Particulate matter distribution and 234Th/238U disequilibrium along the Northern Iberian Margin:: implications for particulate organic carbon export

Light beam attenuation and scattering profiles were obtained from a total of 94 stations from the shelf edge and slope of the Iberian Margin in the early summer of 1997 and winter of 1998. Hydrographic and satellite data suggest that the summer cruise took place just prior to the seasonal upwelling period present along this margin. Our results from the slope show well-developed, high-concentration, intermediate nepheloid layers (INL) present in North Atlantic Central Water, underlain by relatively clear Mediterranean Water, which, at its salinity maximum, shows an off-shelf decrease in particle load to clear water at the outer edge of the water mass. Only low-concentration INL and weak bottom nepheloid layer (BNL) activity is observed in North Atlantic Deep Water. The presence of low BNL activity suggests weak off-shelf transport of material and weak bottom current resuspension, Good agreement between particulate matter concentration estimated from the empirical calibration of beam attenuation and scattering is found in clear water, while differences in instrumental response within the BNL and surface layer are most likely related to the differing nature of the particles present. In the upper ocean particle load calculated from beam attenuation shows a considerable excess over that calculated by scattering. Offshore spatial and temporal differences in the relationship between both measurements are consistent with a response to large particle sized biological material present in the surface layer. The upper ocean(234)Th activity was measured at a total of 23 stations and particulate organic carbon (POC) and particulate organic nitrogen (PON) at each station occupied during the summer cruise. The broadly uniform distribution of total(234)Th, and more variable particulate(234)Th, suggests that during pre-upwelling conditions on the Iberian Margin,Th-234 distribution is strongly influenced by particle type. As a consequence, particle residence times derived from Th-234/U-238 disequilibria show a considerable range between 4 and 193 d. Estimates of export flux of POC and PON from the upper 50 m were made using the Th-234, POC and PON results. POC and PON export ranged from 1-56 mmol C m(-2) d(-1) and 0.15-7.7 mmol N m(-2) d(-1) respectively, showing a broad offshore decrease. Comparison of the POC export with estimates of carbon fixation measured during the same period suggests that an average of 19% of the primary production is lost as a sinking particulate flux. Overall, the pre-upwelling situation observed during CD-105 was characterised by low phytoplankton biomass, primary production and low POC export fluxes. (C) 2000 Elsevier Science Ltd. All rights reserved.