Download - Release of dissolved organic carbon (DOC) from sediments of the N.W. European Continental Margin (Goban Spur) and its significance for benthic carbon cycling

Progress in Oceanography 42 (1998) 127–144

Release of dissolved organic carbon (DOC)from sediments of the N.W. European

Continental Margin (Goban Spur) and itssignificance for benthic carbon cycling

S. Otto, W. Balzer*

FB2-Marine Chemistry, University of Bremen, PF 330440, 28334 Bremen, Germany

Abstract

Pore water samples from N.W. European Continental Margin sediments (49°–48°N; 16°–10°W) were analyzed for dissolved organic carbon (DOC) using a high-temperature-combus-tion method. Two transects across the margin were investigated, a gentle undisturbed slope(Goban Spur: 670–4800 m) and the centre of a nearby precipitous canyon (Whittard Canyon:180–3680 m). Concentrations of pore water DOC were typically an order of magnitude greaterthan those from the overlying water. Therefore, the sediments appear to act as a DOC sourceto the bottom water. Conservative estimates (ignoring possible bioirrigation) of the DOC-fluxes from the sediments gave daily fluxes of 0.09–0.15 mmol m−2 d−1 for the Goban Spursediments and 0.05–0.16 mmol m−2 d−1 at the canyon transect. These relatively low variationsin DOC concentrations and fluxes under widely differing environmental conditions suggest thatproduction and consumption of labile DOC components proceed at similar rates irrespective ofwhat the overall benthic activity is. To the total benthic degradation rate of organic carbonthe DOC-fluxes contribute 2–32% (median 11%), a part that is missing when the carbon degra-dation rate is solely based on determinations of the oxygen consumption rate or on pore watermodeling of the reaction. Thus, DOC-effluxes are a significant component of benthic carbonbudgets in continental margin sediments. 1998 Published by Elsevier Science Ltd. Allrights reserved.

* Corresponding author.

0079-6611/98/$ - see front matter 1998 Elsevier Science Ltd. All rights reserved.PII: S0079 -6611(98)00031-7

128 S. Otto, W. Balzer /Progress in Oceanography 42 (1998) 127–144

1. Introduction

Dissolved organic carbon in the ocean is one of the largest pools of organic matterin the world (Toggweiler, 1988, 1992; Hedges, 1987). It consists of a number ofdifferent components including aminoacids, peptides, nucleotides, carbohydrates, lip-ids, aromatic and non-aromatic hydrocarbons and high molecular humic substances(e.g. Lee & Wakeham, 1988, 1992; Lee & Henrichs, 1993; Williams & Druffel,1987; and literature cited therein). DOC in the water column is thought to consistof reactive and refractive portions: while the refractive fraction is thought to beubiquitous in the water column, the reactive portion of DOC is restricted to the upper500–1000 m (Williams & Druffel, 1987; Druffel, Williams, Bauer, & Ertel, 1992;Bauer, Williams, & Druffel, 1992; Carlson, Ducklow, & Michaels, 1994; Carlson &Ducklow, 1995; Hansell, Nicholas, & Gundersen, 1995).

The role of dissolved organic matter during early diagenetic processes is poorlyunderstood. Until recently there were only a few investigations of DOC in marinepore waters (Krom & Sholkovitz, 1977; Orem & Gaudette, 1984; Orem, Hatcher,Spiker, Szeverenyi, & Maciel, 1986; Henrichs & Farrington, 1984; Heggie, Maris,Hudson, Dymond, Beach, & Cullen, 1987). One reason could have been the relativelygreat volume of pore water needed for DOC determinations when using the tra-ditional wet chemical oxidation (WCO) method (e.g. Menzel & Vaccaro, 1964).After the establishment of the high-temperature-combustion method (HTC) for DOC-analysis (Sugimura & Suzuki, 1988) and the renewed interest in DOC as a majorcarbon pool, some new investigations of DOC in interstitial waters were publishedmost of which, however, deal with shallow water sediments (Burdige, Alperin,Homstead, & Martens, 1992; Alperin, Albert, & Martens, 1994; Burdige &Homstead, 1994; Martens, Haddad, & Chanton, 1992; Skoog, Hall, Hulth, Paxe´us,van der Loeff, & Westerlund, 1996). The oceanic depth range exceeding 100 m isonly covered by studies of Henrichs & Farrington (1984), Heggie et al. (1987),Martin & McCorkle (1993) and Hulth, Tengberg, Lande´n, & Hall (1997). The latterstudy included also flux determinations obtained during laboratory incubations ofdeep sea cores.

All these studies show that the concentrations of DOC in deeper pore waters areat least an order of magnitude higher than those of ocean waters (Burdige et al.,1992; Martin & McCorkle, 1993), thus driving a considerable diffusive flux of DOCto the overlying water. This loss of DOC from the sediment represents a missingfraction of the total particulate organic carbon (POC) rain rate to the sea floor whenbalanced only against organic carbon degradation and burial. DOC with an oxidationnumber close to zero, that escapes to the deep sea before being oxidized concomitantwith oxidant consumption, is not accounted for in the determination of the rate oforganic carbon degradation based on a measured or modeled benthic oxygen con-sumption rate. The significance of DOC fluxes in carbon budgets of deep sea sedi-ments (Martin & McCorkle, 1993; Burdige et al., 1992) has been criticized recently(Jahnke, 1996) on reasons of a missing deep DOC concentration difference betweenthe Atlantic and Pacific (refractive DOC) and of the balance between the POC rainrate and its benthic degradation rate leaving no room for major effluxes of reactive

129S. Otto, W. Balzer /Progress in Oceanography 42 (1998) 127–144

DOC. Pore water DOC reported to be an order of magnitude higher than in the deepsea is not likely to suffer from analytical blank problems in the same way as theearly determinations of DOC by the HTC-method in the water column did (seeMar-ine Chemistry, Vol. 41, special issue, 1993; Sharp, 1993). Thus, the role of DOCduring early diagenesis and the significance of DOC-effluxes remains unresolved(Hedges & Keil, 1995).

Here we present new measurements of DOC in marine pore waters from twotransects across the European continental margin (Goban Spur and Whittard Canyon)ranging from the shelf-break to the adjacent deep-sea basin. Release fluxes of DOCestimated from concentration profiles will be compared to the rate at which organiccarbon is oxidized to CO2 as its inorganic end product.

2. Study area and methods

The study area is shown in Fig. 1. The Goban Spur is a relatively undisturbedgentle slope without any fissures. Whittard Canyon, less than 120 nautical milesaway from Goban Spur, is located inside an area of the Celtic margin which isintersected by several canyons. During the cruises M 30/1 (September 1994) and M36/4 (August 1996) of R.V.Meteor, sediment samples at transects along the GobanSpur and the Whittard Canyon, respectively, were taken with a multicorer (Table 1).

Immediately after recovery, the sediment cores having a seemingly undisturbedsurface were brought to a laboratory refrigerated at14°C. All procedures to obtainpore water were performed inside the cooled laboratory to maintain in situ tempera-ture conditions as far as possible. The cores were sectioned into 0.5–2.0 cm intervals.During the M30/1 cruise (Goban Spur) pore water samples were obtained by porewater squeezing using nitrogen, while during M 36/4 (Whittard Canyon) the porewater was separated through centrifugation of sediment slices in a cooled centrifuge(5 min; max 5000 rpm). Samples of the overlying water were taken from each coreapproximately 1 cm above the sediment surface (in the following called bottomwater). Pore water and bottom water samples were filtered through membrane filters(PTFE, pore size 0.45mm). Samples for DOC andSCO2 (1 ml each) were transferredinto cleaned 4 ml glass vials with teflon-lined screw cap and stored at1 4°C untilanalysis. Analyses were performed on board within 0.3–2 days.

Immediately before the HTC oxidation of DOC, the removal of inorganic carbonwas accomplished by adding 30ml of hydrochloric acid (25%) and purging thesample for 4 min with a stream of pure argon. DOC was oxidized to CO2 in aslightly modified Dimatoc-100 HTC-instrument (Dimatec Company, Germany) usingplatinum on an alumina-support as the catalyst operated at 750°C. CO2 was determ-ined via a built-in IR-detector followed by computer evaluation of the output. Anultrapure argon–oxygen-mixture (95/5%) was used as the carrier gas. Prior to analy-sis the blank of the instrument was determined through injection of 75ml of ‘zerowater’. ‘Zero water’ was produced by adding potassium peroxodisulphate and phos-phoric acid to Milli-Q water and refluxing this mixture several hours followed bydistillation. This blank was determined several times during a complete analysis run.


Fig. 1. Study site at the European continental margin and location of sediment sampling positions.Roman numbers indicate the stations of the project OMEX, where moorings with sediment traps weredeployed.

Standard solutions of glucose in seawater (0.2, 0.5, 1.0 and 2.0 mmol l−1) were usedfor calibration. To obtain the final result 75ml of samples, zero water and standardswere injected at least three times. The sample concentration was calculated by sub-tracting the measured blank from the sample value and subsequent division by theslope of the calibration curve as described in the IOC-manuals and guides (IOC,1994) for DOC determinations in the water column. Several standard solutions atthe end of the analytical run revealed that drift of blank and slope was less than 3%.


Table 1Location and date of sampling: Goban Spur and Whittard Canyon

Station number Abbreviation Depth (m) Position Date

Goban Spur (M 30-1)M 425-94 IOS 4805 48° 58.39 N 16° 28.39 W 12.09.94M 426-94 OMEX IV 4500 48° 59.19 N 13° 45.19 W 13.09.94M 427-94 OMEX III 3665 49° 05.59 N 13° 24.89 W 14.09.94M 430-94 OMEX II 1526 49° 11.19 N 12° 51.09 W 16.09.94M 434-94 OMEX I 674 49° 24.19 N 11° 32.19 W 17.09.94Whittard Canyon (M36-4)M 281-96 F 3677 48° 09.29 N 10° 15.09 W 29.08.96M 296-96 E 3242 48° 21.39 N 10° 24.09 W 01.09.96M 278-96 D 2330 48° 31.09 N 10° 30.19 W 28.08.96M 275-96 C 1510 48° 38.09 N 10° 29.49 W 27.08.96M 271-96 B 806 48° 42.99 N 10° 22.79 W 26.08.96M 286-96 A 177 48° 57.19 N 10° 50.09 W 30.08.96

The analysis forSCO2 was performed with the inorganic channel of the Dimatoc-100 HTC-instrument, which consists of a silica-support coated with phosphoric acidand operated at a furnace temperature of 160°C. The inorganic channel was checkedfor blank values in regular intervals using acidified Milli-Q water. No CO2 originat-ing from the catalyst or from memory effects has been ever detected. Standard sol-utions for inSCO2 were prepared by dissolving sodium carbonate in Milli-Q water(1.5–6 mmol l−1) under a nitrogen atmosphere. The porosity (w) used for flux calcu-lations was obtained from sediment weight loss upon drying.

3. Results and discussion

3.1. DOC in pore water

Starting from concentrations of 0.06–0.12 mmol l−1 in the overlying water, thepore water DOC concentration significantly increased immediately below the surfaceand showed less variation at depth (Figs. 2 and 3). At the Goban Spur DOC concen-tration ranged from 0.27 to 1.00 mmol l−1 at the deeper stations (IOS, OMEX IVand III) of the transect, while higher concentrations of up to 2.61 mmol l−1 werefound at the shallower sites (Table 2 and Fig. 2). The steep increase in concentrationat mid depth of station OMEX I (4 cm) and OMEX II (15 cm) might be related tobeginning sulphate reduction (Alperin et al., 1994) which was indicated by nitrateexhaustion and the onset of the ammonia increase (data not shown). At the WhittardCanyon pore water DOC rose more or less continuously, reaching concentrationsbetween 0.2 and 0.9 mmol l−1 at depth (Fig. 3 and Table 3). Both sets of DOCdeterminations were close to recently reported results on Antarctic bottom waters(0.06–0.10 mmol l−1) and Antarctic interfacial pore waters (0.35–2.00 mmol l−1) atwater depths ranging from 280 to 2514 m (Hulth et al., 1997).


Fig. 2. Pore water concentrations of DOC in sediments of the Goban Spur transect. Horizontal linesmark the position of the sediment–water interface.

Fig. 3. Pore water concentration of DOC in sediments of the Whittard Canyon. Horizontal lines markthe position of the sediment–water interface.

In most cases at Goban Spur (Fig. 2), but only once at the Whittard Canyon, theDOC-concentration(s) in the pore water sample(s) of the first centimetre were higherthan in deeper layers. Such near surface maxima in DOC were also found in othersediments at all water depths (Henrichs & Farrington, 1984; Heggie et al., 1987;Martin & McCorkle, 1993; Burdige & Homstead, 1994), but it is unclear whether


Tab

le2

Por

ew

ater

data

ofD

OC

atG

oban

Spu

rob

tain

edby

sque

ezin

g.D

OC

was

dete

rmin

edby

high

tem

pera

ture

com

bust

ion

IOS

OM

EX

IVO

ME

XIII

OM

EX

IIO

ME

XI

Dep

th(c

m)

DO

CD

epth

(cm

)D

OC

Dep

th(c

m)

DO

CD

epth

(cm

)D

OC

Dep

th(c

m)

DO

C(m

mol

l−1)

(mm

oll−1

)(m

mol

l−1)

(mm

oll−1

)(m

mol

l−1)

BW

a0.

07B

W0.

07B

W0.

11B

W0.

07B

W0.

070–

0.5

1.00

0–0.

50.

530–

0.5

0.86

0–0.

51.

420–

0.5

1.17

0.5–

10.

570.

5–1

0.39

0.5–

10.

530.

5–1

1.57

1–2

0.92

1–2

0.57

1–2

0.57

1–1.

50.

541–

20.

732–

31.

262–

30.

762–

30.

251.

5–2

0.50

2–3

0.80

3–4

0.93

3–5

0.42

3–5

0.23

2–3

0.36

3–4

0.82

4–5

2.61

5–7

0.50

5–7

0.31

3–4

0.46

4–5

1.16

5–6

2.00

7–9

0.52

7–9

0.55

4–5

0.74

5–7

0.70

6–7

2.05

9–11

0.36

9–11

0.53

5–6.

50.

497–

90.

607–

81.

6113

–15

0.49

11–1

30.

336.

5–8

0.64

9–11

0.80

8–9

1.84

17–1

90.

3813

–15

0.43

8–9.

50.

7711

–13

0.77

9–11

1.54

21–2

30.

3315

–17

0.46

9.5–

11.5

0.39

13–1

51.

0611

–13

0.92

25–2

70.

2717

–19

0.33

11.5

–12.

50.

4715

–17

2.20

––

aB

otto

mw

ater

,i.e

.ov

erly

ing

wat

ersa

mpl

ed1

cmab

ove

the

sedi

men

tsu

rfac

e.


Tab

le3

Por

ew

ater

data

ofD

OC

andSC

O2

atW

hitta

rdC

anyo

nob

tain

edby

cent

rifug

atio

n

M28

1(F

)M

296

(E)

M27

8(D

)M

275

(C)

M27

1(B

)M

286

(A)

Dep

thD

OC

SC

O2

Dep

thD

OC

SC

O2

Dep

thD

OC

SC

O2

Dep

thD

OC

SC

O2

Dep

thD

OC

SC

O2

Dep

thD

OC

SC

O2

(cm

)(m

mol

(mm

ol(c

m)

(mm

ol(m

mol

(cm

)(m

mol

(mm

ol(c

m)

(mm

ol(m

mol

(cm

)(m

mol

(mm

ol(c

m)

(mm

ol(m

mol

l−1)

l−1)

l−1)

l−1)

l−1)

l−1)

l−1)

l−1)

l−1)

l−1)

l−1)

l−1)

BW

a0.

062.

46B

W0.

112.

36B

W0.

102.

48B

W0.

082.

36B

W0.

122.

35B

W0.

072.

370–

0.5

0.16

2.72

0–0.

50.

332.

490–

0.5

0.21

2.68

0–0.

50.

232.

740–

0.5

0.21

2.63

0–0.

50.

162.

430.

5–1

0.18

2.71

0.5–

11.

222.

730.

5–1

0.28

2.66

0.5–

10.

272.

890.

5–1

0.25

2.70

0.5–

10.

232.

441–

20.

212.

941–

20.

533.

011–

20.

302.

971–

20.

303.

051–

20.

27–

1–2

0.32

2.48

2–3

0.37

2.97

2–3

0.31

3.15

2–3

0.32

3.19

2–3

0.34

2.94

2–3

0.54

2.66

2–3

0.36

2.40

3–5

0.31

3.39

3–5

0.37

–3–

50.

483.

473–

50.

393.

113–

50.

562.

693–

50.

382.

385–

7–

–5–

70.

61–

5–7

0.36

3.84

5–7

0.40

3.05

5–7

0.66

2.78

5–7

0.38

2.22

7–9

0.27

3.74

7–9

0.56

–7–

90.

463.

977–

90.

612.

997–

90.

68–

7–9

0.32

2.22

11–1

30.

293.

9011

–13

0.60

–11

–13

0.55

4.83

9–11

0.59

2.66

9–11

0.52

2.99

––

–15

–17

0.39

3.91

15–1

70.

74–

15–1

70.

635.

52–

––

11–1

30.

333.

21–

––

19–2

10.

424.

8119

–21

0.68

–19

–21

0.75

5.61

––

–13

–15

0.33

3.26

––

–23

–25

0.50

5.03

23–2

50.

64–

23–2

50.

735.

97–

––

––

––

––

27–2

90.

63–

27–2

90.

86–

27–2

90.

836.

28–

––

––

––

––

a Bot

tom

wat

er,

i.e.

over

lyin

gw

ater

sam

pled

1cm

abov

eth

ese

dim

ent

surf

ace.


this is real or caused by a sampling artifact (Burdige et al., 1992). The pressuredrop and/or the warming of the sediment during recovery of the core often result inconcentration increases of pore water constituents within the first 1–1.5 cm (e.g.nitrate: Balzer, 1989; Balzer, Helder, Epping, Lohse, & Otto, 1998 or ammonia: e.g.Aller, Hall, & Rude, 1987; Glud, Gundersen, Jørgensen, Revsbech, & Schulz, 1994).Up to now such elevated pore water concentration in the top sediment layer havebeen observed for nitrogen bearing components only, and never for phosphate orsilicate (Balzer, 1989). Although there is no sound explanation for such artifacts, arelation to labile organic matter appears likely, because such artifacts seem to beconfined to the top 1–1.5 cm, which contain much more living and non-living organicmaterial than deeper layers (Lohse, Helder, Epping, & Balzer, 1998; Pfannkuche &Soltwedel, 1998). Since organic pore water constituents seem to be more susceptibleto sampling artifacts than inorganic species (Howes, Dacey, & Wakeham, 1985;Martin & McCorkle, 1993), we consider these elevated concentrations in the uppersediment centimetre of the Goban Spur to be unreliable, possibly resulting from theapplication of the squeezing technique. In the reduction of potential artifacts certainprocedural factors may aid: in addition to extremely rapid separation of pore waterfrom the wet sediment at in situ temperature, centrifugation should be applied prefer-entially (Martin & McCorkle, 1993). Because centrifugation was used only duringthe work-up of the canyon samples while pore water from Goban Spur sedimentswere obtained by squeezing, we ignore the DOC concentration from the first centi-metre for Goban Spur samples during flux calculations. Instead, we used a lineargradient from the 1–2 cm horizon to the bottom water. Calculated DOC fluxes atGoban Spur, therefore, represent lower estimates and could have been greater (Fig.4 and Table 4).

3.2. Flux of DOC out of the sediment

Assuming diffusive transport across the sediment–water interface, we calculatedthe DOC efflux from porosity (w) and the sediment diffusion coefficient (DSed) byapplying Fick’s First Law

Jz 5 2 w·DSed·dCdz

(1)

For Goban Spur sediments linear gradients (DC/Dz) were used (Burdige et al.,1992; Martin & McCorkle, 1993; Burdige & Homstead, 1994). For Whittard Canyonsediments, where the DOC-profiles showed a continuous increase with depth, gradi-ents were obtained from a fit of the data to the exponential function (Berner, 1980)

cz 5 c` 1 (c0 2 c`)*exp( 2 az) (2)

in which cz, c0, c` are concentrations at depthz, at the sediment surface (in ourcase: the bottom water concentration) and at depth, where dc/dz approaches zero,respectively. The use of the Eq. (2) instead of a linear gradient (DC/Dz) reduces the


Fig. 4. DOC release fluxes from Celtic margin sediments along the transects at Goban Spur (a) and inthe Whittard Canyon (b).

potential error by basing the flux calculation on more than just one pore water con-centration.

The sediment diffusion coefficient (DSed) for DOC is a matter of uncertainty,because DOC is the sum of a countless number of individual components exhibitinga wide range of molecular weights and chemical properties. Each of these compoundshas its own specific diffusion coefficient (at standard conditions:Do in units of 10−6

cm−2 s−1) which is accessible from the molecular weight (M) by using the empiricalrelationship (Burdige et al., 1992)

logDo 5 1.722 0.39*logM (3)


Table 4Diffusive DOC-flux from the sediments and its percentage on the total rate of organic carbon remineraliz-ation. The negative sign indicates the upward direction of the flux

Station (water depth) DOC flux into bottom Corg-remin DOC(DOC 1 Corgremin.)water (mmol m−2 d−1) (mmol m−2 d−1)

(%)

(a) Goban SpurIOS (4805 m) 2 0.10 0.34a 23OMEX IV (4500 m) 2 0.09 0.55a 14OMEX III (3665 m) 2 0.10 0.85a 11OMEX II (1529 m) 2 0.15 1.27a 11OMEX I (674 m) 2 0.15 2.10a 7(b) Whittard CanyonF (M 281) (3677 m) 2 0.11 0.61b 15E (M 296) (3242 m) 2 0.07 0.98b 7D (M 278) (2330 m) 2 0.05 0.27b 16C (M 275) (1510 m) 2 0.16 2.63b 6B (M 271) (806 m) 2 0.07 0.15b 32A (M 286) (177 m) 2 0.07 3.80a 2

aAverage value from three different methods of determination of the sediment oxygen demand (pore watermodeling of nitrate; oxygen measurents from lander incubations; oxygen pore water profiles; see Lohseet al. (1998) and Balzer et al. (1998).bCalculated fromSCO2 profiles (Table 3).

The molecular weight distribution of organic compounds in marine pore water iscertainly related to the origin and quality of the sedimentary organic matter and theenvironmental conditions during early diagenesis, but all these factors are unknownat the Celtic margin. The molecular weight distribution of DOC in shallow marineand lacustrine sediments ranging typically from 1 to 10 kDa (Krom & Sholkovitz,1977; Orem & Gaudette, 1984; Orem et al., 1986) is often used for flux calculationsin shallow organic rich marine sediments (Chen, Bada, & Suzuki, 1993; Burdige etal., 1992; Burdige & Homstead, 1994; Alperin et al., 1994). This molecular weightdistribution could be misleading in our case (most samples fromz > 500 m), becauseBurdige, Gardner, & Zheng (1996) recently showed a trend for higher molecularweights with increasing depth (from 12 to 500 m), i.e. the size fraction smaller than3 kDa decreased to about 55%, the rest being in the size fraction greater than 100kDa. Similarly, DOC of sediments from 1400 m depth off Venezuela had 44% inthe fraction less than 10 kDa and 46% above 100 kDa (Rashid, Buckley, & Robert-son, 1972). For our purpose we assumed that the DOC is composed of two fractionshaving fixed molecular weights of 3 and 100 kDa and that a 50/50 mixture (3/100kDa) can be used for the whole depth range from 200 to 4800 m. The molecularweight of DOC has a significant impact on the resulting diffusion coefficient andon the calculated diffusive fluxes. If all DOC compounds at our study site had thesize of 100 kDa or (for the other extreme) of 3 kDa, respectively, the resultingdiffusion coefficient would be the 0.6-fold or the 1.5-fold, respectively, of the coef-ficient we adopted for our study. A diffusion coefficientDo for DOC of 1.553 10−6


cm−2 s−1 was used for flux calculations and appropiate temperature, salinity andporosity corrections were applied to yieldDSed (Li & Gregory, 1974; Ullman &Aller, 1982).

The DOC fluxes from Goban Spur sediments range from 0.09 to 0.15 mmol m−2

d−1 (Table 4), but represent a lower estimate for reasons discussed above. The diffus-ive DOC-fluxes would increase 7–14-fold, if the uppermost DOC concentration werenot a sampling artifact. A similar range of DOC-fluxes (0.05–0.16 mmol m−2 d−1)was calculated for Whittard Canyon sediments (except sta. E) leading credence tothe ‘low’ results at Goban Spur because these canyon pore waters were not affectedby possible artifacts. Since we neglected the possible enhancement of fluxes resultingfrom bioirrigation, the reported fluxes (Table 4) represent lower limits of the DOCflux to the overlying water.

Our results are similar to DOC-fluxes at the Central California margin rangingfrom 0.05 to 0.1 mmol m−2 d−1 (Burdige et al., 1992) although the latter site hasmuch higher carbon contents and higher degradation rates. Slighly higher DOC-fluxes of 0.24–1.98 mmol m−2 d−1 were found at the continental slope off New YorkBight (Martin & McCorkle, 1993) and at the Hatteras continental rise (Heggie etal., 1987). Shallow anoxic sediments seem to release DOC to the overlying waterat greater rates: 9.6 mmol m−2 d−1 (Alperin et al., 1994), 1.4–2.9 mmol m−2 d−1

(Burdige & Homstead, 1994) and 6.0 mmol m−2 d−1 (Martens et al., 1992). Consider-ing the wide range of environmental conditions between the upper slope of GobanSpur and the deep basin, the variation of the DOC-fluxes is remarkably small (Fig.4). The organic carbon oxidation strongly decreases from the upper slope to the basin(see below), but the content of the sedimentary POC increases concomitantly (0.2–0.6%), possibly related to down-slope transport of already degraded ‘old’ carboncausing an over-proportional reduction in its labile portion (Lohse et al., 1998; vanWeering, Hall, de Stigter, McCave, & Thomson, 1998). DOC-fluxes in WhittardCanyon sediments are slightly more variable, probably because erosion/redistributionof freshly sedimented materials results in an inhomogeneous depth dependence ofbenthic activity. Since DOC consists of more or less labile compounds which at leastpartly are taken up by benthic heterotrophs, one explanation for the low variationin DOC concentrations and fluxes could be that production and consumption of DOCcomponents proceed at similar rates irrespective what the overall benthic activity is.This is supported by several studies about sedimentary dissolved amino acids whichshow remarkably small differences in the concentrations if several different environ-ments, including anoxic sites, shallow water samples and fully oxic deep-sea sedi-ments were compared (Henrichs & Farrington, 1979; Henrichs, Farrington, & Lee,1984; Mintrop & Duinker, 1994; Jørgensen, Lindroth, & Mopper, 1981). Anotherexplanation for the lack of great differences in DOC-fluxes could be the adsorptionof sedimentary produced DOM at mineral surfaces as proposed by Hedges & Keil(1995). These authors expect the pore water DOM to be involved in reversibleadsorption/desorption processes with the solid phase. Then, the type of the solidsurface may not only have an impact on the sorption intensity but also act as a buffercontrolling the DOC pore water concentration.


3.3. DOC-fluxes in relation to the rate of organic matter degradation

Because the benthic production of DOC from particulate organic carbon is partlya non-oxidizing process (e.g. via enzymatic hydrolysis) and because DOC shouldhave an oxidation number close to zero, any DOC that escapes from the sedimentinto the bottom water entails an underestimation of the true organic carbon degra-dation rate in benthic budgets. This part of the total degradation of benthic organiccarbon (the efflux of DOC) is included neither in oxygen consumption orSCO2

production measurements by using benthic chambers nor in rate determinations whenbased on pore water modelling of the oxidation reaction (Corg-remin). Steady statemodeling of pore water oxidants (O2, NO3) or remineralization products (SCO2) onlyinclude the degradation of DOC as long as it resides in the sediment. Therefore, theDOC-flux has to be added to the benthic remineralization rate (Corg-remin) to reachthe total sedimentary remineralization of organic matter (Ctot-remin). We use theDOC-fluxes out of Celtic margin sediments (Table 4) in combination with the usualremineralization rates to obtain an estimate for a total remineralization rate. As men-tioned before, the diffusive efflux of DOC, and so the Ctot-remin, represent lowerlimits because additional transport mechanisms (advection, bioirrigation) were neg-lected.

Ctot 2 remin.5 Corg 2 remin.1 DOC efflux (4)

For Corg-remin of Goban Spur sediments three series of determinations along thetransect are available: lander oxygen uptake, evaluation of O2-electrode profiles andresults from the modeling of pore water nitrate (Lohse et al., 1998). Since all threesets yielded similar results, we use the mean value at the individual stations [Table4(a)]. The total remineralization rates for Goban Spur sediments, to which the DOC-flux contributes 7–23%, are depicted in Fig. 5(a). Our data show no trend for greaterDOC fluxes with increasing rate of remineralization. Therefore, the relative signifi-cance of the DOC-flux increases with decreasing Ctot-remin, i.e. in the down-slopedirection.

We calculate the rate Corg-remin for Whittard Canyon sediments from ourSCO2

pore water data, because no other determinations are available (Table 3). Assumingsteady state conditions, the flux ofSCO2 through the sediment–water interface equalsthe integrated remineralization of organic matter to CO2. To obtain the concentrationgradient needed for the diffusive flux, we fitted theSCO2-data to an exponentialfunction as described above for DOC [Eq. (2)].DSed was calculated fromDo ofHCO3

− (Li & Gregory, 1974) including the appropriate corrections as shown above.Estimations of the remineralization of organic carbon by using pore waterSCO2

suffer from the simultaneous dissolution of carbonate by CO2 production duringearly diagenesis (Froelich, Klinkhammer, Bender, Luedtke, Heath, Cullen, Dauphin,Hammond, Hartman, & Maynard, 1979; Emerson & Bender, 1981). We use a stoi-chiometric relationship of Corg remin/SCO2 5 106/207, thereby taking the incom-plete dissolution of the carbonates into account (Emerson, Grundmanis, & Graham,1982) but neglecting a possible decompression artefact (Murray, Emerson, & Jahnke,


Fig. 5. Rates of total benthic degradation of particulate organic carbon at the Celtic margin includingthe rate of remineralization by oxygen and the DOC-fluxes out of the sediments.

1980; Emerson, Jahnke, Bender, Froelich, Klinkhammer, Bowser, & Setlock, 1980).Thus, the rates compiled in Table 4(b) represent rough estimates for the remineraliz-ation of organic matter at these sites. Advective transport and/or bioirrigation wouldenhance the efflux ofSCO2 in the same way as noted above for the DOC-flux thushaving only minor effects on the ratio of the DOC-efflux to the total remineraliz-ation rate.

In Whittard Canyon sediments Corg-remin. is in the range of 0.15–2.63 and 3.80mmol m−2 d−1 at the shelf, respectively [Table 4(b)]. Compared to the Goban Spurtransect, remineralization in the canyon sediments is surprisingly low at 806 m and


much higher at depths exceeding 3000 m. This feature is probably related to erosionand redistribution of sedimenting materials between different parts of the canyon.DOC-effluxes in the Whittard Canyon also show only minor variations along thetransect [Table 4(b)]. Its relative contribution to the total remineralization rate of 2–32% [Table 4(b)] therefore basically depends on variations in Corg-remin.

Summing up, DOC-effluxes from the sediment play a role in early diagenesis, thatcannot be neglected in benthic carbon budgets. Even conservative estimates of DOC-effluxes (see above) add between 2% and 32% to the total degradation of POC. It isnot understood, however, why the relative significance of the DOC-effluxes increasestowards greater water depths and lower carbon remineralization rates.

DOC-fluxes from marine sediments in water depths less than 2000 m introduceglobally 1–7.73 1012 mol C y−1 into the ocean (Burdige et al., 1992) which is inthe same order of magnitude as riverine inputs. The conservative DOC-fluxes of thisstudy and additional data from slope sediments of the North and South Atlanticamount globally to a DOC supply of 2.23 1012 mol C y−1 in the depth range from200 to 2000 m (Otto, 1996), a figure in clear support to the estimate of Burdige etal. (1992). Jahnke (1996) recently questioned the statement that DOC fluxes mightbe significant or perhaps even equivalent in magnitude to remineralization-driveninorganic fluxes (Burdige et al., 1992; Martin & McCorkle, 1993). He concludedthat high DOC-fluxes are not compatible with the DOC concentration along the deepwater path and with the present knowledge about the rates of carbon sedimentation,the apparent oxygen utilization in the deep sea, and the benthic carbon degradation.The criticized high DOC-fluxes, however, are based on high near-surface DOC con-centrations of the pore water which we considered artifacts. Taking all stations inves-tigated in the present study, the median of the relative contribution of the DOC-fluxto the total remineralization is 11%. Thus, Jahnke’s argument (1996) against a 50%contribution seems to be not pertinent to our study. Future research involving directmeasurements of benthic DOC-fluxes by using in situ-lander incubations will shedmore light on the magnitude of DOC-release from deep-sea sediments.

4. Acknowledgements

We appreciate the help of the captains and crew of the R.V.Meteorduring cruiseM30 and M36. This work was supported by the German Research Council (DeutscheForschungsgemeinschaft, Sonderforschungsbereich 261 at Bremen University) andthe European Union in the framework of the MAST programme, contract no. MAS2-CT93-0069 and MAS3-CT96-0056 (Ocean Margin Exchange–OMEX). We arethankful for the suggestions of two anonymous reviewers.

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