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

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<ul><li><p>Progress in Oceanography 42 (1998) 127144</p><p>Release of dissolved organic carbon (DOC)from sediments of the N.W. European</p><p>Continental Margin (Goban Spur) and itssignificance for benthic carbon cycling</p><p>S. Otto, W. Balzer*FB2-Marine Chemistry, University of Bremen, PF 330440, 28334 Bremen, Germany</p><p>Abstract</p><p>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: 6704800 m) and the centre of a nearby precipitous canyon (Whittard Canyon:1803680 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 0.090.15 mmol m - 2 d - 1 for the Goban Spursediments and 0.050.16 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 232% (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.</p><p>* Corresponding author.</p><p>0079-6611/98/$ - see front matter 1998 Elsevier Science Ltd. All rights reserved.PII: S0079 -6611(98)00 031-7</p></li><li><p>128 S. Otto, W. Balzer / Progress in Oceanography 42 (1998) 127144</p><p>1. Introduction</p><p>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 &amp; Wakeham, 1988, 1992; Lee &amp; Henrichs, 1993; Williams &amp; 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 upper5001000 m (Williams &amp; Druffel, 1987; Druffel, Williams, Bauer, &amp; Ertel, 1992;Bauer, Williams, &amp; Druffel, 1992; Carlson, Ducklow, &amp; Michaels, 1994; Carlson &amp;Ducklow, 1995; Hansell, Nicholas, &amp; Gundersen, 1995).</p><p>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 &amp; Sholkovitz, 1977; Orem &amp; Gaudette, 1984; Orem, Hatcher,Spiker, Szeverenyi, &amp; Maciel, 1986; Henrichs &amp; Farrington, 1984; Heggie, Maris,Hudson, Dymond, Beach, &amp; 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 &amp; Vaccaro, 1964).After the establishment of the high-temperature-combustion method (HTC) for DOC-analysis (Sugimura &amp; 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, &amp; Martens, 1992; Alperin, Albert, &amp; Martens, 1994; Burdige &amp;Homstead, 1994; Martens, Haddad, &amp; Chanton, 1992; Skoog, Hall, Hulth, Paxeus,van der Loeff, &amp; Westerlund, 1996). The oceanic depth range exceeding 100 m isonly covered by studies of Henrichs &amp; Farrington (1984), Heggie et al. (1987),Martin &amp; McCorkle (1993) and Hulth, Tengberg, Landen, &amp; Hall (1997). The latterstudy included also flux determinations obtained during laboratory incubations ofdeep sea cores.</p><p>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 &amp; 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 &amp; 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</p></li><li><p>129S. Otto, W. Balzer / Progress in Oceanography 42 (1998) 127144</p><p>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 (see Mar-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 &amp; Keil, 1995).</p><p>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.</p><p>2. Study area and methods</p><p>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).</p><p>Immediately after recovery, the sediment cores having a seemingly undisturbedsurface were brought to a laboratory refrigerated at 14 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 0.52.0 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 0.45 mm). Samples for DOC and SCO2 (1 ml each) were transferredinto cleaned 4 ml glass vials with teflon-lined screw cap and stored at 1 4 C untilanalysis. Analyses were performed on board within 0.32 days.</p><p>Immediately before the HTC oxidation of DOC, the removal of inorganic carbonwas accomplished by adding 30 ml 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 argonoxygen-mixture (95/5%) was used as the carrier gas. Prior to analy-sis the blank of the instrument was determined through injection of 75 ml 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.</p></li><li><p>130 S. Otto, W. Balzer / Progress in Oceanography 42 (1998) 127144</p><p>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.</p><p>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 75 ml 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%.</p></li><li><p>131S. Otto, W. Balzer / Progress in Oceanography 42 (1998) 127144</p><p>Table 1Location and date of sampling: Goban Spur and Whittard Canyon</p><p>Station number Abbreviation Depth (m) Position Date</p><p>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</p><p>The analysis for SCO2 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 in SCO2 were prepared by dissolving sodium carbonate in Milli-Q water(1.56 mmol l- 1) under a nitrogen atmosphere. The porosity (w) used for flux calcu-lations was obtained from sediment weight loss upon drying.</p><p>3. Results and discussion</p><p>3.1. DOC in pore water</p><p>Starting from concentrations of 0.060.12 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(0.060.10 mmol l- 1) and Antarctic interfacial pore waters (0.352.00 mmol l- 1) atwater depths ranging from 280 to 2514 m (Hulth et al., 1997).</p></li><li><p>132 S. Otto, W. Balzer / Progress in Oceanography 42 (1998) 127144</p><p>Fig. 2. Pore water concentrations of DOC in sediments of the Goban Spur transect. Horizontal linesmark the position of the sedimentwater interface.</p><p>Fig. 3. Pore water concentration of DOC in sediments of the Whittard Canyon. Horizontal lines markthe position of the sedimentwater interface.</p><p>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 &amp; Farrington, 1984; Heggie et al., 1987;Martin &amp; McCorkle, 1993; Burdige &amp; Homstead, 1994), but it is unclear whether</p></li><li><p>133S. Otto, W. Balzer / Progress in Oceanography 42 (1998) 127144</p><p>Tabl</p><p>e2Po</p><p>rew</p><p>ater</p><p>data</p><p>of</p><p>DO</p><p>Cat</p><p>Gob</p><p>anSp</p><p>urobt</p><p>aine</p><p>dby</p><p>sque</p><p>ezin</p><p>g.D</p><p>OC</p><p>was</p><p>dete</p><p>rmin</p><p>edby</p><p>high</p><p>tem</p><p>pera</p><p>ture</p><p>com</p><p>busti</p><p>on</p><p>IOS</p><p>OM</p><p>EXIV</p><p>OM</p><p>EXII</p><p>IO</p><p>MEX</p><p>IIO</p><p>MEX</p><p>I</p><p>Dep</p><p>th(cm</p><p>)D</p><p>OC</p><p>Dep</p><p>th(cm</p><p>)D</p><p>OC</p><p>Dep</p><p>th(cm</p><p>)D</p><p>OC</p><p>Dep</p><p>th(cm</p><p>)D</p><p>OC</p><p>Dep</p><p>th(cm</p><p>)D</p><p>OC</p><p>(mmo</p><p>ll-</p><p>1 )(m</p><p>moll</p><p>-1 )</p><p>(mmo</p><p>ll-</p><p>1 )(m</p><p>moll</p><p>-1 )</p><p>(mmo</p><p>ll-</p><p>1 )</p><p>BW</p><p>a0.</p><p>07B</p><p>W0.</p><p>07B</p><p>W0.</p><p>11B</p><p>W0.</p><p>07B</p><p>W0.</p><p>070</p><p>0.5</p><p>1.00</p><p>00.</p><p>50.</p><p>530</p><p>0.5</p><p>0.86</p><p>00.</p><p>51.</p><p>420</p><p>0.5</p><p>1.17</p><p>0.5</p><p>10.</p><p>570.</p><p>51</p><p>0.39</p><p>0.5</p><p>10.</p><p>530.</p><p>51</p><p>1.57</p><p>12</p><p>0.92</p><p>12</p><p>0.57</p><p>12</p><p>0.57</p><p>11.</p><p>50.</p><p>541</p><p>20.</p><p>732</p><p>31.</p><p>262</p><p>30.</p><p>762</p><p>30.</p><p>251.</p><p>52</p><p>0.50</p><p>23</p><p>0.80</p><p>34</p><p>0.93</p><p>35</p><p>0.42</p><p>35</p><p>0.23</p><p>23</p><p>0.36</p><p>34</p><p>0.82</p><p>45</p><p>2.61</p><p>57</p><p>0.50</p><p>57</p><p>0.31</p><p>34</p><p>0.46</p><p>45</p><p>1.16</p><p>56</p><p>2.00</p><p>79</p><p>0.52</p><p>79</p><p>0.55</p><p>45</p><p>0.74</p><p>57</p><p>0.70</p><p>67</p><p>2.05</p><p>911</p><p>0.36</p><p>911</p><p>0.53</p><p>56.</p><p>50.</p><p>497</p><p>90.</p><p>607</p><p>81.</p><p>6113</p><p>15</p><p>0.49</p><p>111</p><p>30.</p><p>336.</p><p>58</p><p>0.64</p><p>911</p><p>0.80</p><p>89</p><p>1.84</p><p>171</p><p>90.</p><p>3813</p><p>15</p><p>0.43</p><p>89.</p><p>50.</p><p>7711</p><p>13</p><p>0.77</p><p>911</p><p>1.54</p><p>212</p><p>30.</p><p>3315</p><p>17</p><p>0.46</p><p>9.5</p><p>11.5</p><p>0.39</p><p>131</p><p>51.</p><p>0611</p><p>13</p><p>0.92</p><p>252</p><p>70.</p><p>2717</p><p>19</p><p>0.33</p><p>11.5</p><p>12.</p><p>50.</p><p>4715</p><p>17</p><p>2.20</p><p>aB</p><p>otto</p><p>mw</p><p>ater</p><p>,i.e</p><p>.over</p><p>lyin</p><p>gw</p><p>ater</p><p>sam</p><p>pled</p><p>1cm</p><p>abov</p><p>eth</p><p>ese</p><p>dim</p><p>ents</p><p>urfa</p><p>ce.</p></li><li><p>134 S. Otto, W. Balzer / Progress in Oceanography 42 (1998) 127144</p><p>Tabl</p><p>e3Po</p><p>rew</p><p>ater</p><p>data</p><p>of</p><p>DO</p><p>Can</p><p>dS</p><p>CO2</p><p>atW</p><p>hitta</p><p>rdCa</p><p>nyon</p><p>obt</p><p>aine</p><p>dby</p><p>centr</p><p>ifuga</p><p>tion</p><p>M28</p><p>1(F</p><p>)M</p><p>296</p><p>(E)</p><p>M27</p><p>8(D</p><p>)M</p><p>275</p><p>(C)</p><p>M27</p><p>1(B</p><p>)M</p><p>286</p><p>(A)</p><p>Dep</p><p>thD</p><p>OC</p><p>SCO</p><p>2D</p><p>epth</p><p>DO</p><p>CS</p><p>CO2</p><p>Dep</p><p>thD</p><p>OC</p><p>SCO</p><p>2D</p><p>epth</p><p>DO</p><p>CS</p><p>CO2</p><p>Dep</p><p>thD</p><p>OC</p><p>SCO</p><p>2D</p><p>epth</p><p>DO</p><p>CS</p><p>CO2</p><p>(cm)</p><p>(mmo</p><p>l(m</p><p>mol</p><p>(cm)</p><p>(mmo</p><p>l(m</p><p>mol</p><p>(cm)</p><p>(mmo</p><p>l(m</p><p>mol</p><p>(cm)</p><p>(mmo</p><p>l(m</p><p>mol</p><p>(cm)</p><p>(mmo</p><p>l(m</p><p>mol</p><p>(cm)</p><p>(mmo</p><p>l(m</p><p>mol</p><p>l-1)</p><p>l-1)</p><p>l-1)</p><p>l-1)</p><p>l-1)</p><p>l-1)</p><p>l-1)</p><p>l-1)</p><p>l-1)</p><p>l-1)</p><p>l-1)</p><p>l-1)</p><p>BW</p><p>a0.</p><p>062.</p><p>46B</p><p>W0.</p><p>112.</p><p>36B</p><p>W0.</p><p>102.</p><p>48B</p><p>W0.</p><p>082.</p><p>36B</p><p>W0.</p><p>122.</p><p>35B</p><p>W0.</p><p>072.</p><p>370</p><p>0.5</p><p>0.16</p><p>2.72</p><p>00.</p><p>50.</p><p>332.</p><p>490</p><p>0.5</p><p>0.21</p><p>2.68</p><p>00.</p><p>50.</p><p>232.</p><p>740</p><p>0.5</p><p>0.21</p><p>2.63</p><p>00.</p><p>50.</p><p>162.</p><p>430.</p><p>51</p><p>0.18</p><p>2.71</p><p>0.5</p><p>11.</p><p>222.</p><p>730.</p><p>51</p><p>0.28</p><p>2.66</p><p>0.5</p><p>10.</p><p>272.</p><p>890.</p><p>51</p><p>0.25</p><p>2.70</p><p>0.5</p><p>10.</p><p>232.</p><p>441</p><p>20.</p><p>212.</p><p>941</p><p>20.</p><p>533.</p><p>011</p><p>20.</p><p>302.</p><p>971</p><p>20.</p><p>303.</p><p>051</p><p>20.</p><p>27</p><p>12</p><p>0.32</p><p>2.48</p><p>23</p><p>0.37</p><p>2.97</p><p>23</p><p>0.31</p><p>3.15</p><p>23</p><p>0.32</p><p>3.19</p><p>23</p><p>0.34</p><p>2.94</p><p>23</p><p>0.54</p><p>2.66</p><p>23</p><p>0.36</p><p>2.40</p><p>35</p><p>0.31</p><p>3.39</p><p>35</p><p>0.37</p><p>3</p><p>50.</p><p>483.</p><p>473</p><p>50.</p><p>393.</p><p>113</p><p>50.</p><p>562.</p><p>693</p><p>50.</p><p>382.</p><p>385</p><p>7</p><p>5</p><p>70.</p><p>61</p><p>57</p><p>0.36</p><p>3.84</p><p>57</p><p>0.40</p><p>3.05</p><p>57</p><p>0.66</p><p>2.78</p><p>57</p><p>0.38</p><p>2.22</p><p>79</p><p>0.27</p><p>3.74</p><p>79</p><p>0.56</p><p>7</p><p>90.</p><p>463.</p><p>977</p><p>90.</p><p>612.</p><p>997</p><p>90.</p><p>68</p><p>79</p><p>0.32</p><p>2.22</p><p>111</p><p>30.</p><p>293.</p><p>9011</p><p>13</p><p>0.60</p><p>11</p><p>13</p><p>0.55</p><p>4.83</p><p>911</p><p>0.59</p><p>2.66</p><p>911</p><p>0.52</p><p>2.99</p><p>151</p><p>70.</p><p>393.</p><p>9115</p><p>17</p><p>0.74...</p></li></ul>


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