Seasonal deposition and reworking at the sediment-water interface on the northwestern Iberian margin

Seabed distributions of Th-234 excess (Th-xs) were determined in the upper centimetres of 38 sediment cores front the north-western Iberian Margin. sampled from 41-44degreesN and from 9-12degreesE during five OMEX 11 cruises. Three main areas, a northern, and at 42degrees38 and 42degreesN. were investigated during representative seasons (winter. spring and summer). Low Th-234(xs) activities in summer 1998 (18-252 Bq per kg) were similar to those measured in summer 1997. In winter Th-234 also showed moderate excess. The highest values were observed in spring with surface Th-234(xs) values Up to 402 Bq kg(-1). Maximum penetration depths of Th-234(xs) ranged from a few mm to 3 cm. Th-234(xs) activities always showed a smooth decrease with depth. without an evidence of non-local mixing. Thus particle mixing, on a short time scale can be described as an eddy diffusive process. and bioturbation rates. calculated on this basis, range from 0.02 to 3.07 cm(2) per year. Data (activities. inventories. bioturbation rates) are discussed in order to relate the observed surface and downcore variations to spatial and seasonal trends. Using Th-234(xs) data in sediment as a substitute for sediment trap estimates. particle fluxes were calculated from Th-234(xs) inventories. The range of Th-234-derived particle fluxes for the north-western Iberian Margin is 16-1418 mg.m(-2).d(-1). Mean values indicate a gradual decrease of mass fluxes from the shelf to the open ocean. On a 100-day scale. the northern area (43-44degreesN) represents a low, sedimentation regime. Further south. around 42degrees-43degreesN. particle inputs are more important. On the middle slope. around 1000 to 2000 in depths high inventories and bioturbation rates indicate enhanced. and probably organic-rich. particle fluxes to the sedfloor. particularly in spring. (C) 2002 Elsevier Science Ltd. All rights reserved.