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Article Dans Une Revue Progress in Oceanography Année : 2002

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

Résumé

Seabed distributions of $^{234}$Th excess (Thxs) were determined in the upper centimetres of 38 sediment cores from the north-western Iberian Margin, sampled from 41–44°N and from 9–12°E during five OMEX II cruises. Three main areas, a northern, and at 42°38 and 42°N, were investigated during representative seasons (winter, spring and summer). Low $^{234}$Th$_{xs}$ activities in summer 1998 (18–252 Bq per kg) were similar to those measured in summer 1997. In winter $^{234}$Th also showed moderate excess. The highest values were observed in spring with surface $^{234}$Th$_{xs}$ values up to 402 Bq kg$^{−1}$. Maximum penetration depths of $^{234}$Th$_{xs}$ ranged from a few mm to 3 cm. $^{234}$Th$_{xs}$ activities always showed a smooth decrease with depth, without any 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 cm2 per year. Data (activities, inventories, bioturbation rates) are discussed in order to relate the observed surface and down-core variations to spatial and seasonal trends. Using $^{234}$Th$_{xs}$ data in sediment as a substitute for sediment trap estimates, particle fluxes were calculated from $^{234}$Th$_{xs}$ inventories. The range of $^{234}$Th-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 (43–44°N) represents a low sedimentation regime. Further south, around 42°–43°N, particle inputs are more important. On the middle slope, around 1000 to 2000 m depth, high inventories and bioturbation rates indicate enhanced, and probably organic-rich, particle fluxes to the seafloor, particularly in spring.

Dates et versions

hal-03127235 , version 1 (28-03-2024)

Identifiants

Citer

S. Schmidt, Tj.C.E. van Weering, J.-L. Reyss, P. van Beek. Seasonal deposition and reworking at the sediment-water interface on the northwestern Iberian margin. Progress in Oceanography, 2002, 52 (2-4), pp.331-348. ⟨10.1016/S0079-6611(02)00014-9⟩. ⟨hal-03127235⟩
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