Spatial and temporal variability of particle flux at the N.W. European continental margin

Deep-Sea Research II 48 (2001) 3083–3106

Spatial and temporal variability of particle flux at the N.W. European continental margin A.N. Antiaa,*, J. Maa
ena, P. Hermanb, M. Vo
c, J. Scholtend, S. Groome, P. Millere Institut fu.r Meereskunde, Universita.t Kiel, Du.sternbrooker Weg 20, D-24105 Kiel, Germany Nederlands Instituut voor Oecologisch Onderzoek, PO Box 140, NL-4400 AC Yerseke, The Netherlands c Institut fu.r Ostseeforschung, Warnemu.nde, Seestra
e 15, D-18119 Rostock, Germany d Institut fu.r Geowissenschaften, Universita.t Kiel, , D-24148 Kiel, Germany e Plymouth Marine Laboratory, West Hoe, Plymouth PL1 3DH, UK a

b

Abstract A synopsis of results from two sediment trap moorings deployed at the mid- and outer slope (water depths 1450 and 3660 m, respectively) of the Goban Spur (N.E. Atlantic Margin) is presented. Fluxes increase with trap deployment depth; below 1000 m resuspended and advected material contributes increasingly to bulk flux. Fluxes of dry weight, POC and diatoms in the traps 400 m above bottom (mab) are smaller than those recorded at the sediment surface due to lateral fluxes in the benthic nepheloid layer. These near-bottom fluxes are larger at shallower water depths. 231Pa/230Th ratios in sedimenting material suggest that boundary scavenging is not significant at the Goban Spur. Fluxes of 210Pb in the intermediate and deep traps are comparable to the 210Pb supply rate at this site. At the outer slope, sediment 210Pb fluxes are similar to those measured in the traps 400 mab; at the mid-slope they are a factor of 2 higher, once again indicating large near-bottom lateral particle input. Based on POC-normalised biomarkers in sedimenting material, we followed changes in the quality of sedimenting material with differing trap depth and on seasonal and eventrelated time scales. In spring fresh, diatom-dominated sedimentation occurs, with progressive degradation of POC with time (to winter) and depth (from 600 to 3220 m). Deeper traps are distinguished on the basis of opal and aluminium fluxes that are dominant in lateral input. A storm event during late September 1993 was clearly reflected in the d15N isotope ratio of sedimenting material, with a time lag of 2–3 weeks. Diatom and opal fluxes were elevated in this storm-related signal, and its biomarker composition in the 600-m trap was similar to that during spring. An estimate made of upward nitrate flux (new production) at the shelf break and at the outer slope indicated a 2-fold higher new (export) production at the shelf break. Particulate organic carbon export from the shelf break to below the depth of maximal seasonal mixing ranges between 3 and 9% of primary production. r 2001 Elsevier Science Ltd. All rights reserved.

*Corresponding author. Tel.: +49-431-597-3865; fax: +49-431-565-876. E-mail address: [email protected] (A.N. Antia). 0967-0645/01/$ - see front matter r 2001 Elsevier Science Ltd. All rights reserved. PII: S 0 9 6 7 - 0 6 4 5 ( 0 1 ) 0 0 0 3 3 - 9

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1. Introduction One of the aims of the Ocean Margin EXchange (OMEX) Project was to determine the exchange of particles at the continental margin in relation to production at the surface and to delineate the transport mechanisms responsible for flux. Physical processes and local topography (summarised by Huthnance, 1995) are largely responsible for the gradients in productivity that characterise the ocean margins, as well as for controlling particle redistribution after settling, through resuspension and advective transport. It thus may be expected that there will be commonality between different margins as well as differences due to topography and surface forcing. During the OMEX I study (1993–1996), the focus of investigation was at the Goban Spur (Celtic Sea), which is characterised by a wide shelf and an extended, gradual slope. This differs in topography from the Mid-Atlantic Bight (Biscaye and Anderson, 1994) and the Bay of Biscay margins (Etcheber et al., 1996), which have relatively steep slopes from the shelf break to the abyssal plain. The effect of this differing topography on particle export is reflected in the relation of flux to distance from shelf break and not to water depth alone (Antia et al., 1999). The most evident hydrographic feature of the Celtic Sea continental margin as seen in satellite images is the band of cooler water centred around the shelf break, which is related to local upwelling of nutrient-rich water. This gradient in physical forcing and nutrient availability is thought to result in a shift in pelagic community structure, favouring larger phytoplankton; and elevated new production; as compared to the outer slope. This gradient is difficult to measure on periodic ships’ cruises, though it is seen in long-term recordings from the Continuous Plankton Recorder as an increase in diatom abundance at the shelf break. In pelagic systems with high new production, diatoms are often dominant among the plankton and can sink rapidly through the formation of phytogenic aggregates (Crocker and Passow, 1995). We thus expect a horizontal gradient in production and export characteristics along a slope-ocean transect. Once exported from the epipelagic zone, particles reaching the seabed of the shelf and shallow slope are redistributed by resuspension and advection, such that measurements of particle distribution and flux have no simple spatial relationship to the area of their production. A key question, however, is to what extent organic matter can be exported below the depth of winter mixing and thus sequestered over climatically relevant time scales. The rapidity and mode of export with respect to the POC degradation rate are thus important in determining the potential for a long-term sink of atmospheric carbon in these regions. Also needed for a calculation of net export is the determination of the magnitude and gradient of surface properties such as primary production and spatial changes in food-web structure. These are difficult to demonstrate on periodic cruises by discrete measurements. Within the OMEX project, however, such gradients have been shown in micro- and mesozooplankton biomass and diatom abundance (Edwards et al., pers. comm.; Batten, pers. comm.), though these measurements are limited in spatial and temporal coverage. Long-term direct measurements of particle fluxes using sediment traps provide the unique possibility of determining quality and composition of sinking particles over seasonal and short-term time scales and over vertical and horizontal gradients across the slope. Within the OMEX project, three moorings were placed at the mid- and outer slope and at the foot of the slope, respectively, equipped with sediment traps and current meters. In this paper we

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synthesize results from these moorings and emphasise the use of multivariate analyses of sinking particles to delineate transport pathways of biogenic and lithogenic material. We present evidence of the rapid off-slope transport of labile, phytogenic material in a seaward direction, and estimate the potential export of organic matter from the shelf break.

2. Materials and methods 2.1. Sediment trap samples Data are presented from sediment trap moorings in water depths of 1440 m (OMEX 2), 3660 m (OMEX 3) and 4500 m (OMEX 4). Traps were placed at depths of 600 and 1050 m (OMEX 2), 580, 1440 and 3220 m (OMEX 3) and 4000 m (OMEX 4). Details of sample treatment and analyses of bulk variables are given in Antia et al. (1999). 2.2. Organic biomarker analyses One-eighth splits of sediment trap samples were concentrated by centrifugation for organic biomarker analyses. To provide sufficient material for detection, samples were pooled as presented in Table 1. Particles were concentrated by centrifugation and trace water removed with 5 ml 0.05molar KCl solution. Ultrasonic extraction was performed first with methanol and then twice with dichloromethane. High molecular weight compounds were removed with dichloromethane through a 100
10 mm column packed with 30% desactivated silica (Merck 63–200 mm). The resulting mixture was transferred into pentane and separated into fractions by normal phase HPLC. Fraction 1 contained the n-alkanes and isoprenoids; fraction 2, the C37 -methylketones. The mobile phase consisted of solvent mixtures with increasing polarity (pentane, dichloromethane and acetone). The HPLC was a Constramatic III with a Rheodyne 7125 injector fitted with a 20-cm Nucleosil 100-5 (4-mm i.d.) column (Macherey-Nagel) and a Spectroflow 757 absorbance detector. Biomarker compounds were identified and quantified by gas chromatographyFflame ionisation detection (GC-FID). The GC was a Carlo Erba 5160 GC fitted with an on-column injector. A 30 m DB-5 WCOT capillary column (0.32-mm i.d.) and a 1-m deactivated fused silica

Table 1 Intervals for sample pooling for biomarker analyses Season

Summer Autumn Winter Storm Spring

OMEX 2

OMEX 3

600 m

1050 m

580 m

1440 m

3220 m

1.7.93–20.9.93 20.9.93–26.10.93 23.11.93–2.1.94 26.10.93–23.11.93 26.4.94–1.6.94

1.7.93–20.9.93 20.9.93–26.10.93 23.11.93–2.1.94 26.10.93–23.11.93 26.4.94–1.6.94

1.7.93–20.9.93 20.9.93–4.11.93 4.11.93–2.1.94

1.7.93–20.9.93 20.9.93–4.11.93 4.11.93–2.1.94

1.7.93–20.9.93 20.9.93–4.11.93 4.11.93–2.1.94

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pre-column (0.32-mm i.d.) were used. The detector port was kept at 2801C. Carrier gas was hydrogen (2.5 ml min1). Two different temperature programming conditions were used for the two investigated fractions: Fraction 1 (ballistic temperature programme): 601C (hold for 1 min), with 101C min1 to 1401C, with 51C min1 to 3101C (hold for 25 min). Fraction 2: 601C (hold for 1 min), with 301C min1 to 2101C, with 131C min1 to 2751C, with 51C min1 to 3001C (hold for 45 min). GC-MS was performed with a Hewlett-Packard HP 5993 in the EI mode (70 eV). Temperatures of injector, transfer line, and ion source were kept at 2801C, 3001C and 2801C, respectively. Carrier gas was helium (2 ml min1) on a 25 m
0.25-mm i.d. SE 52 WCOT column (0.12-mm film). GC retention times and full-scan MS data of standards as well as literature data were used for identification. The reproducibility of the biomarker determinations is better than 4%. 2.3. Statistical procedures Principal component analysis was performed on particulate organic carbon (POC)-normalised concentrations of eight biomarkers (alkanes, isoprenoids and alkenones [37 : 2, 37 : 3; values taken from Elvert, 1995]), bulk concentrations of opal and carbonate (data from Antia et al., 1999), chloropigments and fucoxanthin (Antia et al., unpublished data) and aluminium (Chou, pers. comm.). In principal component analysis (PCA), a normalised set of variables is projected on a small number of axes called ‘‘principal components’’ (PCs) in a reduced dimensional space (Davis, 1973). These PCs are vectors composed of variable contributions (loadings) from all variables describing the maximum object variance. Varimax rotation was used on the PCs in order to move each factor axis to a position where the projection of each variable is either near the extremities or near the origin. Statistical interpretation was performed on a total of 13 marker variables using Systat 5.0 software. 2.4. d15N Sample splits on Whatman GF/F filters were analysed with a CHN analyser that is connected via a split interface (Finnigan ConFlow) with a mass spectrometer (Finnigan Delta S). As reference gas, pure N2 from a cylinder calibrated against air as a standard (Mariotti, 1983) was used. Additionally, every 5th combustion run is an internal standard (Pepton, Merck), as control for the combustion process. The reproducibility of the isotope determinations is better than 0.2m. 2.5. Radionuclides Sediment trap samples were pooled to yield annually integrated values for each trap and depth for thorium isotopes, 231Pa and 210Pb following the procedure given in Anderson and Fleer (1982). 210Pb was analysed via 210Po, which was in radioactive equilibrium with 210Pb at the time of analysis (5 years after sampling). Measurements were performed by alphaspectrometry.

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2.6. Remote sensing The nitrate ‘‘pseudo-image’’ was created by first constructing a temperature vs. nitrate relation: Nitrate ¼ 0:35T 2  11:6T þ 95:2; from in situ data obtained on the RRS Charles Darwin cruise in June 1995. The algorithm was then applied to an Advanced Very High Resolution Radiometer (AVHRR) median SST composite for June 1995, computed from all the cloud-free data obtained during the month (see Miller et al., 1997).

3. Results and discussion Reported patterns of fluxes at ocean margins show certain common aspects that suggest topographic control of the distribution and flux of particles, both in the water column and in the sediments. In this paper, we present an overview based on data from the OMEX I project during which two long-term moorings were deployed at the Goban Spur (Celtic Sea) from 1993 to 1995, equipped with sediment traps and current meters. The OMEX study area at the Goban Spur (Fig. 1) has a wide, gradual slope extending over ca. 200 km. Our moorings were placed at the mid- and outer slope, with traps placed below the depth of winter mixing at ca. 600 m, 400 m above bottom (mab) and, at the outer slope at 1440-m depth, below the intrusion of water mass with a Mediterranean imprint. Bulk particulate flux in the sediment traps shows an increase with depth that is related to the additional input of resuspended and laterally advected material below the surface. A detailed report of bulk particulate fluxes from the OMEX sediment trap moorings is presented in Antia et al. (1999), the salient points of which are summarised here and in Fig. 2: *

Predominantly along-slope currents are seen at OMEX 2 whereas at OMEX 3 and OMEX 4 residual currents leave the slope, which may promote particle export at this site.

Fig. 1. Positions of the sediment trap moorings OMEX 2, 3 and 4 on the Goban Spur (Celtic Sea)

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Fig. 2. (a) Annual fluxes of bulk dry weight, carbonate, opal and POC and (b) of diatom valves, chloropigments [S (chl a, phaeophytins and phaeophorbides)] and the diatom marker pigment fucoxanthin at OMEX 2, OMEX 3 and OMEX 4. *

Traps at 600 m sample export from the mixed layer, above the mid- and outer slope, 60–90 km from the shelf break, which is the region of higher productivity. There is low (OMEX 2) or negligible (OMEX 3) lithogenic input to these traps, and they are taken to represent ‘‘primary’’ flux.

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*

*

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Lithogenic fluxes increase with depth and highest fluxes are found at 1440 and 3220 m at OMEX 3 on the outer slope. The relative increases in dry weight, POC, opal and carbonate differ with depth; whereas POC fluxes peak at 1440 m and decrease thereafter to 3220 m at OMEX 3, opal and carbonate fluxes are highest at 3220 m. Diatoms are primarily responsible for opal export and valve fluxes show the highest increase with depth, peaking at 1440 m. Opal fluxes at 3220 m are seen microscopically to consist of a large component of fragmented diatom frustules. Mid-water particle flux at continental margins increase exponentially with proximity to the shelf break with a simultaneous increase in the contribution of POC to total flux from 5.5% in the open ocean to 13.9% at the margin.

Here we use a multivariate approach with analyses of radionuclides, organic biomarkers and the d15N signature of sedimenting particles to follow processes that lead to the production and flux of particles in the complex shelf-slope environment. We compare these to other key fluxes measured within the OMEX project (atmospheric 210Pb flux, primary and new production, benthic mass accumulation and carbon mineralisation rates, see other contributions in this volume) and show that a consistent pattern emerges. Aside from yielding quantitative estimates of shelf export and flux, this provides a powerful tool that enables us to better understand the fate of particles at the ocean margin. 3.1. Radionuclide fluxes in sediment traps We use the activities of 230Th and 231Pa to compare fluxes between the sediment traps and the underlying sediments and to determine the trapping efficiency of the traps. The 230Thex and 231Paex activities (the 230Thex and 231Paex activities are the activities of the isotopes in excess of the 234U and 235U activities present in the lattice of mineral grains) increase with water depth of the traps investigated, as observed in sediment traps deployed during SEEP (Anderson et al., 1994), and is due to the continuous uptake of 230Th by settling particles. When the 230Thex flux measured in the sediment traps (Fa) is related to the production rate of 230 Th in the overlying water column (Fp), Fa/Fp-ratios between 0.35 (OMEX 3, 580 m) and 1.28 (OMEX 3, 3220 m) were observed. 231Paex/230Thex ratios are 0.058 and 0.087, respectively (Table 2). The Fa/Fp ratios in sediment traps depend both on lateral advection of 230Th in the water column and on trapping efficiencies of sediment traps (e.g., ratios in the range of 1 also could be the result of lateral advection of 230Th due to boundary scavenging in combination with low trapping efficiency). For the eastern North Atlantic, Scholten et al. (2001) suggest that boundary scavenging is of minor importance for the 230Th distribution in the water column. The 231Pa/230Th ratios found in the OMEX traps, which are in the same range previously reported from other areas in the North Atlantic (Yu et al., 1996), further point to a minor influence of boundary scavenging on the 230Th fluxes. As we show below, there are no indications that trapping efficiencies of the deep traps significantly influence the radionuclide fluxes in the deeper traps. Sediment 210Pb fluxes at the OMEX sites, measured by van Weering et al. (1998), are compared to 210Pb fluxes measured in the sediment traps 400 m above bottom in Table 2. At the OMEX 3

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Table 2 Radionuclide fluxes in sediment traps Sample

OMEX-2 OMEX-2 OMEX-3 OMEX-3 OMEX-3 OMEX-4

600 m 1050 m 580 m 1440 m 3220 m 4000 m

Sampling time (days)

Fa/Fpa

363 363 315 432 432 370

0.39 0.90 0.35 1.13 1.28 0.7

S.D.

231 230

0.02 0.08 0.01 0.06 0.07 0.07

Paex/ Thex

n. d.c 5.85E-02 n. d. 6.80E-02 8.70E-02 n. d.

S.D.

9.58E-03 5.70E-03 4.50E-03

210

Pb flux (dpm cm2 a1)

Trapping efficiencyb (%)

0.05 0.15 0.04 0.40 0.40 0.27

40 94 36 116 136 78

210

Pb flux at sediment (dpm cm2 a1)

0.5–1.05

0.3–0.4 0.1–0.43

Fp ¼ 0:07068 (dpm m2 d1) * z (m); z=water depth. Th sediment trapping efficiency from Scholten et al. (accepted). c n.d. not determined. a

b 230

and OMEX 4 sites on the lower slope, there is good agreement between the two, whereas at OMEX 2 trap fluxes are half those measured at the sediment surface. Bearing in mind the difference in time scales over which the two measurements integrate, it appears nonetheless that near-bottom processes, below the lowest trap, are more important at the upper slope, a pattern that is also seen in other variables measured. The supply flux of 210Pb to the OMEX sites depends on the atmospheric and water column fluxes. Thompson et al. (1993) estimated this flux for the area south of the Porcupine Abyssal Plain towards the Hatton Drift to be about 0.3 dpm cm2 a1. Since the 210Pb fluxes measured in these deep traps are approximately similar to both the 210Pb supply rate and the 210Pb flux measured in the sediments, no significant biases in trapping efficiencies are suggested for the intermediate and deep traps at the three sites. This is further corroborated by the 230Th trapping efficiencies calculated by 230Th mass balances for these traps (Scholten et al., 2001). Only 10% of the supply 210Pb flux was measured in the 580-m trap. In addition, the 230Th-based Fa/Fp ratio of 0.35 for this trap is significantly lower when compared with the deeper traps. Since the period over which the samples were pooled from the 580-m trap (315 days) was shorter than for the deep traps (432 days) and, more importantly, the bloom event in spring was inadequately sampled by the 580-m trap due to trap clogging, comparison of average (radionuclide) fluxes between the traps is difficult. Nonetheless, we cannot rule out poor trapping efficiencies for the shallow traps, since these often underestimate the flux when compared with that expected by particle scavenging of Thorium isotopes (Buesseler, 1991). Inefficient trap functioning, possibly due to hydrodynamic bias (highest current speeds) in the 580-m trap (maximum 23 m s1, mean 8.5 m s1, Antia et al., 1999) leads us to treat the absolute values with caution. Nonetheless, strong differences in the qualitative composition of settling material based on lithogenic input, and the d15N isotopic signature and organic marker composition of sedimenting material (see below) emphasise differences between the shallow and the deep traps that are independent of the absolute fluxes and can be related to the different origin and transport of particles.

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3.2. Seasonal and storm-related mixing traced by the d15N signal in sediment traps The d15N: d14N isotope ratio of sinking particulate matter provides a means of tracing seasonal and spatial changes in autochthonous nitrate utilisation, since fractionation during photosynthetic nitrate uptake (Horrigan et al., 1990; Montoya and McCarthy, 1995; Velinsky et al., 1989) generates an isotope signal seen in sinking particles (Altabet, 1988; Vo
et al., 1996). We expect that the seasonal and spatial gradients in nitrate availability across the Goban Spur will be reflected in the d15N of particulate organic nitrogen (PON) in the sediment traps. At OMEX 2 and OMEX 3, the seasonally lowest d15N values are recorded at the end of January at 600 m and in February at 3220 m (Fig. 3), showing the late onset of deep mixing, as recorded in the open North Atlantic by Altabet et al. (1991). Similar to previous findings, the

Fig. 3. Seasonal fluxes of particulate organic nitrogen (PON, histograms) and the d15N isotopic ratio (symbols) of sedimenting material at OMEX 2 and OMEX 3.

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signal in the surface and intermediate depth traps shows a seasonal difference of 7m (Altabet et al., 1991; Vo
et al., 1996). Low d15N values between mid-March and the end of May correspond to high f -ratios (0.6–0.8) of production in the euphotic zone (Joint et al., 2001). Lower f -ratios during summer (0.14–0.33), when recycled production dominates, are reflected in high d15N values in sedimenting material. As may be expected, total particulate organic nitrogen fluxes during winter are low even though new production is taking place. We also note that the spring d15N signature of about 1m is evident in traps at all depths and is visible, with a time lag of 14 days, at 3220 m at OMEX 3. This is the only significant seasonal signal evident at this depth and indicates rapid transport of the spring bloom flux to depth without significant ‘‘contamination’’ by a secondary signal of differing isotopic composition. This emphasises seasonal differences in the vehicles and rapidity of particulate transport, which have implications for its quality as a food source for deep living organisms. On shorter time scales, wind-induced mixing and resulting nitrate enrichment can alter the d15N signal of sinking PON. Especially during summer and fall, when phytoplankton is nutrient-limited, storms can enhance algal production (Harris et al., 1991) and alter community structure, favouring growth of larger plankton (Nielsen and Kiorboe, 1991; Taylor, 1989; Schiebel et al., 1995). We have identified one such storm event on the Goban Spur in September/October 1993, the imprint of which is visible in the d15N signal and composition of sedimenting particles. Fig. 4a shows the evolution of sea surface temperature (SST) between August and November 1993. Following two events of high wind speed in October sea surface temperature drops abruptly. This is followed by a rise in SST due to restratification of the upper water column. In order to roughly calculate resulting changes in the Ekman depth of mixing (DE ) we have used the following formula: 4:3 W DE ¼ pffiffiffiffiffiffiffiffiffiffiffiffi; sinjFj where W is wind speed in m s1 at latitude F. Taking 50 m as the depth of the nutricline during September 1993, we calculated that wind speeds above about 20 m s1 would be sufficient to cause injection of cooler water from below the nutracline to the surface. Huthnance et al. (2001) are similarly able to identify poststorm deepening of the thermocline during September 1995 at the Goban Spur based on numerous CTD profiles. This drop in SST also is seen in satellite images of the Goban Spur (dots in Fig. 4a). There is a significant drop in d5N values of sedimenting PON at 600 m from 6m before the storm to 3m two weeks later (Fig. 4b) in both surface and intermediate traps on the slope and slope-decline, although it is masked at 3220-m depth (Fig. 3). This qualitative signal however, is not accompanied by higher fluxes, but these occur after a further time lag of ca. 2 weeks, when POC/PON flux shows a secondary autumn maximum. An increase in opal and diatom sedimentation rates indicate alteration of community structure and export following the storm (Fig. 4c). Using this multivariate approach, we are able to follow the temporal and qualitative responses of surface plankton to physical forcing.

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Fig. 4. Upper: near-continuous recordings of wind speed and sea surface temperature at a meteorological bouy at the OMEX site. Middle: Particulate organic carbon (POC) flux and the d15N isotopic ratio of sedimenting material in the 600-m trap at OMEX 3 between 01 July 1993 and 31 December 1993. Lower: Flux of diatom valves and opal in the 600m trap at OMEX 3 between 01 July 1993 and 31 December 1993.

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3.3. Vertical and lateral contributions to the d15 N signal in traps d5N values in the intermediate and deep traps reflect the juxtaposition of vertical and lateral components of flux. Annually time-weighted mean d5N values decrease with increasing water depth from 4.8m at 600 m to 4.2m at 1050 m in OMEX 2 and from 5.0m at 580 m to 4.2m at 1440 m and 3.7m at 3220 m in OMEX 3, respectively. This is in contrast to the open ocean where d5N values increase from the surface to below 1000-m depth, as the lighter isotope is preferentially remineralized. We assume that this signal results from the mixing of particles from the surface (with the d5N values of the 600-m traps) and those resuspended and advected (with d5N values from the sediment surface). At the sediment surface (0–2.5 mm) there is an increasing trend with depth from values of 3.4m at 200 m to 5.5m at 1500 m water depth on the slope (Lohse, pers. comm.), remaining constant further down the slope. We thus calculate expected values in the intermediate traps as: d15 Nexpected ¼ d15 Nsurf: ð1  kÞ þ d15 Nsed: k; where d15Nsurf. and d15Nsed. are the measured ratios in traps at 600 m and at the shelf break respectively and k is the lithogenic fraction of bulk flux (values from Antia et al., 1999). The expected d15N. ratios of 4.3m and 4.4m at 1050 m and 1440 m in OMEX 2 and OMEX 3, respectively, agree well with the measured value of 4.2m at both positions. At 3220 m, the measured annual d15N. value of 3.7 is lower than that calculated (4.5m). The low d15N. value below 3000 m compared to shallow and intermediate traps has previously been reported by Altabet (1988). The mechanisms causing this anomaly are unclear but may have to do with the processes of aggregation and disaggregation between sinking and suspended particles, of differing isotopic signature (Altabet et al., 1991). 3.4. Depth-dependent and seasonal differences in the quality of sedimenting material The use of biomarkers provides a means for tracing the pathways in production and fate of organic particles from the epipelagic zone. Sedimenting material in traps consists primarily of microscopically unidentifiable particles, so that measurement of trace quantities of biomarkers in bulk samples can provide a valuable tool to trace the origin and transport pathways that lead to export. We use measurements of eight alkenones, alkanes and isoprenoids as biomarkers to follow the transport pathway of particles settling and advected across the slope. Since these organic biomarkers are pelagic in origin (i.e. absent or present only in trace amounts in the sediment), their presence and ratio to POC can be taken as an indication of the ‘‘freshness’’ of material in the traps. We are thus able to follow changes in the quality of sedimenting POC with season and depth in our moorings. Multivariate techniques such as Principal Component Analysis (PCA) allow a simplified interpretation of the large biomarker data set. The active variables in the PCA were the POCnormalised contents of the eight organic biomarkers, bulk parameters (carbonate and opal), as well as aluminium, fucoxanthin and chloropigments (chlorophyll a and phaeopigments). In Fig. 5 the depth-dependent trends in the principal components are presented for the time period 01.07.1993 to 02.01.1994. Three major groups are seen: The first consists of the lithogenic (Aluminium) and biogenic (opal and carbonate) fractions and the biomarker squalene, which is an

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Fig. 5. Mean POC-normalised ratios of opal, carbonate, aluminium and marker compounds at OMEX 2 and OMEX 3 for the time period 01 July 1993 to 02 January 1994.

isoprene-derived alkene found in low concentrations in algae and bacteria (Tornabene, 1981; Saliot, 1981) and is related to bacterial activity. High concentrations of squalene have been found in the surface sediments of the northern North Atlantic (Maa
en, 1994). Since the highest bacterial activity is seen at the sediment surface, this is consistent with the lateral transport of particle-attached bacteria. There is an increase in this group of markers with depth to a maximum at 3220 m, representing the increase in detrital and sediment-derived material in the deeper traps. The second group consists of biogenic, phytoplankton-derived compounds [n-alkanes n-C15 and n-C17 ; Heneicosa-2,5,8,11,14,17-hexene (HEH) and the di- and tri-unsaturated C37 methylketones] that show increasing ratios in the 1050 and 1440-m traps at OMEX 2 and OMEX 3, respectively, decreasing sharply at 3220 m at OMEX 3 (Fig. 5b). The normal n-alkane distribution from phytoplankton generally shows a distinct predominance of one or both compounds (n-C15 or n-C17 ) over the rest of the n-alkanes (e.g., Blumer et al., 1971). HEH as the marker with the major contributing concentration is reported to dominate largely in marine phytoplankton derived from the corresponding docosahexaenoic acid (Lee et al., 1970; Saliot, 1981) as well as from zooplanktonic sources (Blumer et al., 1971). The di- and tri-unsaturated C37 methyl-ketones are

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organism-specific biomarkers biosynthesised by coccolithophorids such as Emiliania huxleyi, that can develop large annual blooms in the Atlantic Ocean (Marlowe et al., 1990). The depth trend in these components is similar to that of intact diatom fluxes and bulk POC (Fig. 2a and b). The presence of this relatively ‘‘fresh’’ material below 1000 m is evidence of the mid-water off-slope transport of freshly settled labile material from the shelf break. The third group of principal components, consisting of chloropigments, fucoxanthin and pristane, remains constant or decreases with depth (Fig. 5c), reflecting their more labile nature. 3.5. Seasonal differences in ratios Seasonal differences also are seen in the marker: POC ratios in sedimenting material at 580 m at the OMEX 2 site (Table 3). Although there is considerable scatter in the data, they fall into three groups: those that show no significant seasonal change (aluminium, CaCO3), those that show a clear summer maximum (n-C15 ; n-C17 ; HEH and pristane), and those that show highest ratios during spring (opal, chloropigments, fucoxanthin, squalene, C37:2 and C37:3 ). In all cases (except for Al : POC and CaCO3 : POC ratios), the lowest ratios are found during winter, when total sedimentation rates are low (o2 g C m2 d1), and there is negligible production in the pelagial. Winter sedimentation is thus characterised by refractory, degraded material, possibly of low nutritive value. During spring and summer, POC fluxes are of similar magnitude; however, based on these biomarker analyses, we can identify differences in their composition that result from differing processes of production and export. During spring, diatoms are seen to play a prominent role in production and flux, reflected in the 2-fold higher opal, and 10-fold higher chloropigment and fucoxanthin components, suggesting that aggregation mediates to accelerate export by sinking. The highest squalene : POC ratios, also during spring, support this pattern and

Table 3 Marker:POC ratios (CaCO3, Opal mg mg1, all others mg mg1) in 600 m in OMEX 2 Summer

CaCO3 Opal Aluminum C29 Squalene C37:2 C37:3 n-C15 n-C17 HEH Chloropigments Fucoxanthin Pristane

Winter

Mean

SD

5.00 0.40 0.043 2.97 13.19 0.48 0.52 17.91 20.68 63.44 0.013 0.011 36.34

1.98 0.09 0.05 1.37 3.11 0.36 0.37 8.99 9.46 43.27 0.010 0.005 19.65

Spring

(n)

Mean

SD

(n)

Mean

SD

(n)

(4) (4) (13) (4) (4) (4) (4) (4) (4) (4) (13) (13) (4)

5.28 0.68 0.071 0.46 4.36 0.14 0.13 0.66 2.07 1.46 0.007 0.02 9.61

0.59 0.10 0.05 0.20 0.14 0.03 0.04 0.29 0.46 0.66 0.003 0.006 1.81

(3) (3) (9) (3) (3) (3) (3) (3) (3) (3) (9) (9) (3)

5.99 1.31 0.074 9.80 41.02 1.17 2.17 6.13 9.19 12.11 0.110 0.15 29.83

0.95 0.20 0.01 5.67 26.96 0.53 1.07 1.22 4.13 7.13 0.070 0.09 9.31

(3) (3) (3) (3) (3) (3) (3) (3) (3) (3) (2) (2) (3)

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n-C15 n-C17 HEH Squalene C37:2 C37:3 Pristane Chloropigments Fucoxanthin Aluminium CaCO3 Opal

PC1 (26%)

PC2 (23%)

PC3 (20.8%)

0.76 0.79 0.85 0.76 0.24 0.24 0.37 0.04 0.25 0.17 0.01 0.05

0.08 0.06 0.11 0.20 0.74 0.86 0.47 0.48 0.82 0.05 0.44 0.04

0.24 0.01 0.11 0.20 0.00 0.06 0.47 0.74 0.21 0.84 0.62 0.94

could be derived from high bacterial activity that has been recorded on marine snow particles. During summer, production is seen to be dominated by nano- and picoplankton (Joint et al., 2001) that fuel a recycling/retention system. Nonetheless, the phytogenic organic biomarkers n-C15 ; n-C17 and HEH are enriched relative to POC compared to the other seasons, indicating that fresh phytoplankton is exported to depth, and the signal of this ‘‘freshness’’ is retained in the lateral flux below 1000 m. Principal Component Analysis (PCA) was performed for integrated sample intervals (according to Table 1) vs. the POC normalised concentrations of markers. Further differentiation was made for the interval from 26.10.1993 to 23.11.1993 (at OMEX 2) following an autumn storm (see above). This could be identified only at the OMEX 2 mooring site, since at OMEX 3 pooling of samples did not allow resolution of the storm event. Three principal components (PCs) together account for 70% of the variance (PC1 26%, PC2 23% and PC3 20.8%, Table 4). PC1 is dominated by biomarkers such as the phytoplanktonderived n-alkanes n-C15; n-C17 and HEH as well as the bacterial marker, squalene. PC2 is loaded by organism-specific markers fucoxanthin (diatoms) and alkenones (coccolithophorids) as well as chloropigments (total autotrophs) and to a lesser extent pristane (zooplankton derived). PC3 is clearly influenced by the bulk parameters opal and aluminium as well as chlorophyll equivalents and carbonate. Based on loadings for PC1 and PC2, a progression from summer to fall to winter is seen in order of decreased loading for PC1 and increased loading for PC2 (Fig. 6a). We interpret this as documenting the progressively more degraded material sedimenting from summer through winter. A dramatic change in the composition of sedimenting material occurs during spring, when the strong diatom signal is evident. Despite a clear seasonal differentiation, with increasing trap depth material has a larger component of older, degraded material such that the summer samples from 3220 m are similar in composition to the autumn samples from shallower depth, due to the higher lateral input (higher PC2, lower PC1 loadings).

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Fig. 6. Loadings of principal components PC1 and PC2 (a) and of PC1 against PC3 (b).

A clear differentiation between the composition of POC in shallow and deep traps is also evident (Fig. 6b) from the strong lateral signal in loading of opal and aluminium for PC3. Negative loading for fucoxanthin again differentiates spring sedimentation in the deeper trap at OMEX 2, which shows a composition similar to that of the shallower trap. Interestingly, the poststorm flux period in autumn shows loading similar to the spring composition for the 600-m trap and autumn for the 1050-m trap due to the larger contribution of resuspended material in the latter. 3.6. Surface gradients at the continental margin The differing seasonal and depth-related trends in specific biomarkers and bulk variables thus allow us to reconstruct the processes leading to particle production and flux on the slope, and

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underscore the necessity to invoke a horizontal gradient in productivity and pelagic community structure as well as rapid lateral transport from the shelf break/upper slope. There is, however, no direct demonstration of gradients in community structure, largely due to inadequate spatial and temporal sampling at the surface. Similar postulations were made during the SEEP study, where a decrease in bulk C : N ratios with depth led Biscaye and Anderson (1994) to suggest that fresh biogenic detritus from the shelf break was transported downslope more rapidly than particles from the surface could settle to the deep traps. Also similar to this study, the role of diatoms in spring fluxes, both ‘‘vertical’’ and lateral, was documented at the western Atlantic Margin by Falkowski et al. (1994). Since this process is largely topographically controlled, it may be a common feature of continental margins. We next attempt to determine gradients in pelagic production at the Goban Spur. The most obvious feature distinguishing continental margins as seen by satellites is the band of cooler upwelled water at the shelf break. The continual input of ‘‘new’’ nutrients fuels higher production, primarily in summer when nutrients are otherwise limited to below the seasonal mixed layer. Using empirical ‘‘sea-truthing’’ algorithms relating SST to nitrate (Fig. 7) we see that during summer nitrate concentrations are up to 2 m mol at the shelf edge, whereas surface nitrate is depleted on the shelf and the open ocean. This is a persistent feature of the Celtic Sea Margin (Pingree, 1984). We estimate the effect of summer upwelling at the shelf break on annual nitrate availability and new production using data reported by Huthnance et al. (2001) on seasonal evolution of mixedlayer depths, diffusivity and mixing coefficients at the shelf break, and nitrate values from Hydes et al. (2001). We consider new production to be a more appropriate parameter with which to compare export than total primary production. Nitrate availability is calculated for the spring, summer and autumn periods as presented in Table 5. New Production amounts to 539.5 mmol N m2 a1 at the outer slope and 1088 mmol N m2 a1 at the shelf break. Assuming Redfield molar C : N ratios, this corresponds to a new production of ca. 42.9 g C m2 a1 at the outer slope and 86.5 g C m2 a1 at the shelf break. These values bracket the Joint et al. (2001) estimate of 80 g C m2 a1 for the entire region. We think these values are realistic, in view of their agreement with both measured new and annual production at the Goban Spur and the factor 2 difference is similar to that estimated by Sathyendranath et al. (1995). We thus use these to further estimate the amount of shelf-edge production that is seen to be exported to the outer slope. Firstly, using 42.9 g C m2 a1 as new production on the outer slope, and using the formula of Martin et al. (1987) to estimate flux to 600 m (4.2 g C m2 a1), it appears that the shallow traps provide an underestimate (see above) at least partly due to incomplete sampling in spring. Calculating the ‘‘vertical’’ input expected by this method at 1040 m and 1440 m at OMEX 2 and OMEX 3, (2.6, 1.98 g C m2 a1, respectively) and subtracting this from the measured fluxes of 2.3 and 3.7 g C m2 a1, we estimate lateral flux below 1000 m to be 0 and 1.72 g C m2 a1 at OMEX 2 and OMEX 3, respectively, which is somewhat lower than that estimated using measured 600-m flux as a measure of vertical input (Antia et al., 1999). As discussed above, and shown by McCave et al. (2001), the major particle input is thought to occur below 1000 m on the slope, accounting for the lower lateral input at 1050 m at OMEX 2. Since the trap at 1440 m at OMEX 3 was below the main nepheloid layer, where lateral particle input takes place, we use this trap to back-calculate the amount of material that must leave the shelf break to account for this lateral flux. To do this we use the model estimates of Herman et al.

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Fig. 7. Nitrate map calculated for June 1995 at the Goban Spur (for details see text).

(2001) for sinking speeds of 150 m d1, a degradation constant of organic material of 40 a1 at the shelf break and measured mean current speeds of 10 cm s1 (Antia et al., 1999) that are representative of this area (Pingree and Le Cann, 1990). Assuming particles settling from the shelf break are resuspended and injected into the water column through canyons ca. 175 km upstream of the moorings, particles were ca. 20 days in transition to the traps. For 1.72 g C m2 a1 to arrive at the 1440-m trap on the outer slope, therefore, on the order of 15.4 g C m2 d1 (or 18% of shelf break new production) must have left the shelf break. This value is sensitive to the degradability constant used. If we assume that lateral input is of more refractory material (degradation constant 20 a1) originating from the slope at 900-m depth where intermediate nepheloid layers are formed, this estimate is reduced to 5.1 g C m2 a1, or 6% of shelf break new production, exported to the outer slope. These calculations are influenced by a number of poorly defined rates. Despite the importance of key processes such as new and total production, these are practically impossible to

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A.N. Antia et al. / Deep-Sea Research II 48 (2001) 3083–3106 Table 5 Seasonal and annual nitrate fluxes to the euphotic zone at the Goban spur N-Flux to upper mixed layer

Slope

Shelf break

Spring flux (mmol NO3 m2)a Summer flux (mmol NO3 m2) Autumn flux (mmol NO3 m2)d NO3 flux (mmol-N m2 a1) New Production (g C m2 a1)

280 207b 52.5 539.5 42.9

280 756c 52.5 1088.5 86.5

a

spring flux=surface layer depth during spring (40 m, Huthnance et al., 2001) [NO3] (7 mM, Hydes et al., 2001). summer diffusive flux at the outer slope=summer duration (106 s) Kv (104) [NO3]/z (35 m). c summer flux at the shelf break=upward flux (0.9 m d1) summer duration (4 months) [NO3]. d autumn production=deepening of thermocline (15 m between August and September/October, Huthnance et al., 2001) [NO3] (3.5 mM at depth of thermocline deepening). b

measure with adequate resolution over annual cycles; calculations by other methods have potentially large ranges depending on the variables and rates assumed. Assuming an annually averaged f -ratio of 0.5 (Joint et al., 2001), our estimate above translates to between 3 and 9% of primary production from the shelf break exported to below the depth of maximal winter mixing. This is somewhat higher than that estimated during the SEEP study at the Mid-Atlantic Bight (Biscaye et al., 1994), but lower than those initially proposed by Walsh et al. (1981). Clearly lateral fluxes contribute to trap collections below 1000 m at both sites, as seen by the lithogenic contribution to flux that is not seen at 600 m and accounts for 32% of the bulk flux at 1040 m at OMEX 2 and 40% at 1440 m at OMEX 3. For the lithogenic fraction at least there are no constraints on residence time and the transport of fine particles in nepheloid layers may take weeks or even months. Based on the calculations above, lateral fluxes also must bring POC to the trap at 1440 m at OMEX 3. The bulk POC has a signature of ‘‘freshness’’ similar to that in 600 m (similar POC:chloropigment ratios, d15N ratios), which suggests that lateral transport of POC must be rapid compared to its rate of degradation. We suggest that rebound aggregates that may be rapidly resuspended and advected, provide transport vehicles for these fluxes. The increase in aggregate abundance with water depth, recorded at the western Atlantic continental slope (Gardner and Richardson, 1992) indicates their role in slope fluxes. Previous studies at shelf-slope sites suggest that a large pool of old, refractory organic carbon is responsible for much of the carbon inventory and flux at the continental margin (Bauer and Druffel, 1998). A major finding of this study is that the composition of organic material found in our traps retains a fresh pelagic signal even where lithogenic fluxes are highest, indicating that material from the upper mixed layer is rapidly exported, possibly as rebound particles (sensu Walsh, 1992) that have settled from the region of high productivity at the shelf break. In contrast to the SEEP study at the Mid-Atlantic Bight (Biscaye and Anderson, 1994) where a mid-slope maximum was recorded, the highest fluxes are found at the outer slope. This is thought to result from injection of particles at intermediate water depths from a major canyon upstream of the mooring site, as hypothesised by McCave et al. (2001). Since these particles are transported below 1000-m water depth, the trap at 1050 m at OMEX 2 was too shallow to sample this flux. By funnelling particles and concentrating energy, thus leading to higher resuspension and advection, canyons locally can play a major role in rapidly exporting fresh material in a seaward direction.

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3.7. Comparison of trap fluxes to benthic fluxes at the goban spur Results from the sediment traps suggest that particles from the shelf break and upper slope are transported in a seaward direction and deposited in the more quiescent region at the foot of the slope. This conclusion is supported by comparison of water column fluxes with those at the sea bed (Table 6). An excess of bulk flux to the sediments over recent mass accumulation rates on the slope and excess accumulation over flux at 4500 m indicate considerable net downslope transport with the benthic nepheloid layer (Thomsen and van Weering, 1998). This near-bed transport provides a food source for the benthic community. Comparing POC fluxes 400 mab and benthic carbon demand (Lohse et al., 1998; Table 6), it is evident that the deficit of supply over demand increases with decreasing water depth. This POC must be increasingly refractive with each progressive deposition-resuspension cycle. The flux of diatom valves, which shows the greatest increase with depth and distance from the shelf break in the traps (Fig. 2b), also indicates down-slope transport from the shelf break, with a higher contribution of diatomaceous debris at the lower slope. Bao et al. (2000) reporting on diatoms at the sediment surface show a strong seaward increase in both intact diatoms and the contribution of fragmented frustules to total diatoms (from o106 to o1
107 valves g1 dry sediment and from o106 to >3
107 frustules g1 dry sediment at 200 and 3000-m water depths, respectively). Lohse et al. (1998), investigating Si dissolution at the sediment surface, show that biogenic Si is less degradable (i.e., has lower dissolution rates) at deeper stations than at the shelf break, indicating that it is increasingly refractory with distance from the shelf break. The general conclusions from this study are therefore in good agreement with the pattern seen at the sediments. This study emphasises that relatively fresh export characterises fluxes measured at the Goban Spur and that (possibly canyon-mediated) rapid transport of (rebound) particles should be evoked. We also recognise that local topography plays an important role in particle distribution, accounting for the large (10-fold) difference in absolute fluxes measured at similar water depths in a variety of margin environments (Mid-Atlantic Bight, Porcupine Sea Bight, Bay of Biscay, Goban Spur), each differing in shelf width and slope inclination. Antia et al. (1999), compiling presently available data from these studies, show a good correlation between mid-water flux and distance from shelf break. The future challenge is in relating these patterns to productivity

Table 6 Comparison of water column (sediment trap) and benthic fluxes at the Goban Spur Mooring (water depth)

OMEX 2 (1440 m) OMEX 3 (3660 m) OMEX 4 (4500 m) a

Bulk fluxes (g m2 a1)

POC fluxes (mgC m2 d1)

Sediment trap

MARa

Sediment trapc

Benthic carbon demandb

27.9 42.0 40.6

16.5 29.7 50.3

6.3 5.7 6.1

21.4 18.6 11.8

Recent mass accumulation rates from van Weering et al. (1998). Benthic carbon demand calculated from Oxygen utilization rates (Lohse et al., 1998). c Mean annual carbon flux. b

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gradients and, if possible, in establishing regional algorithms that will allow estimation of the sequestration of atmospheric carbon from these environments, which has remained to present the large missing variable in regional and global carbon budgets.

4. Conclusions *

*

*

*

*

The increase in particle flux with depth is related to lateral fluxes of resuspended material from the shelf break and outer slope. The presence of relatively fresh material at 1440 m at the outer slope indicates rapid advected flux. The d15N and biomarker signals in sediment traps identify distinct seasonal and depthdependant differences in the origin and composition of sedimenting material. Spring fluxes are rich in diatoms, have a ‘‘fresh’’ pigment signal, and carry the imprint of a new production system; in summer, production is chiefly from recycled nitrogen sources and the flux of phytogenic and zooplanktonic biomarkers is highest. Winter fluxes are low and show the late onset of deep mixing. An autumn storm event (identified in meteorological recordings) results in changes in pelagic community structure and is followed by alteration in the composition of flux, showing a signal similar to that in spring at 600-m depth. Comparing mass, POC and radionuclide fluxes in the deep traps and at the sediment surface shows that although material is transported near-bottom to the mid-slope and fuels high benthic activity, its final deposition occurs at the foot of the slope and abyssal plain. We estimate surface production at the shelf break and over the outer slope and calculate that between 6% and 9% of shelf break primary production is exported to the outer slope below the depth of winter mixing.

Acknowledgements We thank our OMEX colleagues who in numerous discussions have contributed to the ideas in this paper; in particular J. Huthnance for intensive discussions and suggestions and N. McCave, I. Joint and T. Sherwin for improvements of the manuscript. Data from the Meteorological Buoy was obtained from the U.K. Meteorological Office through the BODC. This work was partly supported by the European Union under Contract MAS3-CT96-0056.

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