Particle flux across the mid-European continental margin

Deep-Sea Research I 46 (1999) 1999}2024

Particle #ux across the mid-European continental margin Avan N. Antia *, Bodo von Bodungen
, Rolf Peinert Institut fu( r Meereskunde an der Universita( t Kiel, Dusternbrooker Weg 20, D-24105 Kiel, Germany
Institut fu( r Ostseeforschung, Warnemu( nde, Germany Received 29 April 1998; received in revised form 8 October 1998; accepted 5 February 1999

Abstract Results are presented from particle #ux studies using sediment trap and current meter moorings along a transect at the European continental margin at 493N within the EU-funded Ocean Margin Exchange (OMEX) project. Two moorings were placed, at the mid- and outer slope in water depths of 1500 and 3660 m, with traps at 600 and 1050 m and at 580, 1440 and 3220 m, respectively. Residual currents at the mid-slope follow the slope contour, whereas seasonal o!-slope #ow was registered at the outer slope. At 600 m on the slope #uxes are similar to those in the abyssal North Atlantic. The #ux of all components (bulk dry weight, particulate organic and inorganic carbon, lithogenic matter and opal) increased with water depth. Highest #uxes were recorded at 1440 m at the outer slope, where o!-slope residual currents mediate particle export. The injection of biogenic and lithogenic particles below the depth of winter mixing results in the export of particles from shallower waters. Calculated lateral #uxes of particulate organic carbon exceed the primary #ux by over a factor of 2 at 1440 m on the outer slope. Estimated lateral #uxes of suspended particulate matter in the water column and intermediate nepheloid layers at the outer slope are potentially large compared to sinking #uxes measured by sediment traps. A comparison is made of particle #ux at three continental margin sites and two sites in the adjacent open North Atlantic, from which it is seen that bulk and organic matter #ux increases exponentially with proximity to the shelf break. The percentage contribution of particulate organic carbon to biogenic #uxes increases from a mean of 5.7% in the abyssal N. Atlantic to 13.9% at the continental margins.  1999 Elsevier Science Ltd. All rights reserved. Keywords: Particle #ux; Continental margins; Lateral #ux; Suspended particulate #ux; Carbon export

* Corresponding author. E-mail address: [email protected] (A.N. Antia) 0967-0637/99/$ - see front matter  1999 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 3 7 ( 9 9 ) 0 0 0 4 1 - 2

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1. Introduction Since Walsh et al. (1981) indicated that continental shelves may function as a sink of atmospheric CO , there has been much interest in determining the extent of this  carbon sink and its regional characteristics. Longhurst et al. (1995) estimate that with a total of 29% of global marine primary productivity at only 11% of the ocean's surface, the continental margins account disproportionately for oceanic CO "xation  from the atmosphere. To what extent these regimes mediate carbon sequestration remains unclear, but if, as Jahnke et al. (1990) estimate for the North Paci"c, about 50% of the organic carbon input to the sea bed occurs within 500 km of the margin, determination of these #uxes may be a key factor in estimating the in#uence of the oceans on global biogeochemical cycling. Bauer and Dru!el (1998) estimate that the ocean margins can contribute signi"cantly to the oceanic pools of dissolved and suspended organic matter in the Atlantic and Paci"c Oceans. An elevation of the organic carbon content of sediments at continental margins (Romankevich, 1984) re#ects their high productivity and suggests that they may be local deposition centres for biogenic carbon. The initial high model estimates of shelf export to the slope and deep ocean (Walsh et al., 1981) have not been con"rmed by measurements during the SEEP-I ((10% of primary production, Biscaye et al., 1988), SEEP-II (;5% of primary production, Biscaye et al., 1994) and ECOMARGE (Monaco et al., 1990) experiments at the eastern Atlantic and Mediterranean margins. However, these and other studies have contributed to an understanding of the factors mediating shelf edge #uxes, primary among them the role of the resuspension of particles from the shelf and slope sediments and its lateral transport in benthic and intermediate nepheloid layers (e.g. Gardner and Richardson, 1992). This juxtaposition of primary #ux from the surface and secondary #ux through resuspension and advection is a common feature at the continental slope and results in an increase in #ux with water depth (e.g. Biscaye et al., 1994; Etcheber et al., 1997). Physical forcing at the continental margin (see Huthnance, 1995 for an overview) controls the spatial and temporal distribution of nutrients by mixing and upwelling and characteristically a band of elevated phytoplankton pigment is seen in satellite images along the margin, making it a region of locally enhanced biological activity (Sathyedranath et al., 1995; Edwards et al., 1999; Batten, 1998). Much of this high production is utilised in shallow waters, and it is only that proportion that escapes mineralisation above the depth of maximal winter mixing that is relevant to sequestration over long time periods. The processes that mediate this export result from a complex interplay of physical resuspension and lateral transport and biological mediation of #ux. The EU-funded Ocean Margin Exchange (OMEX) Programme has focused on particulate exchange at the Goban Spur, where regular biological and chemical measurements were conducted along a cross-slope transect. We hypothesise that particles settling on the shelf will di!er in composition from those on the slope, re#ecting across-slope gradients in pelagic properties. The concentration of wave and tidal energy on the slope will redistribute particles in a way that will determine their

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eventual fate and area of deposition. We use results from 2 moorings along the OMEX transect in water depths of 1445 and 3650 m, which were equipped with sediment traps, transmissiometers and current meters, to investigate seasonal and spatial patterns in particle #ux.

2. Materials and methods Data are presented from taut wire bottom tethered moorings deployed at the western European continental margin between July 1993 and September 1994. Positions of the moorings and depths of instrument deployment are given in Table 1. The shallowest traps, at a nominal depth of 600 m on both moorings, were placed below the depth of winter convection to quantify #ux from the seasonally mixed layer. The deepest traps (1050 m at OMEX 2 and 3220 m at OMEX 3) were situated 400 m above the sea bed, above the benthic nepheloid layer, which extends more than 200 m above the sea bed (McCave et al., subm.). These traps are taken to estimate vertical input to the benthos. At OMEX 3 an intermediate trap was placed at 1440 m, which is below the intrusion of water with a Mediterranean imprint. Current speed and direction, were measured using Aanderaa RCM 8 current meters placed 20 m below each trap. At 1070 m and 1440 m on moorings 2 and 3, respectively, SEATECH 25 cm path length transmissiometers were attached to the current meters. The recording interval of the current meters was 1 hour, enabling resolution of tidal velocities which dominate the current record. Sedimenting particulate matter was collected using large-mouth (0.5 m) conical particle interceptor traps of the `Kiela type. For details of trap design see Kremling et al. (1996). Sampling intervals varied between 9 and 14 days from 01 July 1993 to 03 September 1994. During mooring recovery in summer 1994 there was external growth on the 600 m traps, and the last 3 trap cups were empty, although the trap functioned correctly. It is thought that a mass of material clogged the trap funnels at this time, and the data record is thus incomplete at this depth. Table 1 Positions of the OMEX moorings and depths of instrument deployment. CM"Current Meter; Trans." Transmissiometer Mooring

Latitude

Longitude

Water depth

Depth (m)

Instrument

OMEX 2

49311.20N

12349.18W

1445 m

OMEX 3

49305.0N

13325.8W

3650 m

600 620 1050 1070 580 600 1440 1460 3220 3240

Sediment trap#inclinometer CM Sediment trap CM#Trans. Sediment trap RCM Sediment trap CM#Trans. Sediment trap CM#Trans.

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Sampling cups were "lled with deep water ('1500 m) from the Goban Spur and poisoned with borax-bu!ered HgCl to a "nal concentration of 0.14%. Dissolved  silicate was measured in the seawater used to "ll the cups, and the value obtained was used later for blank subtraction. On recovery an additional 0.07% bu!ered HgCl  was added to the samples to compensate for possible poison loss during deployment. Samples were stored cold and dark until further processing in the laboratory. Supernatant was siphoned o! the trap cups for measurements of dissolved silicate, and, after blank subtraction, these values were used to correct for dissolution of particulate silicate #ux. Samples were picked manually for swimmers under a dissecting microscope at 120X magni"cation. These consisted primarily of copepods, ostracods and pteropods ranging in size from (0.5 mm to several centimetres. Samples were then split, and subsamples of between  and  were used for analyses of total mass (DW),   carbonate, POC, PON and biogenic silica as described by von Bodungen et al. (1991). Particulate inorganic carbon is calculated as 12% carbonate by weight. In the case of biogenic silica, time-course digestions were performed using surface and deep trap samples and sediment surface samples to determine appropriate digestion times for minimal bias from lithogenic material. A digestion time of 2 h was used. Silica concentrations were converted to opal assuming 10% water (Mortlock and FroK hlich, 1989). Lithogenic #uxes were calculated by subtraction of biogenic components from total #ux, with biogenic #ux"carbonate#POC*2#opal. Although this method is used to estimate lithogenic #uxes, it can di!er signi"cantly from values derived from more direct estimates of aluminium #uxes (McCave et al., subm.). Annual #uxes were calculated using data for the 12-month period between 01 July 1993 and 30 June 1994. Suspended particulate matter (SPM) was collected using a CTD rosette. Samples were taken from the surface, intermediate and benthic nepheloid layers, and the clear water particulate minimum. Between 1 and 121 of seawater were "ltered on combusted, pre-weighed GF/F "lters and treated as described in von Bodungen et al. (1991). Particulate dry weight was determined gravimetrically and was used to convert beam attenuation to SPM concentration. The calibration derived was c"0.337;SPM#0.352, n"42, r"0.7, where c is the beam attenuation (m\) and SPM the suspended particulate concentration (mg l\).

3. Results and discussion 3.1. Validity of sediment trap measurements The collection e$ciency of sediment traps has been questioned because of potential erors due to inclination (Gardner, 1985) and hydrodynamic biases at high current velocities (Gust et al., 1994; Gardner et al., 1997). Despite the use of traps for several decades, there is no reliable estimate of their functioning under in situ conditions. Since we use the absolute levels of sedimentation at the di!erent depths to di!erentiate

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between lateral and vertical #ux, we "rst examine the question of trap functioning under the ambient hydrodynamic regime. Table 2 shows the minimum and maximum current speeds recorded adjacent to each trap and the periods for which currents exceeded 15 cm s\, a level above which trap collections may well be severely biased. This is a conservative estimate in light of the recent results of Gardner et al. (1997), who show high collection e$ciencies at current speeds of up to 22 cm s\. For individual trap cups velocities exceeded 15 cm s\ for '20% of the collection time for only three cups in February, May and July 1994. Maximum velocities represent short-term peaks in the tidal cycle. Trap inclination to the vertical is also thought to a!ect collection e$ciency of cylindrical traps (Gardner, 1985) although it is unclear how this applies for conical forms. Inclination of the trap at 580 m on OMEX 2 (data not shown), which was subject to the highest current speeds, follows the tidal cycle but, at a maximum inclination of ca. 43 (mean 2.33) o! vertical the e!ect on vertical positioning of the trap is negligible. Given the state of our knowledge on the collection e$ciency of traps in situ, and the inability to calibrate against a `truea #ux, there can be no unequivocal proof of trap function; however, we assume that hydrodynamic e!ects are unlikely to have signi"cantly biased the collection e$ciency in this experiment. 3.2. Physical setting at the Goban Spur At the Goban Spur a strong cross-slope gradient in sea surface temperature and colour is evident in satellite images (Fig. 1). The principal site of elevated biological activity and particle production is at the shelf break (200 m contour), where doming of the thermocline causes the injection of &&new'' nutrients into the euphotic zone (Pingree et al., 1976). Annual new production here is limited by the topographically controlled rate of upwelling. At the mid and outer slope, where the sediment trap moorings were placed, surface conditions resemble more the open N. Atlantic, where winter mixing determines nutrient supply and limits annual new production. The proximity of the shelf break with elevated production and biomass, however, provides a source of particles that can be exported o!-slope via lateral injection in intermediate water layers and contribute to the particle load and #ux on the slope. It is worth noting that horizontal mixing at the surface, as seen in the width of the cold water band around Table 2 Mean, maximal and minimum current velocities from current meters on the sediment trap moorings Mooring-trap depth

Mean annual current speed (cm s\)

Maximal peak current speed (cm s\)

Min

% deployment time '15 cm s\

2}600 m 2}1040 m

9.8 8.1

22.9 18.7

2.8 1.1

7.5 2.5

3}580 m 3}1440 m 3}3220 m

8.5 6.3 4.7

23 14.6 10

1.3 1.6 1.1

8.4 0 0

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Fig. 1. AVHRR image showing sea surface temperature at the Goban Spur (24.06.95). Also shown are the positions of the OMEX moorings and residual current vectors over the investigation period. Continuous lines (OMEX 2 620 m, OMEX 3 600 m); dashed lines (OMEX 2 1070 m, OMEX 3 1460 m); grey line (OMEX 3 3240 m). Image provided by Steve Groom, NERC Remote Sensing Group, PML, UK.

the 200 m contour, extends to the mid and outer slope at the steeper, south-westerly margin, in contrast to the Goban Spur (Fig. 1). Prevailing currents at the margin are responsible for particle redistribution and will determine both their residence time in the water column and the areal extent of their deposition to the sediments. Residual current vectors from current meters adjacent to the traps are shown in Fig. 1 (arrows). At OMEX 2 the residual current #ows to the north-west, parallel to the slope, as has been reported for other sites at the eastern

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Atlantic margin (Pingree and LeCann, 1990, Huthnance et al., subm.). With increasing distance from the shelf break, the o!-slope component of current grows stronger, and at OMEX 3 the net annual residual #ow leaves the slope. Huthnance et al. (subm.), in a comprehensive analysis of currents over the Goban Spur, conclude that tides are the largest contributor to instantaneous shelf edge exchange at the Goban Spur. With a tidal excursion of 2}4 km and a slope width of several hundreds of km, and with a change in tidal direction every 6 h, there can be little net seaward #ux of particles due to tides. Additionally, assuming an average current velocity of 10 cm s\ and particle sinking speed of 100 m d\ (i.e. 0.1 cm s\) it is clear that particle trajectories are primarily in#uenced by the instantaneous current. Particles intercepted on the slope at OMEX 2 site are likely to be transported along the slope towards the Porcupine Seabight, whereas particles intercepted at OMEX 3 are more likely to leave the slope and be exported from the continental margin. The direction and intensity of the residual #ow clearly control the extent of particle exhange across the margin. 3.3. Annual yuxes Annual #uxes of biogenic and lithogenic particles (dry weight, DW), particulate organic carbon (POC), particulate inorganic carbon (PIC) and opal are shown in Fig. 2. Below the seasonal mixed layer at 600 m annual #uxes vary by less than a factor of 2 between the two sites for all the elements shown, revealing little cross-slope gradient in export. This might also be expected from satellite imagery (see Fig. 1), which shows little variation in surface conditions on the slope. There is no lithogenic #ux at 580 m at OMEX 3; at the same depth at OMEX 2 lithogenic #ux is (20% of total #ux, allowing us to use these values as representative of primary mixed-layer export.

Fig. 2. Annual #uxes of biogenic and lithogenic material (left), POC and PIC (middle) and opal (right) at both mooring sites.

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The increase in bulk #ux with depth is marked, and is accounted for both by the contribution of lithogenic particles and by an increase in biogenic sedimentation, a feature that is characteristic of ocean margins (Monaco et al., 1990; Biscaye et al., 1994; Etcheber et al., 1996). The relative increase in the various components of #ux is, however, di!erent, re#ecting the di!ering sources and transport pathways of the components. The increase in lithogenic #ux with depth is most pronounced at 1440 m at OMEX 3, where it accounts for 40% of total #ux. At 1050 m at OMEX 2, 32% of bulk sedimentation consists of lithogenic material. The lack of increase in lithogenic and total #uxes between 1440 and 3220 m at OMEX 3 suggests that the main particle source to deeper traps is above 1400 m on the outer slope. Particulate inorganic carbon (PIC) #uxes follow the trend in bulk #ux with a 2.7-fold increase betweeen 600 and 3200 m that re#ects the lateral transport of calcareous shells of organisms in resuspended material. Coccolithophorids and foraminiferal tests were abundant in the traps and accounted for the bulk of carbonaceous material. SEM micrographs of particles from intermediate and benthic nepheloid layers also showed the presence of calcareous and siliceous debris that can be resuspended into the water column and, through aggregation and scavenging by sinking particles, can contribute to deep water trap collections. The most pronounced depth-dependant increase is seen to occur in opal sedimentation (from 1 g m\ y\ at 600 m to 4.9 g m\ y\ at 3220 m at OMEX 3). Microscopic analyses of sediment trap material show that opal #uxes are mediated primarily by diatom frustules, with a negligible contribution from radiolaria and silico#agellates. Similar POC #uxes were measured at 600 m both sites (2.1 and 2.2 g m\ y\ at OMEX 2 and 3, respectively). The `statistical funnela of the surface traps (Siegel et al., 1990), taken to denote the source area of collected particles at the surface, does not include the shelf break but lies within more oligotrophic, outer slope waters. At 1440 m at OMEX 3 POC #uxes of 3.8 gC m\ y\ are found, decreasing to 2.4 gC m\ y\ at 3220 m, indicating maximal POC input between 600 and 1440 m. In contrast to PIC and opal #uxes, POC re#ects a more labile particulate component of lateral input, undergoing continuous biological degradation rather than the dissolution of siliceous and carbonaceous material that is physically and chemically controlled. The increase in annually averaged molar C : N ratios from 7.3 at 600 m to 9 at 3200 m, while indicating progressive degradation with depth, suggests that organic material is largely pelagic in origin with no terrestrial input. We have approximated the relative contribution of POC export from the surface mixed layer (termed primary #ux) and that from lateral sources (termed secondary, or lateral, #ux) to measured POC #uxes at both mooring positions. Due to trap clogging in spring the primary #ux values are taken to be minimal estimates. We take #uxes at 600 m to represent mixed layer export (primary #ux) and use the algorithm of Martin et al. (1987) to calculate expected primary #ux in the deeper traps: J "J *(z/600)\ , X  where J and J are organic carbon #uxes at 600 m and trap depth, respectively, and  X z is the depth of trap deployment. The di!erence between this calculated primary #ux and measured #ux below 1000 m is taken to represent lateral #ux.

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The primary and lateral POC #uxes thus calculated are presented in Fig. 3, from which it is apparent that the lateral #ux of POC at 1050 m at OMEX 2 is equal to primary input, whereas at 1440 m at OMEX 3 it is a factor of 2.7 higher. At 3200 m lateral #uxes account for the most POC input. Higher lateral input at OMEX 3 than at OMEX 2 may result from the injection of material in intermediate water layers at the outer slope from the canyons upstream of the traps (McCave et al., subm.). These lateral POC #uxes take place below the depth of winter mixing, and at OMEX 3, they indicate export to the adjoining abyssal plain and thus long-term sequestration. Such a transport must be rapid relative to POC degradation, since there is little change in bulk C : N ratios despite the dominance of secondary #ux at depth. Suspended particles, due to their negligible sinking speeds, are thought to play a secondary role in this process, with rebound particles (Walsh, 1992), in the form of aggregates unaltered in biochemical composition through contact with the sea bed, being more important. POC input to the benthic boundary layer amounts to 2.3 g m\ yr\ at 1500 m on the slope and 2.2 g m\ yr\ at 3600 m. Benthic studies within OMEX indicate that

Fig. 3. Annual POC #ux (in boxes) and calculated vertical (open arrows) and lateral (shaded arrows) #uxes at positions OMEX 2 and OMEX 3. All values in g m\ yr\. For details of calculations see text.

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this vertical POC supply to the BNL is short of carbon demand by a factor of 1.8 at 3600 m and 2.2 at 1450 m water depth (Lohse et al., 1999; Herman et al., subm.). This implies a signi"cant near-bottom lateral supply to the slope benthos that is not measured by the traps. The total export of carbon from the surface mixed layer consists of both organic and inorganic (carbonate) #uxes. Since one mole of CO is released for every mole of  "xed in carbonate the POC : PIC ratio is a better measure of net C-drawdown than Corg alone (Tsunogai and Noriki, 1991). Annual average POC : PIC ratios in sedimenting material at the two sites (Fig. 4) are constant with depth (excepting 3220 m at OMEX 3), in contrast to the decreasing ratios found in open ocean, and re#ect the composition of particles transported laterally to the intermediate traps. We use the calculated vertical POC #uxes (above) and assume that the PIC #ux at 600 m re#ects vertical PIC input to the deeper traps (i.e. no degradation with depth) to estimate the POC : PIC ratios of the lateral component of #ux. The high POC : PIC ratios of lateral #ux provide further indication that these particles (a) are rapidly transported compared to the degradation rate of organic matter and (b) originate

Fig. 4. POC : PIC ratios of vertical and lateral components of #ux. For details of calculations see text.

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from a region of low carbonate } high POC production, presumably the shelf break, dominated by siliceous plankton, as seen in the strong lateral opal #ux signal (see Table 3). The horizontal surface gradient both in productivity and in plankton composition (with higher diatom concentrations at the shelf break, Holligan, 1981) is re#ected in the vertical gradient in composition of sedimenting material. Whether or not the continental margin acts as a net sink of atmospheric CO  depends on the depth to which carbon "xed in the productive surface layers is transported. POC remineralised within the seasonal mixed layer (including benthic mineralization on the shelf ) will come into contact with the atmosphere within an annual cycle, and hence is irrelevant to long-term sequestration. Our results indicate that the net o!-slope transport of organic carbon takes place below the winter mixed layer; the ultimate fate of the products of Corg degradation and Cinorg dissolution (DOC, CO ) will depend on the residence time of intermediate and deep water in  which they are produced. Seidov and Haupt (1997), modelling water transport in the North Atlantic, calculate that particles exported to deeper than 1000 m at the Goban Spur will be sequestered for several hundreds of years, contributing to a sink on climatically relevant time scales. 3.4. Seasonality in yux Seasonality in the sedimentation of biogenic material follows that expected in surface production (Fig. 5). A late summer increase in total mass #ux in July/August 1993 is evident in the 600 m traps, equal in magnitude to spring sedimentation in May 1994. This seasonality is still apparent at 3220 m. Although generally following the same seasonal pattern, biogenic and lithogenic #uxes can di!er in their temporal periodicity on short time scales that re#ect more the physical processes causing resuspension than biological control. Lithogenic #uxes are least during spring ((15% total #ux) but otherwise extremely variable, ranging from 20% to 70% of total #ux between adjacent sampling intervals, re#ecting events of high kinetic energy, resuspension and lateral transport. One such event is visible between 28. Sept. and 26. Oct. 1993 at 1050 m at OMEX 2 (Fig. 5). During this period lithogenic #uxes increased from 5 to 111 mg m\ d\, while biogenic #uxes decreased in magnitude. This event was not registered in the trap 400 m above, indicating a lateral particle source below 600 m at this site. Some of the short-term variability in #ux at 1440 m at OMEX 3 coincides with events of elevated o!-slope currents that are associated with increased sedimentation rates. Between July and December 1994 four such events of 4}14 days duration each were recorded (labelled I}IV in Fig. 6). Periods of o!-slope current coincide with periods of elevation in total bulk #ux, especially during winter. However, the correlation between both is weak, which may be partly ascribed to the limited resolution by the trap collection intervals. It is noteworthy that the maximal variability in #ux is seen in this trap, where the highest #uctuation in velocity and current direction is recorded. POC #uxes (Fig. 7) are higher in late summer (July/August 1993) than during the spring bloom in April/May 1994 at all depths except at 600 m at OMEX 3, where due to trap clogging spring #uxes could not be sampled completely. The composition and

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Fig. 5. Seasonality in biogenic (bold) and lithogenic (shaded) #ux at OMEX 2 and OMEX 3.

quality of spring and summer #uxes is signi"cantly di!erent, as seen by their dN isotope signature, pigment composition and microscopical analyses (Antia et al., subm). This seasonality is also seen in traps at 1050 m (OMEX 2) and 1440 m (OMEX 3) but is diminished at 3220 m at OMEX 3. The contribution of POC to bulk particulate sedimentation decreases with depth from an annual mean of 13% DW at 600 m to 8.6% DW at 1440 m and 5.1% DW at 3220 m. These values are high compared to those found in the NW Atlantic (Gardner and Richardson, 1992; Honjo and Manganini, 1993), the NW Mediterranean Sea (Monaco et al., 1990) and at the western Atlantic margin (Biscaye and Anderson, 1994), even though absolute POC #uxes at the latter sites are a factor of 5 higher than at the Goban Spur. The clearest annual trend in the contribution of POC to bulk #uxes is seen at OMEX 3 at 600 m, the trap that receives negligible lithogenic input. A relatively constant contribution of about 16% to dry weight sedimentation is seen during summer and winter, decreasing rapidly to about 5% during the spring bloom sedimentation. These low POC : DW

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Fig. 6. Polar plot showing events of high current speed (I}IV, upper panel) and bulk #ux (lower panel) at 1440 m at OMEX 3.

ratios during spring possibly represent the high inorganic load of a diatom-mediated sedimentation period, as is evident from opal sedimentation and microscopical analyses of trap material. Seasonal opal #uxes are presented in Fig. 8. Biogenic opal production in the euphotic zone is intimately related to diatom growth (Brzezinski and Nelson, 1995),

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Fig. 7. Seasonality in POC #ux (histograms) and percentage contribution of POC to total #ux (diamonds) at OMEX 2 and OMEX 3.

and its sedimentation re#ects seasonal succession in the autotrophic community. As may be expected, maximum opal sedimentation is during spring, when diatoms are the dominant autotrophs in the pelagial, exceeding by a factor of three opal sedimentation during summer. The stronger spring signal of opal #ux compared to POC re#ects the seasonally early depletion of silicate in the euphotic zone (Hydes et al., subm.). Opal #uxes at 600 m are lower over the outer slope (OMEX 3) than at the mid slope (OMEX 2). This could be a result of the injection of silicate through upwelling at the shelf break, enhancing diatom growth. At OMEX 3 opal #uxes 5-fold between 600 and 3220 m. In the trap at 3220 m opal sedimentation is high through to August 1994, indicating lateral advection of siliceous particles during summer. This signal is consistent with that observed at the sediment surface along the Goban Spur transect, with a downslope increase in the abundance of diatom frustules (Romani, 1994). 3.5. Suspended particulate matter (SPM) yux Lateral #uxes, through resuspension and transport in mid-water layers, mediate the pattern of #uxes at the continental margin (this study: Biscaye and Anderson, 1994;

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Fig. 8. Seasonality in opal #ux at OMEX 2 and OMEX 3.

Etcheber, 1996). There is no direct correlation between #ux measured by sediment traps and suspended particulate load (Walsh and Gardner, 1992; Asper et al., 1992), since traps sample large rapidly sinking aggregates that are relatively rare in the water column and are not adequately measured by transmissiometers (Au!ret et al., 1994). Long-term recordings of SPM concentrations at 1070 m at OMEX 2 are depicted in Fig. 9. A number of short-term events of high particulate load were seen with maximal beam attenuation values of up to 1.5 m\, comparable to those found in surface waters. These values are a factor of 7}8 higher than those found by spot measurements during cruises by ourselves and others at the Goban Spur (McCave et al., subm.). Minimal attenuation values of 0.36 m\ correspond to those seen at the particle minimum in the water column. The duration of these nepheloid events (3}10 days) and their non-simultaneous occurrence at these positions about 60 km apart points to the ephemeral and patchy nature of such submarine particle clouds. They were not related to current speed or direction, suggesting sources remote from the moorings, possibly

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Fig. 9. Transmissiometer beam attenuation at 1050 m at OMEX 2 (a) and at 1440 m at OMEX 3 (b). In Fig. 9b only periods of high beam attenuation are visible, as problems with the interface between transmissiometer and current meter led to values below 0.365 m\ being o! the registration scale.

from the canyons upstream of the Goban Spur (I.N. McCave pers.comm.). None of these events were related to elevations in bulk #ux. A comprehensive summary of SPM distributions at the Goban Spur is presented by McCave et al. (subm.). In Fig. 10 a vertical pro"le of SPM in the water column during Sept. 1995 is shown. SPM concentrations in the clear water particulate minimum and nepheloid layers (15}20 lg l\, 100}120 lg l\, respectively) are higher than those measured in 4500 m water depth several hundred kilometres from the shelf break at this latitude (10 lg l\, 20}100 lg l\, respectively; Ny%er and Godet, 1986). Intermediate nepheloid layers (INLs) of similar magnitude have been recorded at the eastern (Dickson and McCave, 1986) and western (Richardson, 1988) Atlantic margins. Thorpe and White (1988) relate the depths of their occurence to layers where kinetic energy of the M2 internal tide is focused on the slope. A conservative estimate of the lateral #ux of SPM between the 600 and 1440 m traps at OMEX 3 is made using an SPM value of 15 lg l\ (Fig. 10) and average

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Fig. 10. Vertical pro"le of SPM concentration at a water depth of 1520 m at the Goban Spur on 24 Sept. 1995.

current speed of 10 cm s\ (as measured at OMEX 2). The mass #ux passing through this 900 m horizon between the traps as SPM amounts to 109 kg DW m\ d\, several orders of magnitude larger than the maximum sedimentation rate of 0.4 g DW m\ d\ measured in the trap at 1440 m (Fig. 2). SPM transport in a single intermediate nepheloid layer of 200 m thickness (with SPM load of 100 lg l\ and current velocity of 10 cm s\) can transport 170 kg DW m\ d\ in the prevailing current direction. These rough calculations demonstrate the orders of magnitude di!erence in sinking (large particle) and suspended ("ne particle) #uxes and clearly show that, where o!-slope transport occurs, as is seen on the outer slope at OMEX 3, suspended particles potentially dominate export.

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SPM, however, presents a particle pool not directly sampled by the traps. It is through aggregation and scavenging that SPM contributes to sinking #ux (Hill and Nowell, 1990; Stolzenbach, 1993). Aggregates are thus both the primary vehicles for export of organic matter from the surface, and once in intermediate water depths accelerate sinking of suspended particles by scavenging, in the process modifying their biochemical composition and physical properties (Asper et al., 1992). The increase in aggregate abundance with water depth on the slope at the western Atlantic margin (Gardner and Richardson, 1992) points to their role in mediating lower water column #uxes. In interpreting our data in the context of the regional setting at the Goban Spur we make two central assumptions. Firstly, at the mid- and outer slope pelagic production resembles more that of the open North Atlantic, where stable summer strati"cation limits nutrient #uxes, distinct from the shelf break, where upwelling alleviates nutrient depletion and enhances growth during summer. Secondly, we assume that this crossslope gradient is su$ciently strong for biogenic material from the shelf break to be transported laterally over the spatial extent of the Goban Spur and account for the increase in #ux with depth that we measure. Over the outer slope we estimate a minimum annual new production using seasonal nitrate depletion from values reported in Hydes et al. (subm.; 7 lmol l\ in winter, 0 lmol l\ in summer) over the depth of the surface mixed layer (45 m, Huthnance et al., 1998). This provides an estimate of 315 mmol-N m\ y\. Measured PON #uxes at 600 m at both trap sites (17.8 lmol m\ y\ and 17.1 lmol m\ y\ at OMEX 2 and OMEX 3, respectively) indicate that 95% of the sinking organic matter is degraded in the upper 600 m of the water column, within the range of POC degradation indicated by Pace et al. (1987), Berger et al. (1987) and Suess (1980) (93}97%). Similarly, the upper mixed layer seasonal depletion of silicate (calculated from values in Hydes et al., subm.), amounts to 113 mmol silicate-Si m\ y\, or a production of 7.5 g opal m\ y\. Of this ca. 1 g m\ y\ is seen to sediment to 600 m, with 85% undergoing dissolution within the upper mixed layer. This is within the range of opal dissolution reported in the literature (Nelson and Goering, 1982; Brzezinski and Nelson, 1995). We thus have some con"dence that measured #uxes at 600 m relate to overlying pelagic production. Joint et al. (manuscript), using a composite of multi-year measurements spanning the entire Goban Spur from the inner shelf to outer slope, estimate a mean annual primary production of 162 gC m\ y\. A paucity of measurements does not allow distinction of cross-slope production gradients. Numerous other studies, however, show a 3}4 fold gradient in production (Sathyedranath et al., 1995), inorganic nutrient availability (Holligan and Groom, 1986, Pingree et al., 1976), and phytoplankton (Garcia-Soto and Pingree, 1998) and zooplankton (Batten et al., 1998) biomass centred around the shelf break. There are limits to the amount of material that can be transported laterally from the shelf break, depending on the relative extents of the source areas and the degradability of organic matter. We take a high production band of 45 km width at the shelf break and a slope band of similar width between the shelf break and OMEX 2 (see Fig. 1) and 249 gC m\ y\ as the annual primary production localised at the shelf break

Water depth DSBB (km) '500 m '1000 m '3000 m '500 m '1000 m '3000 m '500 m '1000 m '3000 m '500 m '1000 m '3000 m

Position

BDistance from shelf break.

OPAL

PIC

POC

DW

Flux (g m\ y\)

4935N, 13325W 3660 m 91 14.0 43.2 42.0 2.2 3.7 2.1 1.1 1.8 2.5 1.0 3.8 4.9

OMEX 3

1.5 1.7

1.2 1.5

2.1 2.3

49311N, 12349W 1450 m 63 19.3 27.9

OMEX 2

5.9 5.6

1.8 1.85

1.38 1.00

26.9 26.2

483N 213W 4400 m '1000

North Atlantic (1)

2.1 1.15

1.02 1.06

1.3 0.65

13.7 13.8

48347N 203W 4550 m '1000

L2-B-92 (2)

1.7/1.89

1.58/1.59

29.7/32.2

513N, 133W 2000 m 140

Porcupine Seabight (3)

22.6

2.7

8.4

37337N, 74310W 998 11 110

SEEP II (4) Mooring 7

28.5

3.8

12

36352N, 74335W 1001 m 7 166

Seep II (4) Mooring 10

10.2 11.7

280 437

44343N, 2317W 2300 10

M1 (5)

5.1

118

44347N, 2338W 3000 15

M2 (5)

Table 3 Comparison of annual #uxes of dry weight, POC, PIC, and opal at the continental margin and in the abyssal North Atlantic. All traps were situated above the benthic nepheloid layer. (1) Honjo and Manganini (1993). Data from 2000 and 3000 m were selected as follows; (2) Ku{ and Kremling (1999), traps at 2030 and 3530 m; (3) Lampitt et al. (1995), traps at 1750 and 1350 m; (4) Biscaye and Anderson (1994), their moorings 7 and 10, traps at 400 m; (5) Etcheber et al. (1996), mooring M1, traps at 768 and 1190 m, and Mooring M2, trap at 1900 m

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(Sathyedranath et al., 1995). Of this, between 18 and 24 gC m\ y\ can be expected to arrive at 200 m on the shelf and shelf break sediments (Pace et al., 1987; Berger et al., 1987). On the order of 4 to 5% (1 gC m\ y\, Fig. 3) is exported as lateral POC #ux to the mid slope below the depth of seasonal mixing. This crude calculation is dependent on the primary production value chosen, but nonetheless gives a range within which shelf export is seen to take place within the water column. Sediment carbon demand (Lohse et al., 1999) at the shelf break ranges from 28 to 68 mgC m\ d\ during spring and summer, when benthic #uxes are greatest. Within the range of error of such measurements, which include spatial and seasonal variability, our measured lateral export to intermediate water depths is feasible, although clearly benthic consumption dominates the fate of the sedimenting organic material. Nonetheless, as we have discussed above, it is that fraction of POC exported below the depth of winter mixing, and not that remineralised in shallow waters, that is relevant to net export. In making these calculations we recognise that near-bottom #uxes in the BNL can be potentially much larger compared to those in the intermediate water column. 3.6. Comparison to other slope and open ocean sites To examine the contribution of the continental margin to basin-wide particle export we compare our own data with those from the open ocean at similar latitude (Honjo and Manganini, 1993; Ku{ and Kremling, 1999), and from the continental margins at the Mid-Atlantic Bight (Biscaye and Anderson, 1994), in the Porcupine Seabight (Lampitt et al., 1995) and in Bay of the Biscay (Etcheber et al., 1996) (Table 3). For data selection criteria see table legend. Since trap deployment depths and mooring con"gurations vary considerably between studies we group the data into shallow (OMEX at 600 m, SEEP at 400 m), intermediate (1000}2500 m) and deep ('3000 m) #uxes. In the abyssal N. Atlantic no reliable near-surface #uxes are available, but assuming exponential decay in POC #ux with depth (after Pace et al., 1987; Martin et al., 1987; Berger et al., 1987), and taking #uxes measured at 2000 m (Honjo and Manganini, 1993; Ku{ and Kremling, 1999), this will lie between 2.8 and 3.5 gC m\ y\ at 600 m, somewhat higher than our measured #ux of 2.2 gC m\ y\ at this depth at the OMEX sites. Below 3000 m #uxes at OMEX 3 are a factor of 2}3 higher than in the open ocean. Highest #uxes of all components are found at the SEEP site, where high lithogenic input (30}70% of total #ux, Biscaye and Anderson, 1994) indicates a large contribution from resuspended material. In intermediate depths there are consistently higher #uxes at all continental margin sites than in the open ocean (Table 3). The relative enrichment of POC, carbonate and opal (increasing in that order) in resuspended material as observed by Walsh (1992) is consistent with the relative increase in their #uxes with depth that results from the contribution of resuspended material. The POC : PIC ratio of exported material, a measure of the net drawdown of CO is a factor of 2 higher at the continental margin (POC : PIC of 2 : 1 to 3 : 1) than  in the open ocean at comparable depth (POC : PIC of 1 : 1 to 1.5 : 1). The role of the margins in biogeochemical elemental cycling is overridden in such a comparison by the inclusion of lithogenic #uxes of varying magnitude. In Table 4 we

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Table 4 Percentage contribution of POC, PIC and opal to biogenic #ux and lithogenic contribution to total #ux for stations listed in Table 3 Lithogenic #ux (g m\ y\)

OMEX 2}600 m OMEX 2}1050 m OMEX 3}580 m OMEX 3}1450 m OMEX 3}3260 m SEEP-7 400 m SEEP-10 400 m N. Atlantic 2000 m N. Atlantic 3000 m L2}2000 m L2}3530 m

3.9 9.1 0 17.3 12.1 56.1 92.9 0 0 0 0

Biogenic #ux (g m\ y\)

15.4 18.8 14.0 25.9 29.9 53.9 73.1 26.9 26.2 13.7 13.8

Percentage contribution to biogenic #ux % POC

% PIC

% opal

13.7 12.0 15.5 14.4 7.2 14.9 12.9 5.1 3.8 9.5 4.7

7.5 8.0 7.5 6.8 8.3 4.8 4.1 6.7 7.1 7.4 7.7

9.8 9.2 6.9 14.6 16.5 40 30.7 21.9 21.4 15.1 8.7

subtract the lithogenic contributions from bulk #ux and calculate the percentage contribution of POC, PIC and opal to biogenic #uxes. At the continental margins, POC accounts for a mean of 13.9 (#3.7)% of biogenic DW #ux, compared to 5.7$2.5 in the abyssal N. Atlantic. The latter value is representative of the global open ocean mean of 5.4$1.4 (Lampitt and Antia, 1997). As demonstrated in this study, sinking POC #ux at the ocean margin implies rapid transport of particles along the slope. This is underscored by the patterns of labile phytopigment #ux (Antia et al., subm.) that show both a strong seasonal signal in autotrophic export and an increase in intermediate depths similar to that of POC. However, much of the suspended POC at and near the margins consists of material whose *C value indicates that it is highly refractory in nature (Bauer and Dru!el, 1998). This is not contradictory if it is kept in mind that sampling by traps and water bottles collects di!ering particulate pools, and it implies that recent pelagic organic material is rapidly remineralized after export and is not retained against the large background pool of refractory material. Opal sedimentation at the western Atlantic boundary appears signi"cantly higher than at the other sites, possibly because the SEEP traps were placed within the area of local upwelling at the shelf break. Annually averaged silicate concentrations in surface waters are much higher at the SEEP site too (Levitus and Boyer, 1994), which would imply a greater role for diatoms in primary production and particle #ux. The 6 to 10-fold di!erence in measured #uxes between the Goban Spur, MidAtlantic Bight and Bay of Biscay cannot be accounted for by di!erences in primary production alone (Berger et al., 1987; Sathyedranath et al., 1995). In contrast to the Goban Spur, at the SEEP site high lithogenic #uxes are found in the shallow traps. Even at a total water depth similar to OMEX 2 and 3 in the Bay of Biscay there are

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large site-speci"c di!erences in #ux at comparable trap depths (Table 3). These are related to di!erences in the width of the shelf and slope as well as slope inclination, with narrow shelf and steep slope at the Mid-Atlantic Bight and the Bay of Biscay and a broad shelf and gradual slope at the Goban Spur. The implications of this are two-fold; "rstly, the extent of mixing at the sea surface and thus of the band of

Fig. 11. Mass #ux ("lled squares) and POC #ux (open squares) and (b) POC #ux as %DW as a function of distance from the shelf break. Data as shown in Table 3. Line "t for (a) DW"521;DSB , r"0.80 (bold line) and (b) POC"25.6;DSB , r"0.89 (dashed line). DSB; Distance from Shelf Break.

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upwelled water with higher productivity, extends well out to the slope and over the trap moorings at the latter sites. Secondly, physical forcing, largely responsible for the redistribution of particles, is accentuated where the slope is steep (Huthnance et al., subm.). Additionally, a diatom population at a shelf front is likely to export more of its total primary production than will an oligotrophic oceanic phytoplankton community. Distance from the adjoining shelf break is thus more closely related to measured particle #ux than water depth alone. Fig. 11a (based on data summarised in Table 3) illustrates the increase in measured total #ux and POC #ux with proximity to the shelf break. An exponential "t explains over 80% of the variance in the observations. The export of POC from the productive shelf break is also accentuated at the continental margin, with an exponential increase in its contribution to biogenic #uxes with proximity to the shelf break (Fig. 11b). Over a 2}3 fold range in primary production from the abyssal N. Atlantic to its continental margins a 10-fold increase in POC #ux below the depth of winter mixing is seen. It has long been recognised that continental margins are regions of elevated surface productivity (Berger et al., 1987; Sathyedranath et al., 1995) and sediment accumulation (Romankevich, 1984). Increased productivity and #ux are also re#ected in increased benthic activity, which shows an expoenetial increase with decreasing water depth similar to that in Fig. 11 (Lohse et al., 1999). The postulate that continental margins can be deposition centres of atmospheric carbon relies on the observation that POC burial (and thus sequestration on geological time scales) takes place where mass accumulation rates are high (Can"eld, 1989). As we show here, total #uxes increase exponentially with proximity to the shelf break, and this material is higher in organic content than in the open ocean (Fig. 11b). Conditions for locally enhanced burial are thus provided, although the degree to which this takes place is largely in#uenced by remineralisation by the benthic community. With high particle and POC concentrations in the BNL (van Weering et al., 1999), particle transport near the sea bed is potentially large compared to #uxes in the intermediate water column and provides both an important source to the benthic community and a mode of export not quanti"ed in this study. Finally, spatial patterns in benthic deposition and #ux at the Goban Spur, dealt with in detail by others in the OMEX project (see Progress in Oceanography), are consistent with the conceptual model presented in this study. The emergent pattern at the Goban Spur suggests that the foot of the continental slope is the site where particles from the margin may reach their "nal site of deposition. Across-slope patterns of deposition re#ect the #ux patterns in our traps, with maximum accumulation rates, benthic Corg burial (van Weering et al., 1999) and sediment total organic carbon content (Lohse et al., 1999) at the foot of the continental slope.

Acknowledgements We express our gratitude to a number of people whose assistance contributed to this work: T. Kumbier and G. Lehnert for technical support with the moorings:

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M. Krumbholz, P.Berger, E. Breitbarth, T. Marquadt, O. Friedrich and I. Thordsen for assistance in the laboratory. We are grateful to V. Smetach, R. Lampitt and two anonymous reviewers for constructive comments that improved the manuscript. We also express our gratitude to the captains and crews of the FS POSEIDON, FS METEOR, RRV DARWIN and RRV DISCOVERY and to K. Goy for assistance at sea. Steve Groom and Peter Miller of the NERC Remote Sensing Team at PML, UK, have been generous in providing satellite imagery. Financial support for this study was provided by the European Commission MAST O$ce (Contract MAS2-CT93).

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