Advances inMicrobial Ecology, vol. 9. Plenum Press, New York, pp. 293}352.

Revsbech, N.P., Jorgensen, B.B., Blackburn, T.H., 1980. Oxygen in the sea bottom measured witha microelectrode. Science 207, 1355}1356.

Romankevich, E.A., 1984. In: Geochemistry of Organic Matter in the Ocean. Springer, Berlin, pp. 334.Rowe, G.T., Boland, G.S., Phoel, W.C., Anderson, R.F., Biscaye, P.E., 1994. Deep-sea #oor respiration as an

indication of lateral input of biogenic detritus from continental margins. Deep-Sea Research Part II 41,657}668.

Rowe, G.T., Sibuet, M., Deming, J., Khripouno!, A., Tietjen, J., Macko, S., Theroux, R., 1991. Totalsediment biomass and preliminary estimates of organic carbon residence time in deep-sea benthos.Marine Ecology Progress Series 79, 99}114.

Rowe, G.T., Smith, S., Falkowski, P., Whitledge, T., Theroux, R., Phoel, W., Ducklow, H., 1986. Docontinental shelves export organic matter? Nature 324, 559}561.

Rowe, G.T., Theroux, R., Phoel, W., Quinby, H., Wilke, R., Koschoreck, D., Whitledge, T.E., Falkowski,P.G., Fray, C., 1988. Benthic carbon budgets for the continental shelf. South of New England.Continental Shelf Research 8, 511}527.

Ruch, P., Mirmand, M., Jouanneau, J.M., Latouche, C., 1993. Sediment budget and transfer of suspendedsediment from the Gironde Estuary to Cap-Ferret canyon. Marine Geology 111, 109}119.

Silverberg, N., Bakker, J., Edenborn, H.M., Sundby, B., 1987. Oxygen pro"les and org. C #uxes inLaurentian Trough sediments. Netherlands Journal of Sea Research 21, 95}105.

Simon, M., Azam, F., 1989. Protein content and protein synthesis rates of planktonic marine bacteria.Marine Ecology Progress Series 51, 201}213.

Singer, J.K., Anderson, J.B., Ledbetter, M.T., Mc Cave, I.N., Jones, K.P.N., Wright, R., 1988. An assessmentof analytical techniques for the size analysis of "ne-grained sediments. Journal of Sedimentary Petrol-ogy 58, 534}543.

Smith Jr., K.L., 1974. Oxygen demands of San Diego Trough sediments: an in situ study. Limnology andOceanography 19, 939}945.

Smith Jr., K.L., 1978. Benthic community respiration in the N. W. Atlantic Ocean in situ measurementsfrom 40 to 5200 m. Marine Biology 47, 337}347.

Smith Jr., K.L., Cli!ord, C.H., 1976. A free vehicle for measuring benthic community metabolism.Limnology and Oceanography 21, 164}170.

Smith Jr., K.L., Hinga, K.R., 1983. Sediment community respiration in the deep sea. In: Rowe, G.T. (Ed.),The Sea, Vol. 8, Deep-Sea Biology. Wiley-Interscience Publisher, New-York, pp. 331}370.

Smith Jr., K.L., Teal, J.M., 1973. Deep-Sea benthic community respiration: an in situ study at 1850 meters.Science 179, 282}283.

Smith Jr., K.L., White, G.A., Laver, M.B., 1979. Oxygen uptake and nutrient exchange of sedimentsmeasured in situ using a free vehicle grab respirometer. Deep-Sea Research 26, 337}346.


Smith Jr., K.L., White, G.A., Laver, M.B., Haugness, J.A., 1978. Nutrient exchange and oxygen consump-tion by deep-sea benthic communities: preliminary in situ measurements. Limnology and Oceanogra-phy 23, 997}1005.

Stanley, D.J., Addy, S.K., Behrens, E.W., 1983. The mudline: variability of its position relative to shelfbreak.In: Stanley, D.J., Moore, G.T. (Eds.), The Shelfbreak: critical interface on continental margin. SocietyEcology Paleontology Mineralogy, 33. Special Publication, pp. 279}298.

Stanley, S.O., Boto, K.G., Alongi, D.M., Gillan, F.T., 1987. Composition and bacterial utilization of freeamino acids in tropical mangrove sediments. Marine Chemistry 22, 13}30.

Suess, E., 1980. Particulate organic carbon #ux in the oceans - Surface productivity and oxygen utilization.Nature 288, 260}263.

Suzuki, Y., Tanoue, E., 1991. Dissolved organic Carbon enigma: implications for ocean margins. In:Mantoura, R.F.C., Martin, J.M., Wollast, R. (Eds.), Ocean Margin Processes in Global Change. Wiley,New York, pp. 197}209.

Ullman, W.J., Aller, R.C., 1982. Di!usion coe$cients in nearshore marine sediments. Limnology andOceanography 27, 552}556.

Venkatesan, M.I., Steinberg, S., Kaplan, I.R., 1988. Organic geochemical characterization of sediments fromthe continental shelf south of New-England as an indicator of shelf edge exchange. Continental ShelfResearch 8, 905}924.

Walsh, J., 1991. Importance of continental margins in the marine biogeochemical cycling of carbon andnitrogen. Nature 350, 53}55.

Wiebe, P.H., Boyd, S.H., Winget, C., 1976. Particulate matter sinking to the deep-sea #oor at 2000 m in theTongue of the ocean, Bahamas, with a description of a new sedimentation trap. Journal of MarineResearch 34, 341}354.

Wollast, R., 1990. The coastal org. C cycle: #uxes, sources and sinks. In: Mantoura, R.F.C., Martin, J.M.,Wollast, R. (Eds.), Ocean Margin Processes in Global Change. Wiley, New York, pp. 365}381.


Documents

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