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Benthic processes and dynamics at the NW Iberian margin: an introduction
Progress in Oceanography 52 (2002) 123–128 www.elsevier.com/locate/pocean
Editorial
Benthic processes and dynamics at the NW Iberian margin: an introduction 1. Introduction An important part (10–50%) of the global marine primary production occurs at the continental margin, and continental margin sediments play an important role in the cycling of organic matter (Wollast, 1998). Indeed, it has been argued that over 90% of the organic carbon in marine sediments is stored in continental margin settings (Berner, 1982). The upwelling-dominated, narrow Iberian margin was selected as the study area to provide a contrast with the previously studied Goban Spur margin, characterised by a broad shelf and slope without upwelling (van Weering, McCave, & Hall, 1998a; Wollast, Chou, Avril, & Huthnance, 2001). The OMEX I results from the North Biscay (Goban Spur) margin identified no obvious depocentre of organic carbon (Wollast & Chou, 2001). Slightly enhanced remineralisation and accumulation rates, however, do occur at mid slope areas (Thomsen & van Weering, 1998; Lohse, Helder, Epping, & Balzer, 1998; van Weering, Hall, de Stigter, McCave, & Thomsen, 1998b; van Weering, De Stigter, Balzer, Epping, Graf, Hall et al., 2001). On the other hand, the organic carbon required to sustain the benthic fauna on the Goban Spur shelf and upper slope represented a major fraction of the total organic carbon input (Heip, Duineveld, Flach, Graf, Helder, Herman et al., 2001). It was the main objective of the benthic studies included in the EU-funded Ocean Margin Exchange-II project (OMEX II) to contribute to understanding, quantification and modelling of the benthic processes involved in exchange of carbon, elements and energy at the Iberian ocean margin. Comparison and integration of these results and models with results from other well studied European and other continental margins was intended to contribute to the main objective of OMEX II Phase II “to measure and model exchange processes at the ocean margins as a basis for the development of global models to predict the impact of environmental changes on the oceanic system, and more specifically, on the coastal zone”.
2. The Iberian margin The northwestern Iberian margin is characterised by intense, wind-driven seasonal upwelling in summer (Fiu´za, 1983; Fraga, 1981; McClain, Chao, Atkinson, Blanton & De Castillejo, 1986; Frouin, Fiu´za, ˆ mbar & Boyd, 1990) strongly affecting the watermass composition and productivity. Studies of pelagic A productivity of the region were also carried out during OMEX II, including an intensive Lagrangian study of primary and secondary production, sedimentation and physical measurements (Joint & Wassmann, 2001).
∗
Corresponding author. Tel.: +31-222-369395; fax: +31-222-319674.
0079-6611/02/$ - see front matter 2002 Published by Elsevier Science Ltd. PII: S 0 0 7 9 - 6 6 1 1 ( 0 2 ) 0 0 0 0 2 - 2
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The increased productivity is clearly reflected in the character and distribution of surface sediments off Galicia (Lopez-Jamar, Cal, Gonzalez, Hanson, Rey, Santiago et al., 1992). At the Iberian shelf south of Vigo some earlier work on shelf sediments and shelf sediment dispersal by bottom currents and resuspension was available (Dias & Nittrouer, 1984; Dias, Pilkey, & Heiweil, 1984). Transport of matter across the slope and the deeper margin sediments, however, were unknown prior to OMEX-II. Water-mass transport and currents beneath the surface layer are directed to the north and a southerly directed surface flow is associated with the upwelling (Maze´ , Arhan, & Mercier, 1997; Fiuza, Hamann, Ambar, del Rio, Gonzalez, & Cabanas, 1998; van Aken, 2000a,b; Huthnance, van Aken, White, Baton, Le Cann, Coelho et al., 2001). Pingree and LeCann (1992) have shown that filaments and patches of the upwelled productive water may be dispersed offshore in a northerly and northwesterly direction by eddies (Richardson, Bower & Zenk, 2000). These tend to recur every year and may extend up to 250 km offshore (Haynes, Barton & Pilling, 1993). The filaments are believed to foster enhanced production and were also the subject of a Lagrangian experiment during OMEX II (Joint & Wassmann, 2001). Internal waves and tides had been recorded at the shelf edge off Iberia (Jeans & Sherwin, 2001), but their effects were unknown compared to the transfer of energy at the slope and continental shelf edge which results in the formation of nepheloid layers and mass particle transport previously noted off Porcupine Bank and at the Goban Spur (Dickson & McCave, 1986; Thorpe & White, 1988; Inall, Rippeth, & Sherwin, 2000; McCave, Hall, Antia, Chou, Dehairs, Lampitt et al., 2001). Upwelling off Portugal is documented in planktic foraminiferal and diatom species found far offshore in the surface sediments (Abrantes, 1988; Lebreiro, Moreno, McCave, & Weaver, 1995), and there are indications that upwelling was more intense during the last glaciation than at present (Abrantes, 1991). However the amount of carbon in surface sediments of the Iberian margin north of 41°N was nearly unknown and the upwelling-induced carbon exchange processes over the margin were not well constrained. The late Quaternary depositional history of the Iberian margin reflects the major climatic zonations of the Iberian peninsula (Monteiro, Dias, Gaspar, & Possolo, 1980), resulting in Holocene margin sediments of predominant biogenic composition and pre-Holocene margin sediments reflecting increased terrigenous input from local and distant sources (Baas, Mienert, Abrantes & Prins, 1996). Again however, sedimentation off the shelf was not well studied. The shelf sediments were reworked during the last low stand of sea level, and present deposition and accumulation of sands and fines on the shelf is controlled by the geomorphology of the seabed (Drago, Oliveira, Magalha˜ es, Cascalho, Jouanneau and Vitorino, 1998). The western Iberian margin is dissected by deep canyons, that cut through the slope and into the shelf edge. The largest canyons (as for example Nazare´ and Setubal) intersect the entire continental shelf, allowing them to intercept sediment transported over the shelf and upper slope, thus providing a direct conduit of particles and matter from the inner shelf to the deep sea. All canyons were probably more active at times of low sea-level than at present, although this also is not well constrained.
3. The benthic context It is still unclear whether or not continental shelves export detritus from primary production to the continental slopes and the open ocean as was suggested by Walsh, Rowe, Iverson and McRoy (1981). However, this is of prime importance for assessing the role of the ocean margins in the global carbon cycle. To do so, extensive knowledge is required of the processes and fluxes involved in particulate matter transport through the water column to the seabed, of the near-bed advective fluxes and transport/settling processes, and of the fluxes and rates of benthic remineralisation processes at the sediment-water interface and in the surface sediments. The benthic community development and structure ultimately is closely linked to the input of fresh food to the sediment. Most field studies so far indicate that assumed vertical sedimentation determined by sediment-trap analy-
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ses does not balance material and carbon budgets, and lateral advection of organic matter must be invoked (Courp & Monaco, 1990; Monaco, Biscaye, & Pocklington, 1990; Biscaye, Flagg, & Falkowski, 1994; Antia, von Bodungen, & Peinert, 1999; Heussner, de Madron, Radakovich, Beaufort, Biscaye, Carbonne et al., 1999; McCave et al., 2001). In particular the benthic boundary layer processes and fluxes involved in particle transport, resuspension, aggregation, scavenging, disaggregation and sediment accumulation and mixing need to be determined, to complement measurements of remineralisation and diagenesis at the sediment-water interface, and burial fluxes. Also, the role of the benthic faunal and microbial community in the benthic carbon cycle needs to be defined and measured, to allow quantification and modelling of the carbon and element fluxes at the ocean margin, and of the temporal and spatial variability involved. The focus of the studies in the Benthic Processes component of OMEX II was on the understanding and quantification of processes, pathways and fluxes of organic and related particles, in the continental margin area off Iberia, characterised by pronounced seasonal upwelling with the overall aim of contributing to the main OMEX I and II objective “to measure and model exchange processes at the ocean margins as a basis for the development of global models to predict the impact of environmental changes on the oceanic system, and more specifically on the coastal zone”.
4. Key questions The key questions addressed during the OMEX II Benthic Process Study were directed to the understanding of: 앫 the role, importance and dynamics of the bottom boundary layer in driving exchanges of particles, carbon and other components between the water column and the sediment in relation to vertical and lateral fluxes—the role of microbial activity in aggregate formation and organic carbon transfer and accumulation. 앫 organic matter diagenetic processes, products and fluxes at the sediment-water interface and in the surface sediments, defining carbon mineralisation. 앫 the relationship between types of food supply and the benthic community structure and distribution. 앫 spatial and temporal variability of processes and products at continental margins. In the OMEX-II Benthic Studies these questions were approached in three main project areas of work. The papers in this volume are grouped below under the project subcomponents (given here in italics), namely: 1. Particle transport, settling, accumulation, mixing and burial fluxes. Temporal and spatial variability. a) Amount, character, distribution and composition of suspended particles in nepheloid and clear water layers. In this area, the papers of Oliveira et al. and McCave & Hall, together with the published work of Hall, Schmidt, McCave and Reyss (2000) deal with shelf and slope nepheloid layers. Bottom nepheloid layers over the deeper slope and in the Nazare´ canyon are discussed by van Weering et al. b) Spatial and temporal variability of the benthic boundary layer dynamics. Extensive work on benthic dynamics is reported here by Vitorino et al., van Weering et al., Thomsen et al., Huthnance et al. and Davies et al. discuss near bed particle transport. c) Particle fluxes to the seabed, accumulation and mixing rates. Work on fluxes estimated from sediment traps have been reported by Antia et al. (pers. com., 2000), and sediment accumulation rates on the shelf and slope are reported here by Dias et al. (Present day sedimentary processes…), Jouanneau
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et al., and van Weering et al. Short term particle residence times in surface sediments and in the Nazare´ canyon is presented by Schmidt et al. d) Sediment distribution, properties and composition along selected transects. The detailed mapping of shelf sediment distribution is reported by Dias et al. for the Galicia– Minho sector off northern Portugal, its mineralogy and chemistry by Oliveira et al. and Araujo et al., and a slope to rise transect by van Weering et al. e) Modelling near bed-transport and budgets. These aspects are covered by Jouanneau et al. and Davies et al. Having studied only a limited area offshore we do not have a good hold on the sediment budget for the whole slope yet. f) Long term change. Hall and McCave (2000) examined the current control of past slope sedimentation and the PhD thesis of Lucia De Abreu (2000) and De Abreu, McCave, Hall, Higginson, & Thomson (2000) on palaeoproductivity of the northern Iberian margin used OMEX-II cores. 2. Sediment /water exchange processes and early diagenesis, and 3. Role and importance of the benthic community. a) Sediment-water exchange processes and organic matter diagenesis, carbon mineralisation, respiration, and burial is covered by Epping et al., who deal with the central problem of carbon oxidation and burial both on the slope transect and down Nazare´ Canyon. b) Role and importance of bioentrainment and biodeposition. Changes occurring in the boundary layer are examined by Thomsen et al. and interface processes are probed using radionuclide tracers by Schmidt et al. c) The Benthic community structure is treated by Flach et al. and the related area of d) Variability of benthic community structure and functioning at contrasting margins is covered, at least for the megafauna, by the contribution of Lavaleye et al. The papers combined within this issue thus provide a first, comprehensive overview of recent benthic processes acting upon the Iberian margin system, and of the associated particle and carbon fluxes over the margin, inclusive of aspects of the Nazare´ canyon system.
Acknowledgements The guest editors express their thanks to all reviewers for their willingness to critically read and comment upon the manuscripts submitted. We also wish to express our thanks to Dr M. Angel (SOC) and A. Bayfield (PIO) for their constructive support during the editing. K-G. Barthel (EU-DG XII) is thanked for his continuous interest in and support for the OMEX-II Program. The front cover was drawn by Henk de Haas (NIOZ), which we gratefully acknowledge.
b
T.C.E. van Weeringa I.N. McCaveb a Netherlands Institute for Sea Research (NIOZ), P.O. Box 59, 1790 AB Den Burg, Texel, The Netherlands E-mail address: [email protected] (T.C.E. van Weering) Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK
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References Abrantes, F. (1988). Diatom assemblages as upwelling indicators in surface sediments off Portugal. Marine Geology, 85, 15–39. Abrantes, F. (1991). Increased upwelling off Portugal during the last Glaciation:diatom evidence. Marine Micropalaeontology, 17, 285–310. Antia, A. N., von Bodungen, B., & Peinert, R. (1999). Particle flux across the mid-European continental margin. Deep-Sea Research I, 46, 1999–2024. Baas, J. H., Mienert, J., Abrantes, F., & Prins, M. A. (1996). Late Quaternary sedimentation on the Portuguese Continental Margin: Climate related processes and results. Palaeogeography, Palaeoclimatology, Palaeoecology, 130, 1–23. Berner, R. A. (1982). Burial of organic carbon and pyrite sulphur in the modern ocean: its geological and environmental significance. American Journal of Science, 282, 451–473. Biscaye, P. E., Flagg, C. N., & Falkowski, P. G. (1994). The Shelf Edge Exchange processes experiment, SEEP-II an introduction to hypotheses, results and conclusions. Deep-Sea Research II, 41, 231–252. Courp, T., & Monaco, A. (1990). Sediment dispersal and accumulation on the continental margin of the Gulf of Lions: sedimentary budget. Continental Shelf Research, 10, 1063–1087. De Abreu, L. (2000). High resolution palaeoceanography off Portugal during the last two glacial cycles. Ph.D Thesis, Dept of Earth Sciences, University of Cambridge. De Abreu, L., McCave, I. N., Hall, I. R., Higginson, M. & Thomson, J. (2000). Changing climate and productivity: Problems with proxies off Portugal over the past 35 ka. EOS, Transactions of the American Geophysical Union. Dias, J. M. A., & Nittrouer, C. A. (1984). Continental shelf sediments of northern Portugal. Continental Shelf Research, 3, 147–165. Dias, J. M. A., Pilkey, O. H., & Heiweil, V. M. (1984). Detrital mica: Environmental significance in north Portugal continental shelf sediments. Commun. Serv. Geol. Portugal, 70, 93–101. Dickson, R. R., & McCave, I. N. (1986). Nepheloid layers on the continental slope west of Porcupine Bank. Deep-Sea Research, 33, 791–818. Drago, T., Oliveira, A., Magalha˜ es, F., Cascalho, J., Jouanneau, J. M., & Vitorino, J. (1998). Some evidences of northward fine sediment in the northern Portuguese continental shelf. Oceanologica Acta, 21, 223–231. Fiu´ za, A. (1983). Upwelling patterns off Portugal. In E. Suess, & J. Thiede (Eds.), Coastal Upwelling: Its Sediment Record. Part A (pp. 85–97). New York: Plenum. Fiuza, A. F. G., Hamann, M., Ambar, I., del Rio, G. D., Gonzalez, N., & Cabanas, J. M. (1998). Water masses and their circulation off western Iberia during May 1993. Deep-Sea Research I, 45, 1127–1160. Fraga, F. (1981). Upwelling off the Galician coast, northwest Spain. In F. A. Richards (Ed.), Coastal Upwelling (pp. 176–187). Washington: AGU. ˆ mbar, I., & Boyd, T. J. (1990). Observations of a poleward surface current off the coasts of Portugal and Frouin, R., Fiu´ za, A., A Spain during winter. Journal of Geophysical Research, 95, 679–691. Hall, I. R., & McCave, I. N. (2000). Palaeocurrent reconstruction, sediment and thorium focussing on the Iberian margin over the last 140 ka. Earth and Planetary Science Letters, 178, 151–164. Hall, I. R., Schmidt, S., McCave, I. N., & Reyss, J-L. (2000). Particulate matter distribution and 234Th/238U disequilibrium along the Northern Iberian Margin: Implications for particulate organic carbon export. Deep-Sea Research I, 47, 557–582. Haynes, R., Barton, E. D., & Pilling, I. (1993). Development, persistence and variability of upwelling filaments off the Atlantic coast of the Iberian peninsula. Journal of Geophysical Research, 98, 22681–22692. Heip, C., Duineveld, G., Flach, E., Graf, G., Helder, W., Herman, P. M. J., Lavaleye, M., Middelburg, J. J., Pfannkuche, O., Soetaert, K., Soltwedel, T., de Stigter, H. C., Thomsen, L., Vanaverbeke, J., & de Wilde, P. (2001). The role of the benthic biota in sedimentary metabolism and sediment water-exchange processes in the Goban Spur area (NE Atlantic). Deep-Sea Research II, 48, 3223–3243. Heussner, S., de Madron, X. D., Radakovich, O., Beaufort, L., Biscaye, P. E., Carbonne, J., Delsaut, N., Etcheber, H., & Monaco, A. (1999). Spatial and temporal patterns of downward particle fluxes on the continental slope of the Bay of Biscay (northeastern Atlantic). Deep-Sea Research II, 46, 2102–2146. Huthnance, J. M., van Aken, H. M., White, M., Barton, E. D., Le Cann, B., Coelho, E. F., Fanjul, E. A., Miller, P. & Vitorino, J. (2001). Ocean Margin Exchange - Water flux estimates. Journal of Marine Systems. (in press). Inall, M. E., Rippeth, T. P., & Sherwin, T. J. (2000). Impact of nonlinear waves on the dissipation of internal tidal energy at a shelf break. Journal of Geophysical Research, 105, 8687–8705. Jeans, D. R. G., & Sherwin, T. J. (2001). The evolution and energetics of large amplitude non linear internal waves on the Portuguese shelf. Journal of Marine Research, 59, 327–353. Joint, I. E. & Wassmann, P. (Eds) (2001). Lagrangian studies of the Iberian upwelling system. Progress in Oceanography, 51, 215Lebreiro, S. M., Moreno, J. C., McCave, I. N., & Weaver, P. P. E. (1996). Evidence for Heinrich layers off Portugal (Tore Seamount: 39°N,12°W). Marine Geology, 131, 47–56.
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Lohse, L., Helder, W., Epping, E. H. G., & Balzer, W. (1998). Recycling of organic matter along a shelf-slope transect across the N.W. European Continental Margin (Goban Spur). Progress in Oceanography, 42, 77–110. Lopez-Jamar, E., Cal, R. M., Gonzalez, G., Hanson, R. B., Rey, J., Santiago, G., & Tenore, K. R. (1992). Upwelling and outwelling effects on the benthic regime of the continental shelf off Galicia, NW Spain. Journal of Marine Research, 50, 465–488. Maze´ , J. P., Arhan, M., & Mercier, H. (1997). Volume budget of the eastern boundary layer off the Iberian Peninsula. Deep-Sea Research I, 44, 1543–1574. McCave, I. N., Hall, I. R., Antia, A. N., Chou, L., Dehairs, F., Lampitt, R. S., Thomsen, L., van Weering, T. C. E., & Wollast, R. (2001). Sources, composition and flux of suspended particulate material on the European margin 47°–50°N: A synthesis of results from the OMEX I programme. Deep-Sea Research II, 48, 3107–3139. McClain, C. R., Chao, S. Y., Atkinson, L., Blanton, J. O., & De Castillejo, F. (1986). Wind-driven upwelling in the vicinity of Cape Finisterre, Spain. Journal of Geophysical Research C, 91, 8470–8486. Monaco, A., Biscaye, P. & Pocklington, P. (eds) (1990). Particle fluxes and ecosystem response on a continental margin: The FranceJGOFS, ECOMARGE Mediterranean Experiment. Continental Shelf Research, 10, 807–1155. Monteiro, J. H., Dias, J. A., Gaspar, L. C. & Possolo, A. M. (1980). Recent marine sediments of the Portuguese continental shelf. In: Actual problems of oceanography in Portugal., Lisbon, Portugal, pp. 89–96. Pingree, R. D., & LeCann, B. (1992). Three anticyclonic Slope Water Oceanic EDDIES (SWODDIES) in the southern Bay of Biscay in 1990. Deep-Sea Research, 39, 1147–1175. Richardson, P. L., Bower, A. S., & Zenk, W. (2000). A census of Meddies tracked by floats. Progress in Oceanography, 45, 209–250. Thorpe, R. S., & White, M. (1988). A deep intermediate nepheloid layer. Deep-Sea Research, 35, 1665–1671. Thomsen, L., & van Weering, Tj. C. E. (1998). Spatial and temporal variability of particulate matter in the benthic boundary layer at the N.W. European Continental Margin (Goban Spur). Progress in Oceanography, 42, 61–76. van Aken, H. M. (2000a). The hydrography of the mid-latitude Northeast Atlantic Ocean II: The intermediate water masses. DeepSea Research I, 47, 789–824. van Aken, H. M. (2000b). The hydrography of the mid-latitude northeast Atlantic Ocean I: The deep water masses. Deep-Sea Research I, 47, 757–788. Van Weering, T.C.E., McCave, I.N. & Hall, I.R. (eds) (1998a). Ocean Margin Exchange (OMEX I) Benthic Processes Study. Progress in Oceanography, 42, 1-257. Van Weering, T. C. E., Hall, I. R., de Stigter, H., McCave, I. N., & Thomsen, L. (1998b). Recent sediments, sediment accumulation and carbon burial at Goban Spur on W. European Continental Margin, (47°-50°N). Progress in Oceanography, 42, 5–35. Van Weering, T. C. E., De Stigter, H., Balzer, W., Epping, E., Graf, G., Hall, I. R., Helder, W., Khripounoff, A., Lohse, L., McCave, I. N., Thomsen, L. A., & Vangriesheim, A. (2001). Benthic dynamics and carbon fluxes on the NE Atlantic Ocean Margin. DeepSea Research II, 48, 3191–3221. Walsh, J. J., Rowe, G. T., Iverson, R. L., & McRoy, C. P. (1981). Biological export of shelf carbon: a neglected sink of the global CO2 cycle. Nature, London, 291, 196–201. Wollast, R. (1998). Evaluation and comparison of the global carbon cycle in the coastal zone and in the open ocean. In K. H. Brink, & A. R. Robinson (Eds.), The Sea, Vol. 10 (pp. 213–253). New York: Wiley. Wollast, R., & Chou, L. (2001). The carbon cycle at the Ocean Margin in the Northern Gulf of Biscay. Deep-Sea Research II, 48, 3265–3293. Wollast, R., Chou, L., Avril, B. & Huthnance, J. (Eds) (2001). Ocean margin exchange in the Northern Gulf of Biscay: OMEX I. Deep-Sea Research II, 48, 2971–3293.
Editorial
Benthic processes and dynamics at the NW Iberian margin: an introduction 1. Introduction An important part (10–50%) of the global marine primary production occurs at the continental margin, and continental margin sediments play an important role in the cycling of organic matter (Wollast, 1998). Indeed, it has been argued that over 90% of the organic carbon in marine sediments is stored in continental margin settings (Berner, 1982). The upwelling-dominated, narrow Iberian margin was selected as the study area to provide a contrast with the previously studied Goban Spur margin, characterised by a broad shelf and slope without upwelling (van Weering, McCave, & Hall, 1998a; Wollast, Chou, Avril, & Huthnance, 2001). The OMEX I results from the North Biscay (Goban Spur) margin identified no obvious depocentre of organic carbon (Wollast & Chou, 2001). Slightly enhanced remineralisation and accumulation rates, however, do occur at mid slope areas (Thomsen & van Weering, 1998; Lohse, Helder, Epping, & Balzer, 1998; van Weering, Hall, de Stigter, McCave, & Thomsen, 1998b; van Weering, De Stigter, Balzer, Epping, Graf, Hall et al., 2001). On the other hand, the organic carbon required to sustain the benthic fauna on the Goban Spur shelf and upper slope represented a major fraction of the total organic carbon input (Heip, Duineveld, Flach, Graf, Helder, Herman et al., 2001). It was the main objective of the benthic studies included in the EU-funded Ocean Margin Exchange-II project (OMEX II) to contribute to understanding, quantification and modelling of the benthic processes involved in exchange of carbon, elements and energy at the Iberian ocean margin. Comparison and integration of these results and models with results from other well studied European and other continental margins was intended to contribute to the main objective of OMEX II Phase II “to measure and model exchange processes at the ocean margins as a basis for the development of global models to predict the impact of environmental changes on the oceanic system, and more specifically, on the coastal zone”.
2. The Iberian margin The northwestern Iberian margin is characterised by intense, wind-driven seasonal upwelling in summer (Fiu´za, 1983; Fraga, 1981; McClain, Chao, Atkinson, Blanton & De Castillejo, 1986; Frouin, Fiu´za, ˆ mbar & Boyd, 1990) strongly affecting the watermass composition and productivity. Studies of pelagic A productivity of the region were also carried out during OMEX II, including an intensive Lagrangian study of primary and secondary production, sedimentation and physical measurements (Joint & Wassmann, 2001).
∗
Corresponding author. Tel.: +31-222-369395; fax: +31-222-319674.
0079-6611/02/$ - see front matter 2002 Published by Elsevier Science Ltd. PII: S 0 0 7 9 - 6 6 1 1 ( 0 2 ) 0 0 0 0 2 - 2
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Editorial / Progress in Oceanography 52 (2002) 123–128
The increased productivity is clearly reflected in the character and distribution of surface sediments off Galicia (Lopez-Jamar, Cal, Gonzalez, Hanson, Rey, Santiago et al., 1992). At the Iberian shelf south of Vigo some earlier work on shelf sediments and shelf sediment dispersal by bottom currents and resuspension was available (Dias & Nittrouer, 1984; Dias, Pilkey, & Heiweil, 1984). Transport of matter across the slope and the deeper margin sediments, however, were unknown prior to OMEX-II. Water-mass transport and currents beneath the surface layer are directed to the north and a southerly directed surface flow is associated with the upwelling (Maze´ , Arhan, & Mercier, 1997; Fiuza, Hamann, Ambar, del Rio, Gonzalez, & Cabanas, 1998; van Aken, 2000a,b; Huthnance, van Aken, White, Baton, Le Cann, Coelho et al., 2001). Pingree and LeCann (1992) have shown that filaments and patches of the upwelled productive water may be dispersed offshore in a northerly and northwesterly direction by eddies (Richardson, Bower & Zenk, 2000). These tend to recur every year and may extend up to 250 km offshore (Haynes, Barton & Pilling, 1993). The filaments are believed to foster enhanced production and were also the subject of a Lagrangian experiment during OMEX II (Joint & Wassmann, 2001). Internal waves and tides had been recorded at the shelf edge off Iberia (Jeans & Sherwin, 2001), but their effects were unknown compared to the transfer of energy at the slope and continental shelf edge which results in the formation of nepheloid layers and mass particle transport previously noted off Porcupine Bank and at the Goban Spur (Dickson & McCave, 1986; Thorpe & White, 1988; Inall, Rippeth, & Sherwin, 2000; McCave, Hall, Antia, Chou, Dehairs, Lampitt et al., 2001). Upwelling off Portugal is documented in planktic foraminiferal and diatom species found far offshore in the surface sediments (Abrantes, 1988; Lebreiro, Moreno, McCave, & Weaver, 1995), and there are indications that upwelling was more intense during the last glaciation than at present (Abrantes, 1991). However the amount of carbon in surface sediments of the Iberian margin north of 41°N was nearly unknown and the upwelling-induced carbon exchange processes over the margin were not well constrained. The late Quaternary depositional history of the Iberian margin reflects the major climatic zonations of the Iberian peninsula (Monteiro, Dias, Gaspar, & Possolo, 1980), resulting in Holocene margin sediments of predominant biogenic composition and pre-Holocene margin sediments reflecting increased terrigenous input from local and distant sources (Baas, Mienert, Abrantes & Prins, 1996). Again however, sedimentation off the shelf was not well studied. The shelf sediments were reworked during the last low stand of sea level, and present deposition and accumulation of sands and fines on the shelf is controlled by the geomorphology of the seabed (Drago, Oliveira, Magalha˜ es, Cascalho, Jouanneau and Vitorino, 1998). The western Iberian margin is dissected by deep canyons, that cut through the slope and into the shelf edge. The largest canyons (as for example Nazare´ and Setubal) intersect the entire continental shelf, allowing them to intercept sediment transported over the shelf and upper slope, thus providing a direct conduit of particles and matter from the inner shelf to the deep sea. All canyons were probably more active at times of low sea-level than at present, although this also is not well constrained.
3. The benthic context It is still unclear whether or not continental shelves export detritus from primary production to the continental slopes and the open ocean as was suggested by Walsh, Rowe, Iverson and McRoy (1981). However, this is of prime importance for assessing the role of the ocean margins in the global carbon cycle. To do so, extensive knowledge is required of the processes and fluxes involved in particulate matter transport through the water column to the seabed, of the near-bed advective fluxes and transport/settling processes, and of the fluxes and rates of benthic remineralisation processes at the sediment-water interface and in the surface sediments. The benthic community development and structure ultimately is closely linked to the input of fresh food to the sediment. Most field studies so far indicate that assumed vertical sedimentation determined by sediment-trap analy-
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ses does not balance material and carbon budgets, and lateral advection of organic matter must be invoked (Courp & Monaco, 1990; Monaco, Biscaye, & Pocklington, 1990; Biscaye, Flagg, & Falkowski, 1994; Antia, von Bodungen, & Peinert, 1999; Heussner, de Madron, Radakovich, Beaufort, Biscaye, Carbonne et al., 1999; McCave et al., 2001). In particular the benthic boundary layer processes and fluxes involved in particle transport, resuspension, aggregation, scavenging, disaggregation and sediment accumulation and mixing need to be determined, to complement measurements of remineralisation and diagenesis at the sediment-water interface, and burial fluxes. Also, the role of the benthic faunal and microbial community in the benthic carbon cycle needs to be defined and measured, to allow quantification and modelling of the carbon and element fluxes at the ocean margin, and of the temporal and spatial variability involved. The focus of the studies in the Benthic Processes component of OMEX II was on the understanding and quantification of processes, pathways and fluxes of organic and related particles, in the continental margin area off Iberia, characterised by pronounced seasonal upwelling with the overall aim of contributing to the main OMEX I and II objective “to measure and model exchange processes at the ocean margins as a basis for the development of global models to predict the impact of environmental changes on the oceanic system, and more specifically on the coastal zone”.
4. Key questions The key questions addressed during the OMEX II Benthic Process Study were directed to the understanding of: 앫 the role, importance and dynamics of the bottom boundary layer in driving exchanges of particles, carbon and other components between the water column and the sediment in relation to vertical and lateral fluxes—the role of microbial activity in aggregate formation and organic carbon transfer and accumulation. 앫 organic matter diagenetic processes, products and fluxes at the sediment-water interface and in the surface sediments, defining carbon mineralisation. 앫 the relationship between types of food supply and the benthic community structure and distribution. 앫 spatial and temporal variability of processes and products at continental margins. In the OMEX-II Benthic Studies these questions were approached in three main project areas of work. The papers in this volume are grouped below under the project subcomponents (given here in italics), namely: 1. Particle transport, settling, accumulation, mixing and burial fluxes. Temporal and spatial variability. a) Amount, character, distribution and composition of suspended particles in nepheloid and clear water layers. In this area, the papers of Oliveira et al. and McCave & Hall, together with the published work of Hall, Schmidt, McCave and Reyss (2000) deal with shelf and slope nepheloid layers. Bottom nepheloid layers over the deeper slope and in the Nazare´ canyon are discussed by van Weering et al. b) Spatial and temporal variability of the benthic boundary layer dynamics. Extensive work on benthic dynamics is reported here by Vitorino et al., van Weering et al., Thomsen et al., Huthnance et al. and Davies et al. discuss near bed particle transport. c) Particle fluxes to the seabed, accumulation and mixing rates. Work on fluxes estimated from sediment traps have been reported by Antia et al. (pers. com., 2000), and sediment accumulation rates on the shelf and slope are reported here by Dias et al. (Present day sedimentary processes…), Jouanneau
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et al., and van Weering et al. Short term particle residence times in surface sediments and in the Nazare´ canyon is presented by Schmidt et al. d) Sediment distribution, properties and composition along selected transects. The detailed mapping of shelf sediment distribution is reported by Dias et al. for the Galicia– Minho sector off northern Portugal, its mineralogy and chemistry by Oliveira et al. and Araujo et al., and a slope to rise transect by van Weering et al. e) Modelling near bed-transport and budgets. These aspects are covered by Jouanneau et al. and Davies et al. Having studied only a limited area offshore we do not have a good hold on the sediment budget for the whole slope yet. f) Long term change. Hall and McCave (2000) examined the current control of past slope sedimentation and the PhD thesis of Lucia De Abreu (2000) and De Abreu, McCave, Hall, Higginson, & Thomson (2000) on palaeoproductivity of the northern Iberian margin used OMEX-II cores. 2. Sediment /water exchange processes and early diagenesis, and 3. Role and importance of the benthic community. a) Sediment-water exchange processes and organic matter diagenesis, carbon mineralisation, respiration, and burial is covered by Epping et al., who deal with the central problem of carbon oxidation and burial both on the slope transect and down Nazare´ Canyon. b) Role and importance of bioentrainment and biodeposition. Changes occurring in the boundary layer are examined by Thomsen et al. and interface processes are probed using radionuclide tracers by Schmidt et al. c) The Benthic community structure is treated by Flach et al. and the related area of d) Variability of benthic community structure and functioning at contrasting margins is covered, at least for the megafauna, by the contribution of Lavaleye et al. The papers combined within this issue thus provide a first, comprehensive overview of recent benthic processes acting upon the Iberian margin system, and of the associated particle and carbon fluxes over the margin, inclusive of aspects of the Nazare´ canyon system.
Acknowledgements The guest editors express their thanks to all reviewers for their willingness to critically read and comment upon the manuscripts submitted. We also wish to express our thanks to Dr M. Angel (SOC) and A. Bayfield (PIO) for their constructive support during the editing. K-G. Barthel (EU-DG XII) is thanked for his continuous interest in and support for the OMEX-II Program. The front cover was drawn by Henk de Haas (NIOZ), which we gratefully acknowledge.
b
T.C.E. van Weeringa I.N. McCaveb a Netherlands Institute for Sea Research (NIOZ), P.O. Box 59, 1790 AB Den Burg, Texel, The Netherlands E-mail address: [email protected] (T.C.E. van Weering) Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, UK
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