Distribution, composition and flux of particulate material over the European margin at 47 degrees-50 degrees N

In the framework of the Ocean Margin Exchange project, a multi-disciplinary study has been conducted at the shelf edge and slope of the Goban Spur in order to determine the spatial distribution, quantity and quality of partic le flux, and delineate the transport mechanisms of the major organic and in organic components. We present here a synthesis view of the major transport modes of both biogenic and lithogenic material being delivered to the open slope of the Goban Spur. We attempt to differentiate between the direct bi ogenic flux from the surface mixed layer and the advective component, both biogenic and lithogenic. Long-term moorings, instrumented with sediment traps, current meters and tr ansmissometers have yielded samples and near-continuous recordings of hydro graphic variables (current direction and speed, temperature and salinity) a nd light transmission for a period of 2.5 years. Numerous stations have bee n occupied for CTD casts with light transmission and collection of water sa mples. The sedimenting material has been analysed for a variety of marker c ompounds including phytoplankton pigments, isotopic, biomineral and trace m etal composition and microscopical analyses. These samples are augmented by seasonal information on the distribution and composition of fine particles and marine snow in the water column. The slope shows well-developed bottom nepheloid layers always present and i ntermediate nepheloid layers intermittently present. Concentrations are mai nly in the range 50-130 mg m (-3) in nepheloid layers and 6-25 mg m (-3) in clear water. A seasonal variability in the concentration at the clear wate r minimum is argued to be related to seasonal variations in vertical flux a nd aggregate break-up in transit during summer months. It is suggested that the winter sink for this seasonal change in particulate matter involves so me reaggregation and scavenging, and some conversion of particulate to diss olved organic matter. This may provide a slow seasonal pump of dissolved or ganic carbon to the deep ocean interior. Differences in trapped quantities at different water depths are interpreted as due to lateral flux from the c ontinental margin. There is a major lateral input between 600 and 1050 m at an inner station and between 600 and 1440 m at an outer one. The transport is thought to be related to intermediate nepheloid layers, but those measu red are too dilute to be able to supply the flux. Observed bottom nepheloid layers are highly concentrated very close to the bed (up to 5 g m (-3)), w ith a population of large aggregates. Some of these are capable of deliveri ng the flux seen offshore during intermittent detachment of nepheloid layer s into mid-water. Concentrated bottom nepheloid layers are also able to del iver large particles with unstable phytoplankton pigments to the deep sea f loor in a few tens of days. Calculated CaCO3 fluxes are adjusted for dissol ution, which is inferred from Ca/Al ratios to be occurring in the CaCO3-sat urated upper water column where up to 80% of the CaCO3 resulting from prima ry production is dissolved. (C) 2001 Elsevier Science Ltd. All rights reser ved.