Recycling of organic matter along a shelf-slope transect across the N.W. European Continental Margin (Goban Spur)

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<ul><li><p>Progress in Oceanography 42 (1998) 77110</p><p>Recycling of organic matter along a shelf-slopetransect across the N.W. European Continental</p><p>Margin (Goban Spur)L. Lohsea,*, W. Heldera, E.H.G. Eppinga, W. Balzerb</p><p>aNetherlands Institute for Sea Research (NIOZ), Department Marine Chemistry and Geology, P.O. Box59, 1790 AB Den Burg, The Netherlands</p><p>bUniversitat Bremen, Fachbereich Meereschemie, Postfach 330440, 28334 Bremen, Germany</p><p>Abstract</p><p>Within the framework of the Ocean Margin Exchange Programme (OMEX), benthic carbonmineralisation was determined along the Goban Spur shelf-slope transition (2004500 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 sedimentwater 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, &amp; Biscaye (1994). Carbon budget of the Middle Atlantic Bight. Deep-Sea Research I,41, 669703.). 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</p><p>* Corresponding author. Tel: 1 31-222-369448; fax: 1 31-222-319674; e-mail:</p><p>0079-6611/98/$ - see front matter 1998 Elsevier Science Ltd. All rights reserved.PII: S0079 -6611(98)00 029-9</p></li><li><p>78 L. Lohse et al. / Progress in Oceanography 42 (1998) 77110</p><p>fluxes calculated from pore water profiles indicated that within the experimental errors therewas no significant contribution by bioirrigating organisms to the sedimentwater exchangefluxes. 1998 Elsevier Science Ltd. All rights reserved.</p><p>1. Introduction</p><p>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 (Jrgensen, 1983; Middelburg,Soetaert, &amp; 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, &amp; Heussner,1990; Pedersen, Shimmield, &amp; Price, 1992; Anderson, Rowe, Kemp, Trumbore, &amp;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).</p><p>While an earlier paper of ours has highlighted aspects of phosphorus cycling inthe north European continental margin (Slomp, Epping, Helder, &amp; 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, &amp; 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, &amp; Heggie, 1984).</p><p>Given the dominant role of oxygen in organic carbon mineralisation in continentalmargin sediments (Jahnke, &amp; 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</p></li><li><p>79L. Lohse et al. / Progress in Oceanography 42 (1998) 77110</p><p>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.</p><p>2. Materials and methods</p><p>2.1. Study area</p><p>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.</p><p>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.</p><p>Fig. 1. Sampling locations and cross section of the Goban Spur transect.</p></li><li><p>80 L. Lohse et al. / Progress in Oceanography 42 (1998) 77110</p><p>Table 1Characteristics of sampling sites</p><p>Station Sampling Latitude Longitude Depth (m) Bottom [O2] depth-date ( N) ( W) water Bottomwater integrated</p><p>temperature (mmol porosity( C) dm - 3) over the</p><p>oxic layer</p><p>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</p><p>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</p><p>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</p><p>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</p><p>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</p><p>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</p><p>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</p><p>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</p><p>2.2. Sediment sampling</p><p>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 sedimentwater 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.</p><p>During the 1995 cruise, sediment cores were taken with a multi-corer (Barnett,Watson, &amp; 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.</p><p>2.3. Pore water collection</p><p>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,</p></li><li><p>81L. Lohse et al. / Progress in Oceanography 42 (1998) 77110</p><p>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, &amp; de Vries, 1979), nitrate and nitrite (Grasshoff, Erhardt, &amp; Kremling, 1983)and silicate (Strickland, &amp; Parsons, 1972) using TRAACS 800 1 autoanalysers.Additionally, pore waters were analysed for dissolved manganese (Brewer, &amp; Spen-cer, 1971) and iron (Stookey, 1970) after acidification to pH 2, respectively. Theanalytical precision for all compounds was 6 1% relative standard deviation.</p><p>The squeezer cake was stored frozen at 220 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, &amp; 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 50 ml) 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.</p><p>2.4. Oxygen profiling</p><p>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, &amp; Helder (1997). TROL is equippedwith a resistivity electrode (Andrews, &amp; Bennett, 1981) and 6 pressure-compensatedoxygen micro-electrodes. The oxygen-electrodes are of the single cathode type(Revsbech, &amp; Jrgensen, 1986) and have an outer tip diameter of 10 to 50 mm. Theoutput ranges typically between 3001500 pA (100% O2 saturation at 20 C, 35S). Readings stabilised within 3 seconds. A single Ag/AgCl reference electrode isshared by all oxygen electrodes.</p><p>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 100 mm. The signal was allowed to stabilise at each step for 5seconds before the reading was stored in the solid state memory of the profiler.</p><p>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 sedimentwater interface. Themomentum of this magnet was switched from clockwise to anticlockwise rotation</p></li><li><p>82 L. Lohse et al. / Progress in Oceanography 42 (1998) 77110</p><p>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 500 mm. 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, &amp; van Raaphorst, 1996).</p><p>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.</p><p>2.5. Modelling of the oxygen profiles</p><p>The steady-state distribution of oxygen in sediments is governed by a balance ofdiffusion and reaction and can be approximated by</p><p>0 5 Dsd 2Cdz2 2 Rtot (1)</p><p>where Ds is the effective diffusion coefficient for oxygen, C represents the concen-tration of oxygen at depth z, and Rtot denotes the volumetric oxygen consumptionof the sediment. The reaction term R can be specified by</p><p>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 coefficient a. The application of two reactionterms, each identified by their own degradation constant may oversimplify the com-plexity of organic matter mineralisation, but has...</p></li></ul>


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