Material supply to the abyssal seafloor in the Northeast Atlantic

Downward particle flux was measured using sediment traps at various depths over the Porcupine Abyssal Plain (water depth similar to 4850 m) for prolon ged periods from 1989 to 1999. A strong seasonal pattern of flux was eviden t reaching a maximum in mid-summer. The composition of the material changed with depth, reflecting the processes of remineralisation and dissolution a s the material sank through the water column. However, there was surprising ly little seasonal variation in its composition to reflect changes in the b iology of the cuphotic zone. Currents at the site have a strong tidal component with speeds almost alway s less than 15 cm/sec. In the deeper part of the water column they tend to be northerly in direction, when averaged over periods of several months. A model of upper ocean biogeochemistry forced by meteorology was run for th e decade in order to provide an estimate of flux at 3000 m depth. Agreement with measured organic carbon flux is good, both in terms of the timings of the annual peaks and in the integrated annual flux. Interannual variations in the integrated flux are of similar magnitude for both the model output and sediment trap measurements, but there is no significant relationship be tween these two sets of estimates. No long-term trend in flux is evident, e ither from the model, or from the measurements. During two spring/summer periods, the marine snow concentration in the wate r column was assessed by time-lapse photography and showed a strong peak at the start of the downward pulse of material at 3000 m. This emphasises the importance of large particles during periods of maximum flux and at the st art of flux peaks. Time lapse photographs of the seabed show a seasonal cyc le of coverage of phytodetrital material, in agreement with the model outpu t both in terms of timing and magnitude of coverage prior to 1996. However, after a change in the structure of the benthic community in 1996 no phytod etritus was evident on the seabed. The model output shows only a single peak in flux each year, whereas the me asured data usually indicated a double peak. It is concluded that the obser ved double peak may be a reflection of lowered sediment trap efficiency whe n flux is very high and is dominated by large marine snow particles. Resuspension into the trap 100 rn above the seabed, when compared to the pr imary flux at 3000 m depth (1800 mab) was lower during periods of high prim ary flux probably because of a reduction in the height of resuspension when the material is fresh. At 2 mab, the picture is more complex with resuspen sion being enhanced during the periods of higher flux in 1997, which is con sistent with this hypothesis. However there was rather little relationship to flux at 3000 m in 1998. At 3000 m depth, the Flux Stability Index (FSI), which provides a measure o f the constancy of the seasonal cycle of flux, exhibited an inverse relatio nship with flux, such that the highest flux of organic carbon was recorded during the year with the greatest seasonal variation. (C) 2001 Elsevier Sci ence Ltd. All rights reserved.