recycling of organic matter along a shelf-slope transect across the n.w. european continental margin...

Progress in Oceanography 42 (1998) 77–110

Recycling of organic matter along a shelf-slopetransect across the N.W. European Continental

Margin (Goban Spur)

L. Lohsea,*, W. Heldera, E.H.G. Eppinga, W. Balzerb

aNetherlands Institute for Sea Research (NIOZ), Department Marine Chemistry and Geology, P.O. Box59, 1790 AB Den Burg, The Netherlands

bUniversitat Bremen, Fachbereich Meereschemie, Postfach 330440, 28334 Bremen, Germany

Abstract

Within the framework of the Ocean Margin Exchange Programme (OMEX), benthic carbonmineralisation was determined along the Goban Spur shelf-slope transition (200–4500 m waterdepth) at the eastern margin of the North Atlantic. Carbon oxidation rates were derived fromthe pore water distributions of oxygen, nitrate, ammonium, dissolved manganese and dissolvediron in combination with fluxes of solutes across the sediment–water interface. Pore waterprofiles of oxygen were obtained in situ with a benthic lander and on-deck in sediment coresretrieved by multi-coring. With water depths increasing from 200 to 1500 m benthic carbonoxidation rates decreased from 4.3 to 1.5 mmol C m−2 d−1, while the interfacial organic carbonconcentrations increased from 0.2 to 0.7% (wt/wt). At stations deeper than 1500 m, no furthertrends with depth were found. Carbon burial efficiencies in this low-sedimentation continentalmargin were not related to water depth and ranged between 0.8 and 2.3%. We conclude fromthese data that there is no distinct carbon depocenter at the Goban Spur continental slope, thisin contrast to the slope at the western North Atlantic margin (Anderson, Rowe, Kemp, Trum-bore, & Biscaye (1994). Carbon budget of the Middle Atlantic Bight.Deep-Sea Research I,41, 669–703.). Integrated carbon mineralisation rates indicated that oxic respiration accountedfor more than 70% of the total carbon oxidation at all stations. Substantial anoxic mineralis-ation was identified only on the upper slope, while the contribution of denitrification neverexceeded 10% along the entire transect. Benthic oxygen fluxes showed no direct response topulses of organic material settling on the sea floor, as appearing in sediment traps, suggestingthat the organic material deposited is dominated by refractory compounds. This finding wassupported by steady-state modelling of pore water oxygen profiles which showed that theorganic matter being mineralised at stations deeper than 200 m had very low degradation rateconstants (, 1 y−1). Comparison of the measured oxygen and nutrient fluxes with the diffusive

* Corresponding author. Tel:1 31-222-369448; fax:1 31-222-319674; e-mail: [email protected]

0079-6611/98/$ - see front matter 1998 Elsevier Science Ltd. All rights reserved.PII: S0079 -6611(98)00029-9

78 L. Lohse et al. /Progress in Oceanography 42 (1998) 77–110

fluxes calculated from pore water profiles indicated that within the experimental errors therewas no significant contribution by bioirrigating organisms to the sediment–water exchangefluxes. 1998 Elsevier Science Ltd. All rights reserved.

1. Introduction

High rates of particle deposition along the continental margins lead to intensifiedbiogeochemical processes in the underlying sediments. Although continental marginsediments comprise only 11% of the surface area of the world ocean, more than 80%of the global benthic mineralisation takes place here (Jørgensen, 1983; Middelburg,Soetaert, & Herman, 1997). Their contribution in global biogeochemical cycles isundisputed, though considerable debate exists about the spatial variation of benthicmineralisation and preservation processes. Several studies indicate that enhancedrates of carbon mineralisation and burial occur on the upper continental slope com-pared to the continental shelf (Monaco, Biscaye, Soyer, Pocklington, & Heussner,1990; Pedersen, Shimmield, & Price, 1992; Anderson, Rowe, Kemp, Trumbore, &Biscaye, 1994). This argument has been rationalised by invoking hydrodynamicforces which would allow only for temporal deposition of particulate organic matteron the shelf. As a consequence, a significant fraction of organic matter produced onthe shelf is exported towards the continental slope, where more suitable hydro-dynamic conditions allow for deposition, and so-called “depocenters” may be formed(Walsh, 1991).

While an earlier paper of ours has highlighted aspects of phosphorus cycling inthe north European continental margin (Slomp, Epping, Helder, & van Raaphorst,1996), we now focus on benthic carbon oxidation rates along a selected transect inthis region. The principal approach chosen here is to derive benthic mineralisationrates from the depth-dependent production and consumption of redox-indicative porewater components. The well-established sequence of oxygen consumption, nitrate,manganese, iron and finally sulphate reduction reflects the free energy obtained fromeach particular reaction as well as their subsequent disappearance in the sediment(Froehlich, Klinkhammer, Bender, Luetdke, Heath, Cullen, Dauphin, Hammond,Hartmann, & Maynard, 1979). The concentration gradients of these compounds canbe converted into carbon oxidation rates provided that the stoichiometry of organicmaterial undergoing decomposition as well as the diffusion coefficients for eachparticular compound are known (e.g. Bender, & Heggie, 1984).

Given the dominant role of oxygen in organic carbon mineralisation in continentalmargin sediments (Jahnke, & Jackson, 1992) oxygen pore water profiles weredetermined in situ with a profiling instrument, as well as in sediment cores retrievedon-deck. In combination with pore water profiles of the other redox-sensitiveelements and benthic chamber incubations we will provide a detailed picture onthe benthic carbon mineralisation rates along a transect at the north-west Europeancontinental margin. This includes the estimation of the total carbon oxidation rate as

79L. Lohse et al. /Progress in Oceanography 42 (1998) 77–110

well as the contribution of the various electron acceptors to the overall mineralisation.Whether these rates in combination with the solid-phase carbon concentration ident-ify a carbon depocenter in the study area represented a principal question in thisstudy. All investigations were obtained within the framework of the interdisciplinaryOcean Margin Exchange (OMEX) programme of the Marine Science and Tech-nology programme (MAST II), funded by the European Union.

2. Materials and methods

2.1. Study area

The principal research area was located at the Goban Spur, a shelf-slope transitionin the north-eastern Atlantic, located about 250 km to the south-west of Ireland (Fig.1). The morphology of this transect is characterised by gradually decreasing waterdepths from 200 m on the Celtic Shelf down to 1000 m on the upper slope, followedby a steeper lower slope down to 3500 m at the Pendragon Escarpment (station III),an embedded platform on the continental slope. At its western boundary, water depthsharply increases to 4500 m, where the slope transits on to the Porcupine AbyssalPlain.

The transect was visited during three cruises in October 1993, May/June 1994 andSeptember 1995, respectively. Sampling stations were located on the shelf (A), theupper (I, B, II) and lower (C, F, III) slope as well as in the Porcupine Abyssal Plain(E). The entire transect was sampled during the first two cruises, but bad weatherconditions forced us to concentrate on the principal sampling stations OMEX I, II,III and F during the September 1995 cruise. A detailed overview on sampling dates,station positions, and water depths is given in Table 1.

Fig. 1. Sampling locations and cross section of the Goban Spur transect.


Table 1Characteristics of sampling sites

Station Sampling Latitude Longitude Depth (m) Bottom [O2] depth-date (°N) (°W) water Bottomwater integrated

temperature (mmol porosity(°C) dm−3) over the

oxic layer

A 18-10-93 49 28.4 11 12.3 208 10.95 250 0.7123-05-94 49 29.7 11 08.4 208 10.51 274 0.69

I 19-10-93 49 24.8 11 32.0 668 10.05 231 0.7923-05-94 49 24.9 11 31.4 670 9.97 226 0.8021-09-95 49 24.9 11 31.4 650 9.66 222 0.74

B 20-10-93 49 22.0 11 48.1 1023 8.77 208 0.7724-05-94 49 22.4 11 45.1 999 8.69 203 0.83

II 21-10-93 49 11.4 12 49.1 1442 5.89 241 0.8525-05-94 49 11.3 12 49.7 1425 7.00 231 0.8518-09-95 49 11.4 12 44.5 1291 7.06 222 0.85

C 22-10-93 49 09.6 12 59.4 1989 3.50 285 0.82F 25-10-93 49 09.1 13 05.4 2182 3.22 273 0.86

27-05-94 49 09.5 13 05.3 2235 3.20 269 0.8519-09-95 49-09.4 13 05.4 2247 3.24 264 0.85

III 24-10-93 49 05.4 13 25.7 3719 2.53 251 0.8729-05-94 49 05.2 13 25.9 3649 2.50 249 0.8620-09-95 49 05.3 13 26.2 3650 2.50 247 0.83

E 24-10-93 49 02.3 13 42.2 4550 2.52 251 0.8831-05-94 49 02.3 13 42.2 4468 2.53 250 0.88

2.2. Sediment sampling

During the 1993 and 1994 cruises, sediment cores were taken with a cylindricalbox-corer (internal diameter (i.d.) 50 cm) developed at the Netherlands Institute forSea Research (NIOZ). The box-corer retrieves a 30 to 50 cm of sediment columntogether with 30 to 50 l of overlying water. Disturbance of the sediment–water inter-face was prevented by a closing lid on the upper end of the box corer. The lowerend of the box was sealed with a rubber plate which was brought into position duringcore retrieval. Sub-cores were taken by carefully inserting acrylic liners into thesediment which were sealed with rubber stoppers after retrieval.

During the 1995 cruise, sediment cores were taken with a multi-corer (Barnett,Watson, & Connelly, 1984). The sediment cores retrieved by this device (i.d. 62 or95 mm) were processed directly, without further sub-coring. All samples were incu-bated and processed at in situ temperature within a temperature controlled laboratory.

2.3. Pore water collection

Pore water was sampled by sectioning 4 to 6 sediment cores (i.d. 62 mm) in slicesof 2.5 mm in the upper 10 mm of the sediment, 5 mm slices from 10 to 30 mm,


10 mm slices from 30 to 60 mm and 20 mm slices from 60 to 140 mm. The extrudedsediment slices were pooled, homogenised and the pore water was extracted usingsediment squeezers. The squeezers were operated at low pressure (1 to 2 bar) for30 to 45 min and the obtained pore water was analysed shipboard for ammonium(Helder, & de Vries, 1979), nitrate and nitrite (Grasshoff, Erhardt, & Kremling, 1983)and silicate (Strickland, & Parsons, 1972) using TRAACS 8001 autoanalysers.Additionally, pore waters were analysed for dissolved manganese (Brewer, & Spen-cer, 1971) and iron (Stookey, 1970) after acidification to pH 2, respectively. Theanalytical precision for all compounds was6 1% relative standard deviation.

The “squeezer cake” was stored frozen at220°C until the analysis of organiccarbon (Corg) and total nitrogen (Ntot) at the institute. The analysis was carried outon a Carlo-Erba NA-1500 elemental analyser following the analytical protocols ofVerardo, Froelich, & McIntyre (1990). Basically, 10 to 15 mg of homogenised, driedsqueezer cake were filled into small tin cups. The total carbon content was determ-ined by the combustion of an untreated sample. Inorganic carbon was removed byadding small aliquots (10 to 50ml) of sulfurous acid. The ebullition of carbon dioxideafter each acid addition was observed under a binocular in order to verify the termin-ation of the carbonate removal. The inorganic carbon content was calculated as thedifference between total and organic carbon. The reproducibility for inorganic car-bon, Corg and Ntot analysis was 0.1%, 1%, and 1.7% relative standard variation,respectively.

2.4. Oxygen profiling

In situ oxygen profiles were recorded by TROL (Temperature Resistivity OxygenLander), a tripodal free-falling vehicle built at the NIOZ. Details of operation andtechnical instrumentation were given in Epping, & Helder (1997). TROL is equippedwith a resistivity electrode (Andrews, & Bennett, 1981) and 6 pressure-compensatedoxygen micro-electrodes. The oxygen-electrodes are of the single cathode type(Revsbech, & Jørgensen, 1986) and have an outer tip diameter of 10 to 50mm. Theoutput ranges typically between 300–1500 pA (100% O2 saturation at 20°C, 35‰S). Readings stabilised within 3 seconds. A single Ag/AgCl reference electrode isshared by all oxygen electrodes.

The profiling procedure is initiated 30 min after landing on the sea floor by acontinuous downward movement. A 10% increase of resistivity, usually detectedwithin 1 mm above the sediment surface, indicated the position of the sedimentsurface. The profiler then holds and moves the electrode array 10 mm upwards fromwhere the actual profiling procedure starts. Oxygen sensors were moved downwardsin discrete steps of 100mm. The signal was allowed to stabilise at each step for 5seconds before the reading was stored in the solid state memory of the profiler.

Shipboard oxygen profiles were performed in sediment cores (10 cm i.d.) immedi-ately after the temperature of the overlying water had re-adapted to the in situ tem-perature (usually within two hours). The overlying water of the core was stirred bya small rotating magnet, positioned 3 cm above the sediment–water interface. Themomentum of this magnet was switched from clockwise to anticlockwise rotation


every 5 sec in order to minimise the effect of steady pressure gradients caused byone-directional flow patterns. The diffusive boundary layer created by this stirringdevice ranged from 200 to 500mm. The on-deck profiles were made with commer-cially available Clark-type micro-electrodes (Diamond Corporation, type 737).Further details on the electrode characteristics and the profiling procedure are givenelsewhere (Lohse, Epping, Helder, & van Raaphorst, 1996).

Oxygen electrodes were calibrated after determining the oxygen concentration inthe bottom water by triplicate Winkler titrations. The zero signal was obtained fromreadings in the anoxic part of the sediment. When oxygen was still present at themaximum profiling depth, zero-signals were obtained from temperature correctedmeasurements in nitrogen-flushed bottom water.

2.5. Modelling of the oxygen profiles

The steady-state distribution of oxygen in sediments is governed by a balance ofdiffusion and reaction and can be approximated by

0 5 Ds

d2Cdz2 2 Rtot (1)

whereDs is the effective diffusion coefficient for oxygen,C represents the concen-tration of oxygen at depthz, and Rtot denotes the volumetric oxygen consumptionof the sediment. The reaction termR can be specified by

Rtot 5 R1 1 R2exp(2az) (2)

in which R1 represents the organic matter fraction respired depth-independently,while R2 characterises an organic matter fraction which decreases exponentially withdepth, as determined by the attenuation coefficienta. The application of two reactionterms, each identified by their own degradation constant may oversimplify the com-plexity of organic matter mineralisation, but has been applied successfully to a var-iety of deep sea environments characterised by a low input of organic carbon (Hales,Emerson, & Archer, 1994; Hammond, McManus, Berelson, Kilgore, & Pope, 1996).

Eq. (1) can be solved analytically by setting the boundary conditions toC 5 0anddC/Dz 5 0 atz 5 zmax andC 5 Co at z 5 0. Following the procedures as givenin Epping, & Helder (1997) the solution of Eq. (1) is

Cz 5R1

2Ds

(z 2 zmax)2 1R2

Dsa2 (e−azmax(az 2 azmax 2 1) 1 eaz) (3)

The flux (Jo) of oxygen across the sediment–water interface is given by

Jo 5 ø(R1zmax 1R2

a(1 2 e−azmax)) (4)

where ø is the volumetric porosity. The model fit procedure was carried out usingthe Microsoft Excel solver routine in whichR1, R2, a andzmax were varied to mini-


mise the sum of squares between the observed and the modelled oxygen concen-trations.

2.6. Estimation of sediment diffusivity

Ds was estimated from the temperature-corrected free solution diffusion coef-ficients (Do) provided by Li, & Gregory (1974), as

Ds 5Do

øF(5)

whereF is the formation factor. This factor was calculated from the resistivity pro-files recorded in situ by TROL (Andrews, & Bennett, 1981):

F 5Rsed

Rwater

(6)

whereRsedandRwater represent the resistivity in the sediment and the overlying water,respectively. At stations where no TROL deployments were made, resistivity wasmeasured shipboard in sediment cores retrieved by the box-corer. Sediment porosity(ø) is empirically related to theF-factor by the Archie relation

ø 5 F1b (7)

Porosity was determined from the weight loss of known aliquots of sedimentbefore and after drying at 60°C for 24 h, assuming a specific sediment weight of2.65 g cm−3. Fits of Eq. (7) through the data yielded values ofb between 2.8 and3.3. A lower value of 1.4 was estimated for Omex A. The calculated porosity profileswere then fitted with

ø(z) 5 (ø0 2 ø )exp−az 1 ø (8)

where ø(0) and ø(`) represent the porosity at the sediment water interface and atinfinite depthz and a is the depth attenuation coefficient.

2.7. Diffusive fluxes of dissolved manganese and iron, ammonium, nitrate andsilica

Diffusive fluxes at depthz of nitrate, the dissolved fractions of manganese andiron and ammonium where calculated according to Ficks first law.

J 5 2 øDs

dC(z)dz

(9)

in which J represents the flux of the specific redox-component. Nitrate reductionrates were estimated by assuming that the downward gradient below the nitrificationzone equals the depth-integrated nitrate reduction rate. The production rates of dis-solved manganese and iron were derived from the steepest part of the pore watergradient just below the anoxic–oxic interface. Similarly, the contribution of anoxic


mineralisation processes were quantified by estimating the upward ammonium porewater gradient. Pore water profiles of silica were evaluated with the steady-statediffusion first-order dissolution model (Schinck, Guinasso, & Fanning, 1975) follow-ing the procedure of Koning, Brummer, van Raaphorst, van Bennekom, Helder, &van Iperen (1997).

2.8. Sediment-water fluxes

Benthic fluxes of nutrients were determined by shipboard incubation of 3 to 5sediment-cores (95 mm i.d.). Sediment cores retrieved either with box-core or withmulti-core devices were brought together with the recovered water column to thecool-laboratory immediately after the coring device had been secured on deck. Theoverlying water was not exchanged in order to minimise disturbances of the sedi-ment–water interface. The sediment cores were incubated for 3 to 8 h together with500 to 800 ml of overlying water. The water was stirred by a small rotating magnetidentical to that described for oxygen cores in order to avoid the build-up of concen-tration gradients. The changes in concentrations of the nutrients were monitored bytaking sub-samples from the overlying water at 30 to 60 minutes intervals. Eachtime the volume of sampled water was replaced with unfiltered bottom water froma 3 l reservoir which was also monitored for concentration changes. However,changes in concentrations in the bottom water reservoir were usually undetectablefor any nutrients and were, therefore, neglected in further calculations. All sampleswere filtered and analysed within 24 h shipboard. Fluxes were calculated by linearregression of the changes of the concentration as a function of incubation time andmultiplying the slope with the specific volume/area ratio of the core. In 1993 and1994, additional flux data were available from in situ measurements carried out byBOLAS, a free falling benthic chamber lander. This instrument deployed two circularbenthic chambers (i.d. 30 cm) for a period between 4 h and 18 h at the sea floor.The overlying water inside the chamber was stirred by a small rotating magnet andwater samples were taken in regular intervals by motor-driven syringes. Furtherdetailed information on the technical features of this instrument is given in Tahey,Duineveld, Berghuis, & de Wilde (1996).

2.9. Bottom water nutrient concentrations

Bottom water nutrient concentrations were determined from triplicate analysis ofthe overlying water in a box-core and the individual cores of the multi-corer, respect-ively. These values usually did not deviate for more than 5% from the concentrationsmeasured in the deepest CTD-Rosette bottle, which were sampled 5 m above thesea floor.


3. Results and data synthesis

3.1. Bottom water and sediment characteristics

Bottom water temperatures and oxygen concentrations at individual stations werenearly identical during all three cruises (Table 1). Temperatures decreased downslope from 11°C at 200 m water depth (Omex A) to 2.5°C at 4500 m (Omex E).Oxygen concentrations decreased from|260mmol dm−3 to minimum values of|210mmol dm−3 at 1000 m water depth, before increasing again to concentrations between230 and 260mmol dm−3 at greater water depths. The corresponding oxygen saturationvalues ranged between 75 and 95%.

Porosity measurements obtained at individual stations generally varied by less than10% between the three cruises, implying that the diffusional characteristics of thesediments were not varying substantially during the investigated period. Interfacialporosity ranged between 0.91 and 0.97 (calculated from Eq. (8)), revealing no cleartrend with increasing water depth, while the depth-averaged porosities increased from0.70 at 200 m water depth to of 0.85 at 1425 m (Table 1).

3.2. Sedimentary carbon and nitrogen inventory

A selection of representative profiles summarise the essential features of the sedi-mentary carbon and nitrogen inventories at the Goban Spur transect (Fig. 2). Thewater depth related distribution of organic carbon was a more prominent feature thanthe inter-annual variability of down-core profiles at individual stations. Estimatedorganic carbon concentrations at the sediment–water interface increase graduallyfrom about 0.2% at 210 m (Omex A) to maximum 0.7% at 1425 m water depth(Omex II), while slightly decreasing concentrations were found when going deeper(Fig. 3). Our slicing technique resolved steep down-core gradients of organic carbonat the mid-slope stations II, C and F, whereas no or only weak gradients were discern-ible at the lower and the deeper part of the slope (Fig. 2). A similar pattern wasobserved for nitrogen profiles which had the steepest down-core profiles at stationsII, C and F. Extrapolated nitrogen values at the sediment water interface increasedfrom 0.029% at Omex A to maximum values of 0.074% at Omex C. Stations deeperthan 2000 m had relatively uniform concentrations of 0.07%. Atomic C/N ratioswere relatively constant among stations and with depth in the sediment, at valuesbetween 6 and 7.5 (Fig. 2).

3.3. Pore water gradients

Oxygen pore water profiles obtained from in situ profiling by TROL are given inFig. 4. Oxygen penetrates deeper into the sediment with increasing water depth anddistance from the shelf, reflecting the diminishing amount of degradable amount oforganic matter delivered to the sediments. At Omex A, oxygen was undetectablebelow 15 mm sediment depth. The oxygen penetration increased gradually along theupper slope of the transect to 40–100 mm at Omex II. Stations located on the lower


Fig. 2. Downcore profiles of solid phase organic carbon (upper panel) for 1993 (squares), 1994 (circles)and 1995 (triangles) (upper panel), total nitrogen (middle panel) and atomic C/N ratios (lower panel) atGoban Spur. Solid lines represent profile fits by OCz 5 OCzmax 1 (OCsfa. 2 OCzmax) exp−az where OCis the organic carbon concentration at depthz, OCzmax is the asymptotic carbon concentration at depth,OCsfa is the organic carbon concentration at the sediment–water interface, anda is the depth-dependentattenuation coefficient (cm−1). Profiles of total nitrogen were fitted with the same equation.


Fig. 3. Organic carbon concentrations at the sediment–water interface as derived from all organic carbonprofiles fitted (see Fig. 2).

Fig. 4. Representative oxygen profiles obtained from in situ profiling by TROL in 1994 (Charles Darwincruise). Thin lines connecting data points represent the modelled profile.


part of the slope were characterised by comparable penetrations depths, though theasymptotic curvature of the pore water profiles makes it difficult to estimate thepenetration exactly. Diffusive fluxes calculated according to Eq. (4) are given forall individual oxygen profiles in Fig. 5 and are summarised in Table 2. The overalltrend was characterised by high oxygen uptake rates ranging between 3.5 and 8mmol m−2 d−1 on the upper slope (Omex A) which declined to values between 3.1and 0.8 mmol m−2 d−1 at stations located on the lower slope. Most of the decreasein oxygen uptake rates occurred over the upper part of the slope transect spanningan increase in water depth of|1500 m (Omex II). On the lower part of the sloperates showed no obvious trend, despite a further increase in water depth of|3000m down to the abyssal plain. The standard deviation at individual stations rangedbetween 5 and 49%, with 18% as a mean value. Given the magnitude of the temporaland spatial variation of oxygen uptake rates, it is difficult to gauge seasonality fromthe present data set.

Comparing the oxygen fluxes obtained from shipboard measurements and fromTROL deployments revealed no systematic differences between the two methods(Table 2). The fluxes and the maximum penetration of oxygen at individual stationshad a large site-specific heterogeneity which may have masked any systematic differ-ences between shipboard and in situ measurements. Previous investigations haveshown that oxygen profiles measured on-deck may suggest lower penetration depths

Fig. 5. Benthic oxygen uptake rates (Jo) from all individual oxygen profiles measured shipboard and byTROL as calculated from the reaction diffusion model (for explanation see text).


Table 2Benthic oxygen uptake rates (Jo), number of profiles (n), and maximum penetration depths of oxygen(zmax) obtained from in situ TROL deployments and shipboard measurements

TROL-Lander Shipboard

Station Sampling date Jo (mmol m−2 d−1) zmax (mm) Jo (mmol m−2 d−1) zmax (mm)

A 18-10-93 2 4.4 6 0.8 (3) 12.86 1.2 2 5.7 6 0.7 (5) 9.56 0.623-05-94 2 5.4 6 1.0 (3) 7.86 1.7 2 7.5 6 0.3 (3) 4.26 0.0

I 19-10-93 – – 2 5.0 6 1.4 (3) 12.46 5.423-05-94 2 3.3 6 0.0 (2) 23.66 1.7 2 2.3 (1) 19.221-09-95 2 4.4 (1) 14.3 2 1.5 6 0.2 (3) 25.26 4.9

B 20-10-93 – – 2 2.6 6 0.7 (3) 61.96 5.324-05-94 2 2.5 6 0.8 (4) 43.66 22.8 2 1.1 6 0.1 (3) 23.96 2.6

II 21-10-93 2 1.8 (1) 42.7 2 1.2 6 0.1 (2) 55.36 4.525-05-94 2 2.0 6 0.1 (3) 80.56 12.2 2 2.9 (1) 18.018-09-95 2 2.7 6 0.1 (2) 96.46 3.7 2 3.1 6 0.7 (5) 66.86 7.3

C 22-10-93 – – 2 1.3 6 0.7 (2) 84.66 3.1F 25-10-93 2 3.0 6 0.6 (3) 60.46 14.8 2 2.4 6 0.7 (3) 61.86 5.4

27-05-94 2 1.6 (1) 93.0 2 2.1 (1) 87.019-09-95 2 2.6 (1) 94.0 2 2.3 6 0.6 (5) 88.06 8.4

III 24-10-93 2 2.6 6 0.7 (3) 62.66 1229-05-94 2 0.8 6 0.2 (3) 142.66 47.0 2 1.7 6 0.5 (3) 104.16

6.420-09-95 – – 2 3.1 6 0.4 (5) 69.26 6.5

E 24-10-93 2 1.5 (1) 145.631-05-94 2 0.8 (1) 143.0 2 1.5 6 0.2 (2) 84.66 12

compared to in situ measurements (e.g. Glud, Gundersen, Jørgensen, Revsbech, &Schulz, 1994).

While microelectrode techniques visualise pore water gradients of oxygen withoutdisrupting the sediment structure, other solutes so far can only be analysed by separ-ating the pore water from the sediment matrix. Several techniques have been appliedto extract pore water including centrifugation, (Saager, Sweerts, & Ellermeijer,1990), squeezing (Reeburgh, 1967), and whole core extraction techniques (Jahnke,1988). It should be noted that all these techniques are subject to potential errors andproblems (e.g. Berelson, Hammond, O’Neill, Xu, Chin, & Zukin, 1990). Low-press-ure operated sediment squeezers, as applied in the present study, enable extractionof the pore water to be relatively rapid, and avoid a strong physical disruption.

As a detailed evaluation of the nitrate pore water profiles is provided elsewhere(Balzer, Helder, Epping, Lohse, & Otto, 1998), we only focus on the principal fea-tures. All nitrate pore water profiles display clear subsurface maxima (Fig. 6), sug-gesting that nitrification was the primary nitrate source for denitrification rather thanthe overlying water (Lohse, Kloosterhuis, van Raaphorst, & Helder, 1996). The slopeof the nitrate gradient below the subsurface maximum appeared to be a function ofwater depth with sharpest peaks at stations on the shelf stations. Here, nitrate pen-etrated 2 to 4 cm into the sediment, while the penetration at the deepest station


Fig. 6. Nitrate pore water profiles for all stations and seasons (symbols as indicated in legend of Fig. 2).

(Omex E) exceeded the length of our sediment cores. An extrapolation of the linearpart of the down-core gradient suggests a penetration of|40 cm at this station.Assuming that the down-core nitrate gradient equals the depth-integrated nitratereduction rate, we applied Eq. (9) to estimate denitrification rates. Denitrificationrates increase from 0.15 mmol N m−2 d−1 at the shallowest station to 0.19 mmol Nm−2 d−1 at 670 m and then decline rapidly to values between 0.031 and 0.008 N m−2

d−1 at stations deeper than 1900 m (Table 3). As for the oxygen profiles, no clearseasonal trend could be detected from the variability of nitrate profiles at individ-ual stations.

Dissolved manganese and iron profiles are given in Fig. 7. The presence of theseelements in the pore water indicates the initialisation of suboxic mineralisation pro-cesses after the sediment has become depleted of oxygen. Maximum concentrationsof both elements did not exceed 10mmol dm−3 at any of the stations, which arerather low values. The fluxes calculated from the upward gradient below the oxiczone were, 0.03 mmol m−2 d−1 for dissolved manganese and, 0.09 mmol m−2

d−1 for dissolved iron (data not shown).Increasing ammonium concentrations in the anoxic part of the sediment, indicative

for the presence of sulphate reduction, were only observed at the shelf (Omex A)and upper-slope (Omex I and B) stations (Fig. 7). At these stations, down-coreammonium concentrations started to increase gradually after nitrate was depleted,


Table 3Nitrate reduction rates derived from the down-core gradient of nitrate pore water profiles

Station Cruise z1 z2 d[NO3−]/dz DNO

−3

NO3−

(10−2 m) (10−2 m) 10−2 mmol m−4 (10−5m2 d−1) reduction(mmol m−2 d−1)

A 1993 1.75 2.75 2 1.38 14.8 0.1441994 1.25 3.5 2 1.51 14.6 0.153

I 1993 0.675 1.75 2 1.96 13.0 0.2051994 0.875 2.25 2 1.84 12.4 0.1971995 0.875 1.75 2 2.41 10.4 0.185

B 1993 2.25 4.5 2 0.91 12.1 0.0911994 1.75 3.5 2 1.19 12.5 0.123

II 1993 2.75 6.5 2 0.60 10.8 0.0551994 1.75 6.5 2 0.62 11.1 0.0591995 2.75 12 2 0.21 9.42 0.017

C 1993 5.5 10 2 0.38 10.2 0.031F 1993 3.5 10 2 0.20 9.68 0.016

1994 0.875 10 2 0.29 9.42 0.0231995 3.5 10 2 0.21 9.59 0.017

III 1994 3.5 12 2 0.17 9.50 0.0141995 4.5 10 2 0.24 9.59 0.020

E 1994 8 14 2 0.11 9.24 0.008

suggesting the importance of ammonium oxidation at the oxic/anoxic boundary.Maximum down-core concentrations of 40 to 75mmol dm−3 were measured at OmexI at 670 m water depth. Stations located below 1040 water depth showed down-coreammonium concentrations lower than 10mmol dm−3. The frequent presence of smallammonium peaks just below the sediment-water interface may have been caused bytemporary exposure to increased temperatures during the core’s retrieval (Lohse,unpublished). Therefore these peaks are considered as an artefact and will not beincluded in further considerations. Table 4 summarises the anoxic mineralisationrates as quantified by fitting a straight line through the linear part of the ammoniumprofile at depth. Highest anoxic mineralisation rates ranged between 0.041 and 0.082mmol N m−2 d−1 at Omex I. These rates decreased by|90% at 1425 m water depth(Omex II) and did not show any trend with further increasing water depth.

Asymptotic maxima of silica pore water concentrations increase with water depth(Fig. 8), suggesting that the rate of siliceous particle dissolution decreases with waterdepth (Schinck, Guinasso, & Fanning, 1975). The silica profiles also indicate thatthe sediment depth interval at which SiO2 dissolution is virtually complete, increasesfrom 3 to 4 cm at 200 m water depth to 10 to 14 cm at 3500 m water depth. Thediffusive flux of silica across the sediment water-interface was modelled by assumingfirst-order dissolution with respect to the saturation concentration of dissolved silicaat sediment depth (Koning, Brummer, van Raaphorst, van Bennekom, Helder, & vanIperen, 1997). Highest fluxes were observed at Omex I, where they ranged between1.1 and 0.75 mmol Si m−2 d−1 (Fig. 10). Silica fluxes decreased regularly to 0.25mmol m−2 d−1 at Omex F and did not substantially differ further downslope.


Fig. 7. Pore water profiles of dissolved manganese (upper panel), dissolved iron (middle panel) andammonium (lower panel) at Goban Spur stations I, II, and III (symbols as indicated in legend of Fig. 2).

3.4. Benthic chamber nutrient fluxes

Incubations of retrieved sediment cores or deployments of benthic chambersrevealed that nitrate and silica were both being released from the sediments alongthe entire transect. The accumulation of these compounds in the overlying water inshipboard-incubated sediment cores is illustrated in Fig. 9. The increase of silicateand nitrate in the overlying water of a sediment core ranged between 2.5 to 10% ofthe initial bottom water concentration during an 8 h incubation period. During thisperiod concentrations of both compounds increased linearly, although the scatter ofdata points around the regression line tended to be higher at low fluxes. The repro-ducibility at each station, indicated as the coefficient of variation was 45.1–9.3%(mean 24.4%) for silica and 43.8–1.4% (mean 21.1%) for nitrate.


Table 4Anoxic mineralisation rates derived from the build-up of ammonium in pore waters

Station Cruise z1 z2 d[NH4+]/dz DNH

+4

NH4+ production

(10−2 m) (10−2 m) 10−2 mmol m−4 (10−5 m2 d−1) (mmol m−2 d−1)

A 1993 2.75 8 0.27 14.8 0.0281994 2.75 8 0.31 14.6 0.031

I 1993 1.75 4.5 0.79 13.0 0.0821994 1.75 4.5 0.70 13.4 0.0751995 1.75 4.5 0.530 10.4 0.041

B 1993 4.5 12 0.330 12.1 0.0331994 4.5 12 0.290 12.5 0.030

II 1993 0.675 12 0.028 10.8 0.0031994 8 12 0.066 11.1 0.0061995 8 10 0.045 9.4 0.004

C 1993 12 14 0.184 10.2 0.015F 1993 8 14 0.026 9.7 0.002

1994 8 14 0.008 9.4 0.0011995 8 14 0.018 9.6 0.015

III 1994 10 14 0.006 9.5 0.0001995 10 14 0.016 9.6 0.014

E 1994 10 14 0.005 9.2 0.000

Fig. 8. Dissolved silica pore water profiles at Omex stations I, II and III (symbols as indicated in legendof Fig. 2).

Plotting benthic silica fluxes as a function of water depth and investigation periodyields comparable high fluxes of 0.836 0.37 mmol m−2 d−1 at 200 m water depth(Omex A) which decreased to 0.186 0.08 mmol m−2 d−1 at Omex F (2100 m) (Fig.10). Fluxes at greater water depth were comparable to those reported at Omex F. Theshipboard determinations compared well with fluxes measured in situ with benthicchambers (Fig. 10). The variation of benthic silica fluxes at each station did notindicate a coherent trend between the three cruises. However, the close agreementbetween measured silica fluxes and calculated fluxes as deduced from the interstitialpore water profiles indicate that bioirrigating organisms were not significantlyenhancing the silica flux across the sediment–water interface. Additionally, the simi-larity of both fluxes indicates the integrity of the sediment–water interface and


Fig. 9. Representative examples of increases of silica (right panel) and nitrate (left panel) in the overlyingwater of shipboard incubated sediment cores at Omex stations I (upper panel), II (middle panel), and III(lower panel). Data are from 1995. Open and closed symbols represent replicate cores.

implies that no major disruption of sedimentary processes took place during samplingor incubation of sediment cores.


Fig. 10. Calculated and measured sediment–water fluxes of silica (upper panel) and nitrate (lower panel)along the Goban Spur transect.

Nitrate fluxes ranged between 0.016 and 0.245 mmol m−2 d−1 with no obviousspatial or seasonal trend (Fig. 10). This holds also for the very low fluxes ofammonium and phosphate which ranged between –0.02 to 0.12 mmol m−2 d−1 and20.007 and 0.011 mmol m−2 d−1, respectively (data not shown).


3.5. Carbon oxidation rates

Production and consumption rates of oxygen, nitrate and ammonium as compiledin Tables 2–4 can be converted into carbon oxidation rates by assuming that theorganic matter degradation proceeds according to Redfield stoichiometry (e.g. seeFroehlich, Klinkhammer, Bender, Luetdke, Heath, Cullen, Dauphin, Hammond,Hartmann, & Maynard, 1979). Hence, the relation between the consumption of anindividual electron acceptor and the organic carbon oxidation rate is given by

(CH2O)106(NH3)16(H3PO4) 1 138 O2→106 CO2 1 16 HNO3 1 H3PO4 1 122H2O

(oxygen consumption) (10)

(CH2O)106(NH3)16(H3PO4) 1 94.4 HNO3→106 CO2 1 55.2 N2

1 H3PO4 1 177.2 H2O

(nitrate reduction) (11)

(CH2O)106(NH3)16(H3PO4) 1 236 MnO2 1 472 H+→236 Mn2 1

1 106 CO2 1 8N2 1 H3PO4 1 366H2O

(manganese reduction) (12)

(CH2O)106(NH3)16(H3PO4) 1 212 Fe2O3 1 848 H+→424 Fe2 1 1 106 CO2

1 16 NH3 1 1 530 H2O

(iron reduction) (13)

(CH2O)106(NH3)16(H3PO4) 1 53 SO24→106 CO2 1 16 NH3 1 53 S2 2

1 H3PO4 1 106H2O

(sulphate reduction) (14)

The depth integrated carbon oxidation rate can be derived from Eq. (10), pro-vided that:

1. all reduced metabolites produced during suboxic and anoxic metabolism are reoxi-dised with oxygen at the oxic-anoxic boundary and do not diffuse out of the sedi-ment;

2. that CO2 is the ultimate reaction product of all carbon mineralisation processes,rather than the production of intermediates like dissolved organic carbon(DOC); and

3. that burial of reactive reduced compounds can be neglected.

The first constraint is met at all but one station (Omex A), where a smallammonium efflux was detected. The second constraint is subject of discussion, sincepore water concentrations of DOC along the Goban Spur transect exceeded the over-lying water concentrations by one order of magnitude (Otto, & Balzer, 1998). How-


ever, given the uncertain diffusion coefficient for DOC and the methodological prob-lems in determining DOC pore water gradients we consider the contribution of DOCto the total mineralisation as uncertain. The third constraint is met since there is noindication of significant burial of reduced compounds. With these constraints, thecarbon oxidation rate (Coxid) is given by

Coxid 5 O2 uptake1 Cdenit 2 O2 uptake nitrif (15)

in which O2uptakerepresents the oxygen uptake rate of the sediment, Cdenit representsthe amount of carbon oxidised by denitrification and O2 uptake nitrifdenotes the oxygenuptake caused by the oxidation of ammonium. The latter term can be quantified byknowing that 2 molecules of oxygen are necessary to oxidise one molecule ofammonium. Cdenit has to be added to the carbon oxidation rate since the end-productof denitrification, gaseous nitrogen, is not reflected by the oxygen uptake of thesediment (Canfield, Jørgensen, Fossing, Glud, Gundersen, Thamdrup, Hansen,Nielsen, & Hall, 1993). Since the contribution of nitrate-, manganese-, iron and sul-phate reduction to Coxid is constrained by Eqs. (11)–(14), we assume that the differ-ence between these processes and Coxid represents the amount of oxygen used directlyfor organic carbon oxidation by heterotrophic bacteria (Chetero):

Chetero 5 Coxid 2 CMn 2 CFe 2 Canoxic (16)

in which CMn and CFe are the contributions of manganese (IV) and iron (III)reduction, respectively, and Canoxic denotes the amount of carbon mineralised alonganaerobic pathways.

Given the apparent lack of a pronounced seasonality (which we will addressbelow), we have condensed our data set by averaging the annual estimates at eachstation. The results of these calculations are summarised in Fig. 11 and Table 5.Total carbon oxidation rates decline from 4.3 mmol C m−2 d−1 at 210 m water depthto 1.5 mmol C m−2 d−1 at 1000 m. Stations located on the lower part of the slope(i. e from 1450 to 4500 m) had carbon oxidation rates between 1.78 and 0.98 mmolC m−2 d−1 which showed no clear trend correlated with water depth. Oxic degradationrates derived from Eq. (16) account for 68 to 90% of the total carbon oxidation rateson the upper slope, while the percentages increase to more than 90% on the lowerslope. The oxygen consumption by nitrification contributed between 5 and 16% ofthe total oxygen uptake rate at stations located between 200 and 1000 m water depth.At deeper stations, 3% of the oxygen taken up was consumed by nitrification. Thecontribution of sub- and anoxic mineralisation processes to the overall carbon oxi-dation rate are given in Table 5. Denitrification accounts for 0.2 to 0.01 mmol Cm−2 d−1 which corresponds to 9.5 to 1.2% of the total oxidation rates. These estimatesare in close agreement with Balzer, Helder, Epping, Lohse, & Otto (1998) whocalculated carbon and nitrogen mineralisation at Goban Spur from modelling ofnitrate pore water gradients. Manganese and iron reduction represented quantitativelyinsignificant processes (, 1%) within in the total benthic carbon budget at GobanSpur and are therefore neglected in further discussions. Sulphate reduction rates,as deduced from the accumulation of ammonium in anoxic sediment layers, werecomparatively high at stations A, I and B where they ranged between 0.25 and 0.48


Fig. 11. Depth-integrated carbon mineralisation rates specified for aerobic respiration, denitrification,and anoxic mineralisation along the Goban Spur transect. Symbols indicate concentrations of organiccarbon at the sediment-water interface as shown as in Fig. 3.

Table 5Compilation of carbon oxidation rates by individual electron acceptors and total carbon oxidation rate

Chetero Cden Cmn Cfe Canox Coxid

Station (mmol C m−2 d−1)

A 3.89 0.15 0.00 0.00 0.29 4.33I 1.58 0.2 0.05 0.04 0.48 2.36B 1.12 0.11 0.01 0.01 0.25 1.49II 1.70 0.04 0.00 0.00 0.03 1.78C 0.92 0.03 0.00 0.00 0.11 1.06F 1.72 0.02 0.00 0.00 0.04 1.78III 1.47 0.02 0.00 0.01 0.05 1.55E 0.96 0.01 0.00 0.00 0.01 0.98

mmol C m−2 d−1, corresponding to 7 to 21% of the total carbon oxidation rate.Sulphate reduction did not account for more than 5% of the total carbon oxidationrate at stations located deeper on the slope.


4. Discussion

4.1. The geochemical sequence of redox-indicative pore water compounds

The pore water profiles of oxygen, nitrate, manganese, iron and ammonium insediments along the Goban Spur shelf-slope transect document an inverse relation-ship between the input of organic matter to the sea floor and water depth. The depth-depending distribution of these compounds resembles the common sequence in theuse and depletion of terminal electron acceptors (Froehlich, Klinkhammer, Bender,Luetdke, Heath, Cullen, Dauphin, Hammond, Hartmann, & Maynard, 1979). Oxygenis the first terminal electron acceptor in the oxidation of organic matter and is con-sumed within the upper 15 mm of the sediment at the shallowest station Omex A.The remaining degradable part of organic matter is then oxidised by secondary elec-tron acceptors which are used according to the free energy gained during theirreduction. As a consequence, nitrate concentration reaches zero values at depthsslightly below the oxic layer. The subsequent appearance of dissolved manganeseand dissolved iron, particularly at stations A, I and B reflects the use of their oxidesas electron acceptors in the suboxic part of the sediment. The contribution of sulphatereduction at these stations is evidenced from increasing ammonium concentrationswell below the oxic zone of the sediment (Bender, & Heggie, 1984). With increasingwater depth, pore waters become increasingly oxic as corroborated by the consecu-tive disappearance of ammonium, dissolved iron and manganese as well as theincreasing oxygen and nitrate penetration into the sediment.

4.2. Does a carbon-depocenter exist at Goban Spur?

Benthic carbon mineralisation rates decrease by|40% when descending from 200m to about 1500 m water depth (Fig. 11, Table 5). Along the same depth interval,organic carbon concentrations increase from 0.2% to 0.7% (Fig. 3). These gradientsof carbon mineralisation and carbon contents occur on the upper slope over a hori-zontal distance of|140 km and are accompanied by an increase of water depth of1300 m (Omex A-Omex II), whereas no distinct variations were found on the lowerslope which spans a horizontal distance of 80 km and a further increase of waterdepth by|3000 m (Omex C-Omex E). These data do not indicate the presence ofa restricted area with a clear maximum in sedimentary organic carbon concentrationsand elevated benthic carbon mineralisation rates, thus suggesting that there is nocarbon-depocenter along the Goban Spur transect. This conclusion is supported bycomparing our diffusive fluxes as calculated from oxygen pore water profiles withconcurrent benthic chamber measurements performed along the Goban Spur transectby Duineveld, Lavaleye, Berghuis, de Wilde, van der Weele, Kok, Batten, &DeLeeuw (1997). These authors measured benthic oxygen uptake rates of between3.1 and 5.4 mmol O2 m−2 d−1 on the upper slope (Omex A) and the rates decreasedgradually with increasing water depth to| 0.8 mmol O2 m−2 d−1 at 4500 m (OmexE). These rates are not significantly higher than our diffusive estimates (5.7 to 1.1mmol O2 m−2 d−1), implying that enhanced transport by bioirrigating organisms can


not, at least in a statistical sense, be discerned from the comparison of the two datasets. This conclusion is further evidenced by the apparent lack of significant differ-ences between diffusive and measured silica fluxes (see above). Studies performedin the direct vicinity of the Goban Spur transect addressed the short term variabilityof benthic oxygen uptake rates in the Porcupine Seabight (2800 m water depth).Here, Lampitt, Raine, Billett, & Rice (1995) measured rates between 1.8 and 3.2mmol O2 m−2 d−1. These authors also report benthic oxygen uptake rates for thePorcupine abyssal Plain, a station located at 4800 m water depth in the north-easternAtlantic. The rates found there varied around 0.7 mmol O2 m−2 d−1 and were closeto estimates made by Pfannkuche (1993) who reported oxygen uptake rates between0.35 to 0.90 mmol O2 m−2 d−1. Slightly higher estimates were reported by Patching,Raine, Barnett, & Watson (1986), who found oxygen uptake rates ranging between1.4 and 1.8 mmol O2 m−2 d−1. We conclude that our diffusive fluxes compare wellwith oxygen fluxes measured in situ with benthic chambers in our study area.

Additional evidence for the absence of a carbon-depocenter at Goban Spur canbe derived from a comparison of our carbon mineralisation rates with the carbonburial rates as estimated by van Weering, Hall, de Stigter, McCave, & Thomsen(1998). These authors estimated carbon burial rates from biostratigraphically determ-ined sedimentation rates which were, after conversion into mass accumulation rates,multiplied with the refractory background carbon concentration in the sediment.Highest carbon burial rates were reported to be on the shelf, were they were > 0.037mmol C m−2 d−1, whereas they vary further downslope between 0.011 and 0.025mmol C m−2 d−1 but without an obvious trend related to water depth (Table 6). Thesevery low values contrast to the carbon mineralisation rates which were one order ofmagnitude higher at all Omex stations. The carbon burial efficiency (E, in %) wascalculated as

E 5Cburial

Coxid 1 Cburial

100 (17)

Table 6Deposition-(Cdeposition) and burial-(Cburial) rates of organic carbon. Burial efficiencies (E) are defined asCburial/(Coxid 1 Cburial) 3 100. Cburial rates are taken from van Weering, Hall, de Stigter, McCave, &Thomsen (1998). For further explanation see text

Cdeposition Cburial E

Station (mmol C m−2 d−1) (in %)

A 4.37 >0.037 0.8I 2.38 0.023 1.0B 1.51 0.021 1.4II 1.79 0.011 0.6C 1.09 0.025 2.3F 1.80 0.016 0.9III 1.57 0.018 1.2E 1.00 0.023 2.3


where Cburial represents the carbon burial rate (mmol C m−2 d−1). In this approachthe input of organic material to the sediment is defined by the sum of carbon oxi-dation (Coxid) and carbon burial (Cburial), rather than estimates obtained from carbonfluxes in deep sediment traps, because any lateral inputs in the boundary layer wouldcause an underestimation and rebound fluxes at the seabed an overestimation of thetrue settling fluxes (Antia, personal communication). Burial efficiencies calculatedin this way range between 0.8 and 2.3% and appear not to be correlated to waterdepth (Table 6). Also, the order of magnitude of these percentages comply betterwith deep-sea rather than with continental margins sediments (Henrichs, 1992). Forinstance, burial efficiencies along the western North Atlantic continental marginrange between 5 and 88% (DeMaster, Pope, Levin, & Blair, 1994).

The absence of a carbon depocenter along the Goban Spur shelf-slope transitionis in contrast to investigations carried out on the western continental margin of theNorth Atlantic within the Shelf Edge Exchange Programme (SEEP, Biscaye,Flagg, & Falkowski, 1994). Results of this programme showed that a carbon depoc-enter is centred around the upper slope at 1000 m water depth with organic carbonconcentrations of about 2.2% (Anderson, Rowe, Kemp, Trumbore, & Biscaye, 1994).In this area, enhanced benthic oxygen uptake rates of 6.2 mmol O2 m−2 d−1 (Rowe,Bowland, Phoel, Anderson, & Biscaye, 1994) were measured despite the fact that thepredominant part of carbon supplied to the slope had a refractory nature (Biscaye, &Anderson, 1994). Studies carried out on the continental shelf of the North Searevealed that virtually all organic material produced is respired on the shelf and onlya minor fraction is transported to the slope depocenter at its north-eastern boundary(De Haas, Boer, & van Weering, 1997). Despite the presence of fast-accumulating,organic-rich sediments in this depocenter area, benthic rates of oxygen uptake(Lohse, Epping, Helder, & van Raaphorst, 1996), nitrogen mineralisation (Lohse,Malschaert, Slomp, Helder, & van Raaphorst, 1995) as well as cycles of silica(Gehlen, Malschaert, & van Raaphorst, 1995), manganese and iron (Slomp, Mal-schaert, Lohse, & van Raaphorst, 1997) were low and not significantly different fromthe shallow shelf.

Middelburg, Soetaert, & Herman (1997) established a empirical predictiverelationship between literature derived data on benthic carbon oxidation rates andwater depth as the independent variable. Applying this relation to the Goban Spurtransect, it would be predicted that carbon mineralisation rates decrease linearly from12.7 (Omex A) to 0.27 mmol C m−2 d−1 at Omex E. However, down to a waterdepth of 3000 m, the actual measured rates are lower by a factor of 2 to 4 than therates predicted from the empirical equation. Below that depth measured rates aremeasurably higher than the predicted rates. We explain this disparity by assumingthat the deposition of degradable organic matter on the upper slope of the N.W.European continental margin is relatively low because of an energetic hydrodynamicregime which tends to transport organic matter towards the lower slope. This hypoth-esis is in line with studies demonstrating a rapid downslope particle transport throughthe benthic boundary layer (van Weering, Hall, de Stigter, McCave, & Thomsen,1998), which can be as rapid as vertical transport from the euphotic zone to thedeep-sea (Thomsen, & van Weering, 1998). Also, data on densities and feeding type


of the macrofauna community suggest the presence of a strong current regime sincethe predominant feeding guild of benthic invertebrates was filter feeders trappingorganic particles from the benthic nephoid layer which would otherwise have beentransported downslope without incorporation into the sediment (Heip, Duineveld,Flach, Graf, Helder, Herman, Lavaleye, Middelburg, Pfannkuche, Soetaert,Soltwedel, de Stigter, Thomsen, Vanaverbeke, & de Wilde, 1998). The fact that filterfeeders irrigate the sediment considerable less than tube-dwelling deposit feeders(Boudreau, 1997) may also explain our observation that diffusive fluxes of oxygenand silica are not significantly different from fluxes measured whole-core incu-bations.

4.3. Reactivity of the organic material mineralised at Goban Spur

The reaction kinetics of organic matter undergoing decay can be approached fromthe depth-dependent oxygen reaction rates. In the present study, we specified thereaction term by two fractions: The first fraction,R1 is respired independent of sedi-ment depth, while the second fraction,R2, decreases exponentially with increasingsediment depth. The specification of oxygen consumption in the sediment into tworespiration types is based on the theory that the depth-independent fraction,R1,characterises a type of organic matter which is supplied and mixed through the biot-urbated zone at rates that are relatively high compared to the rate of degradation ofthis fraction. This situation has been demonstrated in many coastal and shelf areas,where the respiration of organic matter proceeds uniformly through the sedimentcolumn. (e.g. Hall, Anderson, Rutgers van der Loeff, Sundby, & Westerlund, 1989;Rasmussen, & Jørgensen, 1992; Lohse, Epping, Helder, & van Raaphorst, 1996).Analogously,R2 characterises a fraction of organic matter which decays relativelyfast compared to its supply and mixing into the sediment. Therefore, this fraction ischaracterised by an exponential decrease as it penetrates deeper into the sediment.The representation of respiration rates by these two fractions may represent a simpli-fication of reality, since decaying organic matter consists of many compounds,characterised by a continuum of degradation reactivities (Boudreau, 1997). However,many studies have successfully applied models similar to ours in deep-sea-, slope-and shelf sediments (Hales, Emerson, & Archer, 1994; Hammond, McManus,Berelson, Kilgore, & Pope, 1996; Epping, & Helder, 1997).

Fig. 12 shows oxygen respiration profiles as calculated from the oxygen profilesmeasured by TROL in 1994. Significant values ofR1 can only be found at the shal-lowest station Omex A where this respiration term contributed about 25% to thetotal respiration at the sediment–water interface. At all other stations,R1 did notaccount for more than 2% of the total respiration activity (Table 7). Therefore, theorganic matter decay is dominated by a fraction which decreases exponentially inthe sediment. Estimated respiration rates at the sediment–water interface declinedfrom 14036 278 nmol cm−3 d−1 at 210 m water depth (Omex A) to 516 8.5 nmolcm−3 d−1 at 4500 m water depth, indicating that theR2 fraction decreases with increas-ing water depth. This is corroborated by the depth attenuation coefficienta, whichdecreases accordingly from 9.56 5.4 cm−1 at the shallowest station to 0.86 0.2


Fig. 12. Depth-independent (R1, dashed line) and exponentially decreasing respiration (R2, solid line)profiles as calculated from modelled oxygen profiles. Arrows indicate depth at which theR2-fraction hasdecreased with 95% of its value measured at the sediment–water interface. Please note that the interfacialvalue forR2 at Omex A (1403 nmol cm−3 d−1) exceeds deliberately they-scale.

Table 7Model output parameters for TROL oxygen profiles obtained during the Charles Darwin Cruise 1994

Station Jo R1 (nmol cm−3 d−1) R2 (nmol cm−3 d−1) alpha (cm−1)

A 25.40 6 0.97 4696 156 14036 278 9.56 5.4I 23.29 6 0.04 126 10 5356 99 1.86 0.5B 22.47 6 0.84 , 2 441 6 289 1.66 0.6II 22.05 6 0.11 , 2 315 6 54 1.76 0.4F 21.65 , 2 179 1.2III 21.06 6 0.18 , 2 84 6 21 0.96 0.1E 20.79 6 0.01 , 2 51 6 8.5 0.86 0.2

cm−1 at the deepest station (Table 7). Accordingly, the penetration depth of theR2

fraction (here defined as a 95% decrease of the interfacial value) increases from 0.31cm at 200 m water depth to 4.68 cm at Omex III (Fig. 12). The depth attenuationcoefficient is not exclusively related to reactivity but also to sediment accumulationand mixing rates (Emerson, Fisher, Reimers, & Heggie, 1985; Hammond, McManus,Berelson, Kilgore, & Pope, 1996; Epping, & Helder, 1997):

a 52 w 1 √w2 1 4kcDb

2Db(18)

wherew represents the sedimentation rate (cm y−1), kc represents the first order degra-


dation rate (y−1) and Db represents the sediment mixing rate (cm−2 y−1). Data onsedimentation rates (van Weering, Hall, de Stigter, McCave, & Thomsen, 1998,Table 8) and sediment mixing rates (Soetaert, Herman, Middelburg, Heip, de Stigter,van Weering, Epping, & Helder, 1997; van Weering, Hall, de Stigter, McCave, &Thomsen, 1998) indicate that accumulation is negligible compared to 4kcDb, so thatthe first-order decay constant of theR2 fraction can be assessed by

kR2 5 Dba2.

Using the depth attenuation coefficients of theR2 fraction (Table 7) and theDb

values as estimated from a non-local mixing model (Soetaert, Herman, Middelburg,Heip, de Stigter, van Weering, Epping, & Helder, 1997) applied for210Pb profilesof van Weering, Hall, de Stigter, McCave, & Thomsen (1998) results in first orderdecay constants varying from 0.9 y−1 at Omex I to 0.002 y−1 at Omex E (Table 8).A considerable higher value of 22.6 y−1 was found for Omex A. Comparable decayconstants were calculated using the210Pb derived mixing rates provided by Hall(personal communication) for the Goban Spur. However, it should be pointed outthat these decay constants are subject to considerable uncertainty, since the mixingrates, as derived from any radioactive tracer profiles, depend on the assumption thatorganic particles mixed into the sediment with the tracer considered (Smith, Pope,DeMaster, & Magaard, 1993). Thus, the degradation time (1/kR2) of organic materialshould comply with the half-life of the radioactive tracer, which is 22.3 years in thecase of210Pb. Applying radiotracers with a much shorter half-life (e.g234Th, 22.4days) may result in apparently different kinetics of organic matter decay. Recentstudies in the equatorial Pacific nicely demonstrated that the mixing of a fast decay-ing fraction of organic matter was best represented by234Th decay while the mixingof a slow decaying fraction could best derived from210Pb profiles (Hammond,McManus, Berelson, Kilgore, & Pope, 1996).

Evidence for choosing210Pb as an appropriate radioactive tracer to estimate sedi-ment mixing rates and the first-order degradation constants is provided by the appar-

Table 8Sedimentation rates (van Weering, Hall, de Stigter, McCave, & Thomsen, 1998), depth attenuation coef-ficient alpha,Db values from Soetaert, Herman, Middelburg, Heip, de Stigter, van Weering, Epping, &Helder (1997), and first-order degradation constant kR2

Omex Station Sedimentation rate Alpha Db kc

(cm kyr−1) (cm−1) (cm2 yr−1) (yr−1)

A > 4.3 9.5 0.25 22.6I 2.8 1.8 0.30 0.9B 2.3 1.6 0.14 0.35II 1.7 1.7 0.05 0.14F 3.1 1.2 0.01 0.014III 4.3 0.9 0.07 0.029E 6 0.8 0.003 0.002


ent lack of seasonality in benthic oxygen fluxes at Goban Spur. Sayles, Martin, &Deuser (1994) calculated that the degradability of organic matter arriving at the sea-floor should be at least 5 to 10 y−1 in order to induce seasonal variation in benthicoxygen uptake rates. Similar conclusions were drawn by Martin, & Bender (1988)who pointed out that organic carbon degradation constants in excess of 4 y−1 arenecessary to generate a seasonality in benthic oxygen uptake rates. Such high decayconstants were, however, never observed at stations located below 200 m waterdepth. These results are in line with the benthic oxygen uptake rates measured inMay 1994 at Omex stations II and III. This sampling period coincided with a masssedimentation of biogenic and lithogenic material of the spring phytoplankton bloomApril/May as measured with sediment traps (Antia, van Bodungen, & Peinert, 1998).The in situ oxygen uptake rates of 2.06 0.1 mmol O2 m−2 d−1 at Omex II and 0.86 0.2 mmol O2 m−2 d−1 at Omex III (Table 2, Fig. 5) are not statistically differentfrom oxygen uptake rates measured in October 1993 and September 1995. Concur-rent measurements of oxygen uptake rates measured with benthic chambers at thesame stations and periods confirm the absence of seasonality (Duineveld, Lavaleye,Berghuis, de Wilde, van der Weele, Kok, Batten, & DeLeeuw, 1997). Similar resultswere reported for the nearby Porcupine Sea-Bight, where Lampitt, Raine, Billett, &Rice (1995) showed that benthic oxygen uptake rates in 2800 m water depth did notrespond to the sedimentation of the spring phytoplankton, despite a clear detritalcovering of the retrieved sediment cores.

The absence of seasonality in oxygen uptake rates and in the modelled reactivityof the organic material contrast to the C/N ratios observed in the present study (Fig.2). Sedimentary organic matter, characterised by a C/N ratio ranging between 6 and8 is often considered to be labile. Such low C/N ratios were found in the presentstudy (Fig. 2), although the modelled reactivity of the organic matter and the meas-ured oxygen uptake rates point at the dominance of refractory compounds in theorganic matter undergoing mineralisation. Consequently, low or high C/N ratios donot necessarily provide useful information on the degradability of sedimentaryorganic matter. Their primary value can be seen as an indicator for the presence ofterrigenous material in the sedimentary carbon and nitrogen pool in continental mar-gin sediments, which results in relatively high C/N ratios.

Thus, based on the lacking response of oxygen uptake rates to the deposition ofparticles from the spring phytoplankton bloom and the low first-order degradationrates, we conclude that the organic material undergoing decomposition must be domi-nated by relatively slow degrading compounds.

4.4. Sedimentary carbon inventory in relation to oxic mineralisation

Although the mechanisms which determine carbon mineralisation and preservationare not necessarily coupled (Hedges, & Keil, 1995), it is very tempting to compareboth rates and to draw some tentative implications for the burial efficiency in GobanSpur sediments. The low burial efficiencies are in contrast to the relatively slowfirst-order degradation rates that we calculated for the Omex transect. Low first-orderdegradation constants (, 1 y−1, Table 8) suggest that the organic matter undergoing


decomposition is of rather refractory nature, while a nearly complete mineralisation,as indicated by the low burial efficiencies, cannot be achieved when refractory com-pounds represent a substantial part of the organic material arriving at the sea floor(Hedges, & Keil, 1995). It is conceivable that the predominance of oxic degradationpathways in Goban Spur sediments is capable of almost completely mineralisingeven poorly degradable compounds which otherwise would resist mineralisation andbecome buried. Recent investigations on sedimentary organic carbon accumulationhave reconciled the correlation between organic matter abundance and available sur-face area of the mineral matrix, and proposed the sorptive protection mechanism asa primary factor determining organic carbon concentrations in marine sediments(Mayer, 1994; Keil, Montlucon, Prahl, & Hedges, 1994). These investigations dem-onstrated that at many continental shelf and slope sediments there exists an intrinsicrelation between organic carbon concentrations and available surface area with arelatively constant organic carbon (Corg) to surface area (SFA) ratios ranging between0.5 and 1 mg Corg m−2. Preliminary investigations in corporation with L. M. Mayer(University of Maine, USA) have shown that bulk samples of Goban Spur sedimentshave clearly lower Corg/SFA ratios between 0.30 mg Corg m−2 at Omex I and 0.07mg Corg m−2 at Omex III. These low Corg/SFA ratios may be the result of the pro-longed exposure time of organic carbon to oxic conditions at Goban Spur, whicheventually may overcome the sorptive protection mechanisms (Keil, Montlucon,Prahl, & Hedges, 1994). The exposure time of organic carbon to oxygen can beroughly approximated by dividing the thickness of the oxic zone (Table 2) with thesedimentation rates (Table 8). As a result, organic carbon would be exposed to oxy-gen for | 280 years at Omex I. At Omex II, F and III, the exposure time wouldincrease to| 840, 2200, and 2400 years, respectively. These values corroborate withthe decreasing Corg/SFA ratios along the transect and emphasise the influence ofoxygenated pore waters on carbon burial rates.

5. Conclusions

Benthic carbon mineralisation rates along the Goban Spur transect at the N.W.European margin decrease regularly from the shelf (200 m) to the lower slope (1500m), but no further decrease was found beyond the slope break at 1500 m. The organiccarbon concentrations increased from 0.2% on the shelf to 0.7 (%) at the slope break,while the concentrations beyond that depth were slightly lower. Both carbon mineral-isation and sedimentary organic carbon concentrations are very low compared toother continental margins studied and do not indicate the presence of a carbon depoc-enter along the transect. This finding is supported by the very low burial efficiencies(0.8 and 2.3%) which are not correlated with water depth. The dominance of oxicmineralisation at the upper shelf/slope stations (> 75%) and the lower slope (>90%)indicates that the sediments receive comparatively small amounts of organic matter.The organic material undergoing mineralisation is probably of refractory naturewhich is corroborated by its low first-order decay constants (, 1 y−1) and the lackof response of benthic oxygen uptake rates to pulses of organic material originatingfrom the spring phytoplankton bloom.


6. Acknowledgements

The authors of this study would like to thank the masters and the crews of theresearch vessels R.R.S.Charles Darwinand the R.V.Pelagia for providing hospi-tality and safe cruises. We would like to express our gratitude to Maria BelzunceSegarra (CSIC Vigo, Spain), Marlene Dekker, Henk Franken, and Johan van Heer-waarden for excellent technical support. This manuscript benefited from the reviewof an anonymous reviewer and Wim van Raaphorst. Shipboard nutrient analysis wasperformed by Karel Bakker, Jan van Ooijen and Annette van Koutrik (DepartmentMarine Chemistry and Geology, NIOZ). This research was supported by the Euro-pean Community EC-MAST Omex programme, contract no. MAS2-CT93-0069 andMAS3-CT96-0056. This is publication 3277 of the Netherlands Institute for SeaResearch.

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recycling of organic matter along a shelf-slope transect across the n.w. european continental margin...

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